I am curious what kind of circuit would be needed to control the gate of an SCR (thyristor) to make it so that it would need both the high side of a 3.3v output pin (from an Arduino Due) and the high side of a 5v signal turned on/off by a separate relay.
The relay is essentially a "master on" switch that, when not on, should disable the SCR's gate from being able to accept any sort of high signal from the Arduino board.
I don't understand what you're trying to do... Are you trying to control the relay with an SCR? A schematic or block diagram might help.
What are you trying to "disable"?
I don't understand why you're using one output pin to control/disable another output pin through an SCR.
You'll need a current-limiting resistor in series with the SCR's gate. About 200 Ohm's should work.
In case you don't know this, SCRs are "latching" devices... Once you turn it on it won't turn off until power is removed. Is that what you want? (With TRIACs and AC circuits, the TRIAC get's a chance to turn-off at every AC zero-crossing.)
I have basically a little coil gun, where the SCR will be controlling the release of energy from a large can capacitor to energize an induction coil.
The relay is essentially a "master arm" type switch - if there isn't power moving through it, it shouldn't be powering anything in the trigger/charging circuitry.
The charging circuit is thus very easily disabled - the VCC input on that just connects straight to the output of the relay.
However, with the fact that the capacitor could still be discharged with a signal to the SCR, I want to disable that ability for the 3.3v signal from the Arduino to reach the SCR's gate in the case the relay isn't enabled.
Essentially I need a logical AND gate, but done using the right components, to which I have no idea what to do.
I was looking into a high-side switch, maybe using some sort of MOSFET, to make it so that the signal from the Arduino wouldn't make it to the SCR's gate unless the MOSFET was conducting, but I'm unsure of what I would need to make it work, especially since the output of such thing is only just going to the SCR's gate.
Yes, you can pass the SCR gate signal through a relay - you probably ought to pull the gate
down to the cathode potential with a resistor, perhaps 2k or so, so it is insensitive to noise pickup
when the relay is open. A reed-relay or other small low power relay would be adequate. Something
like 150 ohms current-limiting resistor on the Arduino pin should be present too.
A real fail-safe would disconnect the capacitor from the SCR though, to prevent SCR failure froma
activating the thing. However that's a very high current path and you might need a very hefty relay
there to survive the current discharge.
Attached is my crude (and likely wrong) attempt at trying to explain what I was thinking. Note that the relay in the diagram I have here is its own module and already has its own flyback protection stuff going on. Also note that the amplifier is really just a 300v up-step converter (I didn't know what symbol to use for such).
I was looking into using a high-side switch like setup to drive the SCR's gate pin, but I'm unsure how that all works, and there are likely a few extra resistors needed in there.
The idea is that the relay has to provide the power to the entire system, independently of what the Arduino is spitting out. Without the relay enabled (along with the master arm switch) signals from the Arduino shouldn't reach either the charging circuit nor the trigger circuit.
MarkT:
Yes, you can pass the SCR gate signal through a relay - you probably ought to pull the gate
down to the cathode potential with a resistor, perhaps 2k or so, so it is insensitive to noise pickup
when the relay is open.
Okay, sounds good. I was thinking there would need to be a resistor in there somewhere.
MarkT:
A reed-relay or other small low power relay would be adequate. Something
like 150 ohms current-limiting resistor on the Arduino pin should be present too.
Yup, I have a resistor before the output hits anything.
MarkT:
A real fail-safe would disconnect the capacitor from the SCR though, to prevent SCR failure from activating the thing. However that's a very high current path and you might need a very hefty relay
there to survive the current discharge.
Yeah, that's actually very true - I hadn't considered that scenario.
And yeah, you would need to have a hefty thing there because the capacitors I'm using are quite dangerous, and I have to actually put a 0.33 ohm 100W resistor on the line after the inductor coil otherwise the discharge is in the thousands of amps and after about the 950A mark, you gotta switch to some pretty hefty looking SCRs that are used for arc welding and such.
Love the salt and pepper shakers in the background. Yes, a mechanical disconnect relay on the gate is a good solution. Having another safety device somewhere would always be good.
If that's a 'little coil gun' I wonder what a large one would be like
In theory you could tune the inductor /capacitor to set the maximum current to something safe for
the SCRs, but in practice the slug changes the inductance by a large factor so I guess the resistor is
a reasonable way to limit current. It feels wrong in the sense that you'd prefer that energy to end
up as kinetic energy, not heat.
Paulcs:
Love the salt and pepper shakers in the background.
Haha, yeah, small apartment, my little kitchenette area has become my work table. :3
Paulcs:
Yes, a mechanical disconnect relay on the gate is a good solution. Having another safety device somewhere would always be good.
Yeah, I think I honestly might just go with a reed relay for the SCR's gate line, that way I don't have to fiddle with it.
I don't know how else I can make the current hit the SCR, even with a high-side switch thing, just simply because I do not posses the electronics knowledge necessary to make that work.
The trick with putting a relay or something like that on the main firing lines is that they're going to have several thousand amps of current going through them, at 300v.
MarkT:
If that's a 'little coil gun' I wonder what a large one would be like
Haha, well, honestly the problem with coil guns is that there just isn't a good amount of kinetic energy conversion to the projectile from the amount of power from capacitors. I don't suspect this will be very powerful when it's all said and done.
MarkT:
In theory you could tune the inductor /capacitor to set the maximum current to something safe for
the SCRs, but in practice the slug changes the inductance by a large factor so I guess the resistor is
a reasonable way to limit current. It feels wrong in the sense that you'd prefer that energy to end
up as kinetic energy, not heat.
Yeah, the SCR that I have (S6065J) is rated to 950A burst. Anything above 1kA you have to start switching over to SCR's that look more like they're used in arc welding, such as this one:
The problem with those is after a certain amount (2kA) then the gate current required starts to get up to the 200mA level, which means you can't drive it with the Arduino out and have to put a transistor on it to power the gate even.
In fact, if I may ask, what would be the diagram required to up the current through a transistor to trigger a gate that requires more mA of current than what the Arduino has available?
I keep seeing things like this, where you have the voltage source go through, say, a 150-500 ohm resistor into the SCR's gate, and then also that same line going through, say, a 1k ohm resistor to ground, e.g.
And also similarly with a npn transistor powered from an arduino, e.g.
Is this how one would actually trigger an SCR that has a gate current that requires more than the line current can spit out?
For safety reasons, you need to have a bleed resistor connected across your firing capacitor to discharge it within a reasonable time of cessation of charging.
High reliability, safety critical systems use multiple bleed resistors, mounted in orthogonal axes, to reduce the likely-hood of a complete failure of this safety critical item.
