INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

SCR unwanted spurious firing.

SCR unwanted spurious firing.

(OP)
Hi ppl, need advice to find out, what is causing this unwanted SCR firing.mad

Look:
Link

There u can see the signal at my BT152 SCR gate, that sometimes shows spurious firing,that
looks like a bridge, between two firing points.


If some one can help to discover the culprit, i may upload my circuit diagram.

Thanks a lot.

RE: SCR unwanted spurious firing.

(OP)
Sorry, heres the right link:ponder

Link

RE: SCR unwanted spurious firing.

Since it occurs consistently on turn-off you are probably having dVdT turn on due to the type load you're controlling.

Keith Cress
kcress - http://www.flaminsystems.com

RE: SCR unwanted spurious firing.

Try a snubber across the SCR.

RE: SCR unwanted spurious firing.

(OP)
BTW, MJE1300, was replaced by a bc548 like transistor, that has more Hfe.

RE: SCR unwanted spurious firing.

(OP)
Eureka ppl!thumbsup2

As ITSMOKED and BRIANE22 already pointed, voltage rising rate across SCR anode-cathode,
was re-firing my SCR.

Snubber was there, but no really working...

20 ohms resitor (1 watt,sorry), in series with .1uf capacitor, was indeed open because
of heat.


Will keep testing the circuit to be sure thats is OK now, and

RE: SCR unwanted spurious firing.

Thanks for the feedback! Glad you got it chugging along again.

1W seems too low a rating for that. You might consider bumping it up to 3 or 5W. What happens is the 1W can run within its rating but still be so *(&#*$ hot as to oxidize and kill the solder joints holding it to the board.

Keith Cress
kcress - http://www.flaminsystems.com

RE: SCR unwanted spurious firing.

(OP)
Working OK with 5 watts snubber resistor.

No spurious at all!

Thanks again to eng-tips ppl for your help winkr!

RE: SCR unwanted spurious firing.

That was fun!

Yuck. In some cases dVdT breakdowns can damage the triac and the damage can grow quickly to outright failure.

Keith Cress
kcress - http://www.flaminsystems.com

RE: SCR unwanted spurious firing.

(OP)
That is not reassuring at all.

Now i am a bit concerned about SCR and Triac erratic temper.

An spurious low angle firing, may cause the load to receive a lot of current.

Guess will try to control my motor, with a PWM signal driving an IRF840 at 310Vcc! sadeyes.

RE: SCR unwanted spurious firing.

(OP)
Sorry, dv/dt is 0.25 volts X nanoseconds.

RE: SCR unwanted spurious firing.

So you've ditched the TRIAC and have switched to a FET? How's that working out?

Keith Cress
kcress - http://www.flaminsystems.com

RE: SCR unwanted spurious firing.

(OP)
Yes, the thyristor approach was left aside by now, until I find a reliable
way to detect zero cross, and be sure that unwanted firing is 100% unlikely.
May be, I will try something with a PIC to avoid fake zero cross detections.

i like SCR because of its incredible current withstand capability.

Triac turn-off failure picture, was not from this motor control project, but from a coffee
expresso machine that I am trying to fix.

So far, pwm with irf is ok.Vcc was not raised to 310vdc, but just 160volts
because Mosfet in not mounted on the heatsink yet.

I keep in touch, Thanx.


RE: SCR unwanted spurious firing.

(OP)
Heres a picture of the Drain-Source oscillogram, at 280vdc,
tested during a couple of seconds to avoid mosfet overheating.

Motor is no loaded, but running free.

Would like to know, if rise and fall time are ok, or must be improved.
IRF is working without snubber by now.

RE: SCR unwanted spurious firing.

I'm not sure why you're wasting time on the 'other' method if you want a functional TRIAC method. There are a hundred million products out there using TRIACs successfully. There are scads of schematics and almost every TRIAC maker has extensive application notes on using their products. Find one and use it.

Virtually all of them require properly sized snubbers.

Keith Cress
kcress - http://www.flaminsystems.com

RE: SCR unwanted spurious firing.

(OP)
Thats right, I may even get a working dimmer for pennies,
but the fact is that i want learn something in the way about switching mosfets,
scrs and the like.


You are very kind to help me, thanx a lot!wavey.





RE: SCR unwanted spurious firing.

I'm not sure what *.ms12 files are but if you want suggestions on your circuit then try posting a file type people can open.

That constant ringing on the positive part of the your FET waveforms would be rather concerning to me.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close