Explain Like I'm 5 - Car Electric Noob Questions
Discussion
Apologies if this is the most basic-assed confusion of ideas but I figured the worst thing that can happen is someone calls me retarded... But I'm hopeful someone kind passing can get the sock puppets out and steer me through something really basic.
So, car. More specifically the electrics. You get perm lives (these come from the battery obvs) and ignition lives. These do NOT come from the alternator, the alternator charges the battery with 14v, and the 12v electrics the car enjoys come from the battery.
First question, is the above story correct?
Second of all, what tells and makes and determines the ignition live electrics come on only when the key is turned and not fed always from the battery? Is there a master relay somewhere usually?
I'm braced for impact here. Thanks in advance for any replies.
So, car. More specifically the electrics. You get perm lives (these come from the battery obvs) and ignition lives. These do NOT come from the alternator, the alternator charges the battery with 14v, and the 12v electrics the car enjoys come from the battery.
First question, is the above story correct?
Second of all, what tells and makes and determines the ignition live electrics come on only when the key is turned and not fed always from the battery? Is there a master relay somewhere usually?
I'm braced for impact here. Thanks in advance for any replies.
Warhawk said:
what tells and makes and determines the ignition live electrics come on only when the key is turned and not fed always from the battery?
The ignition switch is literally a switch.On modern cars it only carries enough current to operate a relay (sometimes multiple relays) but on older cars the ignition switched supplies would be fed from the switched output of the ignition switch.
ColourRestorer said:
The positive output of the alternator, and the positive terminal of the battery are connected together - sometimes, but not always, with a large fuse in between.
The alternator output has to produce a larger voltage than the battery in order that current will flow into the battery and charge it.
So anything that receives a nominally 12 Volt supply with the engine not running, will see somewhere between 13 and 14 Volts once the engine is running.
Anything containing digital electronics will have some form of power regulation and conditioning circuit so that it is resilient to fluctuations in supply voltage.
Thus permanent live comes from the alternator/battery, and ignition switched lives, of which there were traditionally two, relating to accessory and ignition positions as the key was rotated, came via the ignition switch.
Arguably when cranking there was a third version of live which pulled in the starter relay, and if appropriate bypassed the ballast resistor feeding the coil.
On older vehicles (very loosely pre 2000) it is likely that the ignition switch will provide a low current feed to control units that then pull in relays of their own from permanent live as required. I can’t remember having worked on a vehicle, other than a fork truck, that had a master relay.
On most modern vehicles the 12V permanent live supply is provided to all the control modules at all times, and they receive ‘wake up’ and ‘shutdown’ messages via a databus from whichever combination of control modules are responsible for implementing security.
Undoubtedly this quick and dirty explanation will aggravate the PH pedants who will be along later to point out where I’m incorrect. In fact you’re probably less at risk of flaming with your question than I am from answering it.
That was really kind of you typing all that thank you! Ok I think you've made the penny drop now.The alternator output has to produce a larger voltage than the battery in order that current will flow into the battery and charge it.
So anything that receives a nominally 12 Volt supply with the engine not running, will see somewhere between 13 and 14 Volts once the engine is running.
Anything containing digital electronics will have some form of power regulation and conditioning circuit so that it is resilient to fluctuations in supply voltage.
Thus permanent live comes from the alternator/battery, and ignition switched lives, of which there were traditionally two, relating to accessory and ignition positions as the key was rotated, came via the ignition switch.
Arguably when cranking there was a third version of live which pulled in the starter relay, and if appropriate bypassed the ballast resistor feeding the coil.
On older vehicles (very loosely pre 2000) it is likely that the ignition switch will provide a low current feed to control units that then pull in relays of their own from permanent live as required. I can’t remember having worked on a vehicle, other than a fork truck, that had a master relay.
On most modern vehicles the 12V permanent live supply is provided to all the control modules at all times, and they receive ‘wake up’ and ‘shutdown’ messages via a databus from whichever combination of control modules are responsible for implementing security.
Undoubtedly this quick and dirty explanation will aggravate the PH pedants who will be along later to point out where I’m incorrect. In fact you’re probably less at risk of flaming with your question than I am from answering it.
Edited by ColourRestorer on Friday 15th October 08:54
Would you field another daft question?
