How to Choose a Temperature Controller (PID) in Pakistan

The short answer

• On/off is enough when the temperature may swing a few degrees, as in a water tank or a cold room. PID holds a tight band and suits moulds, extruders and process heating.
• Set the input to the sensor you have: type K, type J or Pt100 (also written PT100). A wrong setting reads wrong or shows an error.
• Relay output drives a contactor and slow switching. An SSR-drive output drives a solid-state relay for fast cycling. A 4–20 mA output drives a power regulator, not the heater itself.
• Measure the cut-out. Front sizes of 48 × 48, 72 × 72 and 96 × 96 mm are not interchangeable, and the hole is smaller than the front.
• Auto-tune on the real load, with the sensor in its working position. Re-tune if the heater, the load or the sensor position changes.

On/off control or PID

An on/off controller behaves like a thermostat. Below the set point, minus a small gap called the differential or hysteresis, the output is fully on. Above the set point it is fully off. The temperature therefore cycles. A wide differential switches less often and swings further; a narrow one switches more often. That is acceptable when the mass is large, such as a tank of water, or when the product allows a band of several degrees, such as many cold rooms.

PID control calculates an output from three terms. Proportional reacts to how far the temperature is from the set point. Integral keeps acting while any error remains, which removes the steady offset that proportional control alone leaves behind. Derivative reacts to how fast the temperature is changing, which limits overshoot. The result is not usually a smooth voltage into the heater. With a relay or SSR the controller time-proportions: it varies the fraction of each cycle that the output stays on. At 40% of a cycle the heater is on for that fraction and off for the rest. The cycle time belongs in the manual; a relay needs a longer cycle than an SSR, or the contacts wear out.

A small on/off thermostat, such as the STC-100A listing in our temperature controllers range, is a different instrument from a panel PID controller. Use it where a probe and a single relay are the whole job. Use a PID controller where the panel already has a cut-out and the process needs a set point that stays put.

Input types: K, J and Pt100

The controller does not know what is wired to it until you select the sensor in its menu. Type K, type J and Pt100 use different tables. Leave the menu on the wrong type and the display is wrong even when the wiring is sound. Select the type before you judge the reading, and use the lower of the controller’s range and the sensor’s own limit.

Thermocouples

A thermocouple is two dissimilar metals joined at the tip. The controller reads a small voltage and converts it with the table for that type. Polarity matters: reversed wires typically drive the reading the wrong way or raise an error. The extension cable must be the same type as the sensor, all the way to the controller. Copper “to finish the run” adds another junction and a wrong reading. Keep the cable away from heater wiring.

Colour is not a safe way to tell K from J. IEC and ANSI colour codes differ, so read the tag or the plug. A K-type thermocouple in our sensors range is described on the listing as 400 °C: that is the sensor’s limit, not a limit of every type K. The process temperature has to sit inside both the sensor limit and the controller’s range for that input.

Pt100

A Pt100 changes resistance with temperature: 100 ohms at 0 °C. It is more accurate and stable than a thermocouple at moderate temperatures, which is why it is used on tanks, dryers and many process lines. It is not a substitute for a type K on a very hot barrel. Three-wire connection is the usual industrial choice, because the controller can cancel the resistance of the leads. Two-wire connection adds the lead resistance to the sensor and reads high, more so on a long run. Four-wire is the most precise and is less common on a panel controller. Do not select “Pt100” for a Pt1000 probe (1,000 ohms at 0 °C), or the other way round.

Relay, SSR or 4–20 mA output

The output is how the controller applies power, and it is a separate choice from the sensor. The same family is often sold as a relay version and an SSR version. On the site, one Yamatake SDC15 is marked relay and another SDC15 is marked SSR. Read each listing: some of those units are surplus, not new.

Relay output

A relay contact is simple and can switch AC or DC within its rating. The rating is often only a few amps, so it is the wrong place to connect a large heater. As a check on the arithmetic: a 3 kW heater on 230 V draws about 13 A, because 3,000 ÷ 230 is about 13. A contact rated a few amps cannot carry that. Use the controller relay to pull in a contactor sized for the heater, and read both the coil current and the contact rating. For motors, size the contactor by its motor rating; our contactor sizing guide is the chart for that, not for a resistive heater.

SSR-drive output

An SSR-drive terminal is not a power contact. It supplies a low-voltage DC signal, often around 12 V DC, that lights the input of a solid-state relay. The manual states the exact voltage and the current available. The SSR then switches the heater. This is the right output when PID must pulse the heater many times a minute. An SSR needs a heat sink, leaks a small current when off, and can fail shorted, so a heater still needs an independent over-temperature cut-out. Sizing, heat and that failure mode are covered in relay vs contactor vs SSR. SSR units themselves are in relays and accessories, for example the Fotek SSR-40 DA; the suffix is maker-specific, so read the label for control voltage and whether the load is AC.

