New Flyer had a problem on their hands. They had shipped 26 buses to a property in southern California where they had omitted the little vendor-supplied 24V alternator from the Thermo-King air conditioning system, instead opting to use the main 24V bus to operate the fans. Unfortunately, at idle, the big, monster Delco 50DN alternator did not produce enough current to maintain the battery voltage, and batteries were going dead. When New Flyer offered to retrofit the vendor-supplied 24V alternator from Thermo-King, as was originally intended, the customer refused and wanted the little Motorola alternator to charge the main battery.
Unfortunately, parallel alternators do not share well – one will cook itself while the other one loafs. In this case, it’s even worse – the Motorola alternator was puny beside the 50DN, so you know there was going to be trouble.
At first, the New Flyer engineer counselled me to just draw a little current out of the base of the regulator’s drive transistor to keep the Motorola alternator from going over its rating. I protested that this wasn’t likely to work over component variation or temperature, but he insisted. We tried… and it didn’t work reliably. But, now he insisted that I was in the game and had to complete the job!
They brought a 40 foot bus over and parked it in front of Vansco, for me to work on. What fun that was, heh heh. I didn’t realize that buses often have pneumatic start – like an impact wrench – very loud and scary if you happen to be standing by the engine!
Well, I designed a module that used a Hall Effect sensor through which you would run the output wire of the Motorola alternator. We would open up the alternator and tap the base drive of the integrated regulator’s pass transistor. When the output current approached the alternator’s output limit, the circuit would draw away the base drive for the pass transistor, limiting the output current.
The assembly was a bit of a nightmare. It was a potted assembly with a hole in it for the alternator output wire to pass through. The potting would leak into the hole if the seal wasn’t solid, rendering the assembly useless. We had trouble with every batch. Fortunately, there were only 2 or 3 batches ever made!