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Member

Join Date: Aug 2016
Posts: 8

AC Switch & Power Supply Reverse Engineering

03/08/2017 9:33 PM

Below is schematic of self-powered(by passing little current not enough to operate) switch reverse-engineered from PCB.

Zener diode(D7) seems like 1N4700 but not sure.

This is connected in serial with load. (one to mains live and one to load)

I omitted fuse and inrush current limiting varistor

How does this exactly work and is there a name for this circuit?

As far as I understand:

  • R3 and C2 is for smoothing
  • D1, D2, D5, D6 is diode bridge
  • C3 is for smoothing
  • D7 is for limiting voltage
  • When S1 is closed, current flows into R1

I can't figure out the purpose of D3 and D4/C1, R4, D8, D9 and S2/L1 and how it limits current(resistor value seems too small).

Also how can I calculate current?

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Guru

Join Date: Apr 2010
Posts: 5803
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#1

Re: AC switch & power supply reverse engineering

03/08/2017 10:14 PM

I'm just guessing here. The thyristor on the left is the actual solid state switch which switches the load on and off. In series with the load is a transformer (upper right). The voltage across the primary is limited by diodes D3 and D4, so you are just using a little series drop in voltage to provide power for the DC circuit (rectifier bridge and filter components, top and right side). The switches S1 and S2 are probably momentary and serve to turn on and off power to the load by controlling voltage to the thyristor (left). C3, C4 and L1 provide filtering for DC circuit, D7 is zener diode to regulate voltage.

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Guru

Join Date: Apr 2007
Location: Hyderabad, India
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#2

Re: AC switch & power supply reverse engineering

03/09/2017 4:46 AM

1. There is no thyristor in the schematic; they are Triacs

2. R3 & C2 are phase angle shifting.

3. 1N4700 is a zener diode. One digit after a hyphen gives the value of zener voltage. It is 0.5 watt zener.

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Guru

Join Date: Apr 2010
Posts: 5803
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#3
In reply to #2

Re: AC switch & power supply reverse engineering

03/09/2017 8:44 AM

You are correct, Triacs should not be called Thyristors.

Just curious, how did you get the zener diode part number from the schematic?

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Guru
Engineering Fields - Power Engineering - New Member

Join Date: May 2007
Location: NYC metropolitan area.
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#4

Re: AC Switch & Power Supply Reverse Engineering

03/09/2017 12:52 PM

Look here...

Call me "old school", but two pushbuttons and a latching relay seem a lot easier to manage.

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Guru

Join Date: Apr 2010
Posts: 5803
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#5
In reply to #4

Re: AC Switch & Power Supply Reverse Engineering

03/09/2017 1:58 PM

Methinks our OP might have posted the question there also. I've seen that happen more than once.

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Power-User

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#6

Re: AC Switch & Power Supply Reverse Engineering

03/10/2017 1:09 AM

The circuit looks like some kind of chopper, probably adjustable by the internal logic of the circuit. This type of hardware is not seen very often since it onvolves touching close to the AC mains.

If you want to know what the circuit does and how it works, go get some simulatiin software and try it. If you get it to work, you will know the principle.

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