Electronic – Using AC rated relay in DC applications

The environment in a car is quite different from that experienced by electronic components in a box on a table in your office. There are several characteristics in particular that stress electronics components more in a car than a office environment:

1. Temperature. A car still has to start on a cold winter morning in Nome or after baking in the hot July sun until mid afternoon in Yuma. Not only do these extremes pose a challenge to operating ability, but the daily thermal cycling causes mechanical stresses.

2. Vibration.

3. Noisy power. "12V" car power is nowhere near as nice as what a 12 V regulated power supply provides. It can drop significantly during starting, goes to 14 V when the engine is running, and can have 10s of volts of nasty spikes on it. For example, the normal 7805 regulator can be damaged by car power, even though it is rated to 35 V. There is a special regulator series that is rated for car power, which of course costs more.

4. Humidity. The relative humidity is unpredictable and can be at one extreme or the other for extended periods of time.

In addition to this, the end users generally don't tolerate failures as well in cars as they do in ordinary office appliances.

I can't say how much of this is relevant to your situation since you haven't told us what you are really trying to do. However, keep in mind that there are millions of cars sold every year, so the volumes are high and there is considerable competition. If manufacturers didn't have to pay for the extra robust characteristics of the components, they wouldn't. The fact that a car company is willing to pay the extra $.50 for a relay that goes into a million cars, should tell you that it matters. That's $500k they could save if they didn't. Put another way, that's about 3-4 engineers for a year to come up with a cheaper alternative.

Your problem is very simple. You are using your breadboard incorrectly. The top pins (pins 1 and 8) are being shorted out by the contact strip. In this case, the breadboard is incompatible with the relay. The spacing between rows of contacts is 0.3 inches, which is a standard DIP IC pin spacing. Your relay, however, has a lateral contact spacing of 0.2 inches, and so cannot straddle the row separation. Since it didn't fit that way, you plugged the relay in to a set of holes that did fit, and that is the source of your trouble.

To confirm this, set up an LED and resistor to monitor your +12 volts. Unplug the relay and you will see that the LED lights up. Now plug the relay in and the LED will go out.

With this breadboard you really have no good way to mount the relay. If you have access to a soldering iron you can make extenders for the relay pins with short wires and plug them into the breadboard, but other than that I'm afraid you're out of luck.