Calculating source capability of Darlington pair

[j777]

Hi,

I have a little circuit board that drives some solenoids that keeps going bad. It belongs to a scale that is made by a company that is now bankrupt so they aren't much help. They used a photocoupler (Sharp PC844 - IRED coupled to phototransistor) and an NPN transistor (2N4401) to create a Darlington pair. I think it's the 2N4401 that keeps going bad but it may be both the photocoupler and 2N4401. The first thing that I noticed that might be causing some of the problems is the fact that they are using an unregulated 12VDC supply. Maybe there are significant voltage spikes on the supply that are damaging the transistors...?? The other thought I had was that maybe the solenoids require too much current to be sourced by the Darlington pair. The solenoids require 12VDC and 1.8W. How do you calculate the current source capability of a Darlington pair?

Thanks

 

[berkeman]

150mA is a bit much for sustained current in the 2N4401. What package is it in? Does it get hot? Is there a catch diode across the solenoid coil (anode on the collector of the 2N4401, cathode to 12V)? What is the value of the resistor from the 2N4401 base (and photodiode collector) up to 12V?

 

[j777]

The 2N4401 is in a TO-92 package. They do get warm. There isn't a catch diode and there isn't a resistor from the 2N4401's base to 12V. The circuit is as follows:

Phototransistor collector connected to 12V and emitter connected to 2N4401 base. 2N4401 collector connected to 12V and emitter connected to solenoid V+. There is a diode (1N4005) with cathode connected to 2N4401 emitter and anode connected to GND.

 

[berkeman]

How warm does it get? What is the typical coil voltage (measured)? Any chance the catch diode has gone bad? What is the catch diode part number -- is it fast and fairly beefy? You could put a small value capacitor across the catch diode if it is not fast enough to catch the spike. Have you watched the turn-on and turn-off transients with an oscilloscope?

 

[j777]

The diode is a 1N4005 and it is warm to the touch (you can hold your finger on it). The coil voltage with the unregulated 12VDC supply is 14.89V and 11.35V with the regulated 12VDC supply. I haven't looked at things with a scope yet. I've used a darlington pair to sink current for some solenoids before but never to source current. Isn't it more common to sink current in such a circuit? The diode seems beefy enough. Could the unregulated supply they are using make the circuit more prone to voltage spikes?

 

[berkeman]

That's weird that the catch diode is warm to the touch. What is the ON/OFF timing for this circuit? Is there a way you can test the catch diode alone?

And yes, I am also more used to driving solenoids on the low side. You might be able to just convert it to a low-side switch with some rewiring.

 

[j777]

I'm sorry...I meant that the 2N4401 is warm to the touch.

Do you think the unregulated 12VDC they are using might be a problem?

 

[berkeman]

I'm sorry...I meant that the 2N4401 is warm to the touch.

Do you think the unregulated 12VDC they are using might be a problem?

Not offhand. I don't remember the exact BVCEO of the 2N4401, but I'd guess in the 40V range. If you can do some poking around with an oscilloscope, that will probably turn something up.