Delphi and Creative Commons Mitchell and Creative Commons SSR's (Solid State Relay) The most recent trend with relays (Figure 8) is to see them go without contacts. SSRs (Solid State Relays) while still referred to as relays are actually transistor circuits which act as remote driver modules. Similar to an automatic HVAC blower module (but in a relay sized package) these gems are slowly coming down in price and making their way into more and more vehicles. No voltage spikes, no parts to wear out, and they often are fault protected (will reset when shorted output condition is gone) and have no problems with turning on and off in PWM applications (try controlling a dash light dimming circuit with a relay instead of a solid Figure 10: The two devices at the top of this chart are traditional bipolar state component at 128 Hz. You'll wear the re- transistors. The terminology applied to FETs (a.k.a. JFET and MOSFET) comparable to their close relative - the transistor. FETs call their 'base' lay out in no time if you don't smash it with isa gate. The FET terms for collector and emitter are drain and gate respeca hammer from all the horrible buzzing it will tively. decades, the appearance of their symbols in vemake cycling that fast!). hicle schematics is becoming more prevalent. While some OEMs still illustrate their operation Who REALLY Controls the Relay? in schematics by showing a mechanical switch Various relays on today's vehicles are mainly symbol, more OEMs are starting to show us the controlled via solid state logic devices. Solid real deal. (Figure 9) Understanding terms like state means no moving parts, of course which F.E.T.s (Field Effect Transistors) are not that hard leads us to the discussion of transistors. Al- (Figure 10) when you break down exactly what though transistors have been around for many a transistor does. Whether in a blower module, Figure 11: Transistor symbols are showing up in many service manuals (usually inside of modules) so it's better to know how they work so you can follow schematics. Breaking it down they are not hard to understand. They basically work like a relay only w/o moving parts and contacts that can go bad. (lower schematic) A small signal voltage with low current (1) is sent to the base of the transistor by a switch or relay. The transistor internally connects the collector (2) to the emitter (3) allowing a larger amount of voltage / current to flow through the transistor. Types of transistors vary by the job they do - switch power, (upper left) switch grounds (top middle) or allow the flow of current to ramp up or down in a variable manner (upper right) such as F.E.T.s do when their base current varies. Transistors can fail due to a shorted output (too much current flowing through them) or from an unsuppressed voltage spike that hits them. Reverse polarity is a common cause for failure. Be extremely cautious when using devices like PowerProbes to source the wrong polarity into a circuit that connects to a module with an internal transistor. January 2017 6 MACS Service Reports