Controlling heaters with solid state relay and analog output

[wbadgers12]

Hello,

I have three heaters that I would like to use to control the temperature of air at a point in my experiment. Each heater is controlled by a solid state relay. I have a thermocouple that I would like to use as the process variable input to a PID controller. 

I have been reading about using Pulse Width Modulation to control the duty cycle of solid state relays, but I do not have access to hardware with a counter. Also, I am using an NI 9264 DAQ to send an analog voltage to the relays to trip them. The examples of software timed PWM that I have seen require digital output.

Would anybody be able to give me some suggestions for setting up a PID controller with the hardware that I have available?

Thanks.

[johnsold]

I think you will need a buffer or amplifier.  The specs on the NI 9264 indicate that each output can drive up to 4 mA.  That is on the low side for reliable drive of solid state relays (SSR).  A standard Crydom D2410 for example has a typical input current of 3.4 to 20 mA.  A driver like the ULN200x series of devices should work nicely.

If you are controlling heaters via solid state relays, the temperature will change slowly enough that you can calculate your PWM in software. The SSR turns off at power line zerocrossings and may turn on at zero crossings also, so the PWM timing resolution will be in increments of a half period of the power line frequency or slower.

The NI 9264 is an output module. What are you going to use for measuring the thermocouple output? Typical thermocouple outputs are in the mV range and you typically need an amplifier or signal conditioner between the thermocouple.

Lynn

[wbadgers12]

Hi Lynn,

Thanks for the advice on this. I have the PID working, but now I am having issues with drawing enough current to switch the relays like you warned. I am trying to drive 6 relays, and each relay needs about 7 mA. If I use a ULN2003 driver, how should it be wired? I have access to a 24 volt power supply in the box that I am using. 

Thanks,

John

[johnsold]

John,

I do not have time to draw up a diagram right now so I will attempt a verbal description.  And I am working from memory on th eULN2003 so I may not have the terminology exactly correct.

For each relay use a separate circuit (assuming the relays need to operate independently). The circuits are identical.  Connect an output from the DAQ device to the input (or base) of one section of the ULN2003.  I do not recall whether the ULN2003 requires a series resistor.  Check the data sheets. Connect the relay coil from the output (collector) of the driver to +24 V.  Connect the common negative terminal (emitter) of the driver to the DAQ ground and to the negative side of the 24 V supply.  The driver may have protective diodes internally.  The anodes must be connected to the +24 V supply.

Lynn