What are the various firing circuit techniques available for firing the SCR circuits. Which is the most used one and which is better to use when we are using microcontroller signals to drive the firing circuits? Need help Thank you.

 

A simple RC network, where the leading edge of a square wave or pulse is used to trigger the SCR. You don't need more than that, since once triggered, an SCR stays conducting until the polarity reverses across the Anode/Cathode.

This is the wrong place to post these questions BTW, this really should be in the Electronics Chat. I suspect a moderator will move it as soon as they see it.

 

What Bill describes is used in many devices. For example, check out capacitor discharge ignitions, particularly the Hall-sensor controlled ones.

You don't mention your application. For high-current applications, motor control, etc. other methods of firing may be desired.

Here are two references I have found useful:

http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF
http://www.littelfuse.com/data/en/Application_Notes/AN1003.pdf

John

 

Thank u. The firing circuits i hav asked is for speed control of a dc drive with normal 1500rpm, 220V ratings. A normal RC circuit can work but still compared to 555 timer firing circuits, which one would be reliable...

 

Not sure what you mean by "more reliable."

If you mean which approach is more likely to fire the gate, that would depend on how each approach was designed. Gates do have specific requirements for firing. The datasheet for your device should give that information. Here is one such sheet from a motor control SCR:

There are two areas outlined: BMZ and BSZ along with a line for a reference application (20V; 20 ohm line). The BMZ area defines conditions in which the SCR MAY fire; the BSZ outlines conditions in which the SCR is SURE to fire. (I am not sure BMZ and BSZ are really defined using those words, but it an easy way to remember.)

John

 

Definition of the terms BMZ and BSZ:

It has always puzzled me what the meaning of the acronyms BMZ and BSZ is. It is pretty self explanatory once you translate it from German.

BMZ is German for "Bereichen möglicher zündung" which is translated as:
"area of possible firing"

BSZ is German for "bereichen sicherer zündung" which is translated as:
"area of safe firing".

 

Not sure what speed control configuration you are using the SCR in, but for those who are focussed on mains AC applications then the most reliable practical technique used for industrial applications has been to repetitively trigger the gate with the 'picket fence' technique, rather than rely on just one pulse.

Maybe in some DC control applications there is still a recommendation to use multiple triggers to reduce the possibility of 'missing a beat'. It will depend on the particular circuit configuration - do you know the circuit?

Ciao, Tim

 

Did you notice the date on this thread? The last post by the OP was March, 2009. There is probably little change of more dialog at this point.

John

 

I received e-mail notification of a response to this thread. It presented an interesting approach to high-side control with an H-bridge. While it perhaps is not absolutely OT, why does it not appear?

Here is the comment pasted from the e-mail:

You may wish to look at this combination of SCR and MOSFETs used in an H-bridge that doesn't require a high side bootstrap circuit.
See US patent # 6617913.
http://www.google.com/patents/about?id=c4kMAAAAEBAJ&dq=6617913

Duane

I deleted the e-mail address of Duane.

John

 

John, the patent application is for motor direction control. It doesn't discuss any mains or faster switching - probably because of the deadzone control to manage shootthrough. I also couldn't identify that they included a clamp to limit Vrgm - the maximum reverse gate voltage.

Ciao, Tim

 

True, but that real question is what happened to the post?

So many times we see OP's try to use high-side control with N-channel mosfets. This is a way around that driver problem that has not been discussed here before, as I recall. Sure, there are going to be some timing issues, but so many of the OPs's are much more versed in digital than in analog that I don't think that will be a major stumbling block.

I am still wondering why the post vanished.

John