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Doubt on snubber circuit for an scr for dv/dt protection

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electricallov
#1
Sep15-12, 07:48 AM
P: 6
The main aim of turning on/off an scr is to disconnect source from load. I read in dv/dt protection of scr using snubber circuit, the scr may turn on when dV/dt is very high as the charging current i=cdV/dt will be high enough to turn on scr. So we are using a snubber circuit to divert this high current. With high switching currents, the capacitor(shown in the attachment) will be having low impedance and hence it give an easy path for current and hence protect the scr.

My doubt is this: While using the above protection circuit, even when the scr is off the source voltage will be connected to the load(Always!) right? Then there will be no use of using the scr.

And i have seen a similar problem while learning series operation of scrs. There to eliminate problems we connect resistances in parallel with scr??? I don't understand. I am confused. I hope my question is clear.
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uart
#2
Sep15-12, 09:20 AM
Sci Advisor
P: 2,751
Quote Quote by electricallov View Post
With high switching currents, the capacitor(shown in the attachment) will be having low impedance and hence it give an easy path for current and hence protect the scr.

My doubt is this: While using the above protection circuit, even when the scr is off the source voltage will be connected to the load(Always!) right? Then there will be no use of using the scr.
Your attachment is not showing but I'm pretty sure I know the circuit to which you refer. You will find that when the SCR is off that the load is only "connected" through a series capacitor (typically < 0.1uF, though it's exact value will depend on several things). In a DC circuit this is, to a very good approximation, an open circuit.

And i have seen a similar problem while learning series operation of scrs. There to eliminate problems we connect resistances in parallel with scr??? I don't understand. I am confused. I hope my question is clear.
Those are voltage sharing resistors and, by design, are a high enough resistance to only allow several times the maximum leakage current (read "off state current") to flow. These resistors are a very much higher impedance than the load, so again it is approximately an open circuit.
electricallov
#3
Sep15-12, 11:58 AM
P: 6
Thanks a lot. This clears my doubt.

FOIWATER
#4
Sep15-12, 02:54 PM
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Doubt on snubber circuit for an scr for dv/dt protection

Yeah I would associate myself with uart's comments..

The snubber circuit is essentialy connected back to the source through the load, but th spike is supressed by the large values of resistance. As you said, the cap looks like a short to large frequencies in your circuit, but then the resistance of THAT circuit is high enough to supress the spike.

Glad you brought this up, I forgot about those circuits, I used them at work alot.. heres a pic of the SCR in a heat sink and the snubber circuit below on the same card as the gate driver.

The pics attached show the SCR's in the heat sinks, they are hockey puck style SCR's

These SCR's convert 600 volts AC to DC to power motors used in the motion of a large mining shovel.. before I left work as an electrician there to come here to engineering school I really enjoyed my job there.

There was a case where the shovel shut down, and usually there is a system whereby these SCR's are connected to pule transformers and large 900Volt capacitors which force commutate the SCR's that power the motors and also turn on another set connected the motor armature to a resistor grid to dissipate the collapsing magnetic field. One of the bridges failed and the cabinet completely burned, me and my journeyman had to clean it all up, use thyristor testers etc to make sure the SCR's were ok and find the bad/failed ones. It was very interesting... had to apply a compound to the SCR when replacing it in the heat sink to ensure it sits flush in the heat sink, so as not to arc as well. it was a very tedious process.. at those high voltages with that pwoer transfer, theres a lot of room for error...

What was I talking about again? oh yeah the snubber circuit... there was a lot of speculation about whether a failed snubber circuit was the cause of the problem, turns out it was a shorted SCR,

You can actually see the snubber wires on the first picture, one connected to each side of the heat sink )across the anode-cathode junction of the SCR, that is the snubber connection. Had to test all that as well.
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