# REWIRING A MOT (Microwave Oven Transformer)



## petcnc

A friend of mine seeks a Power Supply Unit (PSU) capable of 13.8V at 50 Amps. As the ready-made ones are a) rather expensive and b) unreliable (Chinese)  he asked if I could make one for him.

I love challenges and I could not resist this one!  So I sought information on Low voltage High current PSUs.
I found out that the most expensive part to make a high current PSU is the transformer as you need at least 800Watt power out of it! Thank God there are plenty of 800 watt transformers available for free in old microwave ovens. In a day I found a microwave unit with an as-good-as-new transformer.




These transformers have a 230V (Europe) thick wire primary and 2000V thin wire secondary to produce the high voltage needed for the microwave magnetron!

As I only need the primary I cut and removed the secondary using my hammer and an old wood chisel.







After removing the secondary wires (hammering them out of the transformer) I had a transformer body with a primary winding ready to accept my new High Current-Low voltage secondary.




I started by winding 5 turns of an insulated piece of wire for a secondary and after connecting the primary to mains I measured the voltage it gives. It gave 4.5V AC. That means my transformer’s secondary winding gives 0.9 Volts per turn.




Then it was easy to calculate the number of windings for the needed voltage.
I needed a voltage around 16 Volts (to have a margin for rectification diode drops) so 17 to 18 turns were enough.

*Next thing was the calculation of wire thickness.*

I need a wire capable of *60 Amps* to play safe, so I consulted the AWG tables to have an idea of proper thicknesses.




The closest wire, suitable for my needs, was a 7core 2 AWG (6,5mm) capable of 66A. With 1mm insulation the wire was 8.5mm diameter, too thick to be of use for the transformer I had!




It would have space just for 5 turns giving, as per test wires,  5 x 0,9 = 4,5 Volts!!!!

*A different approach needed that:*

1 would make use of all available space without any air gaps
2. give me the 17 to 18 turns I need for the voltage
3. make use of material I already have

I have inherited a few copper sheets 200 x 60 cm and 0.6mm thick that collect dust for more than 30 years in the basement. So I calculated if strips from the copper sheets could be used to rewind the secondary.




Some calculations later, I estimated that the 0,6mm thickness strip will be more than adequate for my needs, As:
a)    Fits in the required space




b) Is capable to withstand some 100Amps (26.5 x 0.6 = 15.9 mm^2, better than 1 core 6AWG as you can see below)




Apart from the copper strips I needed some heat resisting insulation to put between the strips. I found the best insulating material in my wife's kitchen. It is a roll of “silicon impregnated non stick oven paper” (I don’t know the proper english name of it)!




It withstands temperatures up to 220 C, it is very strong although it has a thickness of 0.04mm, so I will put 4 layers of it just to play safe.




Next step was to cut the foil in 27mm strips. That was done with some help from my trusty mini lathe, a steel core and a razor blade.











*How much length of copper I needed?*

Calculating the core of the transformer and adding the thickness of each strip together with its insulation made a nice spreadsheet with all the details.




To cut the copper strips I used an old drill powered sheet metal cutter.
Not the best way to do it but it did the job with just a few areas to file.




Two strips 2m x 26.5 mm after an hour of measuring, marking and cutting were lying on the floor.




The right angle extension marks the starting point of the “wire” to make the connections later.




Rewinding was a matter of careful folding the copper strip over the 4 insulation strips around the center of the transformer.




Wooden wedges helped pressing the sides to keep the windings from unfolding





An hour or so later the first 2 meters strip was in place.




Time to folder and solder the next one.




Another hour or so the rewinding finished, new windings were tested for continuity, short circuits etc. and as the results were fine, Coil winding “cool cure” varnish poured throughout the windings and the transformer was ready.

Transformer back side after varnishing.




Transformer front side. You can see start and finish ends of the secondary high current coil.
At the bottom the 2 wires that go to mains, to test the transformer




Testing the transformer a few hours later. All is OK.
15.63 Volts AC




Plenty of Current also (estimated around 50 Amps)!
Unfortunately my 100A amp meter is on its way from far east and I do not have the equipment to measure the amps produced.
A 40 amps circuit breaker connected to the secondary, goes off instantly while I short the low voltage circuit through it.




