Voltage Limiting Circuit

[grapefruit]

Howdy,

I'm using some rail to rail op amps for a project so I can get almost 9V headroom from a standard pedalboard PSU. The op amps I've chosen have a recommended maximum supply voltage of 10V (12V absolute max). I chose these op amps because they have low noise and distortion. The circuit will be pulling possibly more than 40mA.

My problem is I want to limit the input voltage to 10 volts so that if an unregulated or higher supply voltage is used the op amps are not damaged. I've had three ideas but they all heve their downfalls...

1. Series polyswitch and 10V zener diode.
For the lower trip current devices the resistance gets fairly high, dropping the supply voltage. And with a 10V zener the dissipation would be high once it started to conduct. Similar thing goes for resistor - zener regulator. For dissipation to be low under a fault condition the resistor needs to be large enough that it would drop too much voltage under normal operation.

2. Low droput regulator.
You'd have to lose about a volt to ensure stable operation

3. Using a P channel mosfet switched by an NPN transistor and 9.1V zener diode. so that the mosfet is fully on when the input voltage is less than 9.8V but swtches off above this.
It seems a bit over complicated/expensive. I tried the circuit using an N channel mosfet and PNP transistor, switching the GND supply line and it worked well, but V+ needs to be switched or it will pull GND through signal GND and P channel FET's are not so common and more expensive.

Does anyone have another ideas?

Cheers,
Stew.

[Sir H C]

LDOs at 40mA can often work with about 100mV across them.  You have to check the specs to be sure, but with 9 volts Vgs available, you can turn a pass device on quite well and get a lot of drive.

[George Giblet]

I've used 3 before.  It's ok you just want protection but it will cut-off under over voltage which you might not want.  In fact it can cause strange behaviour on unregulated inputs when the ripple goes above and below the threshold.

2 doesn't have any funny behaviour.

An alternative is to use a low drop-out regulator but set the regulated voltage to 10V, or some safe tolerance below 10V.  When the voltage attempts to go over 10V the circuit still functions but clamps the output to 10V. When the input is below 10V the output voltage is the input voltage less the small dropout.  I would still refer 2 if the input is unregulated simply because it produces the cleanest supply over the widest range.

[grapefruit]

Thanks for the replies. I'll do some experimenting with LDO regs. I've got some 5V ones here. Any other suggestions are still welcome!

Cheers,
Stew.

[grapefruit]

I was under the impression that you would never want the input voltage to go below the output voltage plus dropout voltage and a bit extra, but I guess if the input is regulated it doesn't matter??

Stew.

[Sir H C]

When you drop below the minimum input voltage for regulation the LDO acts as a small series resistor, no longer regulating, but keeping the voltage as close to the desired on as possible.