# Protection circuit to be used for a gate pulse of SCR

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Hello, I am doing a project on a temperature control system.My controller has been designed in LabVIEW and I get a control signal of 0-5V output from DAQ which needs to be given as gate pulse to SCR.We are using an industrial circuit board which consists of pulse transformers in between our SCRs and our gate pulse input. Since the control signal is being given through a DAQ, a protection circuit is required in order to protect the DAQ from reversal voltage which might damage the equipment.I was advised by few people to us opto coupler for isolation,but the inputs to the optocoupler should be in frequency so that I would be able to get a 0-5V varying output.Inorder to use frequency I would still require f to v converter and a v to f converter.When I checked the circuit for a f to v converter using 555 timer,i see that the output is square pulses.My question is whether it is possible to give a square pulse as gate pulse. If not, what are the remedies you can propose and also if other cost-effective and simple protection circuits are available please feel free to share. Thanks Winnie

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• posted

This must be a school project. Do you understand how an SCR works, and how that functionality is changed by having the pulse transformer in there?

The SCR is going to fire if it has the right voltage on it, and if the gate gets some current. Then it's going to keep conducting for as long as it has voltage on it and current is flowing. Gate current is only going to flow if the pulse transformer is receiving or has received a pulse recently.

Just converting the control voltage from the DAQ is going to result in a kinda-sorta monotonically increasing duty cycle command to the SCR, but it's going to be scruffy, nonlinear, generally weird, and (assuming you mean that the board has TRIACs, or complimentary SCRs) a tendency to average DC currents on the AC line induced by the unbalanced firing of the device(s) in the two directions.

Converting the control voltage to a slow, duty-cycle controlled square wave, then using that to turn a square-wave oscillator on and off, may work. So basically you'll be turning the heater full on, then full off, for quite a number of cycles of AC. When I say "slow" here, I mean less than 1 Hz -- and I'm assuming that whatever you're controlling the temperature of responds very ponderously. If you need better control than that you'll probably need to phase-control the SCR, and that's not something easily done.

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I have tested my circuit board with a 0-5V DC using an RPS and I was able to get variable AC at my output. If you say that the square wave generated from my 555timer will be sluggish, what other simple cost effective methods can you suggest so that I will be able to give a dc voltage itself and also protect my circuit.

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