# Adjustable Power Supply- amps or volts?



## GunsOfNavarone (Aug 23, 2020)

I"d like to get a bench power supply and I'm a bit lost on this option, Volt or amps? That is, if you had the option of 120v but only 3 amps OR 30v bot 10amps, which would you find more desirable. I;m sure the follow up question, "what are you using it for"? As I am starting to build more electrical things in the CNC world, I have been in situations that I wish I had one. No one will be flamed for giving me an answer I find not ideal for me a year down the road.
Honestly, I'm leaning 30v 10 amps for whatever that's worth.
As always, thank guys for your input!


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## Ulma Doctor (Aug 23, 2020)

most things cnc will work from either 24v or 5v signal respectively.

i use variacs and inexpensive regulated power supplies for bench testing
and would use regulated power supplies when dealing with anything CNC

like these...









						12V 24V - 2A to 30A Amp Switching Power Supply Adapter for LED Strip   | eBay
					

DC 12V 30A Size 215 115 50mm With Internal Fan. DC 24V 2A Size 110 78 36mm. DC 24V 10A Size 198 110 50mm With Internal Fan. DC 24V 15A Size 215 115 50mm With Internal Fan. DC 24V 20A Size 225 125 50mm With Internal Fan.



					www.ebay.com
				





the price is low, so i keep a few on hand


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## GunsOfNavarone (Aug 23, 2020)

Thanks doc. I'm actually looking at the ones with digial readout for both volt/amps. Apple to apples I guess but more idiot proof when your not paying attention, not to mention overload protection and such. Those I don't believe, can you adjust volts/amps independently.


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## Ken from ontario (Aug 23, 2020)

I have seen  5 amps China made variacs dirt cheap but the vreviews are mostly good:






						Amazon.com: YaeCCC Auto Voltage Transformer AC Variable Voltage Converter Transformer 110VAC Input, 0-130VAC (500W): Industrial & Scientific
					

Buy YaeCCC Auto Voltage Transformer AC Variable Voltage Converter Transformer 110VAC Input, 0-130VAC (500W): Power Converters - Amazon.com ✓ FREE DELIVERY possible on eligible purchases



					www.amazon.com


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## GunsOfNavarone (Aug 23, 2020)

Would those VarI Acs be any good for anodizing? (Different subject)


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## ahazi (Aug 24, 2020)

Ken from ontario said:


> Doc, what brand is the variac? I have seen  5 amps China made variacs dirt cheap but the vreviews are mostly good:
> 
> 
> 
> ...



For variable AC output get the biggest variac (2,000 watt) the price difference is small and it is useful to have more capability. I have one like this on my bench.

For a bench use, a DC variable power supply with proper protection etc I use this model. It is very hefty (30 volt, 5 amp) and the price is very good. 
https://www.amazon.com/gp/product/B00ZBCLJSY There are other DC power supplies that "look the same" but they are a cheaper switching (light weight, no transformer), without a linear regulators and are more noisy and not reliable. The one above is very good and built well. I designed and built many power supplies over the years, I know the difference...

Glad to help with electronics as I am a relatively novice machinist but electronics is what I am doing for almost 50 years.

Ariel


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## MSBriggs (Aug 24, 2020)

Most electronic devices need a specific DC voltage, and draw up to a certain current (amps).   So you select a power supply that supplies that voltage, with a current rating of the required amps or higher.  If you are using a variable power supply, you have to be sure to set the voltage at the designated level so as not to damage item that you are powering.  Many adjustable supplies also have fixed voltage outputs of common values (5 V, 12 V, ...).   Some have "crowbars", where for safety you can prevent the supply from exceeding an adjustable maximum voltage, so as to protect whatever you are powering.  I prefer adjustable DC power supplies that have both voltage and current controls.  In typical operation where you want a particular voltage, you set the current setting high so that it is not controlling, and adjust the voltage to the desired value.  For electrolysis rust removal, I prefer to operate in current limiting mode, at much lower amps and voltage than it seems most people are using.


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## SLK001 (Aug 24, 2020)

GunsOfNavarone said:


> I"d like to get a bench power supply and I'm a bit lost on this option, Volt or amps?



