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Transformerless power supply output help

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 28 November, 2008 » Transformerless power supply output help « Previous Next »

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Just joined
Username: mgb

Post Number: 1
Registered: 11-2008

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Posted on Monday, 10 November, 2008 - 04:38 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

A movement detector on an outside light failed recently which I replaced and then thought I would have a look at the old one and found that most of the rectifier diodes and the LM324 were faulty.
The original diodes in the bridge were one IN4148 diode and one marked 228 towards the DC positive and a similar pair to the DC negative, I tried replacing them all with IN4148 but I only get 9 volts output and nothing happening in the circuit.
I then tried replacing the diodes with one of the modular type bridge rectifiers and after that the detector apears to be working when connected to the mains but hardly detects anything at all.
I found it works perfectly if I connect 16v DC to the output points of the rectifier (with the mains off!)
When 240vAC is first applied to the detector and the relay latches I get 27 volts at the output of the rectifier but when the relay opens this goes to 40 volts.
When on DC the circuit draws 14 mA with the relay closed and 2 mA when it is open but on AC it draws around 23 mA all the time .
So I wondered if anyone might have any ideas about getting this working, info I have found on this type of power supply always has a zenner in the output but I can't find one in this.
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Username: hackinblack

Post Number: 320
Registered: 09-2006

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Posted on Tuesday, 11 November, 2008 - 07:08 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

the other two diodes in the bridge marked 228 where probably zeners,a crude regulator/ rectifier circuit.(normal diode back to back with a zener on both sides of the 'bridge')

the mains voltage is more than likely dropped down using a mains rated capacitor to cut the expense of a mains transformer.this is then 'regulated' in a rough fashion by the bridge regulator.
this is common practice in cheapo remote switches,dimmers etc.

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