Issue: Power Supply voltage increase on its own

Hi there,

How are you all doing?

-Circuit-

I have built a power supply using a transformer that has multiple outputs (7), ranging from 2.5v to 35v.
I use an 8 way switch to switch between the 7 available voltages and then run them through a full wave rectifier and a capacitor(10uF) to smooth out the DC output.
Parallel to this I have an AC out for which I just dont run the current through the rectifier.
The LED is connected to the 2.5v out so I do not have to deal with a changing voltage in the case I connected it to the DC_OUT
*The transformer, again is multitap. I didnt have the symbol in Eagle thats why there are so many cables running to its center. "Ground" comes out from the secondary coil.*



-Situation-

Recently I acquired a DMM and when measuring the voltage out, I noticed the voltage went up on its own. It kept rising and rising, say at the 2.5v setting it was going up to 15v.
However when I applied a load to the output, the voltage wennt quickly back down to its intended level.
(I had previously used an analogue multimeter and never noticed this rising in voltage.)

-Question-

My question is, does the rising of the voltage have anything to do with the DMM or is there something else causing this? In that case, what could it be?

Thanks in advance,

GranTotem


DMM - Digital Multimeter

I think it will be helpful if you post the circuit diagram you used but with all indications your dmm seems to be faulty try using another dmm and see what happen.

I have found this effect many years ago. the problem is that any spikes or noise on the mains gets through the diodes and pumps up the capacitor. With a 10 Megohm DVM, there is no load on the capacitor so the peak voltage is indicated, however if it this effect a small load, say a 10K resistor wired in should stop this effect. FYI the circuit where I found this effect was for a rectified mains supply. The original design had a selenium rectifier, this transmitter was a "high reliability" version where the diodes were changed to Si avalanche types, the DC then went up to 460 V. wiring in a 220K resistor then gave me 230V DC.
Frank

open circuit voltage will be high, you need to give a bleeder resistor load on the DC output side like 10Kohms to get a stable output
the DMM is all right