This mostly depends on what you are going to use the power supply for.  Do you want a linear supply or a switching supply?  Your 30V @ 10A supply, if linear, would be quite heavy (300 WATTS).  A switching supply is generally a noisy supply - a lot of voltage "hash" on the output, while a linear supply is generally very clean.  A lot of circuits that require 30A can get by with a switching supply.  Circuits that require 12V @ 1A usually require a more stable and cleaner supply.

If you're beginning to experiment with electronics, then a clean, linear 30V @ 2A supply is an overall good pick.  These supplies usually have both CV and CC controls (constant voltage and constant current).  There are versions out there that are dual 30V @ 2A supplies, which means that there are two independent supplies neatly contained in one box.  I think that you want a dual 30V @ 2A supply, with CV and CC controls.


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## Ken from ontario (Aug 24, 2020)

GunsOfNavarone said:


> Would those VarI Acs be any good for anodizing? (Different subject)


That's a good question, I don't know anything about anodizing but wouldn't mind to know the answer.




ahazi said:


> For* variable AC output get the biggest variac (2,000 watt)* the price difference is small and it is useful to have more capability. I have one like this on my bench.


I mainly want to use it for a   3 AMPS, 230 Watts ,brushed motor, I would consider the bigger variac if I could set the output not to go beyond 3 AMPS.


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## markba633csi (Aug 24, 2020)

You might find you need both: A variac for variable ac and a dc bench supply with volt and current meters/adjustments.  
If you do electrical stuff enough you will find uses for one of each (or more!)
-Mark


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## ahazi (Aug 24, 2020)

I will answer both questions:



Ken from ontario said:


> That's a good question, I don't know anything about anodizing but wouldn't mind to know the answer.


*Theoretically* you can use a variac followed by a rectifier (bridge) to supply DC current to a chemical process. But... be careful, a variac is an autotransformer which means the secondary is NOT isolated from the mains voltage 110 or 220 volt. Also the voltage range is rather high. You can use a variac followed by a step down transformer (lower voltage, higher current) followed by a bridge rectifier. By adjusting the variac output you can control the current into your process. Depends on how much current you want/need, you might be better off with just buying the variable power supply that I recommended as it comes with current limiting in addition to the variable voltage control which is something that you will not have with the variac.



Ken from ontario said:


> I mainly want to use it for a   3 AMPS, 230 Watts ,brushed motor, I would consider the bigger variac if I could set the output not to go beyond 3 AMPS.


Is it a DC motor? 230 watt, 3 amp motor requires 230/3=76.6 volt nominal. For that the absolute minimum variac size is 3 amp x 115 volt ~= 350 watt. But... a smaller variac will not limit your current, it will just burn itself up. Use a bigger variac, add a current meter in series and monitor both voltage and current not to exceed whatever the maximum is.

I hope this helps.

Ariel


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## Bi11Hudson (Aug 24, 2020)

There are several possibilities depending on what you are going to be doing. For low voltage work, 5 or 12 volts, nothing beats an old computer supply. Two connected in series will yield 10 or 24 volts. I use a couple of computer supplies on my bench. Not so much price as having test leads *exactly* +5 and +12 volts with a few amps capacity just by connecting. But my work is model building in the 12 volt range, using 5 volts as a test bed for motors.
Two on the bench in series would provide *either* 10 or 24 volts. Set up for an installation, especially in a metal enclosure would be a little more difficult. The low voltage ground is actually a "common" and must be isolated when doubled. The 5 and 12 volts share a common , care must be taken to use one *or* the other. They both are not accessable together.

A variac is a variable autotransformer, from naught to 130%(M/L). Actually a trademark by the primary manufacturer, it has become the general reference sort of like "Cresent wrench" for adjustable spanners. The key factor there is that a "variac" *cannot* be used with a switching supply. And neutral connection must be observed, the "120volt line" can be easily connected to the low voltage load. They are *great for linear supplies if* there is a transformer between the line and load. I have a couple on my bench, mostly used for homemade resistance soldering devices. A variac is a *very* useful device to have when you need it, but there *are limitations* on using one.

For more "Lab" quality output, a 30 volt variable supply will give good service. But most are quite touchy about adjusting to within less than half a volt. They can be modified to give more easily adjustable voltage ranges down to the m-volt range. I have used them in an industrial environment but for home shop use the computer supplys have what I need. On the rare occasion that I need some esoteric voltage, I do have a variable 20 volt linear supply, preset to 18 volts. But it sits on the shelf most of the time.

