compressor unload control

[innercalm]

Hi People,
Hope you can help me. We have a bank of compressors running off a proprietary controller. The trouble is that on one of the compressors the on-board controller has no input to allow control of its load/unload state. This on-board controller relies only on the pressure transducer input (0-5vdc) to decide whether to open or close the load/unload solenoid. My idea is that I can build a little voltage regulator using an LM117 whose output is paralleled with the pressure transducers output and switched on from the bank controller to "trick" the on-board controller it has a high pressure state and will unload. My question is; will this work, and is there any danger I will back feed the pressure transducer and damage it?
Any help would be mightily appreciated.

 

[itsmoked]

Yes there is a good chance you will either damage it or freak it out and have issues with it incorrectly running the compressor when it normally needs to operate from the transducer.


To prevent damage to the compressor and to allow the transducer to unload the compressor when the pressure has reached it's maximum allowed by the compressor's design you need to do something that doesn't over-ride the transducer.

Two ways to do this.

1) You use an OP AMP wired as a non-inverting summing amplifier then any added voltage will sum with the transducer and that will be what drives the control. That way the transducer's value will always be there.

Google "op-amp summing amplifier" Note you would not want any amplification so you'd choose resistors appropriately.

2) Don't muck about with the transducer just find the unloader valve and parallel your own control power across it. Set up a DPDT relay to disconnect the compressor's own signal to it whenever you "demand unloading". Whenever you don't demand unloading the compressor will unload itself as it always has.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Why do you want to do this though? Seems to me the control system was DESIGNED this way, now you are trying to circumvent it?

Beware the law of unintended consequences.

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[Compositepro]

Is something broken that needs fixing or are you trying to do something counter to the original design of the controller? We don't understand what you are trying to accomplish. It sounds like a bad idea.