Delphi Your "Resistance Knowhow" Rusty? Before going further with the relay's spike suppression resistor, You may be wondering "how do you get 85 ohms when there is a 300 ohm resistor in parallel with the resistance of the relay winding which is at 120 ohms?" Just remember that resistances (any kind of electrical load) are usually wired in parallel in most circuits. Resistances in series simply add together. For example, a blower motor that has .8 ohms of resistance has a blower resistor with 2 ohms of resistance in series with it. The total circuit resistance is 2.8 ohms. In a parallel circuit, the simple math does NOT apply. But there is a simple rule you can put to memory; "a parallel circuit's total resistance is lower than the lowest individual resistor in that parallel circuit." Getting your head around the math for resistors in parallel is hard unless you use the KIS (Keep It Simple) mantra. A shortcut to calculating resistors in parallel can be applied when all the resistors are equal such as with tail lights, older non-sequential MPI fuel injectors and CAN bus termination resistors. Take one of the equal resistances and divide that number Figure 8: SSR's (Solid State Relays) are rapidly gaining popularity. Using transistors and various other electronic components to activate the transistor, these relays can have fault protection and handle PWM frequencies that mechanical relays could not keep up with. Another advantage of the SSR is the elimination of fly-back voltage (counter EMF) that results from the collapse of the magnetic field in a conventional relay's winding when the relay is turned off. Mitchell and Creative Commons by the number of those equal resistances in the parallel circuit. For resistors that are NOT equal which are wired in parallel, the math is made easier by popping up your smart phone's calculator and turning the phone on its side. That simple movement of the phone will change its basic calculator to a scientific calculator that has the inverse (1/X) button. (Figure 7) Figure 9: Kia is an OEM that gives you lots of detail in some of their factory schematics. One of these details is the FET (Field Effect Transistor) driver module. Its job is to control the variable duty cycle to the HVAC blower motor. It get's its commands from the A/C Control Module. FETs and MOSFETs (MO is for Metal Oxide) work similar to regular transistors January 2017 5 MACS Service Reports