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Anonymous Poster #1

Protection Circuit?

04/30/2013 2:54 PM

I've lived a sheltered life and don't think I've ever seen sort of circuit before. Motor is 120V, several Amps. Are these components meant to be EMF protection or...?
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jack of all trades
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#1

Re: Protection Circuit?

04/30/2013 3:21 PM

Where did you find this weird circuit, a text book perhaps?

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redfred
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#2

Re: Protection Circuit?

04/30/2013 3:56 PM

If the diodes were actually a pair of SCR then I could say that it is a block diagram version of a motor control.

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#3
In reply to #2

Re: Protection Circuit?

04/30/2013 7:19 PM

True, true.

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#4

Re: Protection Circuit?

04/30/2013 9:58 PM

But it's weird, and in most places illegal, to have the switching components in the neutral line.

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#5
In reply to #4

Re: Protection Circuit?

04/30/2013 10:02 PM

Never trust a block diagram presentation as the complete circuit.

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#6
In reply to #5

Re: Protection Circuit?

04/30/2013 10:16 PM

Just as in speed races!

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#7

Re: Protection Circuit?

04/30/2013 10:23 PM

It's what you might do in order to measure the motor current perhaps - use the diodes to limit maximum voltage across the shunt during startup. Is there something else that might be measuring across the resistor?

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Anonymous Poster #1
#8

Re: Protection Circuit?

05/01/2013 8:09 AM

Actually, this comes from a circuit board I have. The motor's AC H line is switched on/off via a triac and microprocessor. The capacitor is (I think) a 0.1uF, the diodes are 1N4007 and the resistor is 220 ohm.

Traces DO extend from both sides of the resistor and these run across the board to a few other SMD components - one of which I suspect is a voltage regulator that appears to be related to the tact switches used for micro input. Interesting idea that motor current is monitored - I need to test for that.
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#9
In reply to #8

Re: Protection Circuit?

05/01/2013 8:51 AM

Ah that sounds like a zero crossing detection system. The 220 Ignore isn't small enough for current sensing.

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#13
In reply to #9

Re: Protection Circuit?

05/01/2013 7:05 PM

Ah - I love autocorrect (not). I sent the above from my phone. For "Ignore" substitute "Ohm".

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#11
In reply to #8

Re: Protection Circuit?

05/01/2013 11:03 AM

Not sure what this is for with this being a pair of diodes. My only guess is that the four parts act as a non-linear current snubber to reduce EMI effects.

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#10

Re: Protection Circuit?

05/01/2013 11:03 AM

It's most likely a snubber circuit to reduce the inductive spike from the motor back to the microprocessor when the SCR is commanded to turn off. My guess you found this inside a commercial apparatus or a household appliance that gets code approval as an assembly that normally doesn't get opened; i.e., labeled "No user serviceable parts inside"

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#12

Re: Protection Circuit?

05/01/2013 1:20 PM

Ar you sure the circuit is drawn correctly?

It the top component a contact or a capacitor?

This circuit mimics a standard "bootstrap" used on TRIAC circuity to provide a path for the leakage current across the PN-NP junction that is always present when using TRIACS for voltage control.

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gmphadte
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#14

Re: Protection Circuit?

05/09/2013 2:27 AM

Looks like a DC blocking circuit.

Gajanan Phadte
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Anonymous Poster (1); gmphadte (1); jack of all trades (2); Johny451 (3); RAMConsult (1); redfred (3); SHOCKHISCAN (1); Tornado (2)

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