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Old 14th May 2009, 02:14 PM   #1
diyAudio Member
Join Date: Nov 2008
Default Need help stabilising PSU

I'm helping a friend with a design of his, which is basically a touch sense light dimmer. He has a set of five 25W DC halogen bulbs and a control box, which has a lead coming off with a coil of wire behind a tile. I'm not 100% sure on how it works, but from what I understand when you put your hand near the tile, it uses the capacitance of your body to detect that you're there and dims or brightens the light.

His power supply is quite simple - a mains transformer with 230V primaries, 12V secondaries, rectifier bridge, one 10,000uF capacitor. From what I understand, he uses a JFET to control the current flow to the bulbs. From the 12V secondaries, he uses a voltage regulator to power the 9V logic rail. He's got a 1000uF capacitor from that rail to ground to try to keep it stable.

The whole thing works perfectly in itself, but the issue is that when other devices are turned on (e.g. power shower, ceiling fan, computer) the sensor often trips and turns the light on. When powered from a power pack (high grade switched PSU) it works perfectly with no tripping.

Short of buying a cheap 200W computer PSU and botching the ATX header up to a switch, any ideas?
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Old 16th May 2009, 06:57 AM   #2
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Join Date: May 2008
Add a good mains filter.

So your friend powers 12 V lights from a 12 V transformer + rectifier bridge + 10000 F capacitor -> up to 17 V DC?

By the way, there is no such thing as a DC halogen bulb. Filament lamps work with AC or DC just the same.
If you've always done it like that, then it's probably wrong. (Henry Ford)
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Old 16th May 2009, 01:18 PM   #3
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Sorry, I just assumed that it was a 12V transformer. It's actually 9V, I checked with him.

This look good?
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Old 16th May 2009, 04:30 PM   #4
Bakmeel is offline Bakmeel  Netherlands
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Join Date: Jan 2002
Location: The Netherlands
Originally posted by NinjaKitten
This look good?
This will work fine.
More Power Igor! More Power!
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