Electronic Circuits and Projects Forum

Hi,


1. You might need a resistor in series with the output of the opto to limit current through the opto and the gate.
2. Is that a DC meter there? You'd need a resistor across the meter to act as a decent load for the SCR or you may end up measuring the voltage due to the leakage of the SCR as well as the voltage when it is turned on.
3. Might also need a resistor across the output of the bridge rectifier to make sure the output goes close to zero volts so the SCR can turn off when required.
4. For better control you might be better off connecting the SCR in the ground lead of the bridge rectifier instead of the positive lead. This allows for better on/off control of the SCR, with the SCR cathode connected to ground.

Hi,

Actually you should connect the SCR between the bottom lead of the motor and ground.
Also, since you are using a controller to drive it you can make the drive pulse short. You'll have to check the specs on the SCR you are using though to find out what the pulse width should be and also the minimum drive current to the gate. The shorter pulse width keeps the gate dissipation down.

It's also wise to connect a resistor from the gate to ground. Usually 1k should be good, but remember that the other resistor has to be able to drive the gate up to the required voltage as well as supply the required gate current too.

Hi,

Oh ok, well you seem to be working with a very hefty SCR there, which would have a decent max average gate power rating too. That means you can probably drive it with 200ma continuous and not worry about the pulse duration. That should ensure turn on every time.

Usually the gate power is more important because the SCR packages are smaller. This one is quite a big one :-)

When will you be ready to test this?

Hi,


It sounds like you have the basics there already.

I would think you would trigger each half cycle though, so for any zero crossing (0 degrees or 180 degrees) you would do the very same thing, unless you only want half wave output. Full wave is better though.

The delay the uC waits is the half cycle time minus the required 'on' time.

It's also a good idea to build in some sort of 'over delay' protection, if you intend to fire at small conduction angles. This helps protect against accidentally triggering a bit too late, which instead of a small conduction angle (and small output voltage) you'd get a large conduction angle (and very high output voltage) which would defeat the required time delay setting.

Also, if you are using a positive trigger pulse you have to watch out what triac you pick. The triac you pick has to support a positive trigger with both polarities of MT1 and MT2. Some dont support both polarities with a positive trigger pulse, but most do with a negative trigger pulse.