Electrical - Automotive Relays
I purchased some new industrial automotive style relays for dirt cheap and immediately ran into a strange problem with the starting circuit. I determined that the relays were missing a bleed down resistor. The bleed down resistor is across the coil and it is used to help bleed down the magnetic field after you've stopped applying +12V to the coil. Automotive relays have the bleed down resistor already built in. You can read about the problem here in the starter circuit page.
3 different styles of automotive relays
Relays are used to control high current devices such as horns, headlights, brake lights, etc.. I don't like to run high currents through 50 year old switches in the dash, so I use them to control the relay coils which only need about 1/10 the current of the contacts and let the relay contacts control the high current draw devices.
Relay wiring code
Relay pigtail socket
Automotive relay pins are labelled with numbers which indicate what their purpose is and the relay pigtail has a standard color code as follow:
Coil 85: Ground - Black wire 86: +12V - White wire NOTE: I found that in half of my pigtails, the black and white wires were reversed! It shouldn't make a difference as the coil is not polarized. Contacts 30: Common - Blue wire 87: Normally Open (NO) - Yellow wire 87a: Normally Closed (NC) - Red wire
Relay contact schematic
When the coil is not energerized (+12V is not across pins 85 and 86), there is no magnetic field. The relay switch contacts are in the normal position. Normal means when the coil is NOT energized. The switch contacts are labeled Normally Open (NO) and Normally Closed (NC). When a contact is closed, it means that current can flow (the switch is on).
When the coil is energized (apply +12V to it), the magnetic field pulls in the Common armature. The NO contact closes (turns on) and the NC contact opens (turns off). Having both types of contacts allows the relay to be flexible so that it can turn off and turn on things if powered is present or not.
We can connect the contacts in any way we like as long as one side of the switch is the Common contact and the other is either the NO or NC contact. Also , The contacts are not polarity sensitive - either contact can be +12V or ground which brings up another topic.
Negative or Positive Control
Some devices are controled by applying a ground to turn on the circuit. This is often called a negative output control. An example of this is the dome light - there is +12V connected to one side of the dome light bulb. The courtesy switches in the door jambs ground the other side of the dome light to complete the circuit when the door opens.
Typical courtesy light wiring diagram.
Many devices are controlled by applying +12V to turn them on. This is often called a positve output control. For example: the horn beeps when +12V is applied. The case of the horn is grounded to the chassis through its mounting screw. Unfortunately, it is not as simple as it seems. A relay lies inbetween the steering wheel horn button and the horn. When you push on the horn button in the center of the steering wheel, you ground one side of a relay coil which energizes. This pulls in the NO contact which applies +12V to the horn circuit. It is done this way so that you don't have high currents flowing through the steering column. High currents would spark and pit the horn ring contacts - pretty soon your horn wouldn't work!
Typical horn wiring diagram.
The above horn wiring diagram show the schematic symbol for a relay. The coil is indicated by a coil of wire, the magnetic core is shown by the parallel lines. The magnetic field is shown by the dashed line on the Common armature and NO contacts.
The relay coil can be used in either positive (+12V controls it) or negative (a ground controls it) mode. The contacts can be connected for either positive or negative control also. Relays are quite versatile and can even be used to change the control from a positive on the coil side to a negative control arrangement on the contact side.
Custom Multi-Relay Board
These relays were made to be bolted to the side of a firewall. When I temporarily wired up the bare necessities to get the engine and gauges working, it became terribly obvious what a mess the wiring harness would become. So I went to the wreckers to see what I could find.
I found basically nothing usable. The wiring harness would have to be cut out and spliced together and another messy excuse would result. Next, I checked to see what was available from the aftermarket for relay packs and was quite shocked to see that the prices were in the $200 range and were quite complex. I just wanted 2 to 3 relays on a printed circuit and there was just nothing that simple out there.
A quick list of functions that needed relays are:
- Headlights (+12V turns on)
- Horn (Gnd turns on)
- Back up lights (+12V turns on)
- Ignition (+12V turns on)
- Starter (+12V turns on)
- One spare - maybe for high beams...
I decided that I would need two 3 relay packs. This would allow one pack to be placed close to the engine for the starter/ignition circuit and the other to be placed close to the lighting circuits.
Each relay could be energized either by +12V or Gnd via a hardwired jumper (soldered in place). Standard automotive relays would be used and each contact would be fused using standard automotive ATO fuses.
3 relay pack
A couple hours later, the above 3 relay pack was laid out and the artwork was ready to be sent out to be shot as a negative. Once I received the negative, I had two boards made and I finished stuffing them around 1:00 am in the morning. They look good and work excellently. The cost per board was CDN $19.50:
- Relays - $15 total
- Relay sockets $4.50 total
- ATO sockets - free
- Wire terminals - free
- Resistors - free
- Negative/Artwork - free
- Printed circuit board - free
It pays to have friends and treat people nice. This is a great example of the golden rule. While I expected to pay for everything, my friends charged me nothing. You just can't beat that..