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Electronics Forum / Basics / March 2007Silicon Controlled rectifier
A curcuit I would like to build calls for a 6 amp. 200 volt SCR. I can't seem to find that particular component and was wondering if I could use a SCR that is rated for 200 volts but has a amperage rating of over 6. After reading a little bit about SCRs I think the voltage rating is a trigger voltage so it shouldn't be changed. I'm not quite sure what 6 amp. means. Is it the maximum current a SCR can take? > A curcuit I would like to build calls for a 6 amp. 200 volt SCR. I > can't seem to find that particular component and was wondering if I > could use a SCR that is rated for 200 volts but has a amperage rating > of over 6. After reading a little bit about SCRs I think the voltage > rating is a trigger voltage so it shouldn't be changed. I'm not quite > sure what 6 amp. means. Is it the maximum current a SCR can take? As long as you meet the minimum and it fits it should be fine, so a 400 volt or 10 Amp or both would work OK. There are some sensitive gate models but I doubt yours is one of those. The voltage and current ratings on SCRs are maximums. I.e. in your case, if you put more than 200 volts across the SCR, it will damage the device. Also, if you put more than 6 amps through the SCR, it will damage the device. So a 200V 6A SCR can handle *up to* 200V and *up to* 6A. An SCR with larger voltage and/or current rating should be a suitable substitute. Note that SCRs *do* have a trigger voltage (and current), but it's not the 200V you see listed. It's usually a lot less, like a volt or two. >The voltage and current ratings on SCRs are maximums. I.e. in your >case, if you put more than 200 volts across the SCR, it will damage >the device. Won't it just self trigger? So any damage will depend on whether the rest of the circuit limited the current. Mark Zenier mzenier@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com) >>The voltage and current ratings on SCRs are maximums. I.e. in your >>case, if you put more than 200 volts across the SCR, it will damage [quoted text clipped - 5 lines] > Mark Zenier mzenier@eskimo.com > Googleproofaddress(account:mzenier provider:eskimo domain:com) No, an SCR needs a trigger current on a separate lead. It has an anode, cathode and gate. The trigger is applied to the gate: + --o \S1 o | +----------+ | | | [Load] [R] | a | [SCR] | _ g/ | c +--o o--- | PB | | Gnd ------------+ When the momentary pushbutton (PB) is pressed, current is supplied to the gate until the pushbutton is released. The SCR turns on. It will remain on after the pushbutton is released, until S1 is opened. Closing S1 again will not trigger the SCR. It won't trigger until the pushbutton is pressed again. Ed >Mark Zenier wrote: >> In article <xn3b3r1ih5.fsf@delorie.com>, DJ Delorie <dj@delorie.com> wrote: [quoted text clipped - 5 lines] >> Won't it just self trigger? So any damage will depend on >> whether the rest of the circuit limited the current. >No, an SCR needs a trigger current on a separate lead. >It has an anode, cathode and gate. The trigger is applied >to the gate: That's the ideal case. What happens to an SCR if used outside of its ratings? As I understand it, if you apply too much voltage to an SCR anode, it leaks current, and that current acts the same as the trigger current into the gate, causing the device to flip into conduction. But since that's behavior outside of the device specs, it's not guaranteed at what voltage, just that it's higher than the rating. Mark Zenier mzenier@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com) >>Mark Zenier wrote: >> [quoted text clipped - 22 lines] > Mark Zenier mzenier@eskimo.com > Googleproofaddress(account:mzenier provider:eskimo domain:com) There is a forward voltage at which the SCR will conduct in the absence of gate current - and you might get away with it, without damaging the SCR. But you might not. It is better to treat the ratings as they are intended: they are maximums which are not to be exceeded. That does not invalidate your observation. In fact, they used to provide a breakover voltage level at which conduction would occur in the absence of gate current, and a peak forward voltage rating not to be exceeded. As long as your voltage was between the two levels, you could get the effect you mentioned, without damage. Ed "joe" <jstribrny@gmail.com> **Groper alert ! >A curcuit I would like to build calls for a 6 amp. 200 volt SCR. ** For god's sake, post the post the damn part number !! No-one can advise you unless you do. ....... Phil |
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