SCR unwanted spurious firing. 3

[danielsoy]

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

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.

 

[danielsoy]

Sorry, heres the right link:

Link

 

[IRstuff]

That's IT? No schematic, no circuit description? Are people supposed to read your mind?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[itsmoked]

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

 

[BrianE22]

Try a snubber across the SCR.

 

[danielsoy]

Kind ppl, thanks for your replies.

Heres another video, where fault is more obvious:
Link

Snubber is already there, as u can see in the attached circuit diagram.

Load is 500 watts a hand driller, with 100uf capacitor in parallel.

 

[danielsoy]

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

 

[danielsoy]

Eureka ppl!

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

 

[danielsoy]

A picture of dV/dt between anode n cathode.

 

[itsmoked]

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

 

[danielsoy]

Working OK with 5 watts snubber resistor.

No spurious at all!

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

 

[danielsoy]

Talking about things that should not be, look this Triac commutation failure.

After a couple of positive half cycles, triac cannot block negative half cycles,
So load (DC motor), stops turning and starts oscillating in its place under AC current.
(red window).

 

[itsmoked]

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

 

[danielsoy]

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! .

 

[danielsoy]

another approach.

 

[danielsoy]

dv/dt is like a cloud thats waiting to rain over me, any time a turn a switch off

This time is not that bad, because its inside IRF840 SOA.

look:

 

[danielsoy]

Sorry, dv/dt is 0.25 volts X nanoseconds.

 

[itsmoked]

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[danielsoy]

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.

 

[danielsoy]

A more accurate oscillogram, using another test probe.