LM317 Adjustable Voltage Regulator project

Thread Starter

SamR

Joined Mar 19, 2019
2,243
I put this together on a breadboard some time ago and have found it useful for when I need a second voltage source. The stepdown transformer is a Triad F-25X 120/12/6 1.5A that is plugged into a ground fault interrupter wall wart and 1A fused on the secondary. Since the regulator has been of use I want to put it on a permanent circuit board. Before I do any comments or corrections?

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crutschow

Joined Mar 14, 2008
25,103
R2 goes to the output, not the input.

If you want to draw more than a couple hundred mA from the output, you need to put the LM317 on a heatsink.
The power it has to dissipate is (Vin-Vout) * Iout.

For best regulation, connect the LM317 OUT pin directly to the supply output terminal with a length of heavy gauge (18 or larger) wire, and connect the top or R2 directly to the LM317 OUT pin.
Connect the bottom of R1 directly to the common output terminal.

Since you have a 12V center-tapped transformer here's an interesting option.
Below is an circuit that gives you a full-wave rectified output of both a little over 7V and 15V from a center-tapped transformer and a bridge rectifier (write-up here).
You can select the lower voltage when you just want 5V out, to minimize the regulator dissipation, and the higher voltage when you want up to about 13V out.
Since you are only using one voltage at a time, you need just one output filter (C1) and can switch between the two outputs with a toggle switch.

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Thread Starter

SamR

Joined Mar 19, 2019
2,243
"R2 goes to the output, not the input."

Yes, it does on my breadboard, my mistake in the circuit diagram. Thx

I was going to heatsink the LM317 although not really looking for a lot of mA out. Mostly to adjunct the bench power supply for such as op amp 2nd V supply. Most of what I do is less than 30mA. The bench supply is the workhorse for me but there are times I've need a quick 2nd voltage source and this is handy. 5KuF? Do I really need that much filtering? The trimmer pot is the weak spot it is only 1/2 watt. I was using a regular pot but too hard to nail the voltage down. Wanted to do it in LTspice and analyze, but no model for the LM317 or DB207 that I could find.
 
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crutschow

Joined Mar 14, 2008
25,103
5KuF? Do I really need that much filtering?
It determines the input ripple voltage.
You don't need that much for low currents.
The trimmer pot is the weak spot. I think it is only 1/4 watt.
Why do you think that?
At 5V out, the trimmer is only dissipating about 20mW.
Wanted to do it in LTspice and analyze, but no model for the LM317
See attached.
.sub goes in sub file and .asy goes in sym/Power Products file
 

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Yaakov

Joined Jan 27, 2019
1,612
You might want to use a larger 10-turn pot for easier adjustment, If you mount it in a box you can get cheap indexing dials, but if not, you can still get a low cost, hand operated 10-turn.
 

Thread Starter

SamR

Joined Mar 19, 2019
2,243
Good idea, however I want to keep it small so the multiturn I have will work for me even if a bit harder to adjust. Planning to use a small strip board on stand offs.



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Thread Starter

SamR

Joined Mar 19, 2019
2,243
EZ adjust trim pot on order. Added LED indicator w/ 620Ω resistor. It lights at 2V and didn't burn out at 18V. Sketched out a layout and mocked it up. Pot due Monday to finish it up. Tested w/ 10K pot and getting 1.2 - 18.7VDC out w/ 12VAC in. ~20 mV ripple on output. Good to go. Time to start soldering.

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crutschow

Joined Mar 14, 2008
25,103
I looked trim pot up and the spec. for it is 1/2W. I = W/V = 0.5W/5V = 100mA and for 0.5/12V = 42mA Not a lot, but as I said most of what I do is less than 30mA.
The load current does not go through the pot.
Only the current going through R2 goes through the pot, and that's just 1.25V/240 = 5.2mA (the LM317 regulates by maintaining the voltage at the OUT terminal 1.25V above the ADJ terminal).
 

wayneh

Joined Sep 9, 2010
16,398
I would have been tempted to lay the capacitor on its side to reduce the overall profile. But that depends on your planned use, whether you want an enclosure or so on.
 

Thread Starter

SamR

Joined Mar 19, 2019
2,243
Didn't think of that but Nah it's a stand-alone. Lots of empty space on the perfboard and waiting on the pot to finish it up. Pot arrived early. All done. Getting 17.70 to 1.25VDC out adjustable to within a couple of hundredths with a 20mVDC ripple for 12VAC in. Clamped a 220Ω 1W resistor into the output and let it run to give it a bit of a load test. Even at only 75mA it's all good. Considering painting the bottom with liquid tape since it is an open chassis design.

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Thread Starter

SamR

Joined Mar 19, 2019
2,243
It's a 10KΩ 1/2W trimmer potentiometer with a nice big easy to adjust knob on it. It has some backlash as typical for that type, but many many multiturns end to end. I am able to tweak the output voltage by a couple of hundredths with it. Would prefer 5KΩ but was not available. And this is for very rare occasional use when I need a second voltage supply to my bench power supply.
 
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