Problem- Home made stepper circuit burns.

[boozeboy]

Hello. I have built a simple stepper circuit that also has three relays that draw 60 or 70 ma as far as I remember. I switched on the 12v power supply and the uln2003a blows in a matter of a few seconds. I do not have the stepper motor wired in yet and the parallel cable is not plugged in yet either. The relays are the only thing wired up right now. There are no shorts and everything looks great. What have I done wrong? I am totally stumped; please help.

[pminmo]

You show -12 connected to gnd??? You need a current limiting method unless the motor coil voltage is 12V.

[boozeboy]

Originally Posted by pminmo You show -12 connected to gnd??? You need a current limiting method unless the motor coil voltage is 12V.

OOPS. That was a typo. I am using 12+ and ground. Not -12volts.

[pminmo]

you still need current limiting unless the motor coil is 12V

[boozeboy]

Originally Posted by pminmo you still need current limiting unless the motor coil is 12V

It is 12volts but it is not plugged in circuit right now. The only thing that is is the 12volt relays. Thanks for your help.

[kreutz]

Originally Posted by boozeboy Hello. I have built a simple stepper circuit that also has three relays that draw 60 or 70 ma as far as I remember. I switched on the 12v power supply and the uln2003a blows in a matter of a few seconds. I do not have the stepper motor wired in yet and the parallel cable is not plugged in yet either. The relays are the only thing wired up right now. There are no shorts and everything looks great. What have I done wrong? I am totally stumped; please help.

Solder a UF4004 (or UF4007) diode across each relay's terminals, all cathodes to the respective +12volt terminal. Use pulldown resistors at the input terminals of the ULN2003A if they are not connected to the LPT port.

[pminmo]

Originally Posted by kreutz Solder a UF4004 (or UF4007) diode across each relay's terminals, all cathodes to the respective +12volt terminal.

The uln has built in clamping diodes.

[boozeboy]

Am I having trouble because the parallel cable is disconnected? I am worried about frying my parallel port. The uln2003 chips are cheap but the only ones I have on hand are from old dot printers. I am running out of printers to smash.

[kreutz]

Originally Posted by pminmo The uln has built in clamping diodes.

The built in clamping diodes are good up to 50 volts transients only! inherent clamping reverse current is only 100 uA max. Depending on the inductance of the relays, and the ULN2003A switching OFF time, there could be a lot more than 50Volts during the diode reverse recovery time period, adding a fast 1A diode in parallel will protect the internal clamping diode.

[pminmo]

Disconnect the relays and see if you have a problem

[boozeboy]

Originally Posted by kreutz The built in clamping diodes are good up to 50 volts transients only! inherent clamping reverse current is only 100 uA max. Depending on the inductance of the relays, and the ULN2003A switching OFF time, there could be a lot more than 50Volts during the diode reverse recovery time period, adding a fast 1A diode in parallel will protect the internal clamping diode.

Ok. I already read something about this and was planning to do it. But .... with the parallel cable unplugged all pins should go high and the relays should be all on. They have not be switched off and the chip still overheats and blows.

[kreutz]

Originally Posted by boozeboy Am I having trouble because the parallel cable is disconnected? I am worried about frying my parallel port. The uln2003 chips are cheap but the only ones I have on hand are from old dot printers. I am running out of printers to smash.

You shouldn't leave the inputs floating, they could be changing status quickly due to noise and leakage currents, that will increase the power consumption of the chip. The best solution is adding pull down resistors (4.7K to 10K), so if the cable gets disconnected the outputs will go to High impedance and no currents will flow on the loads.