Power control of a step-down transformer with a 4-20mA output signal

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Hello, I am looking for advice on the best method for controlling the power to the step-down transformer as shown. To avoid switching noise issues, I've been looking at scr modules Fotek SCR-25LA that allow linear control of the power with a 4-20mA signal.

I've also been looking for an SCR module that takes a 0-10 volt signal that I could drive directly from the DAC board. Have not found this yet.

I have breifly looked at PWM but I'm not sure how or what noise issues might be involved with this form power control.

Any advice and/or pointing me in the right direction is appreciated.

Situation:    Converting a vacuum drying kiln for exotic hardwoods from an obsolete control system to DAQ Factory based contols using two Labjack U3-HV units. There are 15 thermistor inputs and one vacuum sensor input. For the outputs, there is control of the vacuum pump and six resistive heating zones, each controlled seperately.


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Well, normally I would say put an SSR on the low voltage side and do a PWM on it with the LabJack, but 200A is a mighty big SSR and you are AC on the low voltage side, so I can see why you'd want to do the switching on the 240V side.  And of course an SSR doesn't switch AC as well because it doesn't phase align like an SCR.  Likewise PWM isn't going to work on the LabJack because it can't phase align either.    But you knew that already.  It seems like you really are just hung up on the fact that the SCR you want takes 4-20mA not 0-10V.  I'm going to let the LabJack folks chime in on possible solutions, but you can always go with a really cheap PLC that outputs 4-20mA like a Click PLC, just for that part.  The PLC is like $100 so really not a big deal, and you can use it with the LabJack for everything else.  It is available with either a serial or Ethernet connection and talks Modbus (so DAQFactory can easily talk to it).   

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Unfortunately, LabJack does not have any robust solutions for 4-20mA output. Sometimes you can use a DAC output for 4-20mA based on the load impedance, but that will not work here. There are some 0-5V to 4-20mA conditioning modules out there, but I think it would be more expensive and overall worse than the PLC idea. There are also some inexpensive temperature PID controllers with 4-20mA output. It sounds like you want to handle the control in software so most PID controllers would not be suitable, but I did find the following controller that appear to have a voltage to 4-20mA retransmission configuration for a bit less than a signal conditioner:


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