Testing displays.

[Sully008]

Hello all,

Does anybody know if you can test the displays while they are off the board? I haven't tried yet, but I've got several displays that look like all the bonds are intact, but does it need to be on a board for it to light all the digits up?

[Synchroserious]

It would be a big project to build a tester like i use.These are custom made displays and need a custom built tester.Not only do you need to test that all segments and digits work at once at equal brightness you also need to test each segment individualy to confirm none of the wire bonds are shorted to another nearby wire bond.

[rewolf]

To build a 100% tester that can also detect shorts is a bigger task, but as a start you can test the segments individually in a very simply way:
Take a 3V battery, solder one wire to its negative end, a 1kOhm resistor to its positive, and another wire to the other end of the resistor.

Then contact a LED Cathode line (there are probably 4 of them) with the tip of the negative wire.
Contact the positive wire tip to one Anode after the other (7 or 8 of them) - segment by segment of one digit should light up.
Repeat this with the negative wire on the next Cathode line.

The 1kOhm resistor makes sure that nothing happens even if you short something. The 3V ensures you won't damage the LED chips if you touch them with wrong polarity.

[Sully008]

That sounds like a pretty easy method to implement. Borrowing this image:



I'm guessing the 4 circular points along the bottom would be the LED cathode line? And on the backside the 13 contact points are the negatives?

To build a 100% tester that can also detect shorts is a bigger task, but as a start you can test the segments individually in a very simply way:
Take a 3V battery, solder one wire to its negative end, a 1kOhm resistor to its positive, and another wire to the other end of the resistor.

Then contact a LED Cathode line (there are probably 4 of them) with the tip of the negative wire.
Contact the positive wire tip to one Anode after the other (7 or 8 of them) - segment by segment of one digit should light up.
Repeat this with the negative wire on the next Cathode line.

The 1kOhm resistor makes sure that nothing happens even if you short something. The 3V ensures you won't damage the LED chips if you touch them with wrong polarity.

[rewolf]

Hm, not sure what you mean with "4 circular points along the bottom".

The horizontal traces connecting the corresponding segments of all four digits with wire bonds are the positive (anode) lines. I can see 5 (and a half) of these on the top side, but there must be at 2 (and a half) more on the bottom side.

Then, there should be one connection per digit (i.e. four altogether) that goes to a separate trace and nowhere else: that's the negative (cathode). It could simply be the copper pad the digit is soldered onto, accessible from the rear side via a copper-plated hole (a so-called "via").

I don't know how the colon is connected, and there could be an exception for the leftmost digit's segments...

With a magnifier it 's quite easy to see where the wires from the segments are connected to - the photo above isn't clean enough to tell for sure.

[Sully008]

Sorry, I was referring to the circular ends of the horizontal traces. So those traces are the +ves, on the backside of the display @ the bottom, there are 13 pads that solder to the main circuit board. I guess those will be the -ves?

Hm, not sure what you mean with "4 circular points along the bottom".

The horizontal traces connecting the corresponding segments of all four digits with wire bonds are the positive (anode) lines. I can see 5 (and a half) of these on the top side, but there must be at 2 (and a half) more on the bottom side.

Then, there should be one connection per digit (i.e. four altogether) that goes to a separate trace and nowhere else: that's the negative (cathode). It could simply be the copper pad the digit is soldered onto, accessible from the rear side via a copper-plated hole (a so-called "via").

I don't know how the colon is connected, and there could be an exception for the leftmost digit's segments...

With a magnifier it 's quite easy to see where the wires from the segments are connected to - the photo above isn't clean enough to tell for sure.

[rewolf]

Sorry, I was referring to the circular ends of the horizontal traces. So those traces are the +ves, on the backside of the display @ the bottom, there are 13 pads that solder to the main circuit board. I guess those will be the -ves?

Sorry, no.
Four of the 13 pads are the digits' cathodes (minus), the rest are segment anodes (plus) - quite probably. The task is to find out which are the cathodes and which are the anodes by following the wirebonds and traces. With a good image of the display's front and rear side (no goo, and wirebonds visible) I could probably help you more. Without that I'm afraid I can't do any more because I've never seen such a display.

[Sully008]

Okay, I'll see what I can do about getting better pictures. I've got a bunch of displays here. My track record with taking good pictures is not very good.

Sorry, no.
Four of the 13 pads are the digits' cathodes (minus), the rest are segment anodes (plus) - quite probably. The task is to find out which are the cathodes and which are the anodes by following the wirebonds and traces. With a good image of the display's front and rear side (no goo, and wirebonds visible) I could probably help you more. Without that I'm afraid I can't do any more because I've never seen such a display.