Overvoltage protection

Mark Ebert wrote: > I'm looking for a method of protecting a circuit from overvoltage > (perhaps by turning off a FET in the presence of higher voltages). > The input voltage is normally +5v, but if plugged into the wrong > system it approaches 30 volts which destroys the downstream > circuitry. I'm trying to find a way to isolate the overvoltage > without shorting the input (so a simple fuse or TVS is out), but > have been unsuccessful so far. My original plan was to use a > comparator to switch a MOSFET off when a pre-determined threshold > is exceeded, but I'm unable to come up with a proper driver for > the FET gate. > Any suggestions would be greatly appreciated! >

A problem with turning OFF devices is it assumes they still work. Series fuse, 5 watt 6.2 volt Zener to ground. Fuse is a 'Dang' indicator Use a self healing poly fuse if you don't want to replace the fuse every time.

I don't get how a fuse shorts the input.

GG

There are things called LDO's. If you can't find one suitable then make one. A PMOS type set to regulate at say 5V +15% will be fully driven to RDS,ON with 5V input/output, but clamp at 5.75V output. There will be thermal protection considerations if the loading is heavy. Or maybe you want to just shut the thing down if 30V is applied, just extend the LDO idea except now it is the input and not output that is sensed.

Please define "a circuit". Is the input a logic signal to the circuit, or does it power it? How much voltage does the circuit actually need to function?

John

John Larkin wrote in news: snipped-for-privacy@4ax.com:

overvoltage

voltages).

circuit,

need

Actually, the "circuit" that is driven by the 5 volt supply is just a series of backlight lamps for a control panel. The normal voltage range is 0-5 volts (dimming), but if the panel is plugged into the wrong product (we have field problems with this), it burns out the bulbs. We have used fuses to protect this in the past, but because of the location of the PCB its a pain to replace so I want to improve on it to provide some sort of overvoltage sense, then open the output in the presence of this. The current draw is 2.5 amps max. and the inout voltage can reach 32 volts, otherwise I would use a commercial overvoltage device, such as a On Semi NCP346 driving a FET.

Polyfuses are near useless in overvoltage situations. When the voltage goes too high and the crowbar fires, a polyfuse will usually be destroyed, unless something else protects it..

Check out the "Max. current" ratings of Polyfuses.

Sounds like you could use a polyfuse followed by a 5-volt power zener or transzorb.

Or just a 5-volt LDO regulator, although the 2.5 amps makes it tricky. If you could split the load into sections, 3 maybe, you could use three small ldo's. Below 5 volts, output would track input pretty well, but above 5v in they'd regulate and eventually thermal limit.

John

In article , Mark Ebert wrote: [...]

How about this:

TIP-36 e c --+-------+------ ----------- To bulbs ! ! \\ / / ! ----- \\ ! ! / 0.5W \\ ! ! 2.2K / 1W \\ zener/-/ \\ 1K / 50R 4.3V ^ ! \\ 2W ! ! 22K ! +-----------/\\/\\-+ ! ! ! ! ! ! ! ! !/ c ! --+------! 2N2222 ! !/ !\\ (2 EA) +---! ! ! !\\ GND / ! \\ GND / 1K ! GND

Maybe a LMV431, BJT, MOSFET + 5R would do the trick.

Best regards, Spehro Pefhany

..........This might help....

Regards

AJ