Taming Lester 4

More Lesterisms.

Yesterday, while I was in town, I did some shopping around for a set of cheapo jumper cables to hack up for heavy cable to use as the output wiring of the charger. Looks like I can get some auto-store cables in 8 gauge (good for 40 amps continuous) for around $14. On a whim, I went into the St. Vincent DePaul thrift store, thinking I might find some used cables. No such luck, but what I did find was a bin with second-hand appliance cords, and specifically, a 50 amp, 240 volt dryer cable, complete with crow's-foot male plug -and- an attached female flush-mount receptacle for $2.99. Wotta deal. At the checkout counter, I learned that red-tag items were 10% off on Tuesdays, and that I got an age 55-and-better 10% discount as well, bringing the total to $2.41. The receptacle alone would have cost me $12 at the hardware store, and the cable probably another $19. Gotta love the throw-away society.

Anyhow, I now have an AC input cable and plug good for the full rated output of the charger, and have modified my long 240 volt extension cord with the receptacle so that it can feed the charger. I am building the charger's systems to carry the full rated output, meaning that the input side of the charger has to carry 30+ amperes, the input cable, fuses, contactor, etc, are all rated for at least that current. Circumstance may dictate that I cut back the current while charging the car due to available current from the source, or while using the long extension cord, but I will have the ability to charge at full output when conditions warrant.

Still thinking about jumper cables for the output, but the cheap appliance cords are tempting, two 6 gauge wires and one 8 gauge, and for pennies on the dollar...

It was a dark and stormy day today, perfect for staying inside and mucking about with electronics, so after lunch, I got busy and breadboarded up the phase control circuit prototype that I'll eventually build to control the charger.

After a few false starts, a capacitor in the wrong place, and an error in the datasheet that gave the value of the ramp capacitor 1,000 time too low, I got the circuit to function on a battery and audio generator. Once that was accomplished, I built a "Lester simulator", which was to use a 12 volt transformer and a couple of low-current SCR devices configured exactly like the big charger for a test. No sense blowing up my entire test bench with 120+ volts at 30 amperes if things go wrong for an instant.

Amazingly, it worked the first time! I used a small pilot lamp as the load, and was able to vary it's brightness with an adjustment of the control pot.

Here's a pic of the setup. I used every clip lead I could find to interconnect the battery, two AC transformers (one for sync reference, and another to operate the SCR's and lamp), the 'scope, multimeter, etc. A cobbled-together mess, but it proved the circuit.

The lamp is glowing at about half brilliance under a red lens, while two traces show on the 'scope. The left half of the screen shows the AC sync sine wave (slightly distorted by clipping at the IC), and the right side shows the output of the SCR's, a bit less than half-power one each half-cycle of the output of the transformer. rectified to DC.

Now that I have proven that it's possible to run the circuit without smoke at low voltage, it'll be time to rig Lester up for a high voltage, high current test. Stay tuned for details (from a safe distance).

Part 5