Double gang Pot wiring

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Hello im currently working on Dual polarity regulated power supply project wich can be found here: **broken link removed**

Schematic: **broken link removed**

I made a PCB for it and it looks like this:

**broken link removed**



The problem is, when i tried to turn it on my dual gang pot smoked. Pot was 0,5W 5k ohm. Iv no idea why it happened, can anyone check if i did anything wrong in designing my PCB ? Thank you very much.
 

I think it is because you haven't put a limiting resistor in series with the pots. When you turn the pot to one end the regualtor will try to make the output voltage infinity. While it can't do this, it will do its best, which will result in your full input voltage across R1+R2. I'm not sure what your input voltage is (35V?) but that could put a few watts into the pots if the value is near the low end e.g. 100 ohms or so.

Keith.
 

    L

    Lockshot

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Thanks for reply.

Im using dual polarity transformer 48VCT (24-0-24).

So, where exactly i should wire those resistors? And isnt R1 and R3 doing that function? How about choosing a 1watt Pot instead of 0,5wat? Also what will happen if i replace R1 and R3 with a ~300omh resistors? will it help?

~Lockshot
 

You need to put a resistor in series with the pots so even when the pots are on the minimum value there is some resistance across the regulator pins. I cannot draw it for you at the moment but I can do it later (I am on my mobile phone at the moment).

Increasing the resistance won't help. It would be ok if you never turned the pot too close to the minimum value.

Keith

Added after 23 minutes:

I am not actually sure my assessment is correct. I need to look at you circuit again but I cannot do that from my mobile - I will need to be at a computer. I will look as soon as I can (unless someone can jump in with the right answer sooner).

Keith

Added after 24 seconds:

I am not actually sure my assessment is correct. I need to look at you circuit again but I cannot do that from my mobile - I will need to be at a computer. I will look as soon as I can (unless someone can jump in with the right answer sooner).

Keith
 

    L

    Lockshot

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In the meantime you could try a fixed resistor in place of the pot and make sure it gives the right voltage out. It will prove everything else is ok (or not).

Keith
 

Well, when i first time plugged it in, pot was regulated that output voltage was ~25V. I tuned it to maximum then ( ~34V ), then tried to tune to minimum , and when it was near 2V it started to smoke. One side of the pot was still fine and output voltage regulated as it should be, but the other side of the pot ( connected to LM337 ) was burnt, so i guess that your theory was right.


Updated PCB image a bit too.
 

Your 5k potentiometer can conduct current of up to 10mA - absolute maximum - and here at the very end the current exceeds that value ..
With additional small resistor in series with it, it should work fine .. see attached picture ..

IanP
 

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    Lockshot

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Wow, thanks alot! So it should be 47-100ohm resistor 0.5W ?


~Lockshot
 

Lockshot said:
Wow, thanks alot! So it should be 47-100ohm resistor 0.5W ?
Click to expand...

0.5W is more than enough ..

Just be aware of one thing: say you use 47Ω, with this resistor in place the minimum output voltage will not be 1.25V but ≈1.75V ..

IanP
 

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    Lockshot

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IanP said:
Your 5k potentiometer can conduct current of up to 10mA - absolute maximum - and here at the very end the current exceeds that value ..
With additional small resistor in series with it, it should work fine .. see attached picture ..

IanP
Click to expand...

Ian, are you sure that is correct? That was my first thought but then I had doubts. The current through the 120 ohms will be roughly 10mA because the voltage between the Vout & ADJ pins will be Vref = 1.25V. That won't change unless the regulator cannot regulate, in which case the current will drop, no increase. The only other current through the pot will be the adjust pin current which is only 50uA.

The circuit is very similar to the data sheet (except the data sheet uses 240ohms not 120ohms).

Did both pots blow up or just one? I am uncertain that the 10uF from the adjust pin is a good idea - try removing it.

Keith.
 

Ian, are you sure that is correct? That was my first thought but then I had doubts.
Click to expand...
Keith, no, I am not sure, I/we can only speculate and try to point out the likely reason or reasons ..
I’d be surprised if both pots were damaged, these are like two independent circuits ..
Maybe one of these pots had “imperfections” ..maybe ..

 

I guess increasing the 120 ohms to 240 ohms wouldn't do any harm and stop the pot dissipation being at the limit.

Keith
 

    L

    Lockshot

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Only one part of dual gang pot smoked. The one thats connected to LM337 if it matters.

here is how it should look with resistor series with pot.

**broken link removed**

Uploaded with ImageShack.us


removed 10uF cap. Now it works good if its regulated to ~35V output ( maximum ). If i try it with ~<3k omh resistors. They just blow up.
 

If i try it with ~<3k omh resistors. They just blow up.
Click to expand...
Cool down, have one half working then do the other half ..
At this stage don’t do everything at once ..

The PCB is under design .. so, how do you test this circuit- do you try it on a breadbord?

:?:
 

The original circuit is exactly corresponding to the suggested National application and is supposed to work without additional resistors.
I don't know, why it's failing. Either something is broken or the input voltage is exceeding the maximum ratings.
 

Yes, on breadbord. Im a bit confused now, i replaced R1 and R3 with 240ohm resistors, and one half seemed to work good. Tried with different resistors to see output voltage, then tried with potentiometer. Everything worked fine. When i tried it with other half, i had the same problems as before. Rechecked schematic like 5 times and everything was connected correctly...
 


Ok. Let’s leave the “working” half and concentrate on the other half ..
First thing first: the LM337 has different pin allocation to the LM317 .. are sure you haven’t wired it the other way around ..

:?:
 

Nope, wiring is correct. Could it be that some part is burnt? When my pot smoked, i saw sparkling between pot pins, and there were still 30V output, even if i unplugged it.
 

If you replace the LM337 connected potentiometer with a fixed resistor, e.g. 120 ohm. Do you get the expected output voltage of about 2.4V?
 

    L

    Lockshot

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Lockshot said:
Nope, wiring is correct. Could it be that some part is burnt?
Click to expand...
Yes ..
Lockshot said:
When my pot smoked, i saw sparkling between pot pins, and there were still 30V output, even if i unplugged it.
Click to expand...
.. i'm lost here ..
 

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