LED Alternator Warning Light Circuit
Discussion
Using Tyco VF4 series 12 volt 40 amp Relay or one of similar specification
Tyco VF4 link - https://www.digikey.co.uk/product-detail/en/te-con...
Coil Resistance - 90 Ohm
Nom Turn On Voltage (Max): 7.2 VDC
Turn Off Voltage (Min): 1.2 VDC
Although the above is named a 12 volt relay its operating voltages are wide apart from 12
12 volt relays have carried on working without problems after withstanding 18 volts for several hours when a vehicles alternator has been overcharging
As can be seen above, the Tyco VF4 series will pull-in at approx 7.2 volts and hold-in down to approx 1.2 volts
Is important to view the chosen relay's specification if not using a Tyco VF4 series as specifications vary from manufacturer to manufacturer
Wiring a 100 ohms resistor in parallel with the Tyco's coil gives
TYCO 90 Ohm Plus 100 Ohm Resistor In Parallel = 47.368 Ohms
Current = 12 divided by 47.368 = 0.2533355 Say 0.25 amps
Wattage = 12 x 0.25 = 3 = Same as the original 3 watt bulb
100 ohm resistor link - https://www.ebay.co.uk/itm/Wirewound-Cement-Resist...
Replacing the original 3 watt bulb with the Tyco relay coil circuit gives approx the same alternator field exciter current
The LED is simply switched to earth through the relay contacts
The relay is energised by forward and reverse voltages
Should an alternator diode fail and apply a voltage to the alternator field with engine not running (ignition off), the LED will warn the driver
The warning lights and gauges circuit's show how the relay earths during a reverse voltage condition with ignition turned off (circuits have been simplified)
Tyco VF4 link - https://www.digikey.co.uk/product-detail/en/te-con...
Coil Resistance - 90 Ohm
Nom Turn On Voltage (Max): 7.2 VDC
Turn Off Voltage (Min): 1.2 VDC
Although the above is named a 12 volt relay its operating voltages are wide apart from 12
12 volt relays have carried on working without problems after withstanding 18 volts for several hours when a vehicles alternator has been overcharging
As can be seen above, the Tyco VF4 series will pull-in at approx 7.2 volts and hold-in down to approx 1.2 volts
Is important to view the chosen relay's specification if not using a Tyco VF4 series as specifications vary from manufacturer to manufacturer
Wiring a 100 ohms resistor in parallel with the Tyco's coil gives
TYCO 90 Ohm Plus 100 Ohm Resistor In Parallel = 47.368 Ohms
Current = 12 divided by 47.368 = 0.2533355 Say 0.25 amps
Wattage = 12 x 0.25 = 3 = Same as the original 3 watt bulb
100 ohm resistor link - https://www.ebay.co.uk/itm/Wirewound-Cement-Resist...
Replacing the original 3 watt bulb with the Tyco relay coil circuit gives approx the same alternator field exciter current
The LED is simply switched to earth through the relay contacts
The relay is energised by forward and reverse voltages
Should an alternator diode fail and apply a voltage to the alternator field with engine not running (ignition off), the LED will warn the driver
The warning lights and gauges circuit's show how the relay earths during a reverse voltage condition with ignition turned off (circuits have been simplified)
Wait, you're using the current through a 100 ohm resistor to generate a sufficient voltage to drive enough current through the coil of a massive 40 amp rated relay, in order to close some contacts to then allow a tiny amount of current to flow through an LED?
Seems a suitable complicated and un-reliable way to light an LED, TVR would approve!
Seems a suitable complicated and un-reliable way to light an LED, TVR would approve!
Please also consider what happens to the resistor should the alternator fail and that resistor has to constantly carry the rotor excitation current for an extended period! ie be careful where and how you mount that resistor!
also note, that an incandescent buld has a very strong Positive Temperature Co-efficient, because its operating temperature is so much higher than it's non operating temperature (off = ambient, say 21 degC, when lit = 3000 degC) That mean's it works really well as an alternator warning light, because initally, when it's been off, it looks like a dead short (very low resistance) which means the alternator gets a nice solid wack of current to excite it, but if that current continues to be drawn (because the alternator has failed or the engine isn't spinning it round yet) then the bulbs filament becomes really quite resistive (a 3W bulb at 12v is equivalent to a 48 ohm dc resistance.
