Darlington Transistors in place of ULN2803A

[LyleCaldwell]

What darlington transistors would be good for use in place of a ULN2803A?  The reason I ask is that I don't need the number of Inputs and Outputs the ULN2803A offers, but I need the same functionality.  Thanks.

[davebungo]

Google search for "Darlington Transistor" returned this:
http://www.chipcatalog.com/Cat/722.htm

[LyleCaldwell]

Thanks, Dave.  Now if all Darlingtons were the same, I'd be good to go.

[Peter Snowberg]

There isn't really anything special about the ULN2803 internal circuit. IIRC, if you Google the datasheet you'll see two Darlington connected transistors, three resistors, and a reverse biased diode. The 1st resistor limits the base current while the 2nd and 3rd resistors make really sure the transistors are off when you want them off. The diode is there to eat the voltage spike produced when power is cut to a coil like a relay or stepper motor.

How much current do you need and what are you driving the 2803 with? I use 2N4401s as my standard generic transistor and they'll switch 600mA so they're a really good replacement candidate. Any NPN big enough to switch your load will work. The other perennial favorites seem to be 2N3904 and 2N2222. You can use a wide variety of values for the resistors. The idea is to make sure the transistors are off with no input and on with the input you're going to give them. If you're switching things with a CMOS microcontroller you can figure +5V for on and 0V for off. When on, we want to make sure the first transistor is saturated and that will happen at about 1V for most small signal NPNs. Looking at the resistors as a divider, if we have +5V across the string and we use 4.7K, 10K, and 4.7K as the three resistors, we'll have roughly 2.5V on the base-emitter junction of the 1st transistor. We're solidly on at that point. That's a simplified view, but it works.  

Oh yes, and the diode. Use any 1N400x or better yet a 1N5818.

If you need more current, a Darlington made from a 2N4401 & a TIP31 works very well.

[LyleCaldwell]

Thanks Peter.  This is for use between a PIC and relays.  5V on, 600ma is more than plenty of current handling.  Thanks very much.

There are things I know very well, and things I'm completely new at.  MIDI switching is something I'm completely new at.  But soon I should know it very well too (or else the headaches are wasted).  I appreciate your explanations a ton.

[Peter Snowberg]

You're welcome.

If you're switching relays, I wuold just use a single 2N4401 with a 10K in series with the base and the reverse diode across the coil. No need for a darlington there. 

[LyleCaldwell]

OK, I'll give it a try.  The schematic I'm borrowing from uses the ULN2803A, so I thought I had to use the Darlington approach.  But so far your advice has never released the magic smoke, so I'll do that.  Thanks.

[LyleCaldwell]

Peter,
You mean like this?

[Peter Snowberg]

Flip the diode and add +5 to the top of the coil and you have it. 

I've used a lot of ULN2803s in the past, mostly as relay or uni-polar stepper motor drivers. They're convienient because they have the diodes built in and the Darlington design gave solid on-off drive with TTL-LS voltage levels for driving.

CMOS makes things much easier. These days I don't see any real benefit to the UNL2803 other than reduced part count. There are better chips now and if you're driving only a couple devices, I would go discerte transistor.

From a quick Google search for "pic i/o pin relay npn":

http://www.rentron.com/Files/Stamp/pc-relay4.gif   

[LyleCaldwell]

Peter,
I put up one version earlier, then put up the second version with the +VCC on top of the coils and the flipped diodes.  Then the server went haywire - I had to hit reload for it to display my second image.  From what you're saying I think you're seeing my first version and the current version should be correct.  But thanks for the confirmation and the link.