Waiting for the rest of the parts to arrive from China to construct the PSU I will leave the transformer in peace for the next weeks!

Thanks for reading.

Petros


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## strantor

I love it! Very meticulous and the copper strip was a great idea to maximize space utilization. Wish I had some strip like that laying around.
 I have a bit of a MOT fetish myself. I will pick up a microwave if spotted at a garage sale for <$15 or so. 
You might be interested in my last MOT project which I posted about before: Show Us your Welders!


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## brino

Nice "roll your own" transformer! 
That copper sheet is a great solution to fitting the required secondary winding.
Great work.
-brino


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## petcnc

strantor said:


> I love it! Very meticulous and the copper strip was a great idea to maximize space utilization. Wish I had some strip like that laying around.
> I have a bit of a MOT fetish myself. I will pick up a microwave if spotted at a garage sale for <$15 or so.
> You might be interested in my last MOT project which I posted about before: Show Us your Welders!



Very nice welder you have got there! 3 MOTs together! I like it


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## brino

It occurred to me that this would be great for a number of other projects too, like a high-current automotive battery charger.

I'm going to have to grab a couple more microwave ovens.......

-brino


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## petcnc

brino said:


> It occurred to me that this would be great for a number of other projects too, like a high-current automotive battery charger.
> 
> I'm going to have to grab a couple more microwave ovens.......
> 
> -brino



I'm not sure you need that high amperage to charge a car battery...
On the other hand it will fully charge a 100AH battery in 2 hours! But is it good for the battery?

You also need High Current secondary. The idea to use copper strips for the secondary came to me as I was looking on how to make a 2 roller system to compress a round wire flat so it could fit the transformer!!!!!


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## ELHEAD

Impressive. Great idea. Nothing like thinking outside the box.


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## RJSakowski

A nice job!  I have fought with winding high current windings in the past and this looks like a much easier approach.  I have some aluminum sheet about 10" wide which was a winding in a distribution transformer using the same design as yours, 1mm x 200mm sheet.  It looks like you have about 1.5 milliohms/m of resistance or about 6 milliohms total resistance so you should be seeing a voltage drop of about .3 volts @ 50 amps which is very respectable.


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## petcnc

RJSakowski said:


> A nice job!  I have fought with winding high current windings in the past and this looks like a much easier approach.  I have some aluminum sheet about 10" wide which was a winding in a distribution transformer using the same design as yours, 1mm x 200mm sheet.  It looks like you have about 1.5 milliohms/m of resistance or about 6 milliohms total resistance so you should be seeing a voltage drop of about .3 volts @ 50 amps which is very respectable.



RJ I noticed that the secondary winding of the transformer were not copper but some silver coloured wire, most likely aluminum.

I suspect it is doable with aluminum sheet but I have no info on resistances and magneticaly induced currents on aluminum.
On the other hand I have no aluminum sheets available to test it!
It would be interesting to have some data on an approach like this using aluminum strips.

As for resistance measurements, I measured a total resistance of 0.2 ohms for all 4.5 meters of copper strip. 
I'm sure I do not have the proper equipment to give accurate results on low resistance measurements so I mention it as a rough measurement only.


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## petcnc

Thats the secondary I removed from the MOT




Copper like paint outside but aluminum inside?


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## strantor

Aluminum windings are common. After seeing what you did with the copper strip I had my own idea to do it common kitchen aluminum foil. Several strip in parallel per turn, to achieve the thickness of your copper strip. It would have slightly higher resistance than your copper, assuming it's actually just aluminum and not something else.


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## RJSakowski

strantor said:


> Aluminum windings are common. After seeing what you did with the copper strip I had my own idea to do it common kitchen aluminum foil. Several strip in parallel per turn, to achieve the thickness of your copper strip. It would have slightly higher resistance than your copper, assuming it's actually just aluminum and not something else.


