Convert a 4-20ma signal to a dc voltage
Convert a 4-20ma signal to a dc voltage
(OP)
I need to convert a 4-20ma signal on a watlow to dc voltage to turn on and off a heating tape. Any simple way to do this?
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Convert a 4-20ma signal to a dc voltage
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RE: Convert a 4-20ma signal to a dc voltage
RE: Convert a 4-20ma signal to a dc voltage
RE: Convert a 4-20ma signal to a dc voltage
http://www.foxtamcontrol.com/Fox_con_current.htm
or the Crouzet EI range of current control relays.
To make your own, one solution would be a resistor to convert the current to a voltage, a comparator to set your switch point which would then drive a relay for your heater circuit.
regards
Pete
(TurboXS)
RE: Convert a 4-20ma signal to a dc voltage
Please check me on this. I dont want to put the Watlow under any kind of stress.
Thanks sooooo Much
RE: Convert a 4-20ma signal to a dc voltage
Scott
In a hundred years, it isn't going to matter anyway.
RE: Convert a 4-20ma signal to a dc voltage
I suggest you use a comparator as this would provide sufficient output drive to switch a relay or a driver and give you a controlled switch point. As a bonus you can add hystersis to the comparator quite easily.
Don't forget some means of controlling the back emf of the relay coil.
regards
Pete
(TurboXS)
RE: Convert a 4-20ma signal to a dc voltage
You cannot take an OUTPUT that is 4-20mA and convert it to any kind of on/off control signal from that unit.
However it seems to have serveral other fine ways to control what you want directly from its rear screws.
Have you checked the manual?
http
And please clarify.. Is this 4-20 the input to the controller or the output? (You called it the process which is usually the input, me confuzed!)
RE: Convert a 4-20ma signal to a dc voltage
RE: Convert a 4-20ma signal to a dc voltage
A comparator would making the tweaking of the resistor value unnecessary.
A temp controller would allow a more steady heat output. There are several to choose from with some with self learning PID control (might be overkill in your case but can just use P or PI control on most as well).
RE: Convert a 4-20ma signal to a dc voltage
RE: Convert a 4-20ma signal to a dc voltage
The OP said "I need to convert a 4-20ma signal on a watlow to dc voltage to turn on and off a heating tape.". The key words being on and off, so I can see where the relay idea came from. This could easily be achieved using a threshold detector.
I totally agree that proportional control would be much nicer from a pure control point of view, although if the thermal lag in the system is long then on-off is still a reasonable possibility. Something like
http://www.eurotherm.com/products/power/7pm.htm
would be a technically good solution, although for making homebrew or moonshine or whatever it might be a bit pricey.
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If we learn from our mistakes,
I'm getting a great education!
RE: Convert a 4-20ma signal to a dc voltage
If bang bang is okay why not just use the alarm output? Set the low alarm and whenever it's in alarm the power is on.
trimix: What is the exact part number for this bugger?
RE: Convert a 4-20ma signal to a dc voltage
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
RE: Convert a 4-20ma signal to a dc voltage
You can feed a voltage from a 4-20 resistor drop and drive a SG3524 with it in proportional mode with only about a half dozen parts. Many times I've thought I could just use a transistor and a couple resistors for a simple control application. Almost always, the time it took tweeking and the crude control just wasn't worth it. There are chips for everything that do a great job. As a practical matter, if you have to ask this question, a circuit development projuct of this sort is not likely something that you should take on. I think I saw some proportional solid state relay blocks on eBay a while ago that were cheap. That stuff comes and goes.
RE: Convert a 4-20ma signal to a dc voltage
Thanks
RE: Convert a 4-20ma signal to a dc voltage
RE: Convert a 4-20ma signal to a dc voltage
I was going to use a DIN-A-Mite Ver C that takes the 4-20ma input. Its 3 phase though. But not a problem.
RE: Convert a 4-20ma signal to a dc voltage
The DIN A Mite controller looks like it was made to operate with your watlow controller. Go to watlow home page or search on Watlow Din A Mite and read the details. If you try to use a 4-20 ma signal you'll need some other circuitry as stated above.
-elf
RE: Convert a 4-20ma signal to a dc voltage
I hope you are more specific in your diving times and depths... :)
954A is not the full model number.
Let me be more specific.
There are eight hundred and forty different 945A controllers.(that's if we leave out communications protocols)
There are no 945A. controllers.
945A-????-?000
RE: Convert a 4-20ma signal to a dc voltage
And the DIN a Mite is DC21-60F0-0000
So now that I got off my A** and got you the numbers.
If I use the output (4-20ma) of the 945A to control the DIN A MITE, do I change the upper and lower limit to get it closer to my setpoint of 53deg? Please explain how I would set it up to give power to a heat pad to maintain 53deg in a 35deg enviroment.
Thanks
RE: Convert a 4-20ma signal to a dc voltage
Hey that bugger does too have an alarm out! It doesn't have a SECOND OUTPUT... But it has a Dual, Mechanical Relay, rated at 6AMPs, Form A or B.
Power it up.
Mess with the LOW alarm setting. Set it to about 80F. It should go into LOW alarm immediately. Next grab the thermocouple, once your fingers heat it up to 80+ the alarm should clear. If this is what happens, get out your ohm meter and check out screws 24,25,26,26. Once you figure out the scheme try your heater with it. Watch your current limit tho.