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Registered Member #120
Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83
I have some basic questions on SCR trigger. They're mainly due to paranoia and the fact that since the circuit is a bit lethal. Therefore, i would like to have someone take a look at the SCR triggering. Basically what I have made is a surge generator. It runs on DC and discharges from 0-350 VDC, the bank is 19.5mF. I'm using electrolytic capacitors too. It consists of two circuit, a high voltage circuit that charges the capacitors and then a low voltage circuit that controls the digi panel meter, fans, charging circuitry, bleed discharger, etc. Now the low voltage circuit triggers both the SCR to bleed the bank (a load of 500 Ohm, 200W metal resistor) and the bank discharge SCR. Both SCR's are connected so that the negative of my low voltage supply is connected to the negative of (cathode of SCR) the high voltage circuit. The positive of my low voltage circuit is then connect to two switches, one for each SCR gate (discharge SCR and bank bleed SCR). So now I would like your input on how safe this is? and would say if I wanted to use a higher voltage how should i make my circuit? use a GDT?
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
It is not a good policy to rely for safety on any one method whether it is SCR's, switches or simply pulling the cord out. Some discharge should be built into the circuit that is not bypassed so the charge will leak. This can be integrated with a high intensity led and resistor appropriate for 350V to indicate voltage present. Is that 19.5mF meaning milli or microfarads?
Registered Member #120
Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83
oh it's 19.5 millifarads. I actually have a 300k ohm resister in parallel with the bank so it can slowly bleed it. I was just wondering if my SCRs are connected all right. How do you connect yours.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
If you exchange the positions of the SCR and the Load that would be good. If you really want the SCR in the +ve supply to the Load then I'd use a pulse transformer. (usually NOT critical, a steel laminate OR powdered iron OR ferrite core will work) Could be as simple as a pushbotton switch, a 9V battery and a 1:1 transformer.
Registered Member #87
Joined: Thu Feb 09 2006, 01:36PM
Location: San Jose
Posts: 191
Well, you've got your trigger pulse going through your load, and your load could be inductive, resistave, whatever, and mess things up. SCR's are current activated, anything and everything can change your current, so it might be a good idea to switch your load and SCR.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Depending on what kind of load you are discharging your capacitors through, it might be a good idea to use a much heftier triggering pulse. For example if this is a railgun or other high current load, you should try to hit the SCR gate with a pulse around 300A to turn it on as quickly as possible. You can do this by discharging a 300V electrolytic into the gate through an IGBT and a small (fraction of an ohm) current limiting resistor. I'll dig out a circuit if you are interested.
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SCR triggering