You are looking to build "C-NC" equipment. I have worked, in the distant past, with "NC" equipment. Resolvers instead of encoders, basically. For resolvers, 24 volts is a standard. With the added "C-NC" being for computer numerical control, a +5 volt supply becomes a standard for the computer equipment. I cannot advise on newer equipment, and don't use NC or C-NC for what I do. You're on your own there.

Bill Hudson​


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## GunsOfNavarone (Aug 24, 2020)

I have a sneaking suspicion, if I faced more volts (DC) over amps, I will find it limiting. I THINK more amps, less volts is the way (for me) to go. 60v/10a will be my route unless there are good arguments for the other. I know the perfect world is more of both gives more options, but I don’t think I’ll be working on things regularly that require more than 60v but would need more than 5a.


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## SLK001 (Aug 24, 2020)

GunsOfNavarone said:


> I have a sneaking suspicion, if I faced more volts (DC) over amps, I will find it limiting. I THINK more amps, less volts is the way (for me) to go. 60v/10a will be my route unless there are good arguments for the other. I know the perfect world is more of both gives more options, but I don’t think I’ll be working on things regularly that require more than 60v but would need more than 5a.




Let us know the prices you find on 600 watt power supplies.  Let us "feel" the sticker shock that you experience.


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## ahazi (Aug 24, 2020)

SLK001 said:


> Let us know the prices you find on 600 watt power supplies.  Let us "feel" the sticker shock that you experience.


I was about to write something similar... Buy the 30 volt, 5 amp variable bench power supply from Amazon for $80. It is a very good unit and you cannot go wrong with it.

I designed, built, bought and repaired power supplies from milliwatts to multiple kilowatts and just like the difference between a two seater roadster and a semi trailer the prices, sizes and weights are very different. A 30 volt/5 amp is your family sedan, can do a lot and priced very competitively.  

Good luck!


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## pontiac428 (Aug 24, 2020)

In a proper bench supply, 5 amps is plenty for testing electronics.  If you need to test motors and other big power consumers, you can hook up a switching power supply (if appropriate) or linear supply, or use whatever supply powers the appliance in service.  Bench supplies don't need much amperage to be incredibly useful.  Fine tuning volts and limiting amps is about all the functionality you would need.  If you want to do electrolysis, plating, anodizing, and the like, get a car battery charger (do not use a bench supply for plating ever).  There may be no one size fits here, so what kind of supply you get depends on how you plan to use it...


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## GunsOfNavarone (Aug 24, 2020)

Aaannnd....I’m not sure now which way to go. You make some sane, rational arguments there Pontiac. Fine tuning adjustments are important you say? TBH, I was just going with course adjustment. Seems like you want 10, 11, 12v not so much x.17v
I might just flip a coin, but it’d probably land sitting straight up on its edge.


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## AGCB97 (Aug 24, 2020)

I never figured out what you want to use it for. Naming a few possibilities would help me understand and help others help you.
Aaron


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## jlesser27 (Aug 24, 2020)

Bench top Lab Power Supplies should allow you to set a desired voltage and limit the current. When developing a circuit or project you want to be able to limit the current on startup which keeps you from releasing the magic smoke. There are quite a few options on amazon for around $80 but I opted for a quality used one from Amazon. Here is a picture of the one I bought for $85 on eBay. This one allows you to have two separate outputs or put them in series for higher voltage. This one also allows you to source both a positive and negative voltage for using operational amplifiers. Most projects for CNC will use 24v and 5v so a 30v 5amp device will be fine. 







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## GunsOfNavarone (Aug 24, 2020)

In the past 3 months, building the ELS for the lathe, building the VFD as well. The last week building a whole circuit for coolant controlled by CNC mill. I was always in situation needing juice to check the the whole circuit was wired correctly. Multimeters give me some affirmation, but sometimes I need DC running to confirm before I connect to a system that is sensitive to bungle headedness. My electrical skills are acceptable with anything to do with cars motorcycles, AC wiring is coming along but the wiring AC to DC of the mil was daunting. The coolant system works flawless and only ended up costing me about $80, but there was a lot of time drawing it out to help myself feel better about flipping the switch. I feel like a bench power supply would have allowed a lot of testing of single pieces as well as whole units before making the leap into the real deal.


