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Old 1st February 2008, 12:02 AM   #1
johnmax is offline johnmax  United Kingdom
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Default Huge AC ripple voltage - Help Please!

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,

John
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Old 1st February 2008, 12:12 AM   #2
GK is offline GK  Australia
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Default Re: Huge AC ripple voltage - Help Please!

Quote:
Originally posted by johnmax
This is without any smoothing caps,

That is the problem.
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Old 1st February 2008, 12:23 AM   #3
johnmax is offline johnmax  United Kingdom
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They make that much difference? I don't have any cheap and nasty ones rated at a high enough voltage so I didn't put any in to begin with. Do those numbers seem to make any sense other than that?

Thanks,

John
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Old 1st February 2008, 12:49 AM   #4
GK is offline GK  Australia
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Quote:
Originally posted by johnmax
Do those numbers seem to make any sense other than that?

Yes they do. You will need some large value electrolytics here to cope with the current drain of your chip amp.
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Old 1st February 2008, 07:30 PM   #5
johnmax is offline johnmax  United Kingdom
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Just wired up the supply with 3200uF per rail (1x2200uF Jamicon 50V, 1x1000uF Jamicon 63V while waiting for some nicer components to arrive in the post!) and the multimeter is now my friend: +-37V and 0V of AC ripple.

Thanks so much for the help

John
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Old 2nd February 2008, 11:54 AM   #6
AndrewT is offline AndrewT  Scotland
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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?
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