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Old 16th March 2004, 04:53 PM   #1
lgreen is offline lgreen  United States
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Default Current Inrush Question - low power xfmr

Here is the situation, a 12 VCT transformer rated at 750 mA. The power supply connected to this is using the only caps I have, which are 4,400 uF for each rail. The circuit itself only uses about 100 mA, so this is all the power that is drawn in normal operation.

I am worried about a current surge to charge the caps when this thing is turned on, should I be worried about damaging the 750 mA rated transformer when powering on where the 4,400 uF caps are initially charging?

If so, what's the easiest way to do a little inrush suppression, connect one of these thermisters
http://rocky.digikey.com/scripts/Pro...S&V=45&M=CL-60

in series with the primary? or secondary?
other ideas?
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Old 16th March 2004, 05:11 PM   #2
HDTVman is offline HDTVman  United States
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I don't think you need to worry about it. The resistance of the secondary will current limit to some degree and the charge time is short for that value cap. The transformer will not heat up at turn on because the charging time is so short.

Later BZ
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Old 16th March 2004, 05:35 PM   #3
Eva is offline Eva  Spain
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50-60Hz transformers are usually very forgiving of load surges

This is due to the thermal inertia of all the copper mass, that allows to operate them even in short-circuit conditions for a fraction of a minute without enough temperature increase to damage anything. 120șC to 180șC are usually required to damage winding insulation

In the other hand, capacitors and diodes or other solid state devices may suffer damage due to these current surges, but this is hardly seen in 10VA applications
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Old 16th March 2004, 05:55 PM   #4
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Default Surges

Your transformer will be fine. Its your rectifier diodes you have to worry about. WIth caps that big, you may blow 1 Amp diodes. I suggest at least 6 amp diodes be used to handle the turn on surge.
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Old 16th March 2004, 06:04 PM   #5
lgreen is offline lgreen  United States
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Default Diodes

FYI, should have mentioned this but the diodes are MUR820, rated at 8 Amps 200V, and are not heatsunk.

While we are at it, what about the on/off switch? This is rated for 120 VAC and 6 Amps.
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Old 16th March 2004, 06:31 PM   #6
subwo1 is offline subwo1  United States
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All should be fine.
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Old 16th March 2004, 06:46 PM   #7
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If it is a toroid it could trip your automatic fuse. You could use two thermistors in series with the primary. Check the power supplies from the Aleph amps from passlabs.
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