Driving large blue 7 segment display

A

AllyCat

Senior Member
Hi Paul,

The NPN transistors in #27 have the 560 ohm resistors in their emitters, but the ULN has all the emitters commoned to ground, so you must put the resistors in the collectors.

Normally a "Darlington" has two base-emitter diodes to ground which would need an input resistor, but I see from the data sheet (foot of page 4) that 2k7 resistors are included within the package, so no, you don't need to add them externally (except for the optional PICaxe "safety" resistors to help protect against wiring errors or "screwdriver" short-circuits).

That power supply looks much more suitable than the previous ebay offering, but I'm not sure I'd use it at a full 10 Amps. The efficiency is probably around 90%, and I wouldn't be very happy dissipating 12+ watts in what appears to be a rather small plastics box. Also, the "specification plate" is rather "suspect". I believe that the stated input current should ba a maximum, but at 100 volts that could only put 100 watts in, so you're not going to get 120 watts out! :confused:

Cheers, Alan.
 
P

PaulRB

Senior Member
AllyCat said:
but the ULN has all the emitters commoned to ground, so you must put the resistors in the collectors.
Click to expand...
Yes, understood, that was what I meant to say when I said "darlington output to pnp's base via 510R resistor" in #37.

AllyCat said:
2k7 resistors are included within the package, so no, you don't need to add them externally
Click to expand...
Excellent, I will give it a try then.

AllyCat said:
That power supply looks much more suitable than the previous ebay offering, but I'm not sure I'd use it at a full 10 Amps. The efficiency is probably around 90%, and I wouldn't be very happy dissipating 12+ watts in what appears to be a rather small plastics box. Also, the "specification plate" is rather "suspect". I believe that the stated input current should ba a maximum, but at 100 volts that could only put 100 watts in, so you're not going to get 120 watts out! :confused:
Click to expand...
Err... was that comment referring to another thread on the forum? I haven't been looking for PSUs on eBay. But I did buy the 7-seg displays on eBay.

Cheers,

Paul

UPDATE: Yes! Works perfectly with the darlingtons instead of the npn transistors. That leaves only 1 darlington unused, and two npns go back in the box for another project, and simplifies the schematic and final build. Thanks for the reassurance Alan.
 
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