Keep in mind that the resistivity of aluminum is about 50% higher than for copper.  You would want to go down two wire gauges if you substituted aluminum for copper.  Aluminum foils is quite thin, around 1 mil.  To get equivalent current capability to Pete's .6mm copper, you would need somewhere around 40 layers of foil.  

If it were me, I would use aluminum flashing from the DIY.  It is about 8 - 10 mils thick and comes in rolls up to 50' long and in widths of 6" and 12" and is fairly reasonably priced.  Actually less expensive than aluminum foil, pound for pound.  I would pop it in the oven on high for a half an hour to anneal it.  It takes around 650 -700ºF to anneal.

Copper flashing is also available from the same sources.


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## RJSakowski

Digital multimeters are notoriously bad at measuring low resistances.  Without special instruments, the best way to get accurate low resistance measurements is to apply a known current and measure voltage.  I will apply something like 12 volts to a series string of a precision resistor and the unknown resistance and measure the voltage drop across the known resistor to calculate the current and measure the voltage drop across the unknown resistance.  The current is Vprec./Rprec. and the unknown resistance is Vx x Rprec./Vprec. I would use a known resistance of something like 1 ohm so the current is equal to the voltage drop. I have a number of 50 watt  1% resistors which will take that kind of current overload for a short period.  Even the low cost multimeters do a fairly good job of measuring millivolts.


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## strantor

RJSakowski said:


> Digital multimeters are notoriously bad at measuring low resistances.  Without special instruments, the best way to get accurate low resistance measurements is to apply a known current and measure voltage. ...  Even the low cost multimeters do a fairly good job of measuring millivolts.


Yes this is a really good way and I have done it also. DMM measures Ohms with .1Ω resolution, but measures millivolts to .001mV (.000001V) resolution, where with some 3rd grade math you can turn it into a .000001Ω value. If you do it often enough to where keeping up with a separate corded power supply and meter is cumbersome, you can get a cheapo milliohm meter. I have this one and its not the best but it's worth $82. Comes with kelvin clips and all. It's a chinese clone of a $600 meter


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## MetalMuncher

Interesting project!


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## hman

Petros, that's an absolutely wonderful and innovative solution you've come up with.  Congratulations!


RJSakowski said:


> Digital multimeters are notoriously bad at measuring low resistances.  Without special instruments, the best way to get accurate low resistance measurements is to apply a known current and measure voltage.  I will apply something like 12 volts to a series string of a precision resistor and the unknown resistance and measure the voltage drop across the known resistor to calculate the current and measure the voltage drop across the unknown resistance.  The current is Vprec./Rprec. and the unknown resistance is Vx x Rprec./Vprec. I would use a known resistance of something like 1 ohm so the current is equal to the voltage drop. I have a number of 50 watt  1% resistors which will take that kind of current overload for a short period.  Even the low cost multimeters do a fairly good job of measuring millivolts.


Rick Sparber (a member of this forum) has done quite a bit of R&D on measuring low resistances and small changes in near-zero resistance, in connection with his design of touch-down probes for machine tools.  He's also come up with a relatively simple stand-alone milliohm meter.  It's based on a "free" Harbor Freight multimeter and a quad op amp.  Here's a link to his article:
http://rick.sparber.org/electronics/ramp.pdf


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## petcnc

hman said:


> Petros, that's an absolutely wonderful and innovative solution you've come up with.  Congratulations!
> 
> Rick Sparber (a member of this forum) has done quite a bit of R&D on measuring low resistances and small changes in near-zero resistance, in connection with his design of touch-down probes for machine tools.  He's also come up with a relatively simple stand-alone milliohm meter.  It's based on a "free" Harbor Freight multimeter and a quad op amp.  Here's a link to his article:
> http://rick.sparber.org/electronics/ramp.pdf



Thank you for your kind words!
Rick's article is very impressive! It goes to the "to do list".
Petros


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## gonzo

*If you are able to save the secondary you can make a great de-magnetizer out of it.
I use about 24 volts AC on it but it will probably take much more.
Just slip the magnetized object through the center. *


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## RJSakowski

I just began rewinding a salvaged microwave transformer.  Unfortunately, I am looking for 100 volts d.c. and figure that I will need about 80 turns.  I am looking to have a a 1 KW transformer so I am winding with 12 AWG wire.  Magnet wire is scarce as hen's teeth so I opted for using THHN plenum wire.  It has two layers of insulation and a diameter of about 3mm.  If I can keep the windings tight, I can get about 90 turns.  I am about 10% into the winding.


