As the days get shorter it is harder to get all the yard work done before the sun sets, leaving me with a half finished lawn. Once we roll back an hour with daylight saving time, it is almost impossible to get anything done after work. The ‘lights’ on my lawnmower are nothing more than a couple 1156 bulbs behind a plastic cover that blocks most of the light; these lights are not effective in dim or dark conditions, hence my need for a better lighting solution.
If you are looking to add some lights to your lawnmower please read on, perhaps my project can be of some help to you.
The first thing I needed to do is determine the type of electrical system my mower uses. I have a Briggs and Stratton engine so the resources I used were:
Using these resources I was able to determine I had a dual circuit charging system, with 12 VDC output to charge the battery and unregulated AC power for the headlights. This complicated matters slightly because now I could not simply connect my new light to the existing circuit; any automotive LED light would require 12VDC not 14VAC, but more on that later.
The information on the current output of the AC lighting coil was not published by Briggs and Stratton so I needed to use some reverse engineering to get a safe maximum wattage. Based on the two 1156 bulbs (which each consume around 27 watts @ 12VDC) I expected the circuit to be able to supply around 50W but because they were driving the bulbs with unregulated AC power, I needed to take some measurements.
To determine the power consumed by a single bulb I needed the voltage across the bulb and the current through it. This required the following two measurements:
I recorded 14VAC across the bulb and 1.54 A current through it using a standard hand-held multimeter. This means the power consumed is :
We have two of these bulbs so that works out to 43.12W of power available.
Shopping for Lights
I went looking for an LED light that requires 43W or less, I found this light on eBay that requires 18W and was reasonably priced.
Back to the AC voltage issue……
The LED light is designed to run on DC not AC so we need to convert it. There are a number of ways to do this but the most cost effective and practical would be to use a full bridge rectifier which looks like this.
I am not going to explain how a full bridge rectifier works in detail because there are plenty of resources on the web for this. Here is a link to a Wikipedia article if you really want to know:
What you will be mainly concerned about is the current capacity of the diodes in the bridge. I selected my diodes based on what I had laying around in the lab; I happened to have a whole box of FR304 Diodes that can handle 3A which works out to be . If you need to buy some diodes I would recommend Digikey , Mouser , or Ebay which all have a great selection. You are looking for a rectifier diode, the forward current capacity is the primary specification you are shopping by and the second point to consider is forward voltage drop which you should keep as low as possible. If you want to order something go with the MUR420G it is cheap, readily available and handles 4A.
**NOTE – You could also order a bridge rectifier as single device (GBU4A) it is cheaper than buying 4 diodes and saves you soldering.
The output from the rectifier will look something like this:
This will work for your lights but the inclusion of a capacitor across the output will make the voltage much closer to constant DC. You can go with almost any electrolytic capacitor that is 4700 uF or above, they are easy to find at the same locations as mentioned above.
So this is the final circuit you will be building :
Test and Install
This circuit will effectively convert the AC voltage from your lawnmowers lighting coil into a DC voltage that your LED lights require. Once you have the circuit all built and installed you should test the output to ensure it is delivering DC. I used an oscilloscope and signal generator to test my circuit but any multimeter will work as well. You want to connect the circuit to your lighting coil, then measure for AC voltage. There should be less than 1V AC, if not you have wired something wrong. Next measure DC voltage, here you should see between 11VDC and 18VDC (depending on engine speed). If everything checks out, attach your LED light(s) and mow away.