Motorcycle Charging Light using LM393 Comparator

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Thread Starter


Joined Jan 7, 2015
Hi All,

I'm new to the forum and would be very grateful for some help and advice with a project I'm working on. It is to provide a charging indicator light on my classic motorbike. The idea is the light will come on when the alternator is not charging the battery and extinguish once it is producing sufficient voltage to provide a positive current flow and charge the battery.

The attached diagram shows the circuit I have built so far.

There is a 0.01 Ohm shunt resistor on the output from the alternator (regulated and rectified) which measures the current flowing into the system (all loads including the battery)

The 2k resistors are precision items to give a close tolerance input to the LM393 comparator. The current flow from alternator will vary between zero and +ve 25 amps but will never go negative due to the diodes in the rectifier.

It works but I have the the following problems that I am hoping someone can give me some help with:

1: I need to "offset" the inputs in someway so that under zero current flow conditions the + input to the comparator is biased high by say 5mV so that the charge light is ON. When current flows the voltage drop across the shunt should pull this low and extinguish the light.

2: I did manage to introduce some offset by selecting resistors for the potential divider on the input. However there seems to be some sort of latching going on so even it doesn't reliably come back on even when the current drops to zero.

3: Is some sort of feedback from the output desirable to improve the circuit. Perhaps when it is on feedback could be used to further bias the + input higher so it is only once a significant output from the alternator overcomes this (say 5 amps) and pulls the output of the comparator low to remove the feedback.

4: Protection!! I managed to blow one of the LM393 comparators by running without the battery connected. It seems that the alternator may produce voltage spikes if not damped by the load of a battery. This shouldn't happen in practice but the battery is protected by a fuse on the bike and if the fuse blows........ changing a fuse is one thing but changing a IC on a 1950s machine in a layby may raise a few eyebrows!

Any help very gratefully recieved!



Joined Apr 5, 2008

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