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## RJSakowski

As a rule of thumb, 10 AWG has an approximate resistance of 1 milli-ohm/ft.  Resistance doubles for every three wire gauges so 13 AWG will have a resistance of 2 milli-ohms/ft and 7 AWG will have have a resistance of .5 milli-ohms/ft.  Current carrying capacity will double when the resistance halves and vice versa.


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## petcnc

gonzo said:


> *If you are able to save the secondary you can make a great de-magnetizer out of it.
> I use about 24 volts AC on it but it will probably take much more.
> Just slip the magnetized object through the center. *



Nice idea! To do that one has to dismantle the laminated core of the transformer. In my case, as you can see below, I must use a grinder to cut the weldings, remove the bottom part and then I have to reweld it. I'm not sure that such a surgery it will not affect the behaviour of the core!
If you have no intention in reusing the core it's easy to do it.


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## petcnc

UPDATE
After months of use (delivering some 40 Amps)  the transformer works as designed without any problems of overheating!


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## middle.road

Good read over a cuppa java. Enjoy the resourcefulness. 
/me wishes that I fully understood electricity.


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## petcnc

middle.road said:


> Good read over a cuppa java. Enjoy the resourcefulness.
> /me wishes that I fully understood electricity.



He he he Electronics is easy if you relate it to some things you know!
For us the mechanically minded is easy to relate to ...water




This comes from an excellent book I have bought some 30 years ago!



Petros


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## easttex

petcnc said:


> middle.road said:
> 
> 
> 
> Good read over a cuppa java. Enjoy the resourcefulness.
> /me wishes that I fully understood electricity.
> 
> 
> 
> 
> He he he Electronics is easy if you relate it to some things you know!
> For us the mechanically minded is easy to relate to ...water
> 
> View attachment 253329
> 
> 
> This comes from an excellent book I have bought some 30 years ago!
> 
> View attachment 253330
> 
> Petros
Click to expand...

Coincidentally enough, I just bought that book at a used book shop. I bought all Forrest Mims' mini notebooks back in the 90's but did not buy this one. I am happy to have most of his books now.

Sent from my SM-G950U using Tapatalk


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## middle.road

Guess I now know what I am going to keep my eyes peeled for!


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## Tony Pisano

Around here  old microwaves for free for the asking. I posted on facebook that I was looking for one and had 3 offers in a matter of hours.


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## strantor

Tony Pisano said:


> Around here  old microwaves for free for the asking. I posted on facebook that I was looking for one and had 3 offers in a matter of hours.


Enjoy it while it lasts. I've been a microwave hound for years, wishing death on my friends' & family's microwaves so that I can harvest the precious transformer bounty within. But lately I've been getting "inverter" microwaves which have no 60hz transformer, and value to me.

Maybe I need to adapt and start building high power switching power supplies from the new inverter microwaves. Adapt or die!


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## Bi11Hudson

Really behind the "eight ball" on this one. But it's still active, so I might add some comments~~~

First off, the flat *copper sheet stock is a very elegant* solution. I love it. Having built several "home made" transformers for my models, the wiring was small enough that everything fit so I never pursued things any further in that direction. I may build a couple just to play with the idea. I am "old school" in regards to transformers. I use "fische paper" anywhere something touches the core, the "figure 8" iron in the middle.

Comments now; first off: the insulation paper referred to is called "parchment" paper. Wife has several rolls in the kitchen which may well soon become (several -- 1). When warm weather comes back, it's too cold right now. Since the copper foil is so thin, there may well be a couple or three layers per turn. With the parchment paper between turns.

