Lower output voltage from 19.5V to 8.5V ?

[Gabriel]

Got this HP 19.5V 6.9A 135W adp-135fp built by Delta.

I want to lower it to 8.5 to power a slim ps2 8.5V 5.3A.

I did this on another smaller psu by adjusting the resistors on the TL431 TO-92 package but I see no TL431 on this psu - Any other way to adjust this psu ?

[mariushm]

Re: Lower output voltage from 19.5V to 8.5V ?

See where those two optocouplers are connected to on the secondary side (the one with lower voltages). Follow the circuit and you should reach a chip which does the stuff that TL431 did in the other power supply.

The LM358 on the back side is an operational amplifier (opamp) which MAY play a part in detecting the output voltage and comparing it with a voltage reference and telling the actual controller to "slow down" or increase the frequency to maintain the voltage within defined parameters. Playing with the resistors around the opamp may do something.

The actual controller may be the one under that goop of thermal paste, a bit higher and to the right, compared to the LM358 IC. Try moving that paste a bit to the side to see what's written on the chip. If you're lucky you'll find a datasheet for it which may help you.

I have to say though, it may be the case that the main transformer is not "optimized" to give you such low voltage like 8.5v at that current. I'm a bit reluctant to say you'll be able to adjust it by that much - I think less than 14-15v would be... well, you'd have to try and see if it works right, stable, not overheating etc .

[Gabriel]

Re: Lower output voltage from 19.5V to 8.5V ?

Thanks for your answer.

Removed some of the goop and the other so8 is a das001 from ST.

The two optocouplers are both connected to the das on the secondary side through smd caps and resistors.

I found a very interesting article on the das001 and changing voltage - Link

Anyway, to find the new values of the resistors in the voltage divider they must first be located. This can be done by connecting the output jack to an adjustable external power supply. Then measure the connection to the optocoupler and start changing the voltage of the power supply. The beauty of this method is that the primary of the SMPS is not connected to mains power! If the probe tip of the multimeter slips the power brick won't explode or go phut in some other way.

I agree that lowering the the voltage by 11V is too much - most likely it wont work.

[ben7]

Re: Lower output voltage from 19.5V to 8.5V ?

The 'DAS001' chip appears to be, from my google searches, a TSM103.
It is a dual op-amp with internal 2.5V voltage reference. My guess is that is for the feedback part of the SMPS

[Th3_uN1Qu3]

Re: Lower output voltage from 19.5V to 8.5V ?

This would be a drastic output voltage change, so the power supply will behave erratically unless there is a fairly high and constant load placed on it. Otherwise, it'll pretty much go "tick-tick-tick" with accompanying pulses on the output voltage. Switchmode PSUs lend themselves better to mods that increase the output voltage ather than decrease it.

[ben7]

Re: Lower output voltage from 19.5V to 8.5V ?

This would be a drastic output voltage change, so the power supply will behave erratically unless there is a fairly high and constant load placed on it. Otherwise, it'll pretty much go "tick-tick-tick" with accompanying pulses on the output voltage. Switchmode PSUs lend themselves better to mods that increase the output voltage ather than decrease it.

Yes i do agree with this.

I think it has to do with the winding ratios of the power transformer, correct?
As in, the transformer is designed for, say, 12V output when it is used in a 12V PSU.