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# [SOLVED]scr gate trigger voltage question

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#### speedshop

##### Junior Member level 3
I need to build a circuit where the trigger voltage of a SCR is corrected for a temperature range using a thermistor, so that the voltage at which the scr triggers remains the same over a range of temperatures.

I'm guessing I need to match the V/T ratio of the gate to the r/t ratio of the thermistor. Is there a standard table or method for selecting components?

For example I'd be using a BT151 series sct and a 100R thermistor.

Obviously, device data won't allow to calculate the compensation. I guess, you realized, that the datasheet rather focusses on trigger current, according to common SCR applications. You would need to perform a compensation based mainly on empirical measurements I think.

I assume however, that for sensitive gate devices the systematic silicone diode T.C. of -1.9 mV/K will make the major part of temperature dependency. By the way, what's your application? A crow bar overvoltage protection circuit?

Its purely an exercise I have to complete, to produce a switching circuit that remains stable over a range of temperatures using basic components. The gate is not overly sensitive, hence i choose an SCR that would be used for switching heavy loads such as motors.

The data sheets for thermistors shows groups of R/T characteristics I and wondered if there was some tie up with those for SCRs?

For example these Epcos thermistors are grouped according to their R/T characteristic eg 1203, 1305 etc:

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Its purely an exercise I have to complete
So I can save further arguments, why I don't consider voltage control of a SCR reasonable at all.

NTCs have rather an exponential than linear characteristics and aren't generally well suited for linear temperature compensation. Neverless, you can usually design a resistor circuit (series and parallel resistor), that gives a three-point matching with the intended linear characteristic.

Silicone PTC (e.g. KTY8x) have a more linear temperature characteristic and are often better suited for a wide range linear compensation.

Use a comparator comparing input dividing voltage with a bandgap-based reference voltage. The comparing result will decide if a current is injected into SCR for trigger.

Thanks for your input. I now have the answer. I obtained a general manual for AVX's range of NTC thermistors, it revealed methods for connecting and a formula to select resistors to make the ntc react in a more linear fashion.

I even got on to AVX and they sent me some software to aid. Although I've not tried it as I need to find a PC to run it on.

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