When I make the wraps on the core, I use a wooden "guide" on each end. A piece of dowel rod split down the middle and glued half on each side. This avoids sharp corners on the first layer of "whatever" copper is used, the most likely place for insulation to fail. I usually make what I need on the wood lathe but the basic idea is to split a piece of dowel.

The other comment is to try a copper foil from a surplus house. I use Marlin P Jones, there are many others. The foil is available in several widths, specifically one inch and half inch. Well, just under that at 25mm and 12mm. My biggest concern is low voltage at a moderately low current, just a few amps. But I need several sources that are electrically isolated. To isolate them magneticaly is acceptable, so long as there is no electrical connection. 

Using the foil strips would allow several secondary windings to be wound on a single core. Using the narrow, 12mm, strips would allow me to "stack" more windings on a single core. The end result being one transformer with multiple windings. Instead of multiple transformers having one with multiple windings. Wiring is easy to dress, the cores not so much.

The theory, in my mind anyway, is to take two strips and stick back to back. The parchment paper for insulation on each turn. The 12mm(1/2") strip would make two coils, one on top of the other. Then two more above. All on one core~~~

That's my quarter's worth, here's hoping someone can use the information.

Bill Hudson​


			Electronic Components and Accessories | MPJA.COM
		

.


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## petcnc

Thank you Bill! Valuable information!
Petros


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## Proximus

Thank you for the great guide!

I'm planning to build a 13.8V HAM PSU as well, but I have few things to consider. If I use 0,8mm copper strips for secondary, I'l be probable be limited to 17 rounds. If I get for example 14,7V AC out, would this be enough for 13.8V after bridge rectifier & smoothing capacitors (14.7-1.2)*1,41= abt. 19V. Will the bridge rectifier drop voltage more on high currents? Can I use even less turns on secondary in order to keep temps down? Will this affect the capacitor specs (referring your schematics)? Will leaving shunts in lower temps, because 20mm*0,8mm secondary is capable of handling abt. 100A, so I don't necessarily need to remove them. 

Keep up the good work and thanks in advance,
Pena OH6PP


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## brino

@Proximus 

Welcome to the group!

Have you seen great power supply thread also from Petros?
It is here:
https://www.hobby-machinist.com/threads/a-complete-13-8-volt-50-amp-power-supply.63178/post-520580

-brino


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## petcnc

Proximus said:


> Thank you for the great guide!
> 
> I'm planning to build a 13.8V HAM PSU as well, but I have few things to consider. If I use 0,8mm copper strips for secondary, I'l be probable be limited to 17 rounds. If I get for example 14,7V AC out, would this be enough for 13.8V after bridge rectifier & smoothing capacitors (14.7-1.2)*1,41= abt. 19V. Will the bridge rectifier drop voltage more on high currents? Can I use even less turns on secondary in order to keep temps down? Will this affect the capacitor specs (referring your schematics)? Will leaving shunts in lower temps, because 20mm*0,8mm secondary is capable of handling abt. 100A, so I don't necessarily need to remove them.
> 
> Keep up the good work and thanks in advance,
> Pena OH6PP


Dear Pena,
As far as I know when you rectify the AC to DC the voltage you get is (DC voltage-diode voltage drop)*1,41. In full bridge rectifier  the current flow is through two diodes in series for both polarities. Thus, two diode drops of the source voltage are lost (0,7*2=1,4 V for Si regardless of the current) in the diodes.
In your case is (14,7-1,4)*1,41=18,75. I think you are good to go! 
If you use less turns you will reduce heat but you might have lower volts/instability in power fluctuations
Petros


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## Bi11Hudson

Sir;
I use several microwave transformers as auxillary power supplies for my model trains. Voltages range from 8-24 volts. I don't know how much current one can draw, I've never overloaded one. I used a "fixed" load and a Variac to load the primary to 7 amps. Which is as high as my Variac will handle. None has ever heated up beyond ambient.(room temp ~80 deg) For low voltage, nominal 12 V-AC, 7 amps on the primary is 70 amps on the secondary. I don't work with those current levels so it is a non-issue. I do produce a "welding current" at a low voltage, ~200 Amps at 2.5 volts. Primary current is well under the 7 Amp limit. This tested with a clamp on ammeter into a bolted short, not lab instruments.

