ULN2803A failures often while ON

[Murugesh_89]

Hi,
I am triggering a inductor coil with 24V. The current passing through the coil is 360mA ( measured by keeping coil ON for 2 seconds ). I am doing this with ULN2803A. Also i keep the coil ON for 200ms and OFF for 100ms and this cycle repeat for 3 times.

I am controlling the input of ULN2803A by using the direct output from shift register 74hc595 supplied with 5V.

But some times my ULN failed and overheated and there by showing short circuit between its outputs with ground.

Why it is behaving like this?

Thanks,
Murugesh

 

[deepeshmishra]

Did you connected pin no 10 to VCC as you are with inductive load

 

[zuisti]

Did you connected pin no 10 to VCC (?) as you are with inductive load

Yes, but:
VCC is the +5v, connect the ULN2803A's pin 10 to your +24v.

 

[betwixt]

Yes, always connect the catch diodes to the same supply as the load otherwise you create a current path to VCC which could be disastrous!

Murugesh, tell us is this ULN2803A also driving other loads? Each output has a maximum sink current of 500mA but if you are using all of them at high current simultaneously you may have a heat dissipation problem.

Brian.

 

[darla1]

Hi murugesh

plz check the voltage spikes are coming while ON/OFF

thanks

 

[Murugesh_89]

Attached a image here. Hope this would clarifies the above doubts.

Here for sample i showed single output. But all the outputs of uln are connected to a different coils in the same manner. But only one coil gets actuated at a time.

Since each collector in the uln can capable of holding continuous current of 500mA, how does simultaneous triggering causes heat dissipation issues?

 

[betwixt]

You don't need those resistors from the inputs to ground, the ULN has internal ones.

My point about driving other loads is this:

There are two limits to observe, one is the maximum current allowed into each output pin, the other is the total package dissipation.
Your output current is 360mA and at that current the output saturation voltage is specifed as typically 1.6V so the power dissipated is 1.6 * 0.5 = 0.8W
The maximum dissipation for the whole device is 2.25W so if you had been running more than 2 loads at the same time you would exceed the device rating.

Brian.

 

[chuckey]

LVV-24V ? Polarity?
Frank