# scr triggering circuit

Discussion in 'General Electronics Chat' started by pradhuman, Apr 20, 2012.

Apr 10, 2012
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0
i want to fire the scr whose anode voltage is at 24 vdc and cathode is connected to 24 vdc relay, now i think common value of igt in all scr are around 25 ma and gate voltage of 1.5 volt. now i want to make a voltage divider to get 1.5 v from the same 24 dc applied to scr's anode suppose two voltage divider resistors are r1 and r2 and want to get 1.5 volt acroos r2 second end of r2 is connected to ground now from the another end of r2 at which the voltage drop is 1.5 v i connect it to gate of scr, so what should be the values of resistor, whether i have to connect the resistor in series with gate to limit gate current????
i want to trigger the scr by push button swith placed between 1.5 v and gate
please explain the arrangement of circuit..
if any other better idea please suggest me, i don't want to damage scr

2. ### panic mode Senior Member

Oct 10, 2011
1,330
305
if the anode is at +24V and cathode is connected to relay (other side of relay connects to 0VDC) then the cathode is not at 24V level, it is at 0VDC. when you trigger the SCR, voltage drop across SCR is going to be small and cathode will be just below 24VDC (relay on).

you don't need two resistors for this, one will do.

R=(24V-Vgt)/Igt

If Vgt=1.5V and Igt=0.025A
then R=900 Ohm

but since this is DC, once triggered SCR will stay on as long as power is on

3. ### monishkumar New Member

Apr 20, 2012
3
0
The above solution worked for me!

4. ### RRITESH KAKKAR Senior Member

Jun 29, 2010
2,831
89
So, is there any specif need to use SCR over MOS??

5. ### panic mode Senior Member

Oct 10, 2011
1,330
305
why use MOS if you can use BJT?

6. ### crutschow Expert

Mar 14, 2008
13,509
3,385
The disadvantage of an SCR is that it has a much higher ON voltage drop then a properly sized MOSFET. In this application that's probably not significant.

An SCR is not a BJT.

7. ### panic mode Senior Member

Oct 10, 2011
1,330
305
is that how it sounds?
i never intended to state that. my question was if one is to use transistor, why pick MOSFET over BJT...

8. ### crutschow Expert

Mar 14, 2008
13,509
3,385
Sorry, I missed that transition.

MOSFET versus BJT depends upon the application requirements.

MOSFETs require zero DC control current but have a large input capacitance that needs to be rapidly charged and discharged for high speed switch applications.

MOSFETS require a larger voltage to turn ON (typically 10V for full turn-on but can be as low as 3V for Logic-Level type MOSFETS).

MOSFETs also act as a low ohm resistor when ON and can have a very low ON voltage, depending upon their rating and the drain current.

MOSFETs generally make much faster switches than BJTs.

BJT's only require about 0.7V to turn on but require significant current (for full-on switch, typically the base current should be 1/10 of the collector current). This can reduce switch efficiency.

BJT's have an offset component as well as a resistive component to their ON voltage. This limits the minimum ON voltage to the offset component of typically a couple tenths of a volt.