analog value hold module


Thread Starter


i am in need of an electronic module which can take a 4-20mA signal input and output the same (with / without ground isolation). with an added feature that the output should hold at last value, if the input signal is out of range (<4mA / >20mA). pl let me know any products which meet this requirement.

There may be a device out there to do this, but it sounds like you need a custom device.

What are your accuracy requirements? How would you read the "last out-of-range value"?

If its just for troubleshooting, could you get by with setting up a digital storage oscilloscope to trigger on an over current event? They have cheap computer based (USB) oscilloscopes if you don't need bleeding edge accuracy or speeds.

It's hard to say what would be a good fit because we don't know what the application is. If you are trying to prevent out of range data from being fed to the process controller, then the solution is usually to program the functionality into controller.

If by "hold last value" you mean to maintain the value *prior* to the out of range data, then your process will be running without that data. You may have to somehow handle that situation and the controller can't handle it if it is completely unaware that something is wrong.

If the purpose is for temporary troubleshooting, then as Ken E said, it sounds like you need a storage scope with trigger. There are portable chart recorders with similar functionality (but with more storage), and there are software/hardware combinations that work with PCs that can do the same (although they're not as easy to use). If the event is slow enough, then a portable data logger with trigger might also work.

curt wuollet

Maybe even a free chart recorder with a $16 remote, 4 analog channels and any trigger qualification you can do with up to a dozen digital inputs.

I remember seeing a Honeywell single loop temperature controller, a UDC 3xxx on an aluminum extruder where the AL log furnace heater sensing was done by done by moving the element wires of a K type T-C against the log by use of pneumatic cylinder. The elements were left in place for about 8 seconds, the reading taken, then a contact closure from the logic sequencer told the UDC temp controller to hold its value.
Then the pneumatic cylinder retracted, other events transpired, and about 5 seconds later the temp sensor elements were driven back onto the log and the 'hold' on last value was released.

That worked well enough for that shop, but the key was knowing when to hold the value and when to release it, not expecting to be able to hold a value prior to its going off scale or out of range.