If I were, say, (and I'm not doing this btw, nor do I have any specific project or objective in mind other than undoing my confusion on it all) plugging in a 12v LED the came on when the dash lights came on, I would tap into, say, an illuminated switch wire and ground it, and then expect all to be fine. If I ran two wires from the alternator, I would expect my little 12v LED to go bang.
What exists to supply the little bit of current to some of the accessories in the car, so they don't see the full whack of the alternator's output?
I am exceedingly grateful you un-confused me on the above.
GreenV8S said:
The ignition switch is literally a switch.
On modern cars it only carries enough current to operate a relay (sometimes multiple relays) but on older cars the ignition switched supplies would be fed from the switched output of the ignition switch.
It does seem obvious looking at it, but reading this contributed a bigger piece of my mental puzzle than you probably realise lol. Thank you. On modern cars it only carries enough current to operate a relay (sometimes multiple relays) but on older cars the ignition switched supplies would be fed from the switched output of the ignition switch.
OP, you may also find this website useful, although it quickly heads off into mainly car stereo applications:
http://www.bcae1.com/
Rather less technically than ColourRestorer (who has done a great job above), I'd just say: don't get too hung up about the difference between the (nominal) 12v of the battery, max 14v (ish) of the alternator, and the 13.2 - 13.4v (ish) you'd probably measure when the car's running - parts for wiring into a car should just simply work within those ranges. (Modern car electronics do seem to be susceptible to gremlins when the voltage falls too low, however.)
http://www.bcae1.com/
Rather less technically than ColourRestorer (who has done a great job above), I'd just say: don't get too hung up about the difference between the (nominal) 12v of the battery, max 14v (ish) of the alternator, and the 13.2 - 13.4v (ish) you'd probably measure when the car's running - parts for wiring into a car should just simply work within those ranges. (Modern car electronics do seem to be susceptible to gremlins when the voltage falls too low, however.)
ColourRestorer said:
Think of the alternator/battery power supply as something which can produce a certain electrical ‘pressure’ which will cause a flow of electrons (current measured in Amperes) around a circuit.
Now your various electrical consumers around the car need different amounts of flow of those electrons in order to do their job. For a fixed supply voltage which is provided to all of them i.e. our nominal 12V, they all draw what they need to do their job, and I'll try to explain how:
The starter motor needs a massive flow, so it uses a big pipe (thick cable).
A headlight bulb needs a smaller flow so it can get away with being fed by a thinner pipe.
When I say “need”, what I mean is that the device which consumes, is designed to allow a certain amount of electron flow.
The figure that indicates how much flow is what we call ‘resistance’ – something with low resistance doesn’t do a very good job of resisting the flow of electricity (e.g. the starter motor), whereas something with higher resistance does a better job. An insulator does a perfect job of resisting the flow.
So for a given electrical pressure or Voltage, and a given resistance, the amount of current which can flow is measured in Amperes, and is equal to the Voltage divided by the resistance.
Does that make sense?
Now think about the fact that power is used (electricity turned into motion or heat) and defined by the Voltage multiplied by the Current, and at the end of the day it is power which we actually require to turn the engine over to start it, or to light our way, or to deafen ourselves. So simplistically the design task starts by identifying how much power is needed, which for a fixed 12V supply defines how much current is needed, which defines how thick the pipe (cable) must be, and for the total of all the electrical consumers how big the alternator must be and how much capacity the battery must have for periods when the alternator can’t provide for all the demand.
Hopefully you’re still with me.
Now the LED is a special case, for a typical LED only needs 2 Volts of pressure to make it light up.
But a car electrical system provides 12Volts, so what do we do?
We could connect six of them in series (i.e. one after the other in a line), so that each has 2Volts across it, or what we actually do is to connect (or these days build into the LED) an additional resistance that impedes the flow of electrons, so that 10Volts of the electrical pressure is dropped across the resistor.
So for a simple example: an LED might be designed to operate with a flow of 50milliAmps (i.e. one twentieth of an Amp) when 2Volts is applied.
So we need a resistor to drop 10Volts with a current of 50mA, which means a resistance of 10Volts divided by 0.050 Amps = 200 Ohms.
What you might have missed is that other than the special case of LEDs and other semiconductors, all other equipment on a car such as bulbs, motors, relays etc is designed to take 12V directly.
Also, you need to think of all the electrical consumers being connected in parallel across the battery/alternator, a bit like all the central heating radiators in your home, so that if you close off the valve on one, the others go on working. The valve is akin to the design power hence resistance of the electrical component.
