# Curtis 1231C controller repair



## WSJ (Sep 18, 2008)

I have repaired several Curtis 1204 and 1231C controllers, all had good diodes except one. I sold a tested 1204, which was returned two weeks later with all tops blow off the FET's and CR4180R diodes. If the shorted FET's caused a large motor current pulse, could one inductive current pulse cause the diodes to be blasted apart? Max surge current for 6 diodes is about 1,500 A. More likely when the controller was being installed the battery cables were reversed. 
Am I missing something? Do diodes commonly fail along with the FET's?


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## kennybobby (Aug 10, 2012)

i looked at one in which the nut/bolt making connection to the B- terminal had vibrated loose--it blew all the FETs and all the diodes, there was molten metal blasted everywhere and it did not seem a good candidate for repair...


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## WSJ (Sep 18, 2008)

I discovered a troubling design error/production test error with the Curtis 1205 control board. I measured the thermistor temperature vs. resistance to allow testing with an external pot. The pules frequency and width change to 1 kHz and 14% duty cycle at 190° F (Thermistor resistance 8K ohms) on a Taylor Dunn controller board. The Taylor Dunn board has been redesigned in the area of the thermistor, buy is the same as the 1205 in the FET drive area. I have four 1205 control boards, they do not have functional thermal shut down. The thermistor resistance was tested form open to 0 ohms. All of the 1205 controls had burned FET’s. It doesn’t look like the thermal shout down was tested at the factory, three boards worked without replacing any components, one had a bad FET driver.
How many of the controller failures are a result of this design error and/or lack of production testing?
Top trace gate drive. Bottom trace motor - with load resistor.


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