Huge AC ripple voltage - Help Please!


2008-01-22 8:38 pm
Hi there,

Sorry if this is a very noob-ish question, but I am at my wits end. I have built a transformer to power my chip-amp, but I am a bit concerned before linking it up.

The transformer is a 160VA with 25V dual secondaries, running through 2 bridge rectifiers to produce +-35V DC (ish - 25V*1.4 less a bit of Vf for the bridge).

However. When I hook up my multimeter to it, I measure only 25 volts dc between the +ve and 0V, or -ve and 0V rather than the 35, but I can't understand why this is the case? Surely there is no way it can be producing +-25V, is there?

Secondly, and more worryingly if I change the meter to AC mode, it registers 50V from either rail to 0V, and 104V in ac from rail to rail! It is not the best of multimeters,

This is without any smoothing caps, or any regulator, I haven't connected it up to anything else yet as I don't want to fry components or worse. If you have any ideas, or suggestions then I would be delighted to hear them - I have been googling for hours, and I understand that there is some ripple AC voltage expected, but surely not 100V when the transformer is only 2x25!!!!

Thanks in advance,

Hi John,
some DMMs can measure low values of AC voltage or ripple when connected to high DC voltage.
I wonder if your DMM can do this?

Set DMM to 200Vdc and measure the DC voltage across the smoothing caps.
Set DMM to 200Vac and measure the AC voltage.
Reduce DMM setting to 20Vac and read off voltage. is it still near zero?
If so, then set to 2000mVac and read off AC voltage. is it still near zero?
If so, then set to 200mVac and read off the ripple voltage on the cap.
The meter tries to read the rms voltage and due to the simple circuits in most hand held DMMs does a bad job of reading rms voltage. But if you multiply the indicated reading by 3, you get an approximate peak to peak ripple voltage reading (mVpp). I find this compares well to the Vpp obtained with an oscilloscope with my three very different DMMs.
Now add a load to the smoothing cap. 10k resistor will do. Does the ripple voltage increase? Add a 4k7 resistor as a further load. Has the ripple increased even further?