I'm happy to try to continue explaining, but you may find this Haynes book to be helpful: https://www.amazon.co.uk/Haynes-Car-Electrical-Sys...
If you don't already teach this stuff professionally you really really should. Thank you so much for that! I think I get it.Now your various electrical consumers around the car need different amounts of flow of those electrons in order to do their job. For a fixed supply voltage which is provided to all of them i.e. our nominal 12V, they all draw what they need to do their job, and I'll try to explain how:
The starter motor needs a massive flow, so it uses a big pipe (thick cable).
A headlight bulb needs a smaller flow so it can get away with being fed by a thinner pipe.
When I say “need”, what I mean is that the device which consumes, is designed to allow a certain amount of electron flow.
The figure that indicates how much flow is what we call ‘resistance’ – something with low resistance doesn’t do a very good job of resisting the flow of electricity (e.g. the starter motor), whereas something with higher resistance does a better job. An insulator does a perfect job of resisting the flow.
So for a given electrical pressure or Voltage, and a given resistance, the amount of current which can flow is measured in Amperes, and is equal to the Voltage divided by the resistance.
Does that make sense?
Now think about the fact that power is used (electricity turned into motion or heat) and defined by the Voltage multiplied by the Current, and at the end of the day it is power which we actually require to turn the engine over to start it, or to light our way, or to deafen ourselves. So simplistically the design task starts by identifying how much power is needed, which for a fixed 12V supply defines how much current is needed, which defines how thick the pipe (cable) must be, and for the total of all the electrical consumers how big the alternator must be and how much capacity the battery must have for periods when the alternator can’t provide for all the demand.
Hopefully you’re still with me.
Now the LED is a special case, for a typical LED only needs 2 Volts of pressure to make it light up.
But a car electrical system provides 12Volts, so what do we do?
We could connect six of them in series (i.e. one after the other in a line), so that each has 2Volts across it, or what we actually do is to connect (or these days build into the LED) an additional resistance that impedes the flow of electrons, so that 10Volts of the electrical pressure is dropped across the resistor.
So for a simple example: an LED might be designed to operate with a flow of 50milliAmps (i.e. one twentieth of an Amp) when 2Volts is applied.
So we need a resistor to drop 10Volts with a current of 50mA, which means a resistance of 10Volts divided by 0.050 Amps = 200 Ohms.
What you might have missed is that other than the special case of LEDs and other semiconductors, all other equipment on a car such as bulbs, motors, relays etc is designed to take 12V directly.
Also, you need to think of all the electrical consumers being connected in parallel across the battery/alternator, a bit like all the central heating radiators in your home, so that if you close off the valve on one, the others go on working. The valve is akin to the design power hence resistance of the electrical component.
I'm happy to try to continue explaining, but you may find this Haynes book to be helpful: https://www.amazon.co.uk/Haynes-Car-Electrical-Sys...
Edited by ColourRestorer on Friday 15th October 16:10
Edited by ColourRestorer on Friday 15th October 16:11
So a little LED, which requires the little wire would get borked by the full whack of current coming out of the alternator. The thing that stops it "seeing" the full whack (and therefore only needing a little wire) would be a resistor somewhere upstream of it, which drops the voltage and amps to something it can handle.
I think that's right anyways. If it is my penny dropped and I am truly grateful.
defblade said:
OP, you may also find this website useful, although it quickly heads off into mainly car stereo applications:
http://www.bcae1.com/
Rather less technically than ColourRestorer (who has done a great job above), I'd just say: don't get too hung up about the difference between the (nominal) 12v of the battery, max 14v (ish) of the alternator, and the 13.2 - 13.4v (ish) you'd probably measure when the car's running - parts for wiring into a car should just simply work within those ranges. (Modern car electronics do seem to be susceptible to gremlins when the voltage falls too low, however.)
I'm going to give that a good old rummage and thanks much for the explanation too. http://www.bcae1.com/
Rather less technically than ColourRestorer (who has done a great job above), I'd just say: don't get too hung up about the difference between the (nominal) 12v of the battery, max 14v (ish) of the alternator, and the 13.2 - 13.4v (ish) you'd probably measure when the car's running - parts for wiring into a car should just simply work within those ranges. (Modern car electronics do seem to be susceptible to gremlins when the voltage falls too low, however.)
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