4 to 20 ma signal

Hi

Does anyone have a drawing of a circuit that will reduce the 4-20ma signal coming from a controller into another unit

We have a controller that puts out a 4 - 20 ma signal to a thristor drive, that in turn controls a transformer fed heating bank , I need to find a drawing of a circuit that would go in between the output of the controller and the input of the thyristor that would limit the output when the machine first starts up

I believe you can use a variable potentiometer positioned in between the two units , I just dont know how it should be wired and what value resistance

It would be used as a power limiter on start up , to prevent the heating elements starting too quickly from rest

I did a search but could not find a reference to it



All help greatly appreciated

you already have controller, can't you use it to control it's output? is it plc?

the dirty way is to insert resistor (or potentiometer, 5k will do) to limit current.
you would only need two terminals (wiper and one of the ends) and insert it in series with
your analog signal so that 4-20mA must flow through potentiometer.

next thing would be to short it after while (timer) so system operates as usual.

It was slightly off topic , my apologies

The problem i have is the controller only backs off fully after the set point has been reached

It is a controller unit , not a plc

I want a way of power limiting the 4 - 20 mA signal as it initially heats up , so it doesn't reach set point too quickly

Its a dirty way , but it solves it whilst we wait for someone with pid knowledge and knowledge of this particular controller to have a look at it ( he is away for a week )

thanks for the info

Mike

Wellllllllll.......you can do it, but..........!!!

Without seeing how the whole thing is set up, there is no guarantee that your idea will work, but I'll leave that part to you!!!

To answer your question, you can add a resistor in PARALLEL, not series, across the input terminals to the thyristor unit. This will act as a current divider.

Use a variable resistor (potentiometer) of about 1000 ohms. Make sure it is a linear potemtiometer. A 1/2 watt one will be fine. This should give you a good range of control.

As 'Panic Mode' mentioned, you would be better off adjusting the controller. Most have an "output limit" selection in the setup menu that allows you to program the maximum mA output that the controller is permitted to pump out.

Good luck!

Yes, it will have to be in parallel not series. In series there is still just a single loop and the current everywhere in the loop is the same. What likely would happen is that you would overload the output with too high of a resistance.

With the parallel circuit switch the parallel resister in to scale the signal and take it out for normal operation. Note that it really is a scaling and not a limit so if you set your parallel resistance to the same value as the input resistance you effectively cut the signal in half.

Thanks guys , will let you know how we got on