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Input overvoltage protection

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rakeshm55:
Hi
In one of my projects the input power source is a regulated 5V wall wart supply. I wanted to provide an input protection for over volt condition, For this I have put in place a Tanzorb 5V and PTC. Now breakdown voltage for tanzorb is about 6.4 to 7.25V approx.

The input protection is followed by a boost regulator for 5.2V. Please refer to the attachment. when I apply voltages a near 6.5V to 8V the PTC is not tripped though Tranzorb heats up (200mA current flow i could measure). The o/p voltage of boost regulator  at this high voltages are same as i/p 6.5 to 8V .

i have ckts at the o/p of boost which gets damaged if stressed above 5.5V

How to provide a proper input protection???
Can I shut down the regulator??
Please help

bilko:
At this low voltage level classic designs use a crowbar protection circuit consisting of basically a zener diode and thyristor. When input goes over voltage the zener triggers the thyristor blowing a fuse disconnecting the supply

rakeshm55:
Sorry that I forgot to mention

The input regulated 5V powers up a boot regulator and a Li ion battery charger. Charger Max input voltage is 6.0V So protection needs to be at the i/p stage.

--- Quote from: yachtronics on April 27, 2012, 06:22:29 pm ---At this low voltage level classic designs use a crowbar protection circuit consisting of basically a zener diode and thyristor. When input goes over voltage the zener triggers the thyristor blowing a fuse disconnecting the supply

--- End quote ---
what is  crowbar protection circuit used with a current draw more than 1A. My PTC av current is 500mA.

--- Quote from: AcHmed99 on April 27, 2012, 07:18:08 pm ---Usually when I do this I use a quick blow fuse.PTC’s are to slow and the tripping current is highly dependent on the ambient temperature. So if your source for example can’t source enough current to flip the PTC high impedance you just destroyed your zener.

--- End quote ---
Wont this ensure a reliable protection down . Please advice.

jahonen:
Why not just use a P-channel FET which simply disconnects the power to the device when input voltage exceeds some pre-defined limit? Then put a TVS which protects the FET.

Regards,
Janne

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