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soon1
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Join Date: Jun 2009
Posts: 23

Switch Mode Power Supply

12/02/2009 1:14 AM

Hi friends,

I have a switch mode power supply , 12v dc output. when i use meter at dc range give 12v dc, but when i put meter meter to ac range give 25v ac , why ?

It the switch mode power supply faulty ?

Thanks.
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anand.mandarapu
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Join Date: Nov 2009
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#1

Re: switch mode poer supply

12/02/2009 2:36 AM

Hi,

1. The voltage reading showing in AC range selection is the ripple voltage.

2.The reason for showing high voltage is :Most of the digital meters are not shown

the V rms voltage, it shows the Vmax. that is the reason it showing high voltage(Vmax).

In SMPS Vmax is almost 40% more than the rated volatage to maintain the constnat voltage in wider input range/fluctuation.
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soon1
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Join Date: Jun 2009
Posts: 23
#3
In reply to #1

Re: switch mode poer supply

12/02/2009 7:18 AM

thanks.

can the ripple voltage cause my sensor to mulfunction.

thanks.
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redfred
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#9
In reply to #3

Re: switch mode poer supply

12/03/2009 9:55 AM

It might, depends on the sensor.

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Haajee
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Join Date: Feb 2006
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#7
In reply to #1

Re: switch mode poer supply

12/03/2009 4:04 AM

GA

switch mode poer supply if output terminals not properly filtered or an open unit can behave, it is possible that on AC position only 1 test-lead touches the terminal gives indication.

Twisting test leads upto the end to aviod pick-up may help.
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luv123
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#2

Re: switch mode poer supply

12/02/2009 6:15 AM

yes i agree mr.Anand it is Vmax.
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mjb1962853
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#4

Re: Switch Mode Power Supply

12/02/2009 11:45 AM

Meter is reading AC noise ( ripple ) on the output voltage . A properly loaded and running SMPS should have VERY LITTLE output ripple. With NO load or malfunction, noise can be large.

Verify that the supply has the minimum rated load for stability and re-test.

If noise is still present, supply is bad (or design is bad).
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Snaketails
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#5

Re: Switch Mode Power Supply

12/02/2009 11:23 PM

Question, what load are you measuring the output?

Is it O/C voltages?

50% load

100% load?

Try not to measure your PSU at no load conditions, the SMPS is having enough problems trying to stabilise the output (possibility for the high ripple)

Output's of SMPS's should be measured while loaded, this will enable the control circuit to do its job.

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redfred
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#10
In reply to #5

Re: Switch Mode Power Supply

12/03/2009 9:59 AM

Also, some switch mode power supplies require and specify an output capacitance added to meet the stated ripple parameters. It is usually best to test these supplies with an oscilloscope instead of a multimeter for often the ripple frequencies are higher than a multimeter can measure.

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Anonymous Poster
#6

Re: Switch Mode Power Supply

12/03/2009 12:06 AM

The output voltage of switch mode power supply is very changable.

The average or mean value of it is measured in the DC range whereas the RMS value is represented in AC range. It is wellknown that RMS value for this kind of signal is higher than that of average value.
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redfred
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#8
In reply to #6

Re: Switch Mode Power Supply

12/03/2009 9:53 AM

No. Any power supply that has a very changeable power supply is a failing power supply. But I'll count this one as an error from English not being your native tongue.

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kaztronics
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Join Date: Aug 2007
Location: Canada
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#11
In reply to #8

Re: Switch Mode Power Supply

12/03/2009 10:37 AM

I've repaired hundreds of SMPS, 5,12,24,..400V etc., any DC output should be 5,12,24..400V etc. regardless of the load (within the specs),there are rare exceptions. Excessive ripple (above the specs) point usually to bad AC filtering (caps). Ripple that's above rated DC output, that's really bad, I'm surprised it's still working at all.
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subhajit
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Join Date: Jul 2009
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#12

Re: Switch Mode Power Supply

12/03/2009 1:31 PM

Hi friend, that problem due to DC filter. check the DC filter capacitor and coil. thanks subhajit
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Andy Germany
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#13

Re: Switch Mode Power Supply

12/03/2009 2:03 PM

If it was definitely volts and not millivolts, you have a problem to fix.

SMPSs are usually very clean voltage wise!!! When installed and working correctly of course!

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Anonymous Poster
#14

Re: Switch Mode Power Supply

12/05/2009 7:37 AM

No Nothing wrong with your Power supply

AC Reading and DC reading are not same

to Clear your Doubt i suggest a small test to perform by your self

Take a Battery Charger Or Simply a Mobile Phone charger and meassure output Voltage by DC scale and AC scale and you can see your problem solved then and there.

Check and reply me your reading for my furhter comments.

also Write to me the Voltage written on the phone Charger which you have taken for test

Harri
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Users who posted comments:

anand.mandarapu (1); Andy Germany (1); Anonymous Poster (2); Haajee (1); kaztronics (1); luv123 (1); mjb1962853 (1); redfred (3); Snaketails (1); soon1 (1); subhajit (1)

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