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

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.

--
http://www.wescottdesign.com

Re: Protection circuit to be used for a gate pulse of SCR

between

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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Re: Protection circuit to be used for a gate pulse of SCR

Use a resistor in series from source to gate and a diode gate to
cathode for reverse polarity  protection.
Be advised that most SCR will tolerate reverse voltages up to 6V-8V
before zenering.