View Single Post
Old 11-23-2010, 04:10 PM   #4040 (permalink)
EcoModding Apprentice
Join Date: Nov 2010
Location: Annapolis
Posts: 159
Thanks: 0
Thanked 32 Times in 27 Posts
Originally Posted by snakub View Post
Well it seems to me that if the Darlingtons couldn't handle the frequency they simply would not have operated correctly and would have been either full on or just off ...
I was traveling and missed this post the first time around.

If the Darlington pair couldn't handle the frequency, or were not driven strongly enough to put the output into saturation, they wouldn't be full on or full off. They would be somewhere in the middle. It doesn't take long for them to effectively melt -- enough internal dopant diffusion happens for them to become low value resistors.

Once this process starts, it continues independent of the control input. The controller can only detect the fault and blow the contactor (if it's designed to do that).

The same things happens to MOSFETs, although they tend to pop their cases apart a more often. That's why there is always a fuse in the system.

I've never seen a battery boil dry from a short. I've seen melted lead posts several times, and the batteries reportedly pumped current through the molten lead. One event, on a friend's new sail boat, lasted for well over a minute until they understood what was happening and physically knocked the cable loose from the battery post. When I saw it the molten lead had solidified into tracks it had melted into the side of the battery, but otherwise the battery was still "good".

Originally Posted by snakub View Post
As the mosfet was switched by the control board it sent a positive +12 volts was sent to the bases of the Darlingtions turning them on and off. I still think it would work do you recommend anymore tests?
Was this the same circuit that you used with the 555 timer? Keep in mind that the original NE555 could drive 200mA, while modern versions can usually source and sink 500mA with some driving over 1500mA. The MIC4451 could probably match that, but the DC-DC converter that the Cougar uses will fall down long before.
  Reply With Quote