4-20mA Controlled Power Supply? Require 20v and 3A (max)


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So the summary says most of it. We have a heating pad that will be monitored by a PD100. The signal from that will be fed to a regulator which will produce a 4-20mA signal to control a power supply, which will drive the heating pad. The problem is, I am not sure what kind of power supply I need to be looking for, that can be driven by this signal.

I am not familiar with these devices so some keywords pertaining to this kind of setup would be much appreciated!

I think your monitoring sensor is a Pt100, commonly known as an RTD, not a PD100. And your 'regulator' is a PID feedback controller with a 4-20mA control output.

You might consider choosing a different regulator/controller that has a switching output, like an electromechanical or solid state relay, that directly switches/controls the electrical source to the heating pad, rather than the one that has a 4-20mA output. Then you can use any suitable power source and the controller does the controlling, not the power source.

A solid state relay will last longer than an electromechanical relay.

Google 'temperature controller' for hundreds of choices.
A couple things...

As someone else mentioned, look for a single loop PID controller. Ideally something with a discrete output (Digital/Relay). That way you can energize the heating pad with a solid state relay, the PT100 RTD gives a feedback for the PID loop, and you can just specify a temperature setpoint on the PID controller and it'll go.

I'd suggest something like the Gefran 650 Series because it is easy to read, has scrolling text, has an autotune feature for the PID and has a 7 step configuration for easy setup. The part number I'd suggest is 650-D-R00-00000-1-G, it runs on 100-240VAC, has two digital outputs and is In stock in Boston. Website with technical data is here: http://www.gefran.com/en/product_ca...s/481-650-pid-1-16-din-temperature-controller

For the solid state relay I'd suggest a simple "hockey puck" style SSR. Gefran offers these in the GQ series from 25-90A. The GQ Product page is here: http://www.gefran.com/en/product_ca...94)/products/237-gq-solid-state-relay-max-90a. Check the heating pad's current draw and give yourself some wiggle room with the solid state relay. It's likely well below 25A, but make sure to check this so you can put a fuse in series with it (More on this in a second). Some notes on these: They have an exposed copper back which is made to draw heat away from the electronics inside, you will want to mount this with thermal compound to a back panel or heat sink to wick the heat away from it. This is especially important if you're switching close to the maximum current rating of the relay. As I mentioned earlier, you will likely want to put a fuse in series with the SSR and the heating pad in case of a short or a heater failure. Watlow has a guide on fuses and fuse holders for heater applications here: https://www.watlow.com/downloads/en/training/fusefaq_a.pdf

The cliff note's is that you want to rate a fuse 125-160% of the load and make sure the I2T of that fuse is below the rating of the SSR. You can get the SSR I2T ratings from the datasheet at the link above.

I also drew a quick diagram for how the SSR would wire up:
Full Disclosure: I am the Rep and a distributor for Gefran out of the Southwest United States. That is how I know what stock levels are and why I'm so familiar with the product and the application. If you need additional help, don't hesitate to reach out to me via PM or email.