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SCR and Zener regulator question

Bobby G

Aug 31, 2015
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Aug 31, 2015
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Hey guys hoping you can help me..a friend helped me build this circuit for rectifying 3 phase AC from an engine stator into regulated 13/14 volt DC. The issue I have is the SCRs are triggering after the AC rises past a certain point and are dumping off voltage instead of maintaining a constant DC voltage. Is the Zener possibly undersized, I'm in need of a gate cathode resistor, or the gate trigger resistors are wrongly sized to begin with? Thanks in advance!
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
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what do you mean "dumping off voltage"?

an scr is either on or off, and once turned on will stay on until the current through it falls to zero.

Using an act as a regulating device will allow overshoot. So perhaps what you're saying is the voltage overdoing then falling then overshooting again.

note that without a filter capacitor or something similar this circuit won't work
 

TedA

Sep 26, 2011
156
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Sep 26, 2011
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156
If this works at all, it is as a shunt regulator.

It is possibly that the three phase AC source has a high enough impedance that a shunt regulator is satisfactory.

When the zener diode conducts enough, all three AC phases are shorted to the + bus. Each AC phase is released when that phase's current nears zero.

Note that the diagram utilizes triac symbols, not SCR symbols, for the three power devices. The BTA26 is, in fact, a triac:

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00002264.pdf

Perhaps triacs might be the thing to use, as most SCRs wll not turn on to negative gate signals.

I believe that the TIP120 NPN transistor is shown connected backwards: The collector is shown going to the - bus, the emitter is positive. It is unlikely to work very well connected this way.

The TIP120 will live longer with a resistor in series with its base. The zener diode would also be happier. As the circuit stands, there is a low impedance path through the zener and transistor between the + and - buses.

The circuit may have difficulties if the AC frequency very much exceeds 50 to 60Hz. Most triacs are intended for AC line/mains frequency operation. Also, we might expect the impedance of each AC phase to be very inductive. So, there may be problems getting the triacs to "let go" once they are fired.

Is this a circuit "off the internet"? You might provide a link to any source for it.

It would be interesting to know something about the "engine stator", and the intended DC load.

Ted
 
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