Relay wiring help?

[BearDodge67]

A diode is like a check valve. power only goes one way,the direction the arrow points. Try to reverse it and it gets blocked. That's how they are designed. It's actually how you test it with a ohmmeter/circuit tester.. One way works, other doesn't.. good.. Neither work, bad.. Both work, bad. I tested enough of them on old submarine sonar systems to have it burned in my head.

To get back to the OP's question.. here's how I did a electric fan I pulled out of a fwd Ford in boneyard. The big connector at top was wired directly to the batter for both 12V and ground. The coiled blue wire went to a distribution block (you'll see later), that was fed from a 12 ignition switch source (think of this as your toggle switch). The blue wire provided power to the adjustable temperature probe. When it hit the temperature I wanted, 12V would get sent to the coil which would energize and connect the 12V from the battery to both fans via the fuse block on the housing. The gaggle of wires in the middle acted as a ground for the relay and both fans that went back to the battery. Worked great, later on I added a second relay and wired it to a idiot light temperature sender. One fan ran when it got warm, and second would engage when it got too hot.

This is an example of a MSD using a circuit breaker and a relay. The distribution block metioned above was fed from a 12V ignition switched source, soon as key went to RUN, it had power. This energized the relay, which had 12V fed directly from the battery, which went thru a circuit breaker and then the MSD.

This setup is long gone once I went to a MSD distributor, except for the distribution block, that feeds my ECU and the fuel pump as well as gauges.

 

[72RoadrunnerGTX]

Adding diodes across the primary side of automotive Bosch style relays is nothing new. Dates back to the start of high-end automotive audio/stereo, for the same reasons posted here but to address relay “popping” being induced into line level components, pre-computers. Yes, when the diode is internal to the relay, as are most all European makes OE relays, the primary connections are polarized. 85 is always negative.

 

[72RoadrunnerGTX]

A diode is like a check valve. power only goes one way,the direction the arrow points.

Technically, for the diode symbol, current is blocked in the direction of the arrow (indicated by the perpendicular line at the tip of the arrow), as DC current flows from negative to positive, current is allowed to flow opposite the indicated arrow.

 

[Geoff 2]

The 'logic' for using diodes on 30 amp relays in pre-computer era cars...is missing.
I have measured the coil res of numerous Bosch 30 amp relays & they are in the 70-80 ohm range. With a 12v system, they draw about 1/6 amp...about 160 milli amps. How much back EMF do you think that is going to create...that will be a problem??
The ign coil draws amps, not milliamps, creates 200-400 v back EMF....& does not have a diode across it.
Then there is the big brother of back EMF, the starter sol which draws 30-60 amps....& doesn't have a diode across it.
Adding unnecessary parts just creates another failure point...

 

[Charrlie_S]

The 'logic' for using diodes on 30 amp relays in pre-computer era cars...is missing.
I have measured the coil res of numerous Bosch 30 amp relays & they are in the 70-80 ohm range. With a 12v system, they draw about 1/6 amp...about 160 milli amps. How much back EMF do you think that is going to create...that will be a problem??
The ign coil draws amps, not milliamps, creates 200-400 v back EMF....& does not have a diode across it.
Then there is the big brother of back EMF, the starter sol which draws 30-60 amps....& doesn't have a diode across it.
Adding unnecessary parts just creates another failure point...

You are partially corrct. I did state "on our old cars, it really does not make a difference which relay to use".
But your statement about coil resistance and amp draw is not relivant. It is not the coil resistance but coil impedance. It is not the "back" current that damages components or creates electrical "noise", it is the back EMF (electro motive force= voltage). This back EMF can be several hundred volts, and it can break down the internal insulation of electronic components.

The only point I was trying to make about the polarity wireing of these relays is/was:
Even though our old cars do not require paying attention to polarity, it is a good idea to do so. As noted in this thread, many people are/were not aware of these different relays (and they look the same) and if the incorrect relay was used it could cause a problem.

 

[72RoadrunnerGTX]

The 'logic' for using diodes on 30 amp relays in pre-computer era cars...is missing.
I have measured the coil res of numerous Bosch 30 amp relays & they are in the 70-80 ohm range. With a 12v system, they draw about 1/6 amp...about 160 milli amps. How much back EMF do you think that is going to create...that will be a problem??
The ign coil draws amps, not milliamps, creates 200-400 v back EMF....& does not have a diode across it.
Then there is the big brother of back EMF, the starter sol which draws 30-60 amps....& doesn't have a diode across it.
Adding unnecessary parts just creates another failure point...

Well now that’s “nonsense”, the logic of using diodes pre-computer was explained fairly well in post #22 I believe. Have personally installed many hundreds of diodes across added automotive Bosch, or Bosch style, relays, if they didn’t already have one internally, for decades to keep popping noises from being induced into high-end very sensitive aftermarket audio/security systems over decades, computer or pre-computer, no matter. Never saw one diode failure. If it opens, wouldn’t stop the function of the relay anyway. All stock, no computers or high-end audio, no matter.

