Stereo & Electronics - Standard Relay question

Okay correct me if i'm wrong concerning relays cause I want to make sure I've got this correct on wiring these things when it comes to fuel pumps, water pumps, electric fans, etc....

There is 5 posts on most bosch relays 4 of which look like they are used (the last time i checked)...

1. Ground for the relay itself
2. Power wire for relay
3. Wire running to + connection of what you are running (ie fuel pump)
4. Switched power that will turn on component

reason i ask this is because I'm going to run my fuel pump with a bosch relay. I'm going to ground it, run it directly to the battery (that covers #1 and #2).... I'll then run it to the positive connection on the pump (#3)... Then for the last wire I'm going to connect it to the fuel pump wire coming from my painless wiring harness that goes directly to my Painless switch panel mounted on the rollcage.... So that'll become my "switched" power cause when i switch it on it will provide 12 volts, completing the circuit and power up the pump.... does this sound right????? If I'm correct it will solve pretty much all my wiring ?'s cause i'm planning to run the fuel pump, water pump, and fan with my switch panel and all of these will be wired the same....

http://alanhorvath.com/54chevy/chevypics/mad_electrical_relay-a.jpg

Don't forget to install a fuse or two in there.

http://alanhorvath.com/54chevy/chevypics/mad_electrical_relay-a.jpg
Whoa!! That diagram is only useful for something like a fuel pump because it has the relay's coils always energized. For general use you want to put a switch in either the power line (pin 86) or the ground line (pin 85). Also, although those are the most common uses for those terminal numbers, they don't have to be "always" connected that way.

A relay is a device that lets you control a high current circuit from a low current circuit. The idea is that you can have a device like fog lights that has a high current draw without having to run that high current through your switch (or even inside the car).

Pins 85 and 86 of the relay are the coil contacts. They are interchangeable but one must have power and the other ground. One of these contacts you normally connect to your switch. For example, you could connect pin 85 to a dashboard switch that provides power when on and pin 86 directly to ground. Or, you could connect pin 86 to a power source and pin 85 to a switch that goes to ground when turned on. Either way, the idea is to have a control circuit that runs power through the relay coil only when you want your high current device on. For something like a fuel pump you can just use an ignition-switched power source without a separate switch since you want it on whenever the ignition is on.

Pin 30 is generally used for the high current power supply. It can come from any fused source (even battery) since it won't pass the power to the device unless the control circuit is on. In some cases, pin 30 is used for output (when you want the output switched between two different sources) but normally it is used for power.

Pins 87 and 87a are refered to as "normally open" and "normally closed" respectively. That means that pin 87a is connected to pin 30 when the relay is at rest (not energized) and pin 87 isn't connected to anything. When the relay is energized (power is applied to the control circuit through pins 85 & 86) then the relay "flips" and pin 87 is connected to pin 30 while pin 87a is disconnected. This allows you to have either a device that's normally on or a device that's normally off or two devices that you can alternate between. It just depends on which pin (87 or 87a) you connect your device to.

Note: this discussion refers to SPDT relays (the most common kind for automotive use). SPST relays have the same five pin shape but have two pins labeled 87 (instead of 87 and 87a) connected to each other so they work together.

This diagram shows when each contact has or doesn't have power and possible uses for it.

Some relays have polarized coil inputs. So pin 85 should always be (-) and 86 should always be (+). Just to be safe.

Some relays have polarized coil inputs. So pin 85 should always be (-) and 86 should always be (+). Just to be safe.
Yes, you're right. Some recent relay designs have incorporated an internal diode requiring pin 86 to be positive and pin 85 to be negative. They're not too common yet but it's best to use the terminals that way to avoid problems.

The point I was trying to make was that a relay is a switching device but the diagram showed no switch or other control. It just has notes like "this wire always goes here". This would be confusing to someone unfamiliar with the reasons for using relays.

Not initiating an argument or anything, but doesn't the diagram show 85 (-) and 86 (+) ?

Not initiating an argument or anything, but doesn't the diagram show 85 (-) and 86 (+) ?

It does but that is not what they are saying.

Relays are used primarily for two things.

1. Getting power to an electrical component when a smaller wire wont supply enough power.

2. Allowing you to switch on an object without running high current though a switch.

The diagram you showed everyone shows the relay is always open which defeats the purpose of having a relay.





http://www.aeicomp.com/Image2/303.113_BU5084W_PI.jpg

This diagram will hopefully help some of you understand what is going on inside a relay.

1. Your main power wire (fused) is 30
2. 85 goes to chassis...(ground)
3. 86 goes to the SWITCHED source. Could be on a physical switch or hooked up to an ignition wire
4. 87a will have power when 86 does NOT have power. Most of the time nothing is hooked up to this side.
5. 87 WILL have power when 86 has 12v.

Obviously there are lots of ways/configurations that you can use with relays but the way I just showed is the most common.

WOW, thanks for all that info.... Totally made sense and corresponded with what an auto electrician told me in that he said that the 85 and 86 pins are usually dual polarity and either one can be used for ground or power... interesting that the middle post has power when 85/86 (depending on which was used) does not have power.... never knew that.... again thanks for the info...