TRIAC in place of SCR

Thread Starter

MGPERF

Joined Jun 8, 2021
52
Hello,

I am wondering about using a TRIAC in place of an SCR. It is DC voltage 390 volts and has a trigger circuit of around 50 ma delivered to the gate of the SCR. Even though it is DC power a TRIAC would be beneficial as it would allow an easier path for excess current to pass through and be reused vs. the primary of the transformer. I cannot seem to find any data that would give me an idea of whether or not this would work or using a TRIAC in place of an SCR.
 

Thread Starter

MGPERF

Joined Jun 8, 2021
52
When ground is broken from pins 5 and 6 it discharges the capacitor into an ignition coil. I am thinking a TRIAC would allow the leftover current from the ignition coil to travel back into the capacitor without first going through the transformer windings.
 

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Ramussons

Joined May 3, 2013
1,082
A triac may not solve your problem because it needs to be triggered again to allow current flow in the opposite direction.
 

Ian0

Joined Aug 7, 2020
3,459
Hello again! Is this a different CDI circuit?
Whether it's a triac or a thyristor, I'm not too sure that it is going to enjoy shorting out 0.47uF charged to 390V!
 

Thread Starter

MGPERF

Joined Jun 8, 2021
52
Hello again! Is this a different CDI circuit?
Whether it's a triac or a thyristor, I'm not too sure that it is going to enjoy shorting out 0.47uF charged to 390V!
Ian0, this is more or less my own circuit. I got the first one to work however this one is much more compact and uses a flyback as a power source :) I am also hoping a flyback with an IC will allow the transformer to keep adding energy/electrons to the spark increasing duration/adding energy. Regarding shorting C5 it is a smoothing capacitor and I was under the impression that it would be ok to short it because by design more or less all CDI's dead short the capacitor right? If I just deleted C5 from the PCB it would still work right? By delete I just mean not solder in. The track lines are larger than C5 I am also guessing I could fill the holes in with solder.
 
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Thread Starter

MGPERF

Joined Jun 8, 2021
52
A triac may not solve your problem because it needs to be triggered again to allow current flow in the opposite direction.
Ramussons, the TRIAC will be turned on for 400us the capacitor will fully discharge in 100-200us I am thinking the extra 100-200 us current is flowing will allow the excess current to be recycled into the system. Is my thinking sound? Does a triac only allow current to flow in one direction each time it is triggered? I was under the impression it allowed current to flow through it in each direction every time it is triggered. Thank you for your response.
 

Ian0

Joined Aug 7, 2020
3,459
I was under the impression that it would be ok to short it because by design more or less all CDI's dead short the capacitor right?
Often it looks that way from the circuit diagram, but you'll find that the primary of the coil is in the discharge circuit.
 

Thread Starter

MGPERF

Joined Jun 8, 2021
52
Ian, pin 3 is the output to the coil and pin 4 is the ground to the coil. Would this cause a short? It is my understanding that it technically wouldn't however C5 technically could right? Could I replace C5 with a diode in order to recycle excess energy without causing a short?
 

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crutschow

Joined Mar 14, 2008
27,891
pin 3 is the output to the coil and pin 4 is the ground to the coil. Would this cause a short?
No.
But C6 can do nothing since it is connected in series with diode D3 (what's the purpose of D3?).
The diode allows C6 to charge but not discharge.
C5 technically could right? Could I replace C5 with a diode in order to recycle excess energy without causing a short?
Yes, C5 is a short across the TRIAC and does nothing useful.
Indeed, you can replace C5 with a diode to conduct in the reverse direction (anode to ground).
You can then use an SCR instead of a TRIAC.
 

Thread Starter

MGPERF

Joined Jun 8, 2021
52
Crutschow, ok so something like this? So this would allow leftover current to flow back into the capacitor/coil positive right? Also I took out the diode.
 

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Thread Starter

MGPERF

Joined Jun 8, 2021
52
Crutschow, thanks for the response I will try it with my scope and let you know how it goes. I am also considering using a diode bridge instead of the 2 diodes in a row. This would allow power to flow back through the circuit without going through the transformers coil. I have already had the PCB made unfortunately but I'll just put a jumper wire where D3 should go and ask them to leave D3 and C5 off.

Thanks,
Jon
 

Ian0

Joined Aug 7, 2020
3,459
Are you using the LT3751 (for which the transformer was designed)?
If you use a four-diode bridge, there will be a load on the secondary during the MOSFET ON-phase, this will reduce the energy stored in the flyback inductor, so probably isn't such a good idea.

On thing bothers me about this circuit - and I'd probably have to get it into SPICE to check - each output pulse from the flyback inductor puts a voltage across the ignition coil. Does that result in enough secondary voltage on the ignition coil to cause spurious sparks?
 

Thread Starter

MGPERF

Joined Jun 8, 2021
52
Ian, interesting you say that because when I put the diode bridge together in Spice it took longer for it to fully charge the capacitor not by much at all but still longer. The LT3751 operates at 100KHZ so I think that would cause no response from the ignition coil but I'd have to check that to be sure. With the circuit below it is my understanding that unless the points are open and the SCR is conducting any energy from the transformer can't make sparks at the coil. Once the SCR is conducting however it would be advantageous and add significant spark energy right? Also I am thinking the diode D3 will extend the spark by re-using excess energy that was sent to the coil and would otherwise flow through the transformer secondary. Would it hurt to have the current flow through the transformer secondary? I am guessing it would eat up energy output. I don't currently have the PCB with that diode in it but I could use the holes that were originally from C5. Regarding the original diode on the output I am just going to use jumper wire to connect one SMT pad to the other.
Thanks,
Jon
 

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Thread Starter

MGPERF

Joined Jun 8, 2021
52
This article says what can be done to work around a system where you don't have bridge rectifier. I am not quite sure what they mean though any ideas what this would look like in the circuit.
 

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Orson_Cart

Joined Jan 1, 2020
76
Just remember than triacs are usually slower to turn off than SCR's, you can use a triac in place of an SCR - but it should not get hotter than 80 degC, and you need to be careful with the gating to ensure you are not trying to use it in quadrant 3.
 

Ian0

Joined Aug 7, 2020
3,459
Just remember than triacs are usually slower to turn off than SCR's, you can use a triac in place of an SCR - but it should not get hotter than 80 degC, and you need to be careful with the gating to ensure you are not trying to use it in quadrant 3.
It will need a negative voltage to trigger it. Otherwise, it might not trigger with the MT2 voltage negative of ground, in which case you are in exactly the same situation as with a thyristor.
How about an IGBT or a 600V MOSFET? They weren't around in the heyday of capacitor-discharge ignition. Even IGBTs switch faster than a triac!
 

Ian0

Joined Aug 7, 2020
3,459
This might be a daft idea, but perhaps not - if you use a MOSFET or IGBT perhaps it can be switched off at just the right point on the ringing waveform to leave the maximum amount of voltage on the capacitor. Sort of like a Quasi-resonant flyback converter in reverse.
 
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