The Importnace of Polarity

[SpacerM]

You guys have been a great resource as I work through all that goes along with being a new Healey owner and without the wealth of information here I never would have been able to rebuild my carbs, clean my trafficator, tune the engine or understand and maintain my BJ8- a big thank you. But I’m hoping you can help me understand one thing I just can’t get my head around even after reading hundreds of posts on the subject- the importance of polarity.

I understand that for some things like incandescent lights polarity doesn’t matter- electricity can flow through the filament either way and the light will work.

I understand for things like diodes in the fuel pump, polarity matters and current must flow in the correct direction- so when reversing a car’s polarity the fuel pump will not work unless you also reverse the diode.

What I can’t understand is why on items that care about polarity (like an electronic ignitor) you can’t simply reverse the wires on the item and be done with it- it seems to me that reversing the leads when converting from pos to neg ground would cause the electricity to flow the intended way through the item without blowing the device or needing to fuss with diodes. Clearly I am missing a basic principle here, but I just don’t know what. I get that if one item is pos. ground internally and another is neg. ground internally that having the grounds touch can create fireworks (such as is discussed in all the “isolating a cigarette lighter” discussions) but I don’t get why you simply can’t reverse the leads when going from pos to neg ground- so long as the power and ground are on the wires that the device was built to carry them on I would think there would be no issues and the device would just work when the car is fully converted to neg. ground.

I know I’m missing something really basic here, but I’m no electrician and I’m barely a car mechanic, so any tips on how to get this straight in my head so I don’t fry something important would really be appreciated.

Thanks,
Chris.

 

[BigGreen]

Not to give you the perfect answer, but just to make something more clear
Electronic circuits can behave like doors, they open to let you thru from either side, but sometimes you have to step back
Going with the flow is the most easy way

 

[Bob_Spidell]

I think what you're missing is the concepts of 'common' (usually, but less correctly, called 'ground'). Common is the more appropriate term because none of the electrical current on a car goes into the ground, unlike on a house which has actual ground terminals (in this instance, the Earth is one large 'common'). In cars, the chassis is used for common since, obviously, just about everything bolts up to it and steel is a reasonably good conductor, so it's easiest--and saves a lot of extra wiring--to provide a source of current to an electricity consumer (aka a 'sink') and to use the chassis to return the current to its source: the battery (yes, the generator provides current, but for the sake of simplicity all current starts at the battery, and goes back to the battery, hence the term 'circuit').

So, the chassis is part of the circuit, and if a source of current is applied to chassis current will flow without resistance (aka a 'load'), and bad things will happen; mainly, whatever part of the circuit has the least capacity for carrying current--usually the wiring--will melt (aka a 'dead short'). From this comes the concept of 'isolation,' meaning the device, or whatever, doesn't have an external need or capability to conduct current through its container. This is why you can reverse a diode: there are two leads going to it; one positive and one negative, and the diode is isolated by its plastic (or ceramic) shell. Many devices, including the engine, are connected directly to chassis common. Many devices in a car are contained in a shell that is also tied to the common circuit within the device, so if the device is of different polarity than the car and chassis are rigged for, and if the device's shell comes in contact with the common--the chassis--current will flow and you'll have a short.

In addition, many devices have components--diodes, for instance--that are polarity sensitive. Since a diode will flow current in one direction and block it in another, if you reverse polarity then current will be allowed to flow in the wrong direction and bad things happen (diodes on printed circuit boards are about the size of a large grain of coffee, and soldered to the surface of the board, so it's not feasible to reverse them). If the device is completely isolated, then you can rig it so it will work on a chassis with different polarity. I rigged-up a cigarette lighter-style charging socket for my positive-ground BJ8, and as long as all devices are isolated--plastic case--things worked fine until I was charging my cellphone and its metal case came in contact with the metal on my console (good thing I had a fuse in the socket's supply line). The phone itself was not damaged, as the current flowed through its case and not through the internal circuitry.

As for the Ignitor, they have two leads: one for a source of current, and one to supply that current--properly timed and switched--to the coil. I'm pretty sure it needs reference to common, which is likely provided through its metal baseplate and through the distributor. If you've ever installed points, and neglected to isolate the contacts from the baseplate--usually this is a fiber or plastic washer--then you know your engine will not fire if the points are not properly isolated from ground and current is only allowed to flow through the contacts on the points set.

 

[SpacerM]

Thanks, Bob- this was really helpful (as was the door analogy BigGreen). It seems I made a bad assumption about the ignitor ground- two wires does not mean power and common, but in the case of the ignitor, the second wire is current, as well.

So can I ask when converting from positive common to negative common you cannot simply reverse the leads to make a device like the fuel pump work? It seems to me that reversing the leads in conjunction with changing the polarity of the car would still have current comimg in on the designed wire and out to the common via the designed wire (or through the case to the chassis), and current would flow through the diode in the correction direction, as well. Yet that is clearly the wrong approach (https://mgaguru.com/mgtech/electric/fp101.htm ) but I just don't see why?

 

[Bob_Spidell]

The diode is a door; it allows current to only flow in one direction. The diode in an SU fuel pump is known as a 'flywheel diode' because it shunts current produced by the collapse of the electromagnet field coil back into the coil, instead of letting it arc across the points and gradually moving the points metal (probably tungsten) from one point surface to the other, causing pitting and general erosion of the points. Connected correctly, when the points are closed the diode 'door' is closed, allowing current to only flow into the coil. When the points open, current is produced in the reverse direction due to the collapsing magnetic field in the coil and the diode allows current to flow in that direction. If you want to change polarity on a fuel pump with a diode, reverse the red and black leads; i.e. move each lead to the other lead's terminal. Pumps with no diode or capacitor don't care which terminal is hooked to what--except there is some internal shunting which might have a slight effect on efficiency--so you can hook them up either way. If you switch polarity and don't reverse the diode, current will flow through the diode to common without flowing through the pump coil and producing an electromagnet: no pumping.

 

[RAC68]

Hi Bob,

That was a very nice explanation.

All the best,
Ray (64BJ8P1)