roll your own 4-20ma

Hello all.

I am locked into a small budget to do a small hydraulic press. It was to be straight forward, all digital off / on with no analog proportional stuff. We are talking simple relay logic, no PLC.

Hydraulic design has thrown in a proportional solenoid that needs a 4-20MA signal. In the past I would have used a signal card but I don't want to increase my cost.

I am thinking of using a pot with appropriate resistors to control current to desired levels. I probably would add a current limiting fuse inline to avoid potential problems.

My question is if this is common? I can do the math for resistance needed and I know to include resistance of load. Anybody have cons or reason why I should not do this?

IF the system was suppose to be just ON/OFF then why was the proprotional valve used. I think someone may have changed their mind on what they wanted the press to be capable of OR is this a situation where they used a part they had in stock?

If the process needs more than ON/OFF then I think that blew the budget. Its hard to say without knowing all the details.

The question is how many different settings do you need? 4, 5, 8, 200? You can build something but by the time you get it working you could have spent way more then going with a small PLC. If your setup is REAL SIMPLE..an AD DL05 and an analog out would cost about $300.00, and greatly simplfy the entire project. Time is money. Unless you work for free, theres no way they are going to save a buck here by going all hardwire.

DL05: http://www.automationdirect.com/adc/Shopping/Catalog/PLC_Hardware/DirectLogic_05/PLC_Units

Analog out: http://www.automationdirect.com/adc...are/DirectLogic_05/Option_Modules/F0-4AD2DA-1

What is the proportional valve used for?

If it is a proportional pressure reducer used to manually set the pressure for the press before pressing and you don't need much accuracy maybe you could use a pot and some other resistors.

But if it is a proportional directional valve for positioning or paralellity then using a PLC gives much more options at a lower cost.

Are you sure that it is a 4-20 mA signal and not a PWM current signal. If 4-20mA the valve has some kind of onboard electronics or integrated signal card.

I think someone may have changed their mind on what they wanted the press to be capable of OR is this a situation where they used a part they had in stock?

Click to expand...

Exactly (the later for hyd dept.)

If your setup is REAL SIMPLE..an AD DL05 and an analog out would cost about $300.00,

Click to expand...

Thanks, but I could get a signal card for 1/2 that.

The question is how many different settings do you need? 4, 5, 8, 200?

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everything between 4 ma and 20 ma. The application is not critical. The result of 4 - 20 input is aproximate 500 to 3000 PSI.

If it is a proportional pressure reducer used to manually set the pressure for the press before pressing and you don't need much accuracy maybe you could use a pot and some other resistors.

Click to expand...

That is the app dead on. I could put together a pot and some resistors in 5 minutes for < $20.00 in parts.

Anyone see any problem with pot and resistors in this generec application? If this is sound possibillity, I make all parties happy with very little time, money wasted.

Keep it simple and cheap!

Go for it Robert, resistors just love to control current! LOL
Should be fine in the case you describe. Automation addicts like us tend to want to push for automation, but sometimes, it is just what is needed to go with the old-fashioned hand method.

You probably wont need to build anything, if using a pot/rheostat you should be able to predetermine setpoints per number on a locking wheel.

They also make selector switches that have linear resistance per set point.

It shouldnt be hard to get 10 divisions of 250lbs...ie 500, 750, 1000, 1250, 1500 etc etc or whatever you need.

Simple is good sometimes.

Omega used to have frequency to analog converters like these http://www.omega.com/pptst/DRP-8500.html but I don't see them. You could also call a company called Calex that has a mopdule that converts frequency output from a PLC discrete to analog signals. GE has an MSP module that can do the same, but is even more flexible and doesn't need a high speed output.