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| SCR Gate Drive Transformer |
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| ZeroResistance:
I am trying to trigger an SCR using a gate drive transformer. The SCR datasheet says Igt (gate trigger current) as Typ 2mA Max 15mA Vgt (gate trigger voltage) as Typ 0.6V Max 1.5V Currently I have hand wound a ferrite toroid core with 30Turns primary and 10 turns secondary and am giving a 10uS @ 12V pulse to the primary via a transistor, I also have a free wheeling diode across the primary. However at the secondary output I get around 8uS @ 0.5V. Its seems the upper limit I get for the pulse width is around 8uS no matter how long I stretch the pulse to the primary, so probably I'm hitting saturation limits of the toroid. The toroid cross sectional area is around 15sq.mm. I was expecting around 4V at the secondary due to the 3:1 turns ratio, so I'm wondering what's going wrong here, I'm just measuring the open circuit voltage across the secondary. Probably a hint or 2 would help me to proceed in the right direction. TIA |
| T3sl4co1l:
Right, 15 mm^2 * 0.3T * 30t = 135 uV.s, so at 12V and 10us you're very close to that. Give or take remenance, exact point you're calling saturation, and so on. I don't know why you're not measuring 4V on the secondary. That's when it's not connected to the gate, right? Also, you have a series current limiting resistor on the gate? Tim |
| ZeroResistance:
As of now I am meauring open circuit voltage across the secondary. Not connected the gate yet. I wound another toroid this time 1:1, 30T primary and 30T secondary and this time when I give 12V pulse in the primary I get 12V in the secondary. To clarify regarding the last transformer, I had taken 5 equal lengths of wire and then wound 10 T will all of them then I wired 3 of these in series to get a 30T primary taking care that I wired the end of the 1st to the start of the second. So using this I finally had a 30T primary and 2 10T secondaries. With regard to the resistor for gate i wasn't planning any resistor except for a reverse diode for SCR gate protection. Another point that I'm not clear with is I'm operating this transformer in unipolar pulse mode. Generally speaking transformers are operated in bipolar mode right?. So will this unipolar pulse eventually cause problems in the circuit. how do typical pulse transformers handle this? |
| schmitt trigger:
You very likely are exceeding the core's Volt-microsecond capability. Perhaps remanence is higher and you are reaching saturation earlier than calculated. But even then 135Vus seems to me a little on the low side to properly drive an SCR. If I remember correctly, they are at least 300Vus. |
| T3sl4co1l:
Without secondary side current limiting, you need it on the primary side then. Which is even better, as that reduces EMF --> more microseconds during the pulse. Tim |
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