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Re: Terry's DRSSTC - All Ready!!

Original poster: "Gerald  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Terry,

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Fix #1

I did add a grounded shield to the CTs during the down time and "special" caps to the protection circuit:

The shield works great and should be grounded the Chassis ground. This stops most of the capacitive noise pickup back to the electronics.


Just a conductive tube between the noisy output wires off the H-bridge and the CTs. This stops most of the capacitive noise pickup back to the electronics. Solved a lot of noise problems right there!!

If you keep this shield, you may want to inject some RTV in the tube to prevent chafting.

Fix #5

So it still did not work... I had begun to worry a week or so back... As I fiddled and mess with things, the level that the current trip activated never changed much... So I tore through every node in the protection circuit with differential probes (I am real good at that now...). It came down to the LM339 was false tripping at 120 amps.

I am using the center tapped CT to get complimentary outputs monitored but the LM339. The LM339 is ground referenced so I chop the negative part of the signal with the 1N4148 diodes.... And there was the error!!!!!!!!!

The 1N4148's forward voltage is about 0.6 volts (-0.55 in my case). The LM339's lower input voltage for ground operation is -0.3 volts... And.. They are NOT kidding!!! Since that was the only thing that could be causing it, I changed the diodes to Schottky 1N5819 with Vf = 0.3.

The clouds cleared, the birds sang, and the darn things stared working perfectly!!!!!!!!!! Too much negative input to the LM339 messed it up and made it false trip. The Schottkys fixed that right up!!!

Great. You still have zero margin (spec wise). Alternatively, you could clamp to +0.6V instead of ground and get the diode drop completely out of the picture. Sorta like the diode compensation for a push/pull audio power amp using complementry bipolar transistors. One extra diode and resistor might do. Or you could bias the CT at Vdd/2 and eliminate need for clamping

Gerry R