Under and Over Voltage Protection

369

04:40:35

This circuit allows the load to be connected to the input voltage only if the input voltage is between 10-15V. This type of protection is useful for circuits like linear regulators where too low a voltage makes the regulator useless and too high a voltage can set the pass transistor on fire. It could have other applications too.
When the input voltage is between 10-15V the 2 op amps' outputs go high, which turns on the PMOS. The BJTs and 10k resistor form an AND logic gate. When both op amps go high, the BJTs turn ON and the output of the AND gate is equal to logic 0 and the PMOS turns on. When either of the op amps output 0V, the corresponding BJT(s) turn(s) off and the output point is pulled high by the 10k pullup.

Note:
The odd values of resistance (2k & 5k) can be obtained using potentiometers.
Op amp output is set to 19V (20V - 1V dropout). And real world op amps are not magically powered by a fixed voltage.
A zener may be needed to protect the gate of the MOSFET from over-voltage.
MOSFET used: IRF4905 http://everycircuit.com/circuit/5282567524450304

published 6 years ago

kiani

6 years ago

Excellent,,,, your link bombs out,, i think its spacebar missing......i have to chk cct. In more detail later..in general (as am sure you know) under voltage csn be problematic on startup. As thr o/p voltage rises sees undervoltage detect low voltage and shuts the osu... Needs to be blanked out for that situation, some kind of negative hysterisis.

Bushmills

6 years ago

Cool stuff 👍

PrathikP

6 years ago

@kiani see if you can improve it @bush thanks

PrathikP

6 years ago

Actually @kiani i didn't know about the startup problem. But startup happen so quickly it may not matter

kiani

6 years ago

That would be something to aim for.

kiani

6 years ago

To make it miss if the voltage is rising,

kiani

6 years ago

I got it now its not a latch type,, so no problem, i don't see how it can be improved,,... The problem is born eith latch type under voltage.

PrathikP

6 years ago

Oh yes of course... Then it would stay off even though there is no real problem

kiani

6 years ago

Yes it will trouble at start up. So undervoltage must be disabled when o/p rising from zero (or there abouts), and then become activated when o/p stablished. There are solutions,, i have to think to remember,, but you can do it, i am sure.

PrathikP

6 years ago

Actually it can be seen as a feature. The output voltage is initially off and a button must be pushed to turn the output on

kiani

6 years ago

Yes but when you switch a psu on, you wouldn't expect to press button as well everytime you switch on.

PrathikP

6 years ago

Actually i prefer that. I have a SEPIC DC-DC converter whose output is disabled on startup. I have to push a pushbutton to enable it.

kiani

6 years ago

If you are doing a custom design, for a customer their system might need start up on power up. As often a psu. is an afterthought. Snyeay the button could easily be replaced with a delay auto swich.

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