Twelve Volts, Cheap

When I was working on the Volvo M41 overdrive transmission as documented in the Beck TD blog posts (Parts 51 through 57), I had need of a power supply that supplied a LOT of current - 20 to 25 amps at a nominal 12 volts - to actuate the solenoid that made the overdrive engage. To get there, I just used the battery from the car, which worked great but is an obvious inconvenience to remove and carry over to the workbench. I resolved to find a better solution...

Way back in the 1970's, when I was an undergraduate music student, I also studied vocational electronics. I did this via a correspondence course (via mail - there was no such thing as "online" then) from National Technical Schools. It was just a hobby, but oddly enough, it led to a good job with a computer company later. Anyway, a big part of the hands-on training was building a number of pieces of electronic equipment, starting with this Heathkit bench power supply. 

Obviously, I still have it. I can't get rid of it, even though it hasn't worked for years. "Someday" I'm going to get it going again, just for nostalgia. But for now, I mention it to give a brief history of power supplies. Back when I was trained, power supplies had three main components: a transformer that stepped the 110 volt AC line down to the desired output voltage, a set of four diodes that created a "bridge" to turn that AC power into DC power, and a hefty capacitor (the silver tube above) to smooth out the power. That Heathkit was extra-special for 1975 because it had extra circuitry to "regulate" the power to a precise 13.8 volts, the output of an automotive alternator.

The Heathkit supply was limited to 2 amps of current, not nearly enough for my overdrive experiment. A power supply made to handle 30 amps using those techniques would have a large, heavy transformer, diodes the size of your pinky or larger, and maybe a capacitor as big as a beer can. That would weigh 40-50 pounds, and cost $$$. 

But these days, everything has changed with the advent of "switching power supplies." These take the input voltage and chop it into little bits, and then put it back together as a new voltage, with no transformer required. Yes, I know that sounds weird, but it works! Incidentally, that's also why we now have power supplies that can plug into either 110 volts or 220 volts without damage.

I did some searching, and found that very inexpensive supplies were available to power strings of LED lights. This 30 amp supply cost only $17 on Amazon, and has features that my old supply never dreamed of, like overcurrent protection, short-circuit protection, cooling fan with automatic shutdown for overheating, etc. I hooked it up for a test, and it worked just fine with that overdrive. The photo below shows the supply, with the meter showing an interesting characteristic of the overdrive solenoid. While it takes a lot of current to initially actuate it (20-25 amps), once it is in position, it only takes an amp (1.01 amp here) to hold it in place.

The second picture shows the "business end" of the supply. The positive and negative terminals have three screws apiece, to carry enough current on the circuit board. In this case, I just bridged my wires across the three terminals. The rightmost three terminals are for the AC line with ground, and all the way to the left is the potentiometer to adjust the voltage. I set it to 13.8 volts, the standard for automotive applications.

Of course, I could have used the supply just as shown in my test, but it made me nervous because that box has so many openings for cooling. Very easy to get some contraband in there and cause a fire or short. I cast around, and found a wooden cigar box that was just the right size to make a case. I made a template for the mounting holes, and drilled the box. Why is it that anytime I have a fabrication project like this, I have to spend time shortening screws to fit?

Once the supply was mounted in the box, I removed the lid and drilled it for the cooling fan. Pretty good fit, but that gives me a wide open hole again!

I found a round scrap of aluminum and bored it on the lathe for a 2" hole, and then mounted it with screen to give a bit of protection. I actually felt very vindicated in that decision later, because when I was re-mounting the lid, I managed to drop a brass screw right down that big fan hole. It was unplugged, so no harm done, but proof that I needed some protection there.

There was more whittling on the box, and then I wired a switch and a pair of banana sockets for the output power. After I took this photo, I added a screw to hold the box shut so I wouldn't shock myself someday.

Better make sure it still works... and it does! The switch switches, the power outputs, the fan spins, and I'm all ready for high-current automotive projects now. Twelve volts, cheap!