SCR controlled by A microcontroller

[john120]

Hello can you plz help me to think on how I can control and visualize the current and voltage signals on the oscilloscope using microcontroller PIC16F877A of three phase rectifier with scr and half wave rectifier using scr.

Thanks

 

[schmitt trigger]

John;
what I have suggested to anyone attempting to control a three phase bridge, is to experiment with it at both low power and low voltage.

Purchase three transformers, rated (240 or 120v) primary and a secondary of 12 v, 1 amp. Fuse each output individually with a fast fuse.

There are many, many textbooks and internet resources indicating how the pulses should be timed....bear in mind that timing is very critical, and if you mess up a pulse and then cross conduction ocurrs............booom!

Unfortunately, the way to actually understand a circuit is to have a go at it...... read thru textbooks, simulate, write some code, build a prototype, and apply power to it.
Hopefully it will work without sparks flying. But if a mistake occurs, and most likely it will, you and your circuit will be safe.

 

[albert22]

There are two topologies for three phase rectifiers. Fully controlled which uses 6 thyristors. And semi controlled which uses 3 thyristors and 3 diodes.
To do it you need to detect the zero crossings of the AC voltage and drive the gates at the right times by means of a opto coupler or pulse transformers. The semi controlled is simpler to implement because you need to fire just 3 thyristors and you may find a way for directly firing the gates without using isolators.
schmitt trigger
I dont see how you can get cross conduction in any of them. As the thyristors will only conduct in one sense. So I think that you are safe with both topologies. Probably your are thinking about a phase selector not a rectifier. That uses triacs. I have blown several triacs while designing a phase selector and they explode very hard. But they were connected directly to the 380Vac mains. John probably requires rectifying the output of a transformer at a lower voltage.
Regards

 

[schmitt trigger]

schmitt trigger
I dont see how you can get cross conduction in any of them. As the thyristors will only conduct in one sense. So I think that you are safe with both topologies. Probably your are thinking about a phase selector not a rectifier. That uses triacs. I have blown several triacs while designing a phase selector and they explode very hard. But they were connected directly to the 380Vac mains. John probably requires rectifying the output of a transformer at a lower voltage.
Regards

John hasn't specified the voltage or power level at which he wishes to operate. So, I'm only advising him to learn the basics at low power levels, and not to jump into a 380 volt circuit while he is still learning.

Maybe cross-conduction is not the proper word. But one can, thru improper gate timing, create a major short on a three phase bridge, most likely if the full bridge (6 SCRs) is used. John hasn't explained his load, but if he is powering a DC motor and said motor is runing, firing simultaneously the top and bottom SCRs on any leg will essentially create a short across the armature. Again, an individual who is learning, may make such a mistake.

 

[albert22]

I disagree.
The whole 6 thyristors in a fully controlled three phase rectifier can be ON at the same time. There is no problem with that. In fact that happens all the time specially when you fire them for maximum output of the rectifier. Then the circuit behaves exactly as a full wave rectifier with 6 diodes.
When you need less output you delay the gate of the thyristor respect to the beginning of the half cycle. The thyristor turns on and it stays on until the current thru it goes to zero. As the half cycles for the remaining phases are 120 degrees apart overlap may occur but one of the devices will be reverse biased and will turn off.
Short circuit cannot occur because the thyristor is unidirectional (as I said before) unlike the triac that can conduct in both directions.

I dont think this is the case but working with devices that are connected directly to mains is dangerous and extra precautions should be taken always.
John
this link may help you:
**broken link removed**

 

[schmitt trigger]

Short circuit cannot occur.... [/url]

You are right, I stand corrected.

But my original advice that a newbie should start working with low power AC cicuits while he is learning, still applies.

 

[albert22]

But my original advice that a newbie should start working with low power AC cicuits while he is learning, still applies.

I think that your advice is very good and not only for newbies. I just wanted to comment about the cross conduction.
Probably I didnt express my self correctly when I said:

So I think that you are safe with both topologies.

I meant the misfiring and not the safety of the experimenter. Already addressed by you.
Regards

 

[john120]

Thnks u all for the advice and helpmwhat about creating the commands signal with PIC?May I use Squqre wqve or PWM?How mqy I determine the wero crossing of SCR?

Thnaks.

 

[schmitt trigger]

If you are detecting the voltage zero crossing, the easiest is to use a plain voltage transformer, and monitor the secondary voltage. One transformer required per phase.

Then you can either: a) full-wave-rectify the output and detect (with a comparator) when the waveform almost reaches zero volts, or b) feed the comparator input directly from the output's AC waveform; but the comparator then requires a bipolar supply.
In both instances you must ensure that the wavform's peak does not exceed any of the supply rails, even during high line conditions.

Now, to detect current...there are many options, but hall-cell based sensors provide isolation, can accurately detect distorted or chopped waveforms, and can detect both AC and DC components.
Afterward, you feed to a comparator, as described above.

 

[john120]

Is it possible that you can help me simply with two schematic;1 for current detection another one for zero oltage detection.

Thanks for the help.