TL783: potentiometer keeps burning

Hi Art,

Unfortunately this is a very old circuit that has obsolete parts in its design. We do not have any better understanding of your customer's circuit than they do so we will not be of much help here.

Regards,

Nick

Hi Nick,

Any form of help is highly appreciated. I will direct customer to this thread. If they explain their use case, it might get clearer.

Thanks and regards,

Art

Overview of the circuit I’m going for is a 1.25 to 120 adjustable DC power supply, able to carry up to 10 amps. At this point, I’m just trying to get the adjustable part to work, without smoking my potentiometers. It seems to work fine without the pass transistor circuit, but as soon as I add that in the mix, I let the smoke out. I’m open to try other designs if this one will never actually work, but I’d like to understand why this is happening. 

Hi Art,

I'm not an expert on it, but I guess the triode you omit function as current limit. when the in port of TL783 take lots of current,{ I think I was wrong about this line: [this triode open the main triode.]}

I suggest try to reduce the 1k resistor if the circuit cannot be change, cause it might limit the speed of the current boost part, and normally mosfet open faster than triode make TL783's inside mos take most of the load current. It might be the reason if the potentiometer burn when the load apply.

please let me know the result.

Thanks!

All that sounds legitimate, except that I haven’t even gotten to the point where I’ve applied any load. I’m still in the circuit design phase, just trying to validate the functionality of the circuit under no load conditions. Bypass transistor = burnt potentiometer. Can’t figure out why. 

Yes, R2, which is the 10 k ohm potentiometer. It will burn up when I start ramping down the voltage. 

That’s what I was thinking, just not sure how high a wattage rating would be needed. I tried a 2 watt rated pot and had the same issue. 

Beside, be advise that the power dissipation capacity may have a temperature limit. On your board you may need to derating the max power dissipation.

Agreed. I guess I’m just confused as to why I don’t have the issue until the bypass transistor is introduced. It’ll tank down the voltage till Scott 30-40 v, then burn out. I was thinking the issue wasn’t the wattage rating of the pot, but maybe something else. Guess I’ll try a higher wattage pot, but I’m skeptical until I can understand why it only seems to fail when the pass transistor is integrated into the circuit. 

Is this what you’re referring to? Just re-read this in the dataset.

“To maintain II(ADJ) at a low level, all quiescent operating current is returned to the output terminal. This quiescent current must be sunk by the external load and is the minimum load current necessary to prevent the output from rising. The recommended R1 value of 82 Ω provides a minimum load current of 15 mA. Larger values can be used when the input-to-output differential voltage is less than 125 V (see the output-current curve in Figure 12) or when the load sinks some portion of the minimum current.”

No idea for now. Maybe try to measure the voltage on R1 with transistor  connected to make sure the transistor part working as we expect.

By the way, huge current up to several ampere flow through 10Ω,1Ω, not R1,R2. Typed wrong, sorry.

I reviewed the data sheet and forgot about this: 

“To maintain II(ADJ) at a low level, all quiescent operating current is returned to the output terminal. This quiescent current must be sunk by the external load and is the minimum load current necessary to prevent the output from rising. The recommended R1 value of 82 Ω provides a minimum load current of 15 mA. Larger values can be used when the input-to-output differential voltage is less than 125 V (see the output-current curve in Figure 12) or when the load sinks some portion of the minimum current.”

 I’m testing the circuit design with no load, so I think, based on what the data sheet is saying, all the circuit current, which is considerably more when the pass transistor is introduced, is returning to the output terminal. Think this may be the issue. Unless I’m reading this wrong. 

Maybe you can confirm it by measure the circuit's idle current with and without the transistor.

For now I suggest connect multiple potentiometer in parallel or use other methods to temporarily prevent the potentiometer from burning for debug.

Yeah, I’ll give that a run as soon as I get back. Have to leave for work for a few days. I’ll follow up when I have something solid. Thanks.