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## jlesser27 (Aug 24, 2020)

It would help but more importantly sketching out the circuit even on the back of a napkin is extremely helpful. I would be happy to lend a hand with electrical drawings if needed. 


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## Bi11Hudson (Aug 24, 2020)

Having read your responses to a number of posts, I just have to jump in here again. To say you are comparing Apples to Oranges is an understatement. You seem to be more comparing bricks to bananas. The question of "volts vs amps" is actually asking two entirely *unrelated* questions. It sounds like you need to learn a little more about electricity. I don't have room(books) here, or time(months) to teach electricity. A good(?) starting point is at:


			http://www.hudsontelcom.com/uploads/ShopElex.pdf
		

Just an introduction though, even the listed books just give an overview. Electricity is a deep subject. I am (used to be) an electrical engineer. Yet I only have a passing knowledge of electronics. The freehanded availability of electronic components is a little misleading. Remember Robert Heinlein's books, TANSTAAFL, there ain't no such thing as a free lunch.

VOLTS: Think of pressure. In the old days, it actually was referred to as pressure. Compare a garden hose to a fire hose. they both have the same pressure, the city fire main. Excluding houses with a pressure regulator, of course.

AMPS: Think of those hoses again. The fire hose can deliver a hundred times more water in the same time as a garden hose. And is much more difficult to handle. Look at the size of the two relative to each other.

WATTS: The amount of work that *can* be done. A garden hose can spin up a small fan. A fire hose can push a car down the street. Same pressure, remember. . . Think about how difficult it is to handle a 3 inch hose compared to a 3/4 inch hose.

To consider volts, a microwave oven has a transformer that can deliver 3,500 volts for the Klystron. But only a few *thousandths* of an amp. A car battery, on the other hand, is limited by the number of cells in that battery. It can only deliver 13.2 volts from full charge. But it can deliver several hundred amps for a few seconds. The two, volts and amps, cannot be compared. But they work together to produce watts.

Watts only apply in small devices when comparing name plates. What matters is amps and volts. Watts only matter in large items like motors, comparing output like watts VS horsepower.

To say you are competant in automotive electrical systems is to say you know how to change the fuse or install a radio. What about using an automotive alternator to get 120 volts 3 phase power?

.


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## tq60 (Aug 24, 2020)

For general bench use Astron and many others offer units that have optional interface.

From basic box with binding posts and power switch, then meters added to show volts or amps to the upper end with both being adjustable which is what we have used for 40 years in the trade.

When we worked on portable radios and pagers micro miniature stuff a probe can short out things and smoke things.

Being able to limit the current to just enough to operate but not enough to damage saves a great amount of repairs.

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## GunsOfNavarone (Aug 24, 2020)

Bill, I wasn't comparing amps to volts. What I am getting at is (the original question) is there a good reason on a bench power supply to favor one over the other, that is, would I be in a situation where I'm working with a DC motor and it requires 60 volts at 1 amp or adversely, a circuit that need 12v but 6 amps? 
As far as my hobbies and what I have wired, I have recently built a chopper and had to create the entire wiring harness from scratch...same with a '63 vw bus. This new direction I am seeing a little bit of everything and I'm a firm believer in having the correct tools for the job. If it's of no matter, I'll just buy he purdiest one I find. I'm not sure that the understanding of using an alternator to create 3 phase power is anything I am currently, or in the foreseeable future, needing to do. Now...fighting with a bad contactor and now having a convenient way to test it at my work bench? Yup.
I understand there are many around here with a very strong skill set in specific fields, that's the appeal for me, unfortunately I've got to take the bad with the good I suppose. Many here are learning as they go (I think Practical machinist web site would be the anti version of this website) and I can say in the last 3-4 years, I have expanded greatly in fields I barely dabbled in. I can take an idea, fully mock it up in CAD, then CAM and make working part. I moved from a manual lathe to a manual mill to a CNC mill and am now converting my lathe to CNC. I won't excel in the electrical field, it's not even in my sights, but I WILL competently acquire the skills I need to complete whatever tasks I set my sight to. Track me down in another 3 years and we can discuss all I have achieved....I'm guessing an alternator powered, 3 phase spaceship won't be one, but its good to dream.


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## pontiac428 (Aug 25, 2020)

Oh, you can surely get to space on alternators.  Look, there's room for four more on the other side!


