Heat tape control

[johnsold]

A 4-20 mA current loop is just a means of passing an analog signal from one device to another.  It has the advantage that the current tends to be somewhat less susceptible to interference from common noise sources. The typical way this might be used in a temperture control system is to have a sensor module which connects to the thermocouple and a control module which regulates the power to the heater.  The current loop connects the two modules.  This allows the thermocouple (which generates very small signals) to be far from the power regulator without worrying about transferring mV signals through a noisy industrial environment.

Since you presently have temperature measurement and power control in one box, it seems unlikely that you would gain any advantage to using a 4-20 mA system.

To answer your specific question: You would need a temperature measurement module with a 4-20 mA output.  You would also need a process controller which can handle 120 VAC at 5.2 A and which uses a 4-20 mA input.  The power source would be the 120 VAC you have avaliable in your facility.

Since you are considering Compact DAQ devices, I suggest the 9211, 9212, 9213 group for thermocouple measurements and the 9477 or 9478 Digital Output devices driving a solid state relay to control your heater.  I see no advantage in trying to make the gas controller also control the heater.

Lynn

[booxcar]

Ah, Lynn, that seems to make sense to me!  Let me see if I am getting this setup straight.  By using a digital output to a solid state relay, a small voltage output is the signal is sent to the relay and depending on the magnitude of this signal, the relay outputs a specific voltage to the heat tape to control temperature.  Is this correct?

[johnsold · Accepted solution]

OK except for one detail. A relay is a switch which is controlled electrically. So the small signal going into the relay (typically a few volts DC at a few milliamperes) controls the output switch.  Electromechanical relays use the magnetic field generated by the coil current (input) to move a metallic contact to open or close the output circuit.  A solid state relay (SSR) uses optical isolation and semiconductor switching devices to perform the same functions.  The major advantage of the SSR for temperature control applications is that it has no moving parts and can last a very long time as long as it is never overloaded.

For your heat tape application you would connect the 120 VAC power line to one of the "contact" output terminals of the SSR. The other output terminal goes to the heat tape.  The return side of the heat tape is wired to the neutral power line. The temperature control is done by varying the percentage of time which the SSR is on or conducting.  Since heaters typically change temperature slowly, the thermal inertia of the system averages the fully on or fully off switching of the relay to get the desired temperature.

If you are not familiar with this kind of wiring, I recommend you hire someone competent to do the wiring to make sure that all the appropriate safety requirements are met.

Many controllers switch about once per second.  With the solid state relay you could switch as often as 120 times per second if needed.

Lynn

[booxcar]

Thanks for clearing it all up!  One last thing, I was looking through some cabinets the other day and found an NI 9263 module lying around.  Is it possible to use this analog output instead of the digital one??  I believe the output is +/- 10V, 0-20mA.  As long as I get the appropriate relay, it should work?

Thanks,

Justin

[johnsold]

Justin,

When I looked up the NI 9263 it shows the maximum current drive  on an output channel to be 1 mA (pg 13 of the NI 9263 Operating Instructions and Specifications). I did not see any reference to 20 mA. This will be too low for most SSRs.

A typical SSR such as the Crydom D1210 requires 3.4 mA at 5 VDC. The Input control range is 3-32 V.  The output switch is rated 24-140 VAC at .04 to 10 A.

Lynn

[booxcar]

You are right.  For some reason, on the first bullet of the NI 9263 Data sheet and specs it says '±10 V and 0 to 20 mA analog output ranges, 16-bit resolution, 100 kS/s simultaneous update rate' but in the comparison tables, it clearly states that the current drive of the module is 1mA.  

[booxcar]

Alright, so I believe I understand everything about the modules required but still am having some questions about the relay.  Could you tell me if I am getting the right idea about the functionality of the relay?  From what I have read and understood, a relay controls how much voltage is applied from the power supply to the load depending on a smaller signal sent to the input of the relay.  This is done by configuring the switches (number of terminals depending on the number of throws and poles) in the relay using an input signal and the configuration of the switch will determine the load voltage.  Having said this, this means that there are set load voltages that can be used when a relay is utilized, correct or way off?

[johnsold]

I have been away for a few days.

A relay is just an on/off switch.  It will connect your load to the power source or disconnect it.  It is exactly the same concept as the switch on the wall which turns on or off the lights in a room. It is NOT a dimmer!

You only have 120 V AC available.  You do not have 93 V or 77 V or 52 V.  So you cannot change the votlage to the heat tape by selecting different voltages.  Because heated systems usually chnage temperature rather slowly, what is often done is to switch the power on and off much more rapidly than the temperature can change and reduce the average voltage to the level required to maintain the temperature. For example turning the power on for one cycle of the power line frequency and then turning it off for one cycle result in the average being 1/2 of the full voltage.  The solid state relays can switch that fast reliably. This is called duty cycle control or modulation.

Typically you will use a solid state relay which has a single pole, single throw, normally open configuration.  Three pole devices for three phase loads are also available. Other devices exist but are rarely used.

Lynn

[booxcar]

Alright!  Thanks a lot Lynn for your help.  Much appreciated! 🙂