driving a dual alphanumeric led display with uC w/o transist

I prototyped a circuit which uses a pic16lf628A uC to drive an alphanumeric LED display.

The PIC uC can't source/sink the amount of current to light up more then like 4 segments at full brightness.

Two questions...
1) Does it damage the uC to attempt to drive more current then it is supposed to? The output voltage on the output pins drops from 3.0V (which is VDD) to about 2.3v. This is because the PIC output pin isn't an ideal voltage source so their is a voltage drop across output resistance of the pic. Therefore, since i'm limited by the amount of current the PIC can supply/source i could eliminate the current limiting resistors since the voltage is already dropped to about 2.2V which is the operating voltage of the LEDs. Over time will this eventually ruin the pic?

2) The PICs can supply/sink about 80mA on PortB. Do you know of any other uC which are in the same price range as PICs which can supply / source more current?

I'm trying to make a circuit which goes on a keychain and runs off a lithium coin battery. I intend to make about 50 of them so the little parts that are neccessary for assembly is important. Therefore i don't want to put 7 transistors on the board.

Thanks!

Re: driving a dual alphanumeric led display with uC w/o tran

DaKandEKid said:

I prototyped a circuit which uses a pic16lf628A uC to drive an alphanumeric LED display.

The PIC uC can't source/sink the amount of current to light up more then like 4 segments at full brightness.

Two questions...
1) Does it damage the uC to attempt to drive more current then it is supposed to? The output voltage on the output pins drops from 3.0V (which is VDD) to about 2.3v. This is because the PIC output pin isn't an ideal voltage source so their is a voltage drop across output resistance of the pic. Therefore, since i'm limited by the amount of current the PIC can supply/source i could eliminate the current limiting resistors since the voltage is already dropped to about 2.2V which is the operating voltage of the LEDs. Over time will this eventually ruin the pic?

Click to expand...

It's certainly very bad practice, and operating the PIC outside of it's specifications, I would expect it to reduce the life of the PIC - but I've no idea how long it might last, you could only try it and see?.

2) The PICs can supply/sink about 80mA on PortB. Do you know of any other uC which are in the same price range as PICs which can supply / source more current?

I'm trying to make a circuit which goes on a keychain and runs off a lithium coin battery. I intend to make about 50 of them so the little parts that are neccessary for assembly is important. Therefore i don't want to put 7 transistors on the board.

Click to expand...

How about using surfacemount driver transistors?, or reducing the current per segment until it's within the PIC's capabilities? - as it is the button cell won't last very long!.

It's certainly very bad practice, and operating the PIC outside of it's specifications, I would expect it to reduce the life of the PIC - but I've no idea how long it might last, you could only try it and see?.

Click to expand...

Its unclear whether it's driving it out of it's specs. It says it can sink.. it can source so it's sinking/sourcing it maximum amount. I dunno enough about the internals of a uC to say what limits this.

How about using surfacemount driver transistors?, or reducing the current per segment until it's within the PIC's capabilities? - as it is the button cell won't last very long!.

Click to expand...

Current per segment isn't bright enough and the display is only used for 10 seconds at a time like once every day. So, the battery should last for a few years considering the majority of the time the pic is in sleeping.

I did some research on uC's with high current output and there are two made by microchip but they only have 9-11 output pins which isn't enough for me. Apparently Motorolla, and STmicroelectronics also make some 8-bit uC's with high current (led direct drive) i/o pins. Anyone who has experience with PICs recommend a vendor which is easy to migrate to. I love microchips documentation and sample policy and community support.
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