Hardware Voltage Clamp Modules ?

[AndyClegg]

Hello,

We have an application using cRIO and associated c-series modules. All analogue output (voltage) modules have fixed ranges of +/-10V, and there is one analogue output that I want to be able to clamp (limit) the analogue voltage output to a lower range (say a fixed +/-5V, or maybe adjustable depending upon the outcomes of our factory testing). This is easy to do in the LabVIEW application, and is done, but I am also looking for a hardware solution to remove the dependency on doing it in software.

I have seen circuits to do this - but we really need a self-contained hardware module that we can easily integrate into the application. Any ideas ?

Hopefully this is the right board for this.

Andy Clegg

[niNickC]

Hello Andy,

As you mentioned, there is a number of circuits which should be able to do this. My first thought would be to use a Zener Diode which will coerce any over voltage to 5V.

With regards to a 'self-contained hardware module', I don't think there is anything NI would offer to help you. Maybe you could expand a little on what you require? Do you want something to plug directly onto the front of your module? and not a separate circuit?

Kind Regards,

[AndyClegg]

Hello Nick,

Many thanks for the reply. Yes a simple zener diode circuit would do the trick - however, we can't seem to find a self-contained, off-the-shelf module to purchase that does this function. Ideally we want such a module to be DIN rail mounted, so that it can be fitted easily into the harware cabinet being supplied to the customer - but I am sure we could cope with any phsyical mouting. The main reason for wanting a pre-built module is that it will have been fully tested, warrantied, etc.. A fixed +/-5V limiter would probably be ok - but ideally adjustable to whatever limit we need after testing (I expect it will be between 3.5-5V).

I would have thought something like this should be available - as it isn't an unusual thing to need - but just can't find anything

Andy

[niNickC]

Hello,

That kind of packaging isn't something I have come across myself, I'm pretty sure you can find IC packages with a circuit built in but that is all. Maybe someone else can suggest something!

Sorry I can't be of any more help, I do hope you find something!

[Henrik_Volkers]

Maybe there are B5 or similar DIN rail modules for galvanic isolation that also provide clamping?

I would directly ask the manufacture of such modules like Weidmueller, Phoenix, Wago etc.

[Buechsenschuetz]

A zener diode would be a quick-and-dirty solution. The breakdown voltage of zener diodes is not very well defined, i.e. it has great tolerances, and they get "somewhat" conductive when the voltage approaches the actual breakdown voltage. 

The problem could be solved with a voltage follower circuit, built with an op-amp which has a rail-to-rail output. So the output of this circuit will be limited to the supply voltage level. With an adjustable power supply the limiting voltage would be adjustable, too. Maybe input overdrive could pose a problem, you have to select an opamp which will withstand input voltages higher than the supply voltage.

However, I am afraid there is no off-the-shelf device you could use.  

[AndyClegg]

Guys - thanks for your inputs to this problem, which has helped confirm that such a simple voltage limiter / clamp isn't readily available as a packaged, standalone module. Since limits are being put in software, and the hardware module is simply to prevent some s/w fault giving rise to a AO signal that is unacceptably big, perhaps the best solution (and one that seems to be available) is to use an overvoltage relay to engage an automatic shut down should a voltage higher than our limit be sensed.

Regards,

Andy