4–20 mA output

A 4–20 mA control output tells another device how much power to apply. At 4 mA the command is at its low end, often 0% power, and at 20 mA it is at the high end. Using 4 mA rather than 0 mA means a broken wire (0 mA) is not mistaken for a real zero. The receiving device, typically a thyristor power regulator, must be ranged the same way. This current cannot heat the load by itself. Some controllers also retransmit the measured temperature as a separate 4–20 mA signal into a PLC analogue input. That is a measurement, not the control output: check which terminals the manual assigns.

Panel sizes: 48 × 48, 72 × 72 and 96 × 96

The front size is the square you see. The cut-out is the hole, and it is smaller than the front. For these three sizes the usual cut-outs are 45 × 45 mm, 68 × 68 mm and 92 × 92 mm. Confirm the figure on the datasheet, then measure the existing hole and the depth behind it. A controller with the right front still will not go into a hole cut for a different size, and a deep instrument will not go into a shallow door.

Other fronts exist, notably 48 × 96 mm. On many controllers in the REX-C family the model number follows the front: C100 is 48 × 48 mm, C400 is 48 × 96 mm, C700 is 72 × 72 mm and C900 is 96 × 96 mm. Treat that as a clue and still measure the hole. Listings on the site include a REX-C400 described as a type K PID controller, a REX-C700 and a REX-C900, plus an Autonics TZ4W and a Fotek MT-48. The sensor and the output are on the label of the exact unit, not in the series name alone.

The supply is the other figure on that label. Many panel controllers accept 100–240 V AC, which suits Pakistani mains better than a unit marked only for a single voltage, because the supply often sits away from a neat 230 V. Others are 24 V AC/DC and must be fed from a control supply; see how to size a 24 V DC supply if that rail also feeds a PLC. Do not assume the listing’s title states the supply. Read the side label.

Auto-tuning

Auto-tune (sometimes marked AT or self-tune) switches the output and watches how the temperature rises and falls, then writes proportional, integral and derivative values for that load. It is the normal way to start. The numbers it finds are only right for the conditions of the test.

1. Select the correct sensor type and confirm the reading is sensible with the heater off.
2. Connect the real output: the contactor, the SSR or the 4–20 mA regulator that will run the machine.
3. Put the sensor where it works in production, not on the bench beside the controller.
4. Run the tune at the temperature you will use, with a normal load: water in the tank, a closed oven, material in the mould if that is how it runs. An empty tank or an open door teaches the controller the wrong process.
5. Leave the set point alone until the controller says the tune has finished. Then watch a heat-up. If it overshoots badly or crawls, check the sensor and the output before you edit the terms by hand.

Re-tune when you change the heater, the load, the sensor position or the set point by a large amount. Auto-tune cannot repair a wrong sensor type, a heater that is too small, or an SSR that has failed short and left the heater on. For a heater, pair the controller with a heating element of the right power and fit an over-temperature cut-out that does not depend on the controller.

A replacement, worked through

A workshop in Lahore has a 48 × 48 mm controller on a small oven. The sensor plug is marked K, and a contactor switches the heater. The temperature must stay close to the set point, so the replacement is a 48 × 48 mm PID controller (cut-out about 45 × 45 mm, and check the depth), with the input set to type K and a relay output used only for the contactor coil. If the old unit was already pulsing an SSR, order the SSR-drive version instead. Note the set point and the alarm levels before you remove the old controller, match the supply on the label, and auto-tune with the oven closed and loaded as it normally runs.

Selection checklist

1. On/off or PID, from how tight the temperature must be and how often the heater may switch.
2. Sensor: K, J or Pt100, including 2-wire or 3-wire for a Pt100, and the same type selected in the menu.
3. Output: relay, SSR drive or 4–20 mA, matched to a contactor, an SSR or a power regulator.
4. Contact or SSR current rating against the real load, not against the controller’s front legend.
5. Front size and cut-out, depth, and supply voltage.
6. Alarms, and for refrigeration a compressor delay. An independent high-limit on any heater.
7. Auto-tune on the real load, then a supervised heat-up before the machine is left to run.

Browse temperature controllers, sensors for the thermocouple or Pt100, heaters and relays and accessories. Brands on the site include Autonics, Omron, RKC, Fotek and Honeywell. Prices are in PKR. Send a photo of the old controller’s label, the sensor marking and the panel cut-out on WhatsApp and our Lahore team will help you match it.