Battery Charging


Posted on Feb 7, 2014

I finally gave up on the LM7805 circuitry that comes with these devices. The Harbor Freight 42292 charger used to be controlled by a LM317 variable voltage regulator a long time ago. They probably changed the regulator at some point to lower cost or overcome a supply problem. However, it originally was a much better device; it even had a


Battery Charging
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variable resistor in it to allow the end user to tweak it a bit to fit his purposes. I got a clue on this during my searches to get ideas on what to do. Here is the link to a schematic of the original device ( link ). Notice how different it is from the current circuitry Just so I have a picture to talk about, here is the schematic from that site: Back in those days, the wall wart was just a transformer so the diodes to rectify the supply were in the little black box. IC1 is a LM317 and notice how they set up the resistor array to limit the range of available voltage and have a transistor to limit current Nice design that you can find all over the web. I still have a diode in series with the output to prevent problems with reversing the connection to the lead acid battery and used different values for all the resistors than the original had. The reason I changed the values was purely practical; it`s what I had on hand. Notice that the parts count is a grand total of five So, why doesn`t the manufacturer do something like this I don`t have a clue. So, my schematic looks more like this: The input on the left is the wall wart`s 19 VDC (apx) output; VR1 is a 1K, 25 turn potentiometer I picked up on eBay; R2 is 1. 2K; and R3 is 180 ohms. This gives the charger a range of about 9. 6 to 16 or so volts. To calibrate it, I put a 1K resistor across the output, set it for 13. 4 volts and then took the resistor off. When I hooked it to...




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