I had some trouble with getting the science fair AM radio I built to work, and it wasn’t helped by not having a real multimeter. I decided I needed to buckle down and buy a new one as the automotive tester I’m borrowing from my dad was only able to crudely measure voltage and I needed more than a continuity checker.
Thus behold the Scope DVM-638 multitester.
I did do something stupid already though. I was using a lawn-mower battery to power a car radio I got for $6 at a farm auction. I wanted to measure the current draw of the radio to see how long I could power it off the car battery, and forgot to disconnect the leads from the battery FIRST before moving the plug to current mode. That shorted about the battery in the unfused 10 Amp range. It made the test leads very hot and melted the insulation. The local TV repair shop took mercy on me with some old test leads they had that fit the meter. Not a perfect fit, but it will be a reminder to look and think before connecting!
With regard to the battery, I found that an unregulated wall adapter would serve as a trickle charger, since my dad didn’t want me to use his good charger. Also it’s for safety since if the charger switch got bumped to normal charge it would quickly overcharge the battery and the battery could explode.
So what I did is take a roll of thin speaker wire from a junked phonograph with remote speaker as a ballast resistor. The wall adapter isn’t enough to power the car radio directly, which draws about an amp with some variation due to volume level and not helped by dial lights. I want to keep the battery at about 12.5-12.6 volts so I’ll experiment with the appropriate ballast wire length vs. amount of 12 Volt stuff I have hooked to the battery. I’d like to find an old marine radio and CB radio to play with and power off this battery.