Which output do I need for my controller?

 6th Feb 2014

Q: Which output do I need for my temperature / process controller?


Four output types are commonly used: Relay contact / Current (typically 4-20mA) / Voltage / SSR (Solid State Relay) drive. For some controllers (like the Fuji PXR) the output type has to be advised prior to despatch.

If you are inexperienced with instrumentation in general, please bear in mind that the output of the controller is used to drive something else (a heater, valve, motor for example).

If you are looking for a temperature controller, you will probably want to control a heater (or a cooler). Typically, you will be looking at a relay contact output or a solid state relay output (SSR).

If you want to drive another device with an analogue signal then 4-20mA output (Current) or Voltage output is most likely to be suitable for you.

However, there are infinite possibilities of processes and the advice above is not necessarily true for you.

We have detailed below the function of each output. If you are still unsure, please do not hesitate to contact us.


Relay Contact Output


A relay contact output works basically like an on/off switch. To simplify, if the output is "off" the circuit will be broken (open). If the output is "on" the contact will be made, completing the circuit. Therefore, the controller does not supply any current or voltage itself. You have to make a circuit with a power supply going through the output pins of your controller in order to power up your heater/pump/device/other.

Also, please note that the current rating of a relay output is too small to switch on and off a heater for - say - an industrial oven. For instance, the rating on our Fuji Controller is 3A @ 220V. It means that, at 220V, you can switch a heater of a maximum power rating of 660W (W=I x V). For a bigger heater, you would need to consider an external relay with a bigger rating. You would then use the output of the controller to energise/de-energise the external relay.

relay contact output
On some controllers, and that is the case of our Fuji PXR controller, the first output is a SPDT (single pole double throw) switch which means that you actually have 3 pins (see below) thus two possibilities of contact for your output:
SPDT relay contact output
This allows a greater flexibility and more possibilities for your wiring.

Limit of the relay contact output: While it will work great in numerous process, you have to bear in mind that a relay contact output is mechanical and quite slow: this is partly due to a setting which limit the switching over a given period of time (time constant) in order to enhance the life time of the relay (the number of operation of each mechanical relay is limited: our relay output are certified for over 10 million operations for example). This is usually fine for temperature because of the temperature lag. However if you require a fast operation on the output you should consider a 4-20mA or a SSR drive output.

Voltage or current signal

A voltage output will typically deliver a 1-5V signal and a current output will typically provide a 4-20mA signal.
Other possible signals can be 0-5V, 0-10V, 2-10V (Voltage), 0-20mA (current).
Voltage output current output

Please note: On our Fuji PXR temperature controller, only 4-20mA output is available. For voltage output signal (such as 1-5V or 2-10V) please see our process controllers PXG and PXH.

Voltage and current output are used to drive a variety of devices in a very accurate way. 4-20mA is an industry standard and lot of devices will accept that as an input (see our Watson Smith I/P converters for example). The signal can vary from 4mA and 20mA and therefore it allows an analogue control much more accurate than that of a simple on/off control you would get with a mechanical relay contact output.

Solid State Relay (SSR) drive output

For an accurate control, SSR control (solid state relay) are preferred over standard relays.

The SSR drive output of the controller will provide a fast Voltage signal. For an "off" signal, the voltage will be closed to 0Vdc. "On" will provide a signal around 24Vdc, depending on the controller. As an example, Fuji Controllers provide 17 to 24Vdc for "on" and 0.5V or less for "off".

This will be the signal for an external Solid State Relay (sold separately in most cases). This SSR will have no moving parts, allowing it to switch on/off extremely quickly and thus following the Voltage signal from the controller. It also results of a greater lifespan.

There are lots of Solid State Relays available with different specifications - the most basic one being the rating (see "current rating" on the "Relay contact output" paragraph). If you want to discuss your application please do not hesitate to contact us.

SSR drive output
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