# What cause Diode bridge to short circuit?

My circuit got

1. 220V to 36V Transformer (Approximately 50VA)
2. KBPC2510 bridge rectifier (1kV, 25A)
3. relay to coltrol motor of/off
4. 36V DC motor (100W, 4A), just drive rotary knife to cut fabric.

At first place, primary side of transformer got short circuit winding and triped fuse. after replace transformer and fuse I found that diode bridge broken as well by 2 diode act like short circuit (negative pole to both AC pole) My question is.

1. Is that normal failure mode of bridge rectifier? How it happen?
2. Is my circuit lead to failure and how to prevent.

-- Edit -- Added schematic. Note: For record I draw this schematic as it's wire in actual mechine, not my design. and I still wonder why it's look so minimal.

simulate this circuit – Schematic created using CircuitLab

1. Transformer has no datasheet or VA rate. I measure E core and got 3"x1.5"x2.5" . As I found similare size transformer It say 50VA.
2. DC Motor nameplate say ZD Motor(Z3D100-36A2), 100W 36V 4.0A But it's used to cut fabric with rotary knife, shouldn't draw much current.
• To give a considered opinion of why your circuit failed we would need a schematic - even if it's simple - ratings for the transformer, rectifier and motor. Links to datasheets are usually a help. Commented Jun 29, 2020 at 21:33
• @Transistor Okay, here the schematic. please consider. Commented Jun 29, 2020 at 22:13
• Your circuit does not show a capacitor. Also, if you rectify 36 VAC using a full-bridge single phase you will get an average dc voltage equal to 36*2*sqrt(2)/pi -2*0.7 =~31.0114 V Commented Jun 29, 2020 at 22:22
• I have considered and I see the schematic but none of the other essential information requested. Commented Jun 29, 2020 at 22:37
• Overcurrent surges can cause diodes to fail short - either charging a capacitor if any, or starting a motor. Check the Issm (single pulse surge) rating in the datasheet and compare with the motor's stall current. Also, over-voltage from motor's inductive spikes. (There appears to be no filtering or spike suppression)
– user16324
Commented Jun 30, 2020 at 9:55

Assuming there is actually no (substantial) filter capacitor, it's possible that an external short circuit or a stalled motor caused the transformer to fail.

Your setup as show, no filter cap, is a reasonably common situation (no filter cap) to run DC motors from an AC source.

A 1kV 25A bridge should not fail easily with a transformer that tiny, I suspect short circuit current is less than the bridge rated current, though without heatsink it might conceivably have eventually overheated and failed. You could probably see some evidence of that (discolored markings, for example).

If that makes sense, I suggest you replace it with the same type and adjust the fuse to a slow-blow type that has a relatively low current rating. Then, if the motor is stalled for many seconds the fuse will open.

The diode bridge is burnt due to the inrush current absorbs by the DC motor at the beginning. To avoid this I can suggest you two methods.

1. Use a NTC thermistor( negative temperature coefficient resistor) in series with DC motor.
2. Use a soft starting circuit to control the DC motor.
• As I found, motor draw 4.0A but rectifier can withstand 25A. Did I miss something? Commented Jun 30, 2020 at 14:58
• May be you measure steady state current. But, the inrush current is more higher than the steady state current. The only factor that limits the inrush current is the total impedance of the circuit. Commented Jun 30, 2020 at 15:11
• infact, I didn't measure. it just labelled on nameplate. but it's good idea to measure real current. Thankyou for your participation. Commented Jun 30, 2020 at 15:25

KBPC2510 bridge rectifier, rectify 36VAC to 36VDC

No, way off - the peak voltage will be more like 50 volts and, if you are using a smoothing capacitor the DC level (ignoring ripple) will be about 50 volts also. Draw a schematic - that is the language of EEs.