Dual supply Opamp missing one half of the supply -- What happens?

[xxxxxxxxxxxxxx]

Hello all,
I have just a simple question:
If I have a dual-supply opamp (used as a preamp) running from +/- 13V, what happens if one of the power rails is disconnected, keeping in mind that the other rail will be on, the ground will still be connected, but there will be no signal at the opamp's input?

I cannot find any information regarding this online or in the datasheet. I am using TI's RC4560 opamp at +/- 13V Datasheet: https://www.electro-tech-online.com/custompdfs/2012/02/rc4560.pdf


Thanks!

 

[crutschow]

That may be a simple question but there's no simple answer. It's hard to determine what happens since it is fault condition and they are not typically stated on a data sheet. Certainly the op amp will not be operating since there is no supply current flowing with one of the rails disconnected, but it's difficult to predict what the output will do. You likely have to try it to find out.

 

[xxxxxxxxxxxxxx]

Oh, I don't mind what the output does, I just want to know if it will damage the opamp..

I only intend to have the opamp "half powered" when the system is in a shutdown state, so the output is unimportant, just want to make sure the opamp will be ok (since it will be in thhis half powered state for many hours or even days)

 

[Nigel Goodwin]

Yes, it should be fine - but it's not much of a 'shut down state' if you're only removing one supply rail - it might even increase current consumption?.

 

[crutschow]

Yes, it should be fine - but it's not much of a 'shut down state' if you're only removing one supply rail - it might even increase current consumption?.

Not in general. Most op amp supply current goes from the positive to the negative terminal. If you open either terminal, there's no path for the current and the op amp becomes unpowered. The exception is, if the output goes to the powered rail, that would give current from any external load resistance.

 

[Nigel Goodwin]

Not in general. Most op amp supply current goes from the positive to the negative terminal. If you open either terminal, there's no path for the current and the op amp becomes unpowered. The exception is, if the output goes to the powered rail, that would give current from any external load resistance.

What about in to the load?, as we've no idea what the circuit is. Instead of having zero volts on the load to chassis, we may now have the +ve supply.

 

[unclejed613]

Analog Devices article about "off-amps".... hope this helps
https://www.analog.com/library/analogDialogue/archives/42-10/off_amps.html

 

[crutschow]

What about in to the load?, as we've no idea what the circuit is. Instead of having zero volts on the load to chassis, we may now have the +ve supply.

I did mention load current in that post.

 

[xxxxxxxxxxxxxx]

That was a good article, UncleJed-- it basically said that if you have the opamp's power rails 'off', then you should try to ensure that the inputs of the opamp are current limited to about 5mA. Since the circuitry providing the opamp's input will not be powered, I think the opamp would be OK, but the article unfortunately did not discuss what could happen with a 'half powered' opamp.

In my application, in a shutdown state, the opamp can either be half-powered or fully powered (but not off) but it doesn't matter what the opamp does either way, but I think i'll just leave it fully powered since getting a definitive yes-or-no answer would be tricky.

 

[unclejed613]

treat a half powered op amp as unpowered. it would be better if you put some kind of switching transistor in the positive rail to kill the positive rail when the negative one is off. with one rail powered and the other one at ground potential, the op amp will still function normally (within the limits of the positive rail) for positive input signals, but could be damaged with non-limited negative signals, and in some cases you may have phase inversion if the input signal is too close to the negative rail (which in this case becomes ground potential).

if this is a guitar effect, and you're trying to switch power with the output jack, use MOSFETS to switch the rails on and off rather than using the jack as a power switch. you will also avoid the nasty "POP" when the center conductor touches the power connection on the way in.