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## jlesser27 (Aug 25, 2020)

I’m sure with all the skill set on this site we can put together some type of inverse flux capacitor and either go to mars or rewind 2020. 


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## whitmore (Aug 25, 2020)

Would those VarI Acs be any good for anodizing? (Different subject)
[/QUOTE]
Yes, but that's not all.   A variac will typically have a current limit (set by the contact resistance of the wiper) of a few amps,
and anodizing requires rather high DC currents at low voltages.   You'd be best off with a fixed transformer to
step down the voltage (the high current output of such a transformer has no wiper problem), in conjunction
with a variac to adjust at the low-current-high-voltage input of the fixed transformer.   

A hundred watts is under 1A at 120V, but is  50A at 2V.   The variac won't like the 50A output condition, 
even though the line cord won't be at risk of catching fire.   So, you use a stepdown fixed transformer,
and the variac feeds 1A/120V or adjusts down to 3A/40V and neither element  has a problem.

Anodizing and plating require DC, so a rectifier after both transformers is required.   Electroplating and
anodizing can use very BIG rectifiers for those currents, with a variety of tricks to keep 'em happy.


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## mksj (Aug 25, 2020)

One of my power supplies is a BK Precision 1761 which has 3 separate DC supplies, two are 32V@3A, one is 2.6-5V@5A. I often need 2 or 3 power supplies when testing circuits, boards, electronics. Each power supply is isolated from the other (floating) so one can either put the two 32V supplies in parallel for 32V@6A or in series for 64V@3A, which is sufficient for most of my low voltage needs. Often when testing a circuit one wants to limit the amperage, so one sets the current limit and then raises the voltage. This can limit damage to the electronics and can also determine the operating current. In some of my control circuits I am current limited by the devices or power supply so it is important determining the actual operating limits of the design.  In other cases if current is not an issue then you can set the current to maximum and then set your voltage. The BK power supply tells you when you are current or voltage limited. In addition, the 1760 series of BK are not a switching power supplies, so the supplies are isolate (use transformers and traditional regulation). This can be important for noise isolation and also can be an issue when using a positive and negative DC supply rail. This is also a common issue in volt/amp meters that the power supply shares a common negative to the voltmeter negative. I tinker with tube equipment and one of my other supplies go to 500VDC@200mA x2 (plate and screen), -100VDC@10mA for the grid, 0-40VDC@1A and fixed center tapped VAC 6.3/12.6 for filament supplies all in one unit.  So the type of power supply depends on the type of circuits you are dealing with.  I recommend getting a dual VDC supply similar to the BK if you can find one at a reasonable price and it meets your voltage and current requirements.

Overall if you can pick up a BK 1760, 1761 or 1762 used in working condition, they are great VDC power supplies for low voltage DC work (mostly solid state).  Motors generally need much higher current and do not need the degree of regulation, so an adjustable high current switching power supply would be more desirable. They tend to be very reasonable. Variac is used for for adjustable AC and they are not isolated, which can be desirable for certain type of equipment. I have several, the larger unit uses an isolation transformer with a variac, it is useful for powering up older tube equipment and also can measure leakage to the chassis. My tube amps put 1100VDC on the plates, I do not like to play around when voltages get that high, it is the kiss of death if you screw up.


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## ahazi (Aug 26, 2020)

mksj said:


> One of my power supplies is a BK Precision 1761 which has 3 separate DC supplies....



This thread is getting "interesting" with such relics as powering filament, plate, grid, screen grid etc... For me this is mostly the past during my younger years...

Mark just expanded the (wide) scope of what (bench) power supplies can be. I have designed, built, repaired, used, owned and still have many of the types mentioned throughout the thread and much more. It is a very wide field and my recommendation to anyone that wants to start and get a first bench power supply is to start simple and unless you have severe budgetary restriction and ability to repair eBay used power supplies - *buy new*. 

Look at my earlier post for my recommendation that I stay firmly behind as this is one of my bench power supplies and it is my go-to one. I have others that are double and triple output, higher voltage and higher current and I keep going back to this one. The prices of good *linear bench power supply* with digital control and display is below the price of components that any of us can source and not much more than a used one.

Ariel


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## hman (Aug 26, 2020)

jlesser27 said:


> I’m sure with all the skill set on this site we can put together some type of inverse flux capacitor and either go to mars or rewind 2020.


I would sincerely appreciate such a rewind!


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