Zener diode as an Off switch in a 12- volt DC circuit?

Thread Starter

Skipper11

Joined Mar 7, 2024
7
I have a vintage motorcycle that originally used a vibrator voltage regulator, but it died and is not manufactured any more. I replaced it with a solid-state regulator which works great, but I now no longer have a feed to my 3-Watt indicator lamp showing when the Alternator is charging. The indicator lamp served as a warning by glowing when no voltage was being provided by the alternator.
Static battery volage may be up to 13 volts but anything over that can reasonably be assumed to show activity from the Alternator.
Thus, I was wondering if a simple circuit using a Zener diode 4743 could be used to cut supply to the 3-watt lamp when voltage exceeds 13-volt DC? This might involve tapping into a "live" feed which operates when the ignition switch is activated, to supply power to the indicator lamp.
Unfortunately, beyond that my knowledge is scant though I have an auto-electrician who could wire this if I could provide a circuit diagram of this control device. Some reading I have seen suggests that a resistor will also be required but just how this all works is beyond my understanding.
The intent is to maintain an authentic function of the indicator lamp to be in keeping with the original design where-in the lamp went out when the alternator was charging but come on in the event no increase in voltage was being provided. The choice of 13 volt cut off point is purely nominal though clearly a fully charged battery may demonstrate up to 13 volts un-assisted.
Any help would be much appreciated.
 

Thread Starter

Skipper11

Joined Mar 7, 2024
7
Hi Irving, thanks for the reply. However, I am looking to turn the lamp off when the set point is reached. The circuit would be live as soon as the ignition switch is turned on. If that can be achieved using your drawing, I would be happy to take advice.
 

Thread Starter

Skipper11

Joined Mar 7, 2024
7
Hi Irving, Ian0, thanks for the replies. However, I am looking to turn the lamp off when the set point is reached. The ciruit would be live as soon as the ignition switch is turned on. If that can be achieved using your drawing, I would be happy to take advice.
How is the regulator wired? What connections do you have to the alternator?
Ian, if I am understanding your question correctly, the regulator was attached to the exciter circuit in the alternator which determines the output of same. This is achieved by two wires. A third wire from the (now removed) analogue regulator used to connect to a "side" tapping off the old plate rectifier, which is still being used. This was sensing voltage or lack thereof, from the alternator which was further connected though the lamp to the ignition system. The lamp operated by this output voltage or lack thereof. As the "other" side of the lamp circuit has a positive feed from the ignition switch I assume the side tapping off the plate rectifier is actually an earthing connection, regulated by power flow though the rectifier? The original wiring for the analogue voltage regulator provided this tapping but when converting to the digital regulator there was no third connection, so the feed from the lamp only goes to the plate rectifier. The two wires from the alternator to the digital regulator provide all the control the alternator requires but with no indication that this is even happening. The analogue regulator was manufactured by Mitsubishi, however, I am told by my electrician that it was similar to one made by Bosch, possibly under license? Add to that it has the name of a third (US) manufacturer also stamped on the cover of the regulator.
 

crutschow

Joined Mar 14, 2008
34,689
Below is the LTspice sim of a circuit similar to the one posted by Irving in post #3, but modified so that the bulb is ON at low voltage and turns OFF at a high voltage:

The TL431 is a programmable precision reference voltage, used here as a voltage comparator, that conducts cathode current, which turns off the transistor, when the Ref voltage exceeds 2.5V.

Pot U1 adjusts the turn-off voltage (here set to 13V).

The bulb model resistance equation is for an incandescent type, emulating the characteristic large increase in resistance with voltage.

The minimum output voltage of this relatively simple circuit is a little over 1V when the battery voltage is high, but that should light the bulb only very dimly, if at all.

1709867266396.png
 

Thread Starter

Skipper11

Joined Mar 7, 2024
7
Thanks to all so far for your contributions - I shall pass the latest circuit diagram on to my electrician for his opinion. Since I am neither electrical engineer or even a competent tradie, my thoughts count for little and my opinion even less.
Any additional ideas to keep this simple gratefully received. It may well be that what we are now looking at is as simple as it gets but all I am trying to achieve is "indication of a positive charge into the circuit via a lamp which will go out.
 

Irving

Joined Jan 30, 2016
3,983
Thanks to all so far for your contributions - I shall pass the latest circuit diagram on to my electrician for his opinion. Since I am neither electrical engineer or even a competent tradie, my thoughts count for little and my opinion even less.
Any additional ideas to keep this simple gratefully received. It may well be that what we are now looking at is as simple as it gets but all I am trying to achieve is "indication of a positive charge into the circuit via a lamp which will go out.
You're welcome, and welcome to AAC.

The circuit suggested by @crutschow is about as simple as you can get if you want to use the existing 3w bulb. It could be marginally simplified if you were to use an LED instead.

A couple of other points:
1. This is for a -ve chassis circuit and assumes one side of the bulb is connected to chassis. A lot of older vehicles had a +ve chassis - if that's the case a little change is needed. Just let us know.
2. The 2N2219A transistor, though you can still buy them, is obsolete. If you do use one will need a small push-on heatsink otherwise if it sits at the on/off transition point for anything but a second or two it will get very hot and will eventually fail. A better choice is a TIP35C which can be used as-is, no heatsink needed.
 

Thread Starter

Skipper11

Joined Mar 7, 2024
7
Back again after a long spell, my auto electrician had a stay in hospital but is now functioning again.
He sent me off to purchase the parts needed to build the circuit using the TIP35C.
The electronics supplier substituted for this item with a ZT2293? And then asked me what item K is in the circuit?
I had no idea but assume from searching Google that it is either a relay or a contactor but know not?
Please advise and we can build this neat little device and move on :)
Much appreciated thanks
 

ElectricSpidey

Joined Dec 2, 2017
2,834
Item "K" is just a reference/test point. (probably means cathode of the TL431A)

I'm sure crutschow will correct me if I'm wrong.

The ZT2293 seems to be a catalog number for the TIP35C.
 

Thread Starter

Skipper11

Joined Mar 7, 2024
7
I appreciate the response, however like you said, and I had hoped, Crutschow would have some contribution.
My auto electrician did not know what the implication of K might be as far as ordering from an electronics store?
Any help gratefully received.
 

crutschow

Joined Mar 14, 2008
34,689
Yes, K is just an electrical node reference designation for the cathode of U2, as ES noted.
It helps identify what voltages are being displayed in a simulation.
It has no meaning otherwise.
 

Thread Starter

Skipper11

Joined Mar 7, 2024
7
This is great, thanks to you both, problem solved and verified for this absolute novice
And my thanks to all who contributed to this thread. Hopefully my Old '69 H1 will have charge indication once more :)
 
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