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Diode that only allows continuity at 13.5v?

vc10786

New Member
Hello, is there such a thing as a diode that only allows current to flow once it has dropped below a threshold of 13.5 volts DC? Or maybe a transistor or other(very simplistic) circuit I can build? I don't need it to pass much current, only enough to trigger a dashboard light on a car project. It's for an alternator charge indicator light. All I want it to do is sample the battery positive system and keep the dash light off if the voltage is above 13.5v to indicate that the alternator is working. Thanks.
 

billybob

Active Member
Hello, is there such a thing as a diode that only allows current to flow once it has dropped below a threshold of 13.5 volts DC? Or maybe a transistor or other(very simplistic) circuit I can build? I don't need it to pass much current, only enough to trigger a dashboard light on a car project. It's for an alternator charge indicator light. All I want it to do is sample the battery positive system and keep the dash light off if the voltage is above 13.5v to indicate that the alternator is working. Thanks.
Look into zener diodes. I don't know too much about them, but I think that you can put them in series or parallel to add the voltage cut off. The reason Im saying this is because I don't know if they make zeners with a breakdown voltage of exactly 13.5 volts.
 

Diver300

Well-Known Member
Most Helpful Member
You will need a transistor or two as well as a zener, so that when the voltage is high enough, the current going through the zener causes the light to turn off, not on.

For more accurate and adjustable voltage measurement, look at the TL431. There is a circuit for undervoltage detection here:- https://ozcott.com/NomadTronics/product/tl431a-adjustable-zener/

Alternatively, a couple of zeners at 6.8 V and two resistors will work but will take quite a lot of power when on. The circuit is here:-
http://www.evconvert.com/article/battery-undervoltage-warning-circuit
(It's set for a lower voltage, so you will need larger value resistors and higher voltage zeners)

You should be aware the some modern cars (<20 years old) will mess around with the alternator output to give more power to the wheels, reduce fuel use or just to reduce apparent brightness changes on lights. Voltages dipping below 13.5 V is not necessarily an indication of a fault.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Hello, is there such a thing as a diode that only allows current to flow once it has dropped below a threshold of 13.5 volts DC?
There are Zener diodes, but they start to conduct at a specific voltage rather than stopping, so need a bit of additional circuitry.

This is the simplest circuit I can find that will turn the LED on below a specific voltage:



That works by the zener diodes diverting the current through the resistors when the voltage increases above the point where the voltage between the two side junctions is less than the LED forward voltage.

The two zeners should be somewhere roughly around half the supply voltage, and total to about 1.8V more than the voltage you need the LED to light.

Two 7.5V rated ones may be suitable; that would give a threshold around 13.2V with a typical red LED.
A simple circuit like that will draw current all the time, LED on or off.

You can use eg. two lower voltage zeners in series to get other voltage steps; low voltage ones are available in smaller increments.
 

Les Jones

Well-Known Member
Most Helpful Member
Are you sure that the original warning light is not needed to provide an initial current through the alternator field before the alternator provides the field current itself. (Via the voltage regulator.) If you remove it there will probably not be enough residual magnetism in the rotor assembly (Field.) to kick the alternator into life.

Les.
 

Diver300

Well-Known Member
Most Helpful Member
Are you sure that the original warning light is not needed to provide an initial current through the alternator field before the alternator provides the field current itself. (Via the voltage regulator.) If you remove it there will probably not be enough residual magnetism in the rotor assembly (Field.) to kick the alternator into life.

Les.
That is correct, but the original post asked about a light that comes on below 13.5 V. The standard alternator warning light is powered when the alternator stops working so it will have no voltage across it when the alternator is working correctly. The OP is wanting something that connects between an ignition switched signal and ground.
 

Les Jones

Well-Known Member
Most Helpful Member
I was assuming that the OP was planning to remove the original warning light.

Les.
 

vc10786

New Member
I would like to keep the original warning light and would like to retain its functionality, but I already replaced the alternator twice and (I guess have I bad luck with alternators), each of them has a bad "L terminal" also known as Charge Light indicator output.

This is a 1991 Subaru 2.2 engine with a standard OEM 3-wire alternator.

The alternator is working and is charging the battery at 14.4v fine while the engine is running.

