0 to 20 Volt, 20 Ampere DC Bench Power Supply

[tq60]

I will look into that. Maybe a SCR/and zener diode would be the ticket...

You find a way to control the regulator.

Something in te current path can be used to monitor current, anything that has a difference in voltage.

This can be used to drive a transistor to feedback to regulator.

Someone else posted about your unit, they may be more familiar with it.

Being able to limit the current to miliamps will save your bacon.

Sent from my SM-G781V using Tapatalk

 

[strantor]

I would definitely consider forced air over those heatsinks. If you are set to provide 5 volts at 20 amps, then the power transistors are dissipating OVER 300 watts across them. That is a LOT of heat for passive heatsinks to get rid of. The closer you are operating to your maximum of 20 volts, the less heat you will have to dissipate.

Otherwise, it looks like a solid classical design. I would suggest moving your LM317T to a different location, where it does not block the top of the heatsink, and also provide less cooling for the regulator. As with any power supply, make sure your fuse is easily replaceable and does not have to be unsoldered to be replaced.

This is said with almost no recent prior research, so probably stupid, but...

From what I remember from the last time I played with linear regulator supplies, the typical setup is: Fixed AC voltage > rectifier> input caps> regulator ( voltage divider (pot) as reference)> output caps>banana jacks.

With the fixed AC input voltage, the power dissipation thing gets worse the lower your output voltage because it's (V_in - V_out) × I_out. So for example your rectified DC before the regulator is 30V and you're outputting 5V @ 2A, the regulator is burning up 50W ((30V-5V)×2A) in order to deliver only 10W (5V*2A).

What if instead of a fixed AC input and variable voltage divider reference (pot), you used a variable AC input and a fixed voltage divider? Speaking in terms of dollars and cents this probably only makes sense if you have a spare variac with no future, but in terms of simplicity of the design maybe it simplifies things and makes a more energy efficient supply? Just set your fixed voltage divider so that there is enough headroom to remove the ripple, and then control the voltage from the variac knob. The voltage divider ratio should scale to whatever the AC input voltage is, so even performance throughout the range, with just slightly more dissipation at higher voltages.

Am I missing something or should that work?

 

[RJSakowski]

Here is the circuit that I used for my constant current anodizing supply. I misspoke previously as it is a 45 amp, not 25 amp supply. I also added a digital volt/ammeter to the supply since.

 

[RJSakowski]

This is said with almost no recent prior research, so probably stupid, but...

From what I remember from the last time I played with linear regulator supplies, the typical setup is: Fixed AC voltage > rectifier> input caps> regulator ( voltage divider (pot) as reference)> output caps>banana jacks.

With the fixed AC input voltage, the power dissipation thing gets worse the lower your output voltage because it's (V_in - V_out) × I_out. So for example your rectified DC before the regulator is 30V and you're outputting 5V @ 2A, the regulator is burning up 50W ((30V-5V)×2A) in order to deliver only 10W (5V*2A).

What if instead of a fixed AC input and variable voltage divider reference (pot), you used a variable AC input and a fixed voltage divider? Speaking in terms of dollars and cents this probably only makes sense if you have a spare variac with no future, but in terms of simplicity of the design maybe it simplifies things and makes a more energy efficient supply? Just set your fixed voltage divider so that there is enough headroom to remove the ripple, and then control the voltage from the variac knob. The voltage divider ratio should scale to whatever the AC input voltage is, so even performance throughout the range, with just slightly more dissipation at higher voltages.

Am I missing something or should that work?

This is a big reason why I have switched (no pun intended) to switching power supplies. When dealing with higher power levels, their inherent efficiency really shines. I can generally pick up 300 - 400 watt supplies for a few bucks at hamfests. Buck/boost constant voltage/constant current supplies have also dropped greatly in price as well so a fairly hefty lab supply could be put together by combining the two. I would replace the on board pots with panel mounted pots and add the digital/ volt/ammeter for a few dollars more to polish it off. Another plus is the greatly reduced weight.

 

[Bi11Hudson]

A well done supply. I have several 'variations' of the same general design that I made from rewound microwave transformers. Most are current limiting supplys with the general voltage range set by the number of turns in the secondary of the transformer. I have a Variac with a rectifier for more touchy adjustments, but the current is limited by the size of (small) the Variac.

+1 on cooling with a fan on the heat sinks. Also watch the inrush current when you increase the capacity of the filters.

.

 

[RJSakowski]

A well done supply. I have several 'variations' of the same general design that I made from rewound microwave transformers. Most are current limiting supplys with the general voltage range set by the number of turns in the secondary of the transformer. I have a Variac with a rectifier for more touchy adjustments, but the current is limited by the size of (small) the Variac.

+1 on cooling with a fan on the heat sinks. Also watch the inrush current when you increase the capacity of the filters.

.

Another benefit of using a switching power supply. They have built-in inrush current protection.

On a recent project, a full wave bridge running off line charging a 3,000 mfd capacitor was capable of surges in the hundreds, if not thousands of amps. In my case it totally welded the contacts on my power switch. A series resistor can be added to limit inrush current to a safe value but it will waste power and generate unwanted heat. My solution was to use a varistor designed specifically for inrush current protection. The one I used had a cold resistance of 10 ohms but in a run condition, the resistance dropped to a few hundredths of an ohm.

https://www.ametherm.com/datasheetspdf/SL3210015.pdf

The only caveat is that the protection only occurs on a cold startup. If the supply is switched off and the capacitors drained and then switched back on, the varistor hasn't had time to cool off and the inrush will be high. In my case, I added a relay in parallel with a time delay of about a second before activation. The relay would short out the varistor after the current dropped to a safe value so the varistor remained cold which permitted a short cycle time without undue consequences.

https://www.google.com/url?sa=t&rct...r/dp/72J6844&usg=AOvVaw2Dji33mq7yUFBNBoPfGgo3​

 

[addertooth]

Strantor,

Actually, you describe a certain type of amplifier that was popular, it would apply just enough DC power to the final stage so that those transistors were nearly in full conduction for the given volume (power) which was demanded. There were some supplies which auto-adjusted as well. Today Switching supplies have eliminated all of those concerns, but require output filtering to get rid of the output spikes (high frequency "ripple") which people complain about from switching supplies which are unfiltered. Even more modern switching designs are 3 phase systems which are even more efficient and with less ripple than the original switcher designs.

There are discrete components which are inrush current limiters, as others have mentioned, or you can build inrush current limiters as well. Believe it or not, but Inductors can make rather excellent inrush limiters as well, but may have to be fairly large to deal with higher currents.

 

[cathead]

Having thought over all the posts and sleeping on it, I decided to make several modifications
to the project. There is a center tap on the transformer so made use of that to make a dual
voltage supply. I added a DPST switch on the front panel to be able to use either full voltage
or half voltage. As long as I was at it, a fan was added as well as another 4700 MFD electrolytic
capacitor. This will make drawing any heavier currents at low voltage less stressing on the
2n3771 transistors. To complete my modifications, I added a single pole switch to control the
fan for lower current applications. Thanks for reading along and all the helpful commentary.
Here you can see the added fan and capacitor as well as moving the lm317 for a little more space.




This is the front panel with the added switches that choose high or low voltage (20 volts or 10 volts)
and a fan switch.