To use an SCR as a rectifier can you just tie the gate to the anode or should you use a diode to prevent reverse current through the gate (in case there is a gate resistor)? Also, would you need to use a resistor between the gate and anode?
"RogerN" wrote in news:2M6dnTxsxcMwlzjRnZ2dnUVZ snipped-for-privacy@earthlink.com:
I imagine nothing is needed, though a single diode ought to be a good safeguard as it has a lower voltage drop so will guarantee the device turns on in forward bias.
Why do you want to do this to an SCR anyway? Convenient high current device?
A guy needs to make a DC supply, about 50A, from 230VAC line. He has an old drive that has the rectifier section but it uses SCR's so it can control power to the DC bus. For the supply he's trying to make he just wants to use the existing SCR's, already mounted to the heat sink, as rectifiers. He found out it has SCR's instead of rectifiers and was about to start over, I told him I thought he could wire the SCR's to act like rectifiers.
A guy I know of needs a 50A rectifier to rectify 230VAC. He has the rectifier section to a motor drive but it has SCR's instead of rectifiers. I think drives use SCR's so they can turn their bus voltage off, or perhaps to soft start the power to the capacitors.
I also have a similar need. A guy gave me some small hockey puck SCR's and I'm wanting to make a rectifier to convert my AC welder into a DC welder. This project has been on back burner since I now have a DC welder.
"RogerN" wrote in news:07GdnYZaFcfbBTvRnZ2dnUVZ snipped-for-privacy@earthlink.com:
I think you could do it. The diode as I said, but perhaps also a 220R resistor in series with it. I remember a triac circuit from Maplin Electronics, driven by an expensive Siemens touch dimmer IC, and that triac often ended up doing things back through its own gate that burnt out IC after IC. I managed to convince Maplin that their app circuit was wrong, and to refund me for several of those IC's, but they never did edit the published diagram with my series 220R fix in the triac gate! Got to feel sorry for all those who assumed it was their error even when they followed the circuit exactly as I did... The point being that if your circuit works with a bit of series resistance in the gate line, add some. It will help protect both ends of that line.
You only need the resistor to limit the current. Some SCR's (or triacs) can be damaged otherwise, or as I said in that other post, they can damage the outputs of a device that controls them. If Roger connects a forward biased (NOT reversed) diode between anode and gate it behaves as he expected, and the whole circuit is protected against reverse bias too, so long as both diode and SCR have a high enough peak inverse voltage value.
I think my first point made that distinction in voltage drops clear enough already.
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