A shock to the circuit
While working on the packaging design for a new magnetic amplifier, a real opportunity for circuit design came along, although it wasn’t presented as such. One of my responsibilities involved a product that had been in production for a number of years. It had never been a problem, so I hadn’t bothered to look at the design. Suddenly there was a problem and it was mine to fix. Production had assembled 600 units before doing any testing, since it had always been trouble-free, but during acceptance testing, every unit failed.
The circuit was simple: a wheatstone bridge drove a sensitive relay, which, in turn, drove a slave relay. The input was one leg of the bridge and the output was the set of slave relay contacts. When the signal was raised to the proper level, the sensitive relay contacts were to close and energize the slave relay. The contacts of the slave relay would then light a lamp, just as in the intended application. The circuit was dead simple; small wonder there had never been a problem.
But now there was. When the slave relay closed, it did so with such force that it sent a shock through the chassis that re-opened the sensitive relay. That, of course, re-opened the slave. In time, the sensitive relay re-closed, causing the slave to re-close, and the process started all over again. The process cycle time was such that the units had become buzzers.
A phone call to the slave relay manufacturer revealed that they had “improved“ the relay so as to give it a more forceful contact closure.
My engineering superiors had suggested possible fixes like adding a vibration isolator to the sensitive relay, or the slave, or both. That would require extensive development and testing of these alternatives with no assurance that any of them would work. Having the relay manufacturer make a special like the old design just for us would drive up cost. In any case, it looked like we would need a new design. The 600 units would have to be scrapped (salvaging just the relays), and our customer would not be happy with the delay in delivery.
I studied the “buzzer action” carefully. It occurred to me that since the slave relay coil was inductive, it might hold enough current long enough to stay closed until the sensitive relay re-closed if an alternate path for that current were provided. At that point, a possible fix became apparent: simply put a diode across the slave coil.
A small, axial-lead diode across the slave relay coil was all it took to get all 600 units to work properly. No redesign, scrapping, or disappointed customer, and minimal cost.
As I prepared to report my success to my bosses, I wondered what their reaction might be. After all, I had thought outside the box to save the company a lot of money, and fixed what had been presented to me as a complex mechanical problem with a simple electrical change. What would be my reward? A bonus? A commendation? Lunch? A pat on the back?
When I reported on my work, my boss said, “OK, let’s get back on that mag-amp job.” Apparently my reward was continued employment.
- Flip the switch
- Controlling big, mean devices
- Out with the new, in with the old
- Drawing out a solution
- Tales from the Cube
Richard Gilbert is a power electronics engineer involved with electric drives and kinetic energy recovery systems for ground vehicles.