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Triggering circuit scheme for a full bridge thyristor converter.

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abe94

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I'm designing a thyristor based fully controlled AC-DC converter. I've got all the ratings and part numbers figured out. What I can't decide is how to connect the triggering circuit to the main thyristors. I'm using four thyristors in a bridge configuration. I know how to apply this scheme to a single thyristor , but there seem to be no resources detailing the scheme for a bridge configuration. I'm using an RC circuit to generate the trigger pulses. The circuit is fed from the same AC source that's feeding the converter bridge. Could someone explain how to connect the triggering circuit to the thyristors ?
 

usually via smallish gate drive transformers, 100kHz, picket fence (50%) triggering... SCr's triggered over entire on time...
 

Easy peasy said:
usually via smallish gate drive transformers, 100kHz, picket fence (50%) triggering... SCr's triggered over entire on time...
Click to expand...
What purpose does the gate drive transformer serve ? Isolation ? Or anything else ? If that's the case , I could use an optoisolator. And how do I wire the o/p of the pulse generating circuit to the scr's ? I just winged it and performed Multisim simulation. Fed the gates of the four scrs from the same point. The simulation showed it behaving as a half wave rectifier. No output for negative cycle. So I guess I'm doing it wrong. Do I need to provide separate trigger circuits for the scr pair which would trigger during successive cycles ?

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Picket fence.. Hahaha
 

pulse transformers are capable of higher trigger current spikes than Opto's thus making the trigger more reliable with dynamic low ESR loads on startup.
 

yes gate Tx's provide isolation and the power for the gate drive, if using two only with two o/p's each then need a gate resistor to guarantee sharing.

Opto's are OK if you use the anode as a power source to feed into the gate, but are current limited, also need a diode to prevent excessive V reverse on your opto output.

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gate drive Tx's are usually used for higher voltage applications (>24VDC)
 

I'm going to post my schematic and simulation results. So you people get an idea about what I'm trying to do.View attachment scgg.bmp
IMG-20151024-WA0001.jpg
 

for a cathode gate SCR (which most modern ones are) the gate current has to go into the gate and out thru the cathode (k) to fire it...
 

scgg.png here's the schematic

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Sorry. I'm posting from my phone and i don't know why it isn't getting displayed as the simulation pic
 
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The circuit can't work. The SCRs would immediately blow in real hardware.

As Easy peasy explained, individual gate transformers respectively isolated output windings are required, only S1 and S2 can share a common winding (with individual gate resistors).

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In addition, the RC circuit is no proper way to generate trigger pulses. There must be at least a trigger device (DIAC, UJT), a discrete transistor circuit or an IC that sends pulses with sufficient current. And remember that SCRs require positive Igk/Vgk polarity.
 

unfortunately your schematic is far too simplistic and appears to be designed without a lot of background info on SCR's...
note that your electrolytic cap in the control will be charged in reverse half of the time :eek:/
time to do some research on the required circuits and how to drive SCR's and how to generate the required control...
 

Thanks ! So I can use two transformers with two scrs sharing one winding ?

Thing is , I did my research from text books. They list R , RC and UJT circuits as triggering methods. RC series circuits have moderate efficiency. I chose it because of ease of implementation. I could use a UJT or an IC , but the problem with all the above methods is that they're explained for half wave configurations using single scrs only. Since this is a bridge configuration , I have no idea how to proceed and how to wire the signals from the trigger circuits to the scr gates. Whether to use separate circuits for two pairs or what Could you give me some pointers regarding the wiring and the scheme ?
 

Windings can be shared for SCRs with common cathode. For your full bridge circuit, three separate secondary windings are needed because S3 and S4 don't have a common cathode.
 

Hmm. So one winding for common cathode scrs s1 and s2 and two for s3 and s4. The pulse generating circuit has to be one , but the trigger signal paths have to be isolated. Is that so ?
 

There's no functional purpose of using 4 SCR for a controlled rectifier bridge, you can use two SCR and two diodes instead. The trigger circuit can be simplified respectively.
 

Okay. So I have to replace s3 and s4 with two diodes and give the same trigger signal to the upper two scrs. Am I right ?
 

Yes, using series resistors (e.g. 100 ohm) to decouple both gates.
 

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