When a technician encounters electronic controls on a system, many times a problem with the control can be diagnosed without knowing how it works internally. If the inputs to a controller are correct, then the outputs should be correct if the controller is operating normally.
For example, a technician encounters a basic electronic temperature controller with one input and one output. To determine if the cause of the system's problem is the electronic temperature controller, the technician must first determine the function of the controller in the circuit. Should the control open or close on a rise or fall in temperature?
Next, the technician needs to determine which leads of the controller are connected to the output (switch) within the controller. Once the switch is identified, the technician needs to determine if the switch is electrically open or closed.
This can be done in the same manner as any mechanical control. If voltage is present on the inlet side but not on the outlet side, the switch is electrically opened. If the switch should be closed, then the problem lies within the electronic controller. The technician would then need to look at the input to the control. If the input signal is correct, then the control is faulty and needs to be replaced.
Types Of InputsControllers can have a variety of different types of inputs. The input can be a varying resistance, voltage, amperage, or a digital closure (an open or closed contact point). In our example, the input to the controller is a thermistor, which is a sensor whose resistance changes based on its temperature.
To determine if the thermistor is operating correctly, a technician should measure the temperature sensed by the thermistor and then its resistance. By comparing the actual resistance measured to the resistance stated by the manufacturer, a technician can determine if the thermistor is OK. For example, if a manufacturer states that at a 70 degree F temperature its thermistor should have a resistance of 10,000 ohms, and if a technician measures a resistance different than 10,000 ohms, the thermistor is faulty and needs to be replaced.
Normally, determining if an input is correct requires obtaining the specifications from the manufacturer of the controller or system. Failure to do so is normally why troubleshooting electronic controllers is at times difficult. Technicians either do not have the time or do not know where to find the information on the controller.
The procedures just described will normally allow a technician to determine if a problem with a system is the result of a faulty controller. Knowing how the electronics within a controller operate is not necessary. All a technician needs to know is the input into the controller and its resulting output.
Joe Marchese is owner of Coldtronics, Pittsburgh. He can be reached at 412-734-4433, www.coldtronics.com, or firstname.lastname@example.org.
Publication date: 05/02/2005