SCR gate control

[KerryW]

Just one question...

Where the fuck are you going to find one?  And, I want some.

Tim

I would go to my parts bins.  You might have to go to Mouser (or Digikey)  Try an LND150, for example.
But for this circuit, it can be replaced by a short.

[T3sl4co1l]

Oh, a MOSFET, not just any FET...

Some of those are pretty interesting for voltage/current limiting purposes.  Haven't put any into use for ESD / EMI / signal path sorts of purposes, but I may have some ideas.

Tim

[Kremmen]

For a quite nice design handbook from On Semi, check this: http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF

[SeanB]

I am also looking for simple SCR speed control, probably will go next week and get some SCR's to sacrifice, as I am running out of triacs. Simple DC motor control, but I need a wide speed variation, and the original controller went and cooked itself properly, and I have not seen one similar to buy.

[Kremmen]

I am also looking for simple SCR speed control, probably will go next week and get some SCR's to sacrifice, as I am running out of triacs. Simple DC motor control, but I need a wide speed variation, and the original controller went and cooked itself properly, and I have not seen one similar to buy.

Check the handbook i linked above. There are many circuit examples including several DC motor controllers. Maybe you will find one that works for you.

[SeanB]

I am looking, and have done some quick and dirty tests so I can see what is needed. Will go get real and not recycled stuff next week, as I ran out of used lamp dimmers and need some real new components. 10A triacs do not seem to like sharp edges and horrible switching, they seem to blow up. Looks like I will have to adapt some of those designs and build up a few GSC switches from discrete transistors to use as triggers.

[Kremmen]

All my real life experience is of the big puck thyristors but there at least it was vital to provide the gate turn-on pulse with a righteous kick. A wimpy slow ramp or dribble-on like most of these resitive divider circuits, would result in a far too slow charge propagation/build up in the gate electrode. That in turn would cause main current congestion and hotspots resulting in thermal breakdown of the entire component. So the order of the day was to discharge a hefty cap into the gate using an auxiliary thyristor or a fast transistor switch. For smaller thyristors a diac as trigger holdoff would allow the driver to build up energy before discharging it into the gate. This is a commonly seen circuit.

[chipwitch]

Wow... take one little nap and... Lots of responses!

Rather than tackle everything one-by-one I'll just address things randomly as I think of them.

The concept of a JFET sounded perfect (increasing voltage pinching off current).  Problem is, Mouser only has them up to 40V.  I did find some convos where people were talking about them (high voltage JFETS) and even linked a couple datasheets.  It seemed finding suppliers was a problem, so admittedly I didn't scrutinize the datasheets too closely.  Could have been MOSFETS they were talking about.

I've never worked with optocouplers, but the concept sounds promising.  Thanks for that suggestion.

Since I was inquiring about SCRs, the voltage output to which I was referring, I thought would be an implied DC voltage as SCRs rectify, no?  Are there non-DC output applications?  Also, I was only referring to half wave.  Again, I  thought that would be implied contextually (references to 0-90 and 90-180 degrees, 0-60V, specifically saying "single SCR solution," in one of my posts).

The Onsemi handbook is where I got 90% of what little knowledge I have of thyristors.  It was solely responsible for helping me select the SCRs and Triacs I ordered from Mouser.  Great resource.  Thanks for reminding me of it.  I'll re-read it for a refresher.

You've all been great.  Thanks for all the help and kindness.

Great thread!

[KerryW]

Actually, I misspoke about the JFET, I was really thinking of a depletion mode MOSFET. (LND150)

[TerminalJack505]

You might play around with this circuit.  (See attached.)  Taken from here.

The output waveforms are for various parametrized values of R2.

Edit: Added png showing a minimum turn-on voltage of about 2V.

This circuit will not work, for reasons already explored: the ensure required gate current at any voltage, the resistance has to be exceptionally small.

Your subsequent simulation showing a 100k resistor is just that, a simulation.  I have no idea what kind of sim model you have there, but SCRs are notoriously difficult to model anyway, and it would hardly be surprising that it might trigger on microamperes.

Tim

That particular SCR is one of the so-called sensitive gate types (datasheet.)  The minimum I_GT is 15uA.  The worst-case I_GT is 200uA.

The simulation model is from the manufacturer.  (Found here.)

Would it behave in real-life like it did in the simulator?  Hard to say.  I don't have one handy to test it out.  Personally, I don't think the simulation would be too far off the mark. 

I didn't make note of it in my earlier posts but the simulator showed that the SCR wouldn't trigger with that particular circuit beyond roughly 130 degrees.

Would I trust that circuit for large production runs or critical systems?  No. 

Would I hook it up and just play around with it to learn about SCRs (like the OP wants to do)?  Sure.