[SOLVED] scr gate trigger voltage question

Status
Not open for further replies.
S

speedshop

Junior Member level 3
Joined
Apr 13, 2010
Messages
26
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,486
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:
**broken link removed**
 
Last edited:

Its purely an exercise I have to complete
Click to expand...
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.
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…