OT- Overvoltage / Overcurrent Protection of Serial Line

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I'm currently in the process of developing a protected 12v- 250mA (current limited) DC supply. The problem is I need to design the supply so that if 240vac (RMS) is connected across its output terminals (ie the 12v250ma output) the power supply circuit is protected.

I'm not talking about surge or transients here either - i do mean 240vac across the outputs for extended periods of time (ie minutes/hours).

After the 240v is removed the supply should go back to normal operation.

I was thinking perhaps a solution could be to use something like a trisil in parallel across the outputs, with a series polyswitch to the load. That way the trisil will take the inital hit, after which the polyswitch will heat up and disconnect the circuit.

Anyone have any tips / solutions. I have attached my simplified serial tx circuit along with the idea i just put foward.

One problem i can see with the polyswitches is that the ones rated at 240vac seem to be few an far between. At the general parts outlets, RS and Farnell, their polyswitch range seem to go up only to 60vac or so.

Look foward to some replies.

Todd

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Does your circuit have to "automatically" recover to normal operation after application of 250VAC ? Backfeeding 250VAC into a 12VDC output does not seem like a typical miswiring error and would not normally qualify for an automatic recovery type of circuit. If user intervention is allowed after a miswire of this type, then a fast acting fuse, a crowbar type of clamp, and a polyfuse should do the trick. Of course, the fuse would have to be replaced after the miswire, but the circuit would be protected.

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You might want to look at isolated DeviceNet examples. There is a nominal 24VDC power wire, but the specs say you need to handle large overvoltages (IIRC it was several hundred VDC).

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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I'm getting a little lost on the Isolated Devicenet examples..... Theusch is it possible to get a link to what you are talking about? (sorry, i did google but it seems to show mostly physical devices, not ICs / components)

In the meantime I guess I will look for a stockist of 240vac rated polyswitches. Still keen to get some feedback on this issue though.

odd.

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What you are asking for is really pretty difficult.

Yes, most polyfuses are rated at quite low voltages.But, I recall seing some higher voltage ones somewhere. DigiKey, perhaps. Look at both Raychem and Bournes.

You know, there are ESD-rated RS-232 and RS-422 transceivers out there. I don't know what their longer-term voltage specs are, but you might find something in that area.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Ok, I had a bit of an idea on how i could disconnect the circuit if 240v was put across the outputs.

Basically was thinking I could use an optocoupler to flick an AC switch.

Does this seem like a valid approach?

Attached is a picture of what i mean. The sensitive side is where the 12v serial input is. The output side (where the idiot accidently connects the 240vac is on the output side)

Please assume that the triac symbol is actually a AC switch.

odd

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