Overvoltage n Overcurrent

[GabrielTYM]

Hi~ i'm designing an overvoltage protection circuit and an overcurrent protection circuit. Been considering mosfet n bjt for those designs, bt nt sure hw to design it~ Is scr usable?

 

[dknguyen]

We need a circuit concept. But no, an SCR is not usable for one simple reason...you can only turn an SCR on, never off. Once it's turned on, it stays on unless something else reduces the current to zero (like a relay, or an AC wave crossing zero). That's the two big reason why SCRs are used for AC and not DC.

 

[Nigel Goodwin]

We need a circuit concept. But no, an SCR is not usable for one simple reason...you can only turn an SCR on, never off. Once it's turned on, it stays on unless something else reduces the current to zero (like a relay, or an AC wave crossing zero). That's the two big reason why SCRs are used for AC and not DC.

SCR's are commonly used for over-voltage, it's called a crowbar circuit.

 

[peufeu]

SCR in a protection circuit is used to short circuit the supply and blow the fuse, not to switch off the load.

What is the load current/voltage and how is it controlled ?

 

[GabrielTYM]

SCR in a protection circuit is used to short circuit the supply and blow the fuse, not to switch off the load.

What is the load current/voltage and how is it controlled ?

Apparently i don't want to use a fuse. I was thinking of cutting off the circuit by using mosfet and BJT. The cutoff voltage would be at roughly 8.5V and the current is at 2A. I wanna make it into 2 separate circuit so it would seem simpler. ^^
Erm, do you have any idea on how to connect the circuit? I thought of making the mosfet and BJT to be switches. But i cant find the suitable components.

 

[GabrielTYM]

We need a circuit concept. But no, an SCR is not usable for one simple reason...you can only turn an SCR on, never off. Once it's turned on, it stays on unless something else reduces the current to zero (like a relay, or an AC wave crossing zero). That's the two big reason why SCRs are used for AC and not DC.

Thx for the info. ^^

 

[GabrielTYM]

SCR's are commonly used for over-voltage, it's called a crowbar circuit.

crowbar circuit would need to insert fuse wanx right?

 

[Nigel Goodwin]

crowbar circuit would need to insert fuse wanx right?

Yes it would, and you really do want to do it that way - over voltage circuits are to protect whatever you're feeding, and crowbar circuits are the best way to do that. You 'could' try and rurn the feed off, but if that circuit doesn't work it will destroy what you're trying to protect.

Crowbars are used because they are simple, cheap, reliable, and provide maximum protection.

 

[peufeu]

Well, if you already use something (usually a MOSFET) to switch your load on/off, you can use that as a protection circuit : sense the current, if it goes above a certain threshold, turn off the MOSFET.
Overvoltage is a bit harder to do. If your power supply becomes crazy and sends high voltage, it could blow your MOSFET driver and the MOSFET could turn on permanently.

An SCR overvoltage protection, on the other hand, is really reliable, because it's really dumb. If you don't like the idea of changing a blown fuse, use a polyswitch.

 

[GabrielTYM]

Yes it would, and you really do want to do it that way - over voltage circuits are to protect whatever you're feeding, and crowbar circuits are the best way to do that. You 'could' try and rurn the feed off, but if that circuit doesn't work it will destroy what you're trying to protect.

Crowbars are used because they are simple, cheap, reliable, and provide maximum protection.

thankx for the idea!! i think i'll stick with the transistor part. Because i'll be turning the circuit on and off plus i don't want to "burn" fuse.

 

[GabrielTYM]

Well, if you already use something (usually a MOSFET) to switch your load on/off, you can use that as a protection circuit : sense the current, if it goes above a certain threshold, turn off the MOSFET.
Overvoltage is a bit harder to do. If your power supply becomes crazy and sends high voltage, it could blow your MOSFET driver and the MOSFET could turn on permanently.

An SCR overvoltage protection, on the other hand, is really reliable, because it's really dumb. If you don't like the idea of changing a blown fuse, use a polyswitch.

Thn i think the current part i can fix in the MOSFET already. Now just left with the voltage part. One question, would the scr circuit which is connected with the polyswitch act the same way as the one with MOSFET?

 

[dknguyen]

Ah that's interesting. I would have thought a transistor for that but I guess the circuitry is simpler with an SCR since you don't have to worry about the transistor turning off prematurely.

 

[GabrielTYM]

Ah that's interesting. I would have thought a transistor for that but I guess the circuitry is simpler with an SCR since you don't have to worry about the transistor turning off prematurely.

ya~ that's true. But by using SCR means that i'll be using fuse already, seems to be a bit troublesome when the fuse "burn". ^^

 

[Nigel Goodwin]

ya~ that's true. But by using SCR means that i'll be using fuse already, seems to be a bit troublesome when the fuse "burn". ^^

You're rather missing the point - over voltage protection is used when the power supply fails (regulator gone S/C or whatever) - it's to prevent damaging whatever the load is. You WANT it to blow the fuse to keep things safe. Trying to switch off the S/C transistor won't make any difference, your load will continue to receive too much voltage and will be destroyed.

 

[indulis]

An SCR overvoltage protection, on the other hand, is really reliable, because it's really dumb.

What's "really dumb" is to make comments like that.

An SCR crowbar works very nicely in circuits that have good current limiting, and there'll be no blown fuses.

 

[GabrielTYM]

You're rather missing the point - over voltage protection is used when the power supply fails (regulator gone S/C or whatever) - it's to prevent damaging whatever the load is. You WANT it to blow the fuse to keep things safe. Trying to switch off the S/C transistor won't make any difference, your load will continue to receive too much voltage and will be destroyed.

Erm, apparently i just want to show an open circuit when it reach certain voltages. That's all. As long the circuit can be open circuited by which ever components(MOSFET/BJT) then can already. It's just testing.

 

[GabrielTYM]

What's "really dumb" is to make comments like that.

An SCR crowbar works very nicely in circuits that have good current limiting, and there'll be no blown fuses.

Thx for the info!! ^^

 

[Nigel Goodwin]

Erm, apparently i just want to show an open circuit when it reach certain voltages. That's all. As long the circuit can be open circuited by which ever components(MOSFET/BJT) then can already. It's just testing.

Sounds like you have no need for over voltage then?. It's mainly a safety device in case of a failure in the PSU.

 

[GabrielTYM]

Sounds like you have no need for over voltage then?. It's mainly a safety device in case of a failure in the PSU.

I think it's not fully consider as an overvoltage. But somehow my friends all called it overvoltage. I just want to perform a scene of "overvoltage" and the presence of components(MOSFET/BJT) to show open circuit. Thats all. ^^