Reducing voltage of TL494 based supply? - diyAudio
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Old 29th July 2007, 06:17 PM   #1
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Default Reducing voltage of TL494 based supply?

I have a donor supply from a car amp that I would like to use with some amplifier modules that take +-35v max. The supply gives dual +-40v output @ 13.8v input. I would like to reduce to closer to +-30v @ 13.8v input.

I would like to know if the voltage can be adjusted without rewinding the toroids? The toroids are a PITA to remove because I only have a low wattage iron.

The chip is actually a KIA and not TL, but I doubt it matters. There is also a KIA4558S dual op-amp in part of the circuit.

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Old 29th July 2007, 06:26 PM   #2
luka is offline luka  Slovenia
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Hi

Yes you can, you will have to change feedback. I assume it is basic resistor devider. It is probably sensed from + to GND, via two resistors, one big, other is smaller(value). Smaller stays the same, you will have to change only upper one.

You could have feedback done too with those op-amps, but I think they are over and under voltage settings. You may need to thange those too.

Could you take one big/high resolution pic of whole board from top and bottom and send them to my mail. I would like to see how they have connected toroids... Thanks..
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Old 29th July 2007, 06:44 PM   #3
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Sure, I'll send some high res pics.

If I can't manage to lower the voltage the correct way do to my lack of skills, would say 8-9 high current diodes in series on each rail work? I know, that's a completely lazy and noobish approach to the problem.
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Old 29th July 2007, 07:04 PM   #4
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Luka, I uploaded the files to a host instead of e-mailing them do to the size.

http://s000.tinyupload.com/?file_id=...97806307010986
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Old 29th July 2007, 07:13 PM   #5
luka is offline luka  Slovenia
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hi

We will do this the right way, no diods in series.
First thimg for you is to find feedback, which probably will be connected to pin1, pin2 will constant voltage, made also by resistor devider.

You could also draw how is 494 around pin1, pin2...
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Old 29th July 2007, 08:17 PM   #6
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I am no good at drawing a schematic.

I hope this is understandable.....

Pin 1 (+IN) ---> 33k (R501) ---> +Vs
Pin 1 (+IN) ---> 1k (R503) ---> 6.8k (R505) ---> Pin 4 (Dead Time)
Pin 1 (+IN) ---> 1k (R503) ---> box cap? (C508) ---> Pin 5 (Ct)
Pin 1 (+IN) ---> 1k (R503) ---> 3.9k (R506) ---> Pin 6 (Rt)
Pin 1 (+IN) ---> 1k (R503) ---> box cap? (C526) ---> Ground
Pin 1 (+IN) ---> 1k (R503) ---> Large 1k (R512) ---> Ground
Pin 1 (+IN) ---> 1k (R503) ---> Transistor A1023 (Q503 & Q502)
Pin 1 (+IN) ---> 1k (R503) ---> 1k (R509) ---> Pin 10 (E2)
Pin 1 (+IN) ---> 1k (R503) ---> 1k (R510) ---> Pin 9 (E1)
Pin 1 (+IN) ---> 1k (R503) ---> Jumper to op-amp circuit

Pin 2 (-IN) ---> 22k (R504) ---> box cap (C507) ---> Pin 3 (FB)
Pin 2 (-IN) ---> 33k (R502) ---> -Vs

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Old 29th July 2007, 08:29 PM   #7
luka is offline luka  Slovenia
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Hi

I will try to decode this and get back at you tomorrow. I will draw shema for myself for better understanding. So until tomorrow...
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Old 29th July 2007, 08:40 PM   #8
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most amps are unregulated. that might be the case in this one.

try running the supply with a variable supply and vary the input voltage from 11-15V and watch the output voltage (use some load of sort like bulbs etc)

if it is indeed unregulated, adding a resistor from the deadtime pin to the +5Vref pin of the TL494 decreases PWM modulation so that should lower the output voltage.

from looking at the pic, try measuring the voltages across R507 (1M) it looks like this component in parallel with C509 appears to be the soft start circuit. if the voltage across this is around 5V, you replace R507 wiith a 1Mohm pot and decrease resistance until you get to the desired output voltage.
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Old 29th July 2007, 09:11 PM   #9
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Thanks for the input djQUAN.

The supply is indeed unregulated. At 12.5v input the output is down to around +-35v. Yes, R507 is in parallel with the 220uf 16v cap. I will measure the voltage across it in a bit.

Is this the best way to lower the voltage, or is it better to change the feedback?

Thank you both for your input and time.
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Old 29th July 2007, 09:21 PM   #10
luka is offline luka  Slovenia
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Hi

Ok now is it regulated or not? If not then there is no feedback to change and only way to change output voltage would be with rewinding trafos.
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