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# 12V LM317 variable power supply, pot got burned.

#### afspiom_

##### New Member
Hi, I'm a complete newbie in electronics so please bear with me. I came across this diagram online. I tried to build the circuit and made some slight changes on the values of some of the components. The changes made are the following:

• Transformer || 15V to 12V - 1 Amp
• Diode 1 & 2 || 1N4001 to 1N5004
• C2 || 10uf to 0.1uf
• All capacitors are change into 50V rating
• POT || 5k ohms to 1k ohms

I tried to simulate it on multisim before testing the actual circuit and changed the power rating of the POT into 1/8 and it turned out okay with an output of 1.25V to 11.7V. Although when I tried the circuit on actual, the POT smoked almost immediately when I turned the circuit on.

Please help me on this, I want to learn and determine the problem. I'll try again after I get some suggestions and solutions.

P.S. I checked the connections on my PCB and nothing are connected wrong. I think the problem lies around the regulator or the POT...

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Post a picture of the built circuit, it sounds mostly like you've made a wiring error.

Wiring error.
If the pot burned, then it's likely connected directly to the output, and not through R2.

Sorry for very poor diagram but I'm currently outside right now and I don't have access on my actual work but this is what it looks like roughly. The curly lines are just wires. Please tell me what's wrong...

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If you meant to say Watt for 1/8 , that was your problem and need 1/4W due to max power at one setting is > 150 mW.

(120ohm/1.25V) ^2 * Radj = Pd

Always derate resistor power 25% or more.

You can also reduce the current by raising the feedback R from 120 to 240 or 330 then 1/8W is OK

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If you meant to say Watt for 1/8 , that was your problem and need 1/4W due to max power at one setting is > 150 mW.

11 mA *2 * R = Pd

Always derate resistor power 25% or more.

You can also reduce the current by raising the feedback R from 120 to 240 or 330 then 1/8W is OK
Thank you so much. I'll try to rebuild my circuit tomorrow and buy a new R2. I'll try to increase the feedback.

You can also reduce the current by raising the feedback R from 120 to 240 or 330 then 1/8W is OK
That won't guarantee no-load regulation with the LM317, since its worst-case minimum output current needs to be 10mA.
That's why 120Ω is normally used, which guarantees 10mA minimum.

That won't guarantee no-load regulation with the LM317, since its worst-case minimum output current needs to be 10mA.
That's why 120Ω is normally used, which guarantees 10mA minimum.

The datasheet is or has been confusing on this point. They often show a 240Ω resistor, which is ok for the LM117, but this "gotcha" detail isn't made clear.

Sorry for very poor diagram but I'm currently outside right now and I don't have access on my actual work but this is what it looks like roughly. The curly lines are just wires. Please tell me what's wrong...
I don't see the grounds tied together. LED - is supposed to be ground, as is Vo -. Yet, you seem to have two different points around the pot (G and Out). Those should be all one ground path. Thus I suspect the pot burned out due to ground return/wiring mistake.

I don't see the grounds tied together. LED - is supposed to be ground, as is Vo -. Yet, you seem to have two different points around the pot (G and Out). Those should be all one ground path. Thus I suspect the pot burned out due to ground return/wiring mistake.
And where has he got In, Out and Ground from on the pot? - Ground might apply but the others don't - no wonder he killed the pot.

However, that's assuming it's correct - I asked for a picture, and got a poor drawing, so no idea if it bears any resemblance to reality.

IL= 5mA with R1 = 240 ohms should be reliable with Vi-Vo=15V on no-load since the spec is rated for max Vio input-out = 40V.

The spec for the high-current LM317 (vs 317L) is 3.5 mA typ 10 mA max for ILmin

...I asked for a picture, and got a poor drawing....

It is not that difficult to draw a proper schematic, either by hand or using a program like EasyEDA from JLCPCB. Maybe this would be a good first step.

A clear, well-focused picture helps too. Then people can compare what you're trying to do with what you actually did.

If you're buying resistors a few at a time, consider buying an assortment kit with many different values.

Change your R2 from 120 to 240 V(R2)=1.3V max
Change your Pot to 2.5 K 1/8W to obtain Vo=15V max

Ensure you have a large heatsink to handle Vdrop * I out so that you don't burn your finger on it.

www.eleccircuit.com is a great site if you are 10 yrs old with a parent but poor if you want to be a super-tech because it lacks mechanical engineering skills and thermodynamics.

If you have a scrap CPU heatsink for free, that works well for a linear regulator to dump all the Vdrop * Imax out. Use thermal grease and a thin electrical insulator to isolate the conductor from the heatsink to avoid shorts if desired.

Here are many TO-220 heatsinks which one can buy from EBay, Mouser, Digikey or any other distributor provided.

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Change your R2 from 120 to 240 V(R2)=1.3V max
Change your Pot to 2.5 K 1/8W to obtain Vo=15V max

Ensure you have a large heatsink to handle Vdrop * I out so that you don't burn your finger on it.
does this still apply even if the transformer supplies 12V with 1A current?

does this still apply even if the transformer supplies 12V with 1A current?
My transformer is tamagawa 12V - 1Amp center tapped transformer.

does this still apply even if the transformer supplies 12V with 1A current?
Of course,
The source of the power has no effect on the dissipation in the LM317.

I chose the parts so you could get 15V but as you should know the LDO needs 2Vdc greater input than the output and 12VAC might give 14% more with large enuf caps and a light load. But remember if you plan to use 1A out, the heatsink must handle the power drop from (Vin-Vout)*Io so use a big heatsink like a CPU heatsink.

Hi, I'm a complete newbie in electronics so please bear with me. I came across this diagram online. I tried to build the circuit and made some slight changes on the values of some of the components. The changes made are the following:

• Transformer || 15V to 12V - 1 Amp
• Diode 1 & 2 || 1N4001 to 1N5004
• C2 || 10uf to 0.1uf
• All capacitors are change into 50V rating
• POT || 5k ohms to 1k ohms

I tried to simulate it on multisim before testing the actual circuit and changed the power rating of the POT into 1/8 and it turned out okay with an output of 1.25V to 11.7V. Although when I tried the circuit on actual, the POT smoked almost immediately when I turned the circuit on.

Please help me on this, I want to learn and determine the problem. I'll try again after I get some suggestions and solutions explore.

P.S. I checked the connections on my PCB and nothing are connected wrong. I think the problem lies around the regulator or the POT...
What is the wattage rating of the pot you are using.

I once smoked one of those tiny little 1/8 watt multi turn pots in a configuration like that.

What is the wattage rating of the pot you are using.

I once smoked one of those tiny little 1/8 watt multi turn pots in a configuration like that.
If you refer back to the drawing of how it's supposedly wired, the entire output current was been passed through the pot.

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