 

[Geoff 2]

Not nonsense at all. If you have installed hundreds of diodes on 30 amp Bosch relays, then you have wasted hundreds of diodes...

 

[Geoff 2]

Charrlie,
I am totally correct, not partially correct. You need to brush up on your reading skills...& electrical theory which is sorely lacking.
[1] I never mentioned ' back' current, only back EMF. That is one you 'made up'
[2] The current draw IS relevant [ which is why I mentioned it ] because it controls the inductance that is produced in the coil. The inductance then factors into how much back EMF is produced.
[3] It IS coil resistance. Impedance is a measure in AC circuitry & this is a DC circuit.

 

[72RoadrunnerGTX]

Not nonsense at all. If you have installed hundreds of diodes on 30 amp Bosch relays, then you have wasted hundreds of diodes...

Nope, each one meant a happy customer and much time saved chasing the occasional transient popping noise problems later in most every installation of high-end audio systems into high-end automobiles of the time I operated a very successful high-end installation shop. Bought them in thousand-unit bags for pennies as I recall, very handy for several other functions as well, worth every cent. Widely used in the industry then, it appears still the case, didn’t take long to find this.
www.youtube.com/embed/JELMaHrzNuk?start=325&end=409

 

[Charrlie_S]

Charrlie,
I am totally correct, not partially correct. You need to brush up on your reading skills...& electrical theory which is sorely lacking.
[1] I never mentioned ' back' current, only back EMF. That is one you 'made up'
[2] The current draw IS relevant [ which is why I mentioned it ] because it controls the inductance that is produced in the coil. The inductance then factors into how much back EMF is produced.
[3] It IS coil resistance. Impedance is a measure in AC circuitry & this is a DC circuit.

Ok, I'll go back to school. Any electrical coil has both resistance, and impedance.
On ignore

 

[65CopCar]

The 'logic' for using diodes on 30 amp relays in pre-computer era cars...is missing.
I have measured the coil res of numerous Bosch 30 amp relays & they are in the 70-80 ohm range. With a 12v system, they draw about 1/6 amp...about 160 milli amps. How much back EMF do you think that is going to create...that will be a problem??

Oh my! Yes.........

So how much voltage can damage a computer input? How much voltage can cause arcing and fail switching contacts? Ever wonder why you get significant arcing when you open a starter relay coil?

I just put a limit switch in my Cheetah shifter for the starter relay. The starter relay draws just under 1/2 amp. Limit switch is rated 1/2 amp DC. You betcha it got a diode across the coil.

But what's as important is putting a diode across the switching contacts. All my inductive loads get a diode across the load to prevent the switching contacts from failing. If it goes into a sensitive input like a data recorder, it gets a diode across the relay coil also.

So how much voltage can the Bosch 30 amp relay produce when the magnetic field collapses? - see image 1 (150 volts+ peak to peak). Image 2 is a starter relay coil (450 volts+ peak to peak), image 3 is the starter relay with a diode across the coil (65 volts peak to peak).

My test setup is not perfect and I think you see switch bouncing in the coil images. Some of it is ringing from the parasitic RLC circuits. I wasn't gonna spend more time on it. But just as bad, these voltages, as expected, have a very high dv/dt. High dv/dt is just as harmful many times as much as high voltage is.

 

[65CopCar]

Charrlie,
I am totally correct, not partially correct. You need to brush up on your reading skills...& electrical theory which is sorely lacking.
[1] I never mentioned ' back' current, only back EMF. That is one you 'made up'
[2] The current draw IS relevant [ which is why I mentioned it ] because it controls the inductance that is produced in the coil. The inductance then factors into how much back EMF is produced.
[3] It IS coil resistance. Impedance is a measure in AC circuitry & this is a DC circuit.

That is not all that accurate...........

 

[Some Car Guy]

Geoff is like having a second engineer on the board. Just can’t get enough of those, can we?

 

[Geoff 2]

I am/have been talking about pre-computer era cars. Not computer era cars.

Popping noises from the radio or audio device.
If your car uses a relay for the headlights [ most do ] & has radio, you can try this 5 min test......
Switch on the radio & then switch on the headlights. There is no 'pop'. Now switch off the headlights...& you will hear a pop.
Now remove the wire on the relay that goes to the h'lights. The relay still operates but the lights do not come on. There will be no pop when the lights are turned on...nor when they are turned off....even though the relay was energised & then de-energised. The pop comes from the opening of the relay contacts, causing arcing [ like ign points ]. The pop comes from opening the load that the relay was switching, NOT the switching on & off of the relay coil.