i tried to extend it to full range, but still cant get +5V measurement at chip level. any way to modify the psu?
Posted on 2004-02-26 19:43:24 by dion
I'm gonna trust you're not extending the range to something the basic unit can't handle. It sounds like you have some sort of voltage drop between the smps and your device (not suggesting this, I'm asking)

OK, there are two ways to adjust the output on any closed loop system such as a power supply: either increase the reference voltage into the error amp, or decrease the measures output back to the error amp.

A well designed supply would have some sort of "remote sense" line that measures the output down at the load end. even a cheapie unit lacking this feature has the same sense inside it, you need to find it, open it, and insert some attenuater to increase the output voltage.
Posted on 2004-02-28 11:45:34 by Ernie

I'm gonna trust you're not extending the range to something the basic unit can't handle. It sounds like you have some sort of voltage drop between the smps and your device (not suggesting this, I'm asking)


yes, it is. some sort of voltage drop.
i need to test it at a real +5V level, so i can tell them (sega's staff) about the matter. if not, then i would blamed for the bad psu. btw, it is daytona machine, if anyone know about it :)

if you not suggest me this, what then?
Posted on 2004-02-29 20:35:45 by dion
I generally advise people against this, but you asked twice.

In a PS, there is a device that contains a reference and an error amplifier. The reference may or may not be available for you to measure. However, the input to the error amplifier is connected to a divider that has the top connected to 5V and the bottom to GND. You will have to increase SLIGHTLY the top resistor of the divider.

Typical PS's use a circuit called TL431, or LM431, or similar (in a plastic TO-92 package, just like a 2N2904 transistor).
The reference is only internal, equal to 2.5V. The divider is connected to pin 1 (with the pins down and the flat portion towards you, it's the leftmost pin). The resistor from that pin to ground is typically 2K49, but other values are possible. For a 5V output the two resistors will be equal (both 2K49 or whatever).

Increase slightly the top resistor (connect 100 ohm or so in series with it). Be careful, since around the TL431 there will be other resistors and capacitors that shape the characteristic of the error amplifier. Leave those alone. Correctly identify the divider.
Generally the only dividers connected to +5V are the one that regulates the voltage (that you need to modify) and possibly another one that protects against overvoltage (if you accidentally modify this one you will not increase the voltage, but worsen the protection).

If you do it right, the output will increase. I would only increase it by 0.1V to 0.2V.

In case your PS uses a different IC, you will still have a divider. The same applies (increase the top resistor), but the values can be different, depending on the reference.
Posted on 2004-03-03 11:59:43 by VVV

I generally advise people against this, but you asked twice.


sorry VVV for i am so forgetful. anyway thanks and actually i had solved it. it just a drop voltage because of cable.
Posted on 2004-03-08 03:09:43 by dion