Convert a 4-20ma signal to a dc voltage

[trimixdiver]

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?

 

[trimixdiver]

Also if 250 ohms gives a 5vdc then 600 ohms should give me 12vdc. 12vdc is better for the relays I have. Am I missing anything here?

 

[imok2]

Go here:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/823

 

[TurboXS]

You can either build a circuit to do this which would be of low material cost but require some time, or buy a ready made solution such as;

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)

 

[trimixdiver]

Well since its just for me brewing beer and the Watlow 945a will do the setpoint, I just need to get a usable voltage from the 4-20ma process output. The resistor idea is the most sound. I was going to put about 600 ohms accross the output (resistor plus relay coil resistance) to get 12vdc. So when the Watlow is programed for heating it should output the full 20ma thus 12vdc to engerzie the relay and turn on the heat tape.

Please check me on this. I dont want to put the Watlow under any kind of stress.

Thanks sooooo Much

 

[ScottI2R]

I like Turbo's "BIY" idea. And if the Watlow gets stressed, heck, just give it a beer. Always worked for me...

Scott

In a hundred years, it isn't going to matter anyway.

 

[TurboXS]

Just in case I misunderstood you. While a 600 ohm resistor will generate the 12V, you would have to take into account the load of the relay coil. This will reduce the output voltage across the resistor. I guess you could use a 600 ohm relay instead of the resistor. But this would be an extrememly crude control circuit and you really wouldn't know when it was going to switch on or off. Remember that the turn on voltage and release voltage of a relay can differ considerably and a 12V relay doesn't actually turn on at 12V!

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)

 

[itsmoked]

I think you are gonna blow it if you are not careful!

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://www.watlow.com/literature/prodtechinfo/files/controllers/945e_a.pdf

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!)

 

[OperaHouse]

I can understand the urge to use something that you grabed from work, but you can buy a new digital controller w/thermocouple for $9.95 on eBay. Personally, I would prefer to use a proportional controller made from a switching regulator chip like a SG3524 or TL494 and a thermistor. Relay heater systems have just too wide a swings in temperature unless you are very careful.

 

[buzzp]

The resistor might work with lots of tweaking to figure out where the relay pulls-in. However, an interface between the resistor and relay coil is needed unless you take into account the resistance of the coil. Since this is for one unit you could probably get it to work. However, the wide variations in temp is likely since your basically making an off-on controller. This is generally not the way temp control is handled. What might help make this work a little better is the fact that your using heat tape to generate the heat and not an open flame system.

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).

 

[itsmoked]

GUYS, Guys, guys... Am I missing something here? This thing puts out and ANALOG 4-20mA current that indicates how much power or flow or whatever that a heater or actuator is supposed to go to. You know, 37.6% heat or something. Why are you talking about hooking an on/off relay to this to control a heater?? I am confused!

 

[ScottyUK]

Hi itsmoked,

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.





----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[itsmoked]

Okay ScottyUK.

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?

 

[ScottyUK]

Absolutely!



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[OperaHouse]

itsmoked makes a good point. Rectify the heater voltage and drive a transistor etc. with the 4-20 ma. If you drop the voltage to a level to just about 30% higher than is needed to maintain the temperature, it wouldn't be too bad. Tha bad I'm talking about is the controller will waste a LOT of heat.

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.

 

[trimixdiver]

Thank you all for the help. Can I use a Watlow DIN a mite to perform the functions? Are they all 4-20ma inputs?

Thanks

 

[itsmoked]

trimix... I/we can advize on all this stuff BUT! These controllers all have UMPTEEN options.. They have terminals dripping off the back that CAN do alllllll sorts of things. BUT! It all depends on which of the 20 options are installed in YOUR controller. So ask anything you want, BUT! tell us the controller's EXACT part number!!

 

[trimixdiver]

Watlow 945A with just 4-20ma output, no alarms no relays
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.

 

[elfgriper]

Trim,
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

 

[itsmoked]

Sorry trimixdiver:

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