I have a circuit with four 0-10 V, 5 mA output channels on a PCB designed to work for ventilation, where the output valves are powered by 24 VAC. It is to resolve a problem we have at work with 350 ventilation units that have issues.
I would like to protect the op-amp outputs from 24 VAC being connected to them in error (quite likely to happen). Each channel needs to output up to 5 mA normally without an error greater than 5% as a start (strictly speaking, as it is a PID controller, I guess the controller would deal with errors even if they were larger than that as long as they were not too non-linear).
In addition I have the usual constraints of trying to minimise number of parts (especially number of specialised parts), board space, and cost. The board is standard double-sdied FR4.
The circuit has a bi-directional TVS for surge with a clamp of +/-70 V but that has no effect on 24 VAC.
I have looked everywhere, and on this site have found some excellent ideas that cover part of the problem, but nothing that completely does it. I wondered if anyone out there had met a circuit in the past that they found ideal for this consideration.
The final stage of the 0-10 V circuit is a TLV274 powered by 15 V and GND. Short circuit limit on this is 100 mA. The datasheet is unclear on what negative or overvoltage the output can take. The inputs can take 0.2 V beyond the rails (i.e. -0.2 V past GND and +0.2 V past supply).
So the problem is in two areas:
- High positive voltage coming back;
- High negative voltage coming back.
Each case needs to be covered, with the op-amp both powered on and off.
Here are some ideas I have that have some chance of success:
An ideal diode based on a FET, or a low voltage drop diode, and live with the drop that it gives.
However, all this is a lot of bits for 4 channels, so please may I ask, am I over-complicating this?
I have tried loads of variants using resistors with Zeners, but remember the incorrect 24 VAC connection could be constant, and so the resistors end up being several watts. I have looked at three-terminal regulators as current limiters, and nothing quite works. I thought of the two-transistor circuit alone (so no diode), but it cannot stand a reverse bias greater than the transistor breakdown or about 5 V.
I appreciate you are busy people and good at what you do, and probably can eat this kind of problem, so just some quick hints would be great. Anything you have seen that may give me another route to follow.