[SOLVED] Unexplained over current HELP!

[bobc896]

Attached schematic is being used to drive an old treadmill DC motor which seems to work fine on an other DC supply.
The symptoms are:
Ran motor under load up to speed Amp-probe on L1 was reading 12.4 amps ran almost 2 minutes before popping breaker
No evident odd noises or smells
breaker popped (Will no longer reset)
C4 & 5 quite warm to the touch
D2 checks OK C 4&5 check OK too.
Circuit is based on app notes for the triac. Both bridge rectifier & triac are rated for 40 amps.

Can anyone provide a clue as to what is causing the problem and if possible a solution?


https://obrazki.elektroda.pl/4362363900_1422040133.jpg

 

[chuckey]

I think its the cyclic inrush current as the bridge rectifiers try and charge the 1000MF smoothing capacitor. This current has overheated the bridge and burnt a diode out. I think you need a current limiting resistor or inductor in series with the DC output of the bridge, or just disconnect one or both capacitors.
Frank

 

[bobc896]

Thank you chuckey! Your analysis was spot on! Ended up reducing capacitance to 25uf. problem solved.
Surge current, one of those pesky things that easily slips out of sight and out of mind. Equipment to allow one to see restively short large current spikes, is not likely to be found in most peoples workshop. (particularly mine)