PWM multiple 7-segment LEDs

[richard]

i see a lot of smoke in your future

apart from the upside down mosfet

pic1f946 max chip current 95mA although each pin can sink or source 25mA equally well
@ 42 leds [or 48] if you use dp's thats 2.2mA per led 2 Full bright , the 270 ohm seg resistors are not going to do that for you

pic1f1947 max chip current 120mA [the industrial ver can sink more] although each pin can sink or source 50mA equally well
@ 42 leds [or 48] if you use dp's thats 2.8mA per led 2 Full bright , the 270 ohm seg resistors are not going to do that for you

[Ioannis]

The sink is always better choice than sourcing.

Then N-channel MosFets along with NPN transistors are much better than P equivalent ones and more choices available.

And finally, even if a PIC can source or sink 20, 30 or 50mA per pin, there is a maximum TOTAL dissipation for the whole chip, so that you cannot source at the same time on all pins a max of 50mA on each pin!

You will melt the chip!

So, yes. I am for the sink topology too.

Ioannis

[Ioannis]

Now, if you look at datasheet, p236, for the 946 chip.

Output Low is 0.6 Volt at only 8.5mA but,

Output High Vdd-0.7 Volt at 3.0mA

So, yes. It is better to sink than source.

Ioannis

[richard]

for my money i would do it the traditional way or if multiplexed display is not bright enough use a bunch of tpic6b595's



note schema is not complete but you get the idea, i was going to try it in proteus then i remembered how crap proteus is at simulating multiplexing
so i gave up

[tumbleweed]

With that many displays I assumed they would be multiplexed as in richard's post #17.
Surely a lot less supply current, and you wouldn't need nearly as many pic's to drive them... maybe get away with one or two 44-pin packages.

[Demon]

for my money i would do it the traditional way or if multiplexed display is not bright enough use a bunch of tpic6b595's...

Thanks, I'm off to google that chip (no idea what it is). I've also seen talk about using a 74HC595 (never met that creature either).


EDIT: Google says TPIC6B595 can sink up to 150mA at 50V. JLCPCB carries them at $1.54.

Google says 74HC595 has a 70mA total limit, so there goes that chip. JLCPCB has them at $0.11 too.

[Ioannis]

The 74HC595 is a shift register. How can this help on 7-seg LED's ?

Dedicated chips like Maxims or the TPIC6B595 I think is the way to go.

Ioannis

[Demon]

The 74HC595 is a shift register. How can this help on 7-seg LED's ? ...

Some people use them to drive 7-segment LEDs. They don't have digit control, only segment control, so you need one per digit.




It turns out my supplier for 7-segment LEDs also print and assembles PCBs. It wouldn't be cost effective to ship my LEDs back, so I might very well do the jump to the MAX7219 if I have to buy more LEDs.

I'm keeping all my options open for the time being, but the MAX7219 does offer one really neat feature; an intensity control. That TPIC6B595 is a close 2nd for common anode LEDs.

[Demon]

(I'll move the PWM LED posts to an off-topic thread in a few minutes to keep this about Chuck's code example. I just want to put down some comments first.)

... apart from the upside down mosfet ...

I was hoping you wouldn't see that. It was late and I went to bed.

i see a lot of smoke in your future ...

Usually I'd agree with you, and yes I'm aware of the current limitations on PICs and pins.

But ... LEDs are special. You can get different performance out of them via PWM. This is identical LEDs:

- left, 5V rail with 270ohm resistor.
- right, PWM without resistor.




I can get identical brightness with 2.5mA less. If I add a 270 ohm resistor to PWM, I get 1/2 the current.




Yeah, I thought I could improve this even further down to 2mA but that isn't gonna happen. The LED won't be bright enough.

And yes, I've been playing with using transistors/MOSFETs and I'll be going back that way again.




I had dropped that idea cause HPWM has lower limits on Frequencies and I didn't like not have total control (64 MHz is 3905Hz).

But I just realized I could pulse the devices using a timer interrupt; switching between different ON-time and OFF-time.