greets,

I've got a EPS-112E PSU from a printer and I want to lower its output voltage from 40 to 30V. I found the schematic diagram online (see attachment). There are seven 5.6V zener diodes connected in series in the output stage, is there a reason why they did not use a single 40V zener instead? Could I just remove the zeners and solder in a single 30V z-diode?

Thanks

 

Why not get a piece of wire and jumper across two diodes?
No need to buy another component.

 

Why not get a piece of wire and jumper across two diodes?
No need to buy another component.

Good idea, thanks!

I'm still curious as to why they used multiple diodes instead of one...

 

By standard zener diodes, temperature coefficient will be close to zero at voltages of about 6 volts.

In some series zener diodes it's 5.6 volts and 6.2 volt in other series.

 

I would replace the zeners with a Tl431 programmable zener, and adjust the voltage with two resistors to fire the opto-coupler.

That means two resistors with a ratio of 11 times, so a 110K for R1, and a 10K R2, will give 30V.

https://www.google.co.uk/search?q=tl431+circuit&client=ms-android-sonymobile&prmd=ivsn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjgwoSjkODQAhXJCMAKHdjDCDAQ_AUIBygB&biw=360&bih=512#imgrc=_RJ1EuhMcU4QAM:

 

curious as to why they used multiple diodes instead of one...

The zeners are part of the error amplifier. Their primary goal is to detect if the output voltage is over a present threshold. If it is, the zeners start conduct and the led light up.

Using multiple zener here could be for power dissipation: high power zeners ate rare and expensive.

 

I would replace the zeners with a Tl431 programmable zener, and adjust the voltage with two resistors to fire the opto-coupler.

Which means that I could make a variable voltage PSU with that, pretty cool.

I was also thinking of just installing a switch that would switch between the zener diodes and change the output voltage in 5.6V increments. The only thing I'm not sure about is the (short) switching period, when the circuit will not get a value from the zener diodes, i.e. as if they were not there, would that be bad for the circuit?

 

No, as the voltage rises the Tl431 is very fast at conduction, they're use extensively in smps for computers.

You can make it variable with a preset resistor in series with the R1 resistor.