The big white wire is supposed to be battery feed I think.
The yellow wire needs IGN 12v+ fed to it or else the alternator does not charge.
The black/white wire is the "L terminal"/charge light indicator, but has zero output at all times..

The black/white wire is supposed to provide 12v+ out when the engine is running to keep the dashboard charge light off, but it's not. It's just providing ground all the time, keeping the charge light on.

Rather than simply bypassing the alternator altogether and hooking up the dash charge light to a 12v+ ignition power source, I would like it to actually sense the alternator output so the functionality is retained. Or else, if the alternator ever does go out, the driver will not have the charge light come on.
 

Attachments

vc10786

New Member
Check the bulkhead connector for corrosion. Check the charging system for loose connections. Been there. Done that.
All connections check out. I even traced the wire all the way from the dash and connected it to a 12v+ source at the connector just before the alternator. The charging light went out. I even tested the terminal at the alternator itsself and verified there is no 12v+ coming out of it. So I am certain the problem must be within the alternator...
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Never getting any output on the warning lamp terminal implies either the rectifiers or regulator are dead..

This is a typical internal arrangement; the warning light bulb connects to the internal auxiliary rectifier, that feeds the field coil in the rotor.
As soon as the alternator starts producing any output that terminal should have a voltage similar to the main output terminal.

 

Les Jones

Well-Known Member
Most Helpful Member
The fact that you did not get a voltage reading at the dash light connector on the alternator is an indication that the fault is EXTERNAL to the alternator. The other side of the dash lights should have 12 volts connected to it when the ignition is on. With the ignition on and the alternator not rotating the current through the dash light provides enough current through field to get it started. Once the alternator is rotating the field is supplied by the diode trio (See post #11) so there is no path to negative through the field for the dash light.

Les.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Read again, Les:
The black/white wire is supposed to provide 12v+ out when the engine is running to keep the dashboard charge light off, but it's not. It's just providing ground all the time, keeping the charge light on.
If there were no power to the lamp, surely it would not be on?
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I don't like https://www.gomog.com/BLAIR/tech/melectrical/ign-light.html this post, but, I think, it adds some insight.

I had one failure that was really really odd. The car just plain old died. The dying was due to a weld on the rotor breaking off complely.

The no indication was due to corrosion in the idiot light circuit. This was a 1974 Chevrolet Chevelle. I did like the drop-in regulator design.
The chevrolet had a test point, but it was not easy to access A screwdriver to the frame would make the regulatorput out full power. I gess that's not a good thing to do these days.

My favorite tools were a scope (checks for bad diodes), a voltmeter (heater, high beams) checks the thing under load and the hy(something) to check specific gravity.

What I did do, when everythig was working is turn the loads on (typically heater and hi-beams) and measure the voltage from the alternator output terminal to the battery. yep, across a big fat cable. That number would be proportional to current.

Kinda concentrating on ripple form (bad diodes), regulation (load and no load).


==

One failure was due to the paint shop not tightening the battery cables. 1982 Toyota. Rebuildable, but only if you could source the parts and had a spot welder.

==

One failure, a 1968 Chrysler was just plain worn out. Winding insulation. Regulator was external and the slip rings were one parallel, one anti-parallel. I liked this alternator because the rectifiers were pressed in and made exceptional thermal contact.
 

crutschow

Well-Known Member
Most Helpful Member
Here's a simple circuit to light an LED below the desired voltage.
It uses a programmable TL431 voltage reference as a comparator to get a precise and stable trip point.
This trip voltage can be adjusted as desired by changing divider resistor values R2 and/or R3.

The TL431 turns off when the Ref voltage from the divider goes below 2.5V, allowing current from R1 to light the LED.
The simulation shows the LED current going on and off (red trace) when the battery voltage (green trace) goes below and above a nominal 13.5V.

1615186550552.png
 
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Just out of curiosity, has the battery been properly load tested and checked for sulfation? Having been a rebuilder of alternators, starters and generators, I can tell you that the number one cause of premature alternator failure is the battery. You may also want to drag test the small connector terminals as those regulators are very sensitive to loose connections. Run the vehicle with all the accessories off and tell us what the alternator output amperage is at idle.
 

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