I use the primary winding intact, removing only the secondary. Using AWG 10 salvaged from my house, insulation class curioslly THHN. I make my own secondaries. The AWG 10 is rated at 30 Amps inside a cable. I think probably 35-40 amps free air exposure. I use high current, high voltage bridge rectifiers because they are cheap. For a linear supply, 60 Hz, the 'eyeball' capacitor is 10K uF per amp for <2% ripple. You may can use less, it's a matter of noise in the load.

The rectifier drops <1.5 volts, about ~1.4. At the line load, 1.5 is close enough. This drop is the "Fermi" level for silicon diodes and is constant until the diode is overloaded. That's another reason to use 40 Amp 600 Volt bridges.

It isn't absolutely necessary, but I highly recommend using a dowel that has been split for each side. This to avoid a sharp bend as the wire comes around the core, giving it a radiused bend. I use "fiche paper" (pronounced fish paper) as an insulator. That is from my electrical background, it need only be a good insulator. Even shoe box cardboard will work, as long as the transformer doesn't run hot.

Adjusting final AC voltage can be varied by adding or subtracting a "half turn", where the wires come out the same side or opposite sides. Closer adjustment is a matter of rewinding the primary which I don't bother with. Final DC voltage can be lowered 0.7 volts by adding a diode in series with the load. 0.7 volts per diode. . . More than 3 or 4 diodes gets cumbersome though.

Most microwave transformers use aluminium wire. The secondary is about useless, just cut it free. The primary is run intact, watch for heat, backing off the load if necessary. I don't like aluminium wire, obviously. It is much less conductive than copper.

.


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## Proximus

Thanks a lot for replies!



petcnc said:


> In your case is (14,7-1,4)*1,41=18,75. I think you are good to go!


Great, so I'll stick at 17 turns, if I get the same voltage/turn.



Bi11Hudson said:


> For a linear supply, 60 Hz, the 'eyeball' capacitor is 10K uF per amp for <2% ripple.



In Finland mains is 230 V ± 10 %, 50Hz, so do I need more capacitance for smoothing? I'd like to have low ripple, so would it be a good idea to invest on this capacitor? I'm not sure what ripple current mean in the specifications, but I think that's probably what I am looking for...

Cheers,
Pena OH6PP


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## B2

Great project, pictures and analysis!

Minor point:  Is it not 14.7*1.41-1.4~19.4Vdc?   I don't think we multiply the diode drop by the root 2 factor.     14.7 Vac (rms measurement).  1.414=root(2) to convert to Vdc (0 to peak voltage).  ~1.4=two diode drops for a full wave rectifier (first order model of a high current Si diode).  

Of course there are also losses due to the copper coil resistance which will lower the output voltage.  If the magnetic core approaches saturation then the Vac is actually not a sine wave so rms is inaccurate as it becomes distorted as the magnetization response is not linear. Of course the magnetic state is a function of the current being pulled so these effects will be load dependent.  The output will be lower when 50 amps are pulled rather than what it will be when 5 amps is all that is being used.  So even if we use a true RMS meter we would still need to measure the transformer output voltage while the full 50 amp load is connected.  

However, this 50Amp PS is going to have a lot of 120Hz ripple on the output even if it is going on to a large filtering capacitor.  I do not know a lot about HAM units, but is this ripple not a problem or is so far down in frequency compared to the modulation frequency that it never shows up? Of course if one is just charging a lead acid battery who cares.   So good, low ripple power supplies start out higher voltage and then are regulated down with active pass transistors.  Or more modern units use switching supplies like you see in computers.... no large transformers at all.  Low loss smaller transformers or flyback circuits that used magnetic materials (ferrites or special amorphous metal alloys that have something called uniaxial anisotropy to minimize hysteresis losses) that have minimized the Eddy currents so that they can be run at 10s of KHz rather than 60Hz. But these are still regulated to give a constant voltage.   Some PC power supplies put out a lot of 12Vdc current and many Amps at 5 volts.


