Pls help me on AC to AC tirac circuit (Constant current source)

[Eric911]

Hi all,

Right now, i met a issue. I should build a 1.8Amp (max voltage 80v) constant current source for a risistive load. I have experimented with PWM on the DC side etc but on small loads <0.1R the circuits did not give out expect results. So i want to use triac for application.

But this is issue, when we give out trigger pluse, we couldn't shut off the triac at half a cycle. So how to manage the trigger pulse when load change or input voltage change?

how to realize the constant current of tirac?

Any help is welcome.

 

[alec_t]

the circuits did not give out expect results.

What results did you get. Can you post a schematic?

how to realize the constant current of tirac?

Not understood. Since current through a triac is AC it is varying continuously, not constant.

 

[panic mode]

"we couldn't shut off the triac at half a cycle"

just reverse the time ;-)
seriously you may want to lookup phase angle control

https://en.wikipedia.org/wiki/Phase_fired_controllers

but probably better would be to first post your DC circuit so that one can be fixed (if something is wrong with it).

 

[Eric911]

Tks for alec_t and panic mode replies.

Allow me explain it again. we have a resistor (constant resistance), this resistor will be series with Tirac. But the input voltage will be change from 108V to 132V. so if we want flow through the resistor of current keep constant (RMS), how to control the triac? how to add the feedback? Pls advice.

Any info is welcome, tks.

 

[alec_t]

But the input voltage will be change from 108V to 132V

Are those AC RMS values?
Is the change random or stepped or .......?
Over what time span does the change occur?

 

[Eric911]

Thank you for your reply! alec_t!

Yes, 108V and 132 are AC input voltage, they're RMS value. same as current, they're RMS too. They're random from theory, but If that, the design become more complex, so we think the input voltage is change step by step. not transient.

Any info is welcome, tks.

Are those AC RMS values?
Is the change random or stepped or .......?
Over what time span does the change occur?

 

[panic mode]

i guess "reversing time" was not clear. as you have found out triacs and scrs do not turn off after they are turned on. that is they stay on until zero crossing. sounds like you are turning it on, then waiting for certain value (current) and trying to turn it off. obviously this does not work. you have to change the order...

detect zero crossing, fire triac with some delay from the moment where you detected zero crossing.
delay has to be adjustable in range that is 0...T/2 (or 0..1/2f).

initially you can start with larger delay (so triac conducts only near the end of AC halfperiod),
you need to measure current and if the measured value is below the setpoint, reduce delay, if it greater increase delay.

and this is how 'time reversal' works, (actually only graph looks time reversed):
you turn triac on mindway into halfperiod, and let it turn itself off at the zero crossing.

 

[alec_t]

you need to measure current

To measure true RMS current will be difficult because of the chopped waveform. An averaged rectified value would be easier to measure, but would that meet your requirements?