A fixed 100R resistor, with 12v across it means a fixed 120 mA of current and a fixed power loss of 1.44 watts! But remember the rotor coil has it's own resistance, typically around 3 ohm. When an incandescent bullb is upstream, with an effectively zero resistance, when you first turn the ignition on, a full 4 amps transiently flows into the alternator excitation coil, rapidly charging the magnetic field, and as the bulbs filament warms up over about half a second, that current falls, and stabalises at some level depending on the total resistance of the system (filament + excitation coil), but lets say it's around 50 ohms, which means pretty much 250mA through the coil to excite the alternator.
also note, that an incandescent buld has a very strong Positive Temperature Co-efficient, because its operating temperature is so much higher than it's non operating temperature (off = ambient, say 21 degC, when lit = 3000 degC) That mean's it works really well as an alternator warning light, because initally, when it's been off, it looks like a dead short (very low resistance) which means the alternator gets a nice solid wack of current to excite it, but if that current continues to be drawn (because the alternator has failed or the engine isn't spinning it round yet) then the bulbs filament becomes really quite resistive (a 3W bulb at 12v is equivalent to a 48 ohm dc resistance.
A fixed 100R resistor, with 12v across it means a fixed 120 mA of current and a fixed power loss of 1.44 watts! But remember the rotor coil has it's own resistance, typically around 3 ohm. When an incandescent bullb is upstream, with an effectively zero resistance, when you first turn the ignition on, a full 4 amps transiently flows into the alternator excitation coil, rapidly charging the magnetic field, and as the bulbs filament warms up over about half a second, that current falls, and stabalises at some level depending on the total resistance of the system (filament + excitation coil), but lets say it's around 50 ohms, which means pretty much 250mA through the coil to excite the alternator.
O mage said:
I don't know but that top line in the diagram is just going to blow its fuse as soon as the relay pulls in as it just goes straight to earth and doesn't seem to do anything else.
It's implicit in that diagram that the led has a (not shown) series current limiting resistor to allow it to be driven with 12v and not go pop! When you buy a "12v led" what you are actually buying is an led (typically 2v forward voltage drop for a red led) with a series resistor to drop the other 10v at a current the led can survive (a modern led is pretty bright with just 5mA, so a 2k resistor would typically be used. In that scenario, it's interesting (or not...) to note that the led is using 10mW, but the resistor that's just there to keep the LED alive is "using" 50mW, meaning you system is really just 16% efficient.... (you don't care at these power levels for a single led of course)Max_Torque said:
It's implicit in that diagram that the led has a (not shown) series current limiting resistor to allow it to be driven with 12v and not go pop! When you buy a "12v led" what you are actually buying is an led (typically 2v forward voltage drop for a red led) with a series resistor to drop the other 10v at a current the led can survive (a modern led is pretty bright with just 5mA, so a 2k resistor would typically be used. In that scenario, it's interesting (or not...) to note that the led is using 10mW, but the resistor that's just there to keep the LED alive is "using" 50mW, meaning you system is really just 16% efficient.... (you don't care at these power levels for a single led of course)
Thanks , could the incorporated resistor in the led explain's why people just swop the bulb for an led and it works just fine?O mage said:
Max_Torque said:
It's implicit in that diagram that the led has a (not shown) series current limiting resistor to allow it to be driven with 12v and not go pop! When you buy a "12v led" what you are actually buying is an led (typically 2v forward voltage drop for a red led) with a series resistor to drop the other 10v at a current the led can survive (a modern led is pretty bright with just 5mA, so a 2k resistor would typically be used. In that scenario, it's interesting (or not...) to note that the led is using 10mW, but the resistor that's just there to keep the LED alive is "using" 50mW, meaning you system is really just 16% efficient.... (you don't care at these power levels for a single led of course)
Thanks , could the incorporated resistor in the led explain's why people just swop the bulb for an led and it works just fine?magpies said:
still begs the question as to why change the simple incandescent lamp that works fine for a complicated and inherently more unreliable set up for an led?
The OP has form posting these dilettantish, Heath Robinson solutions to problems no one is asking. Ask him about his 18V starter system. 
The phrase “just because you can, doesn’t mean you should” is definitely lost on him.
dvs_dave said:
The OP has form posting these dilettantish, Heath Robinson solutions to problems no one is asking. Ask him about his 18V starter system.
The phrase “just because you can, doesn’t mean you should” is definitely lost on him.
That's nothing...……….The phrase “just because you can, doesn’t mean you should” is definitely lost on him.
in the 70's and 80's we had 24v starting systems on our 12v rally cars
rigga said:
Steve_D said:
I'm wondering if someone has hacked Penny's login as only one relay is way out of character.
Steve
Think he must have purchased a job lot at one stage, and is trying to get through them. Also surprised only one on this effort.Steve
dvs_dave said:
magpies said:
still begs the question as to why change the simple incandescent lamp that works fine for a complicated and inherently more unreliable set up for an led?
The OP has form posting these dilettantish, Heath Robinson solutions to problems no one is asking. Ask him about his 18V starter system. The phrase “just because you can, doesn’t mean you should” is definitely lost on him.
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