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## petcnc

Proximus said:


> In Finland mains is 230 V ± 10 %, 50Hz, so do I need more capacitance for smoothing? I'd like to have low ripple, so would it be a good idea to invest on this capacitor? I'm not sure what ripple current mean in the specifications, but I think that's probably what I am looking for...
> 
> Cheers,
> Pena OH6PP


Well, In simple words at startup the circuit will see a LARGE capacitive tank EMPTY and it will have to fill it up instantly (it is like a short circuit for the startup). I think the in-rush current will be too high! So you need some sort of in-rush current limiter to handle it.
Other than that although the capacity is high it is doable


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## B2

Ripple voltage is just the AC change in the DC voltage due to current being removed from the PS storage capacitor and then being replaced.  

Current is the flow of charge.  I=change in charge/change in time.  

When the circuit rectifies the AC to make DC voltage you put the current (charge) on a large storage capacitor.  The diodes (rectifiers) carry current in only one direction to the capacitor, and so as noted, the current flows until the capacitor voltage reaches the peak AC voltage less the diode drops.  If there is initially no voltage on the capacitor the current "surges" to a very high value to try to charge the capacitors up rapidly. After this initial surge there is some charge on the capacitor and so currents after this are somewhat lest.   The charge does not flow back into the AC circuit during the negative half of the AC cycle because the diodes only conduct in one direction.   If there is no load (current/charge leaving the capacitor to be used) the DC voltage on the capacitors reaches the peak and is then the DC voltage is constant at this peak from then on.   There is no ripple voltage, as the DC is constant and there is no current/charge being drawn from the capacitor.  However, as soon as there is a load the charge (current) on the capacitors discharges into the load continuously.  During the period of time while the AC voltage is in the positive half cycle the current through the diode tries to charge up the capacitor and also provides current to the load.  Hence, the voltage on the capacitor increases and simply follows the AC voltage wave to the peak.  However, during the negative half cycle of the AC voltage the current to the load has to come from the capacitor as the diode cannot conduct or provide any current.  Hence, the voltage on the capacitor goes down.  So the DC voltage on the capacitor is not really DC.  It has both a DC level plus it has a smaller voltage on top of the DC voltage which is oscillating.  Think of it as the waves on top of a body of water.  Sure the water has depth (DC voltage) but there is also a ripple (AC voltage) on top.  As the load increases (more current is used) then the waves get higher as the depth of the DC goes down. 

Ripple voltage is usually specified as a percentage.  Ripple/DC *100%.    If the current that is being pulled under full load conditions is large then the ripple can become quite large.   If you had this situation in a stereo amplifier you would hear it as a 120Hz buzz.  If bad enough in some circuits they simply will not work or work improperly.  So decent power supplies are set up so that under the largest (current flow) load conditions, there is worst case DC (water depth) that is going to be reached.  The supplies use active transistor circuits (or tube of old) to just allow the DC part to come out and not much of the ripple.  Hence, ripple is not as much of a problem.  

So think of it this way, in a good PS the peak DC plus ripple on the capacitor under maximum load might be 20 volts, but the ripple is 5 volts of this and the DC is only 15 volts.  (The ripple voltage would be 33%.) The active circuits then output (pass) a constant 15 volts (DC) or less and the output has little to no ripple on it.  Of course the active circuits take up some head room voltage and so the output is always less than the 15 volts.  Now one must power the active circuits and this is commonly done from the DC plus ripple voltage coming directly off the storage capacitor, so some ripple feeds into the active circuits.  Hence the output can carry a bit of this ripple and so is not completely pure DC.  So now the spec of ripple percentage is much small, but not zero.  And of course the improved ripple is still a function of the load.  It is not uncommon in a very good PS that the ripple can be specified at <0.05% at full load current and even less when not at full load!  

There is a lot more to this technology, but hopefully what I have discussed is not too obscure.  

Dave.


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