Hi,
I'm new here but not at all new to electronics. But here is one I just can't get my head around.
I'm looking for a solution to drive 3 inch 7-segment common cathode LED displays with a MAX7219 LED driver IC.
The displays require 9V 20mA per segment. The 20mA is well within the MAX7219's max segment current of 40mA.
However, the MAX only provides 5V to the segments. I've tried this and the only segment that comes on is the decimal point (through a zener diode) as that requires only 4V.
I've done some searches on the internet and I've found several ways to approach this problem but they all don't really apply to my situation.
I've found a MAXIM application note that describes driving displays with higher voltage but they require common anode displays. I have already bought the common cathode displays and wouldn't want to spend more money on common anode displays.
http://www.maxim-ic.com/app-notes/index.mvp/id/1196
The other thing I found is in the MAX7219 datasheet on page 12. It uses the MAX394 quad analogue switch IC to drive a MOSFET per digit. 2 problems with that solution are that a) it seems to be for higher current, not higher voltage (supply voltage still at 5V) and b) I can only find the MAX394 as SMD and I don't have SMD soldering equipment.
http://datasheets.maxim-ic.com/en/ds/MAX7219-MAX7221.pdf
I've been thinking about this for a while but just can't work it out. If this was logic outputs I'd probably find a way. But since both anode and cathode of each segment are connected to the MAX directly in a matrix kind of way I just can't get my head around this.
The best thing I could come up with is to use a matrix of 64 opto-isolators. But that would be slightly exaggerated, I hope.
Does anyone have any further ideas, please?
Chris
I'm new here but not at all new to electronics. But here is one I just can't get my head around.
I'm looking for a solution to drive 3 inch 7-segment common cathode LED displays with a MAX7219 LED driver IC.
The displays require 9V 20mA per segment. The 20mA is well within the MAX7219's max segment current of 40mA.
However, the MAX only provides 5V to the segments. I've tried this and the only segment that comes on is the decimal point (through a zener diode) as that requires only 4V.
I've done some searches on the internet and I've found several ways to approach this problem but they all don't really apply to my situation.
I've found a MAXIM application note that describes driving displays with higher voltage but they require common anode displays. I have already bought the common cathode displays and wouldn't want to spend more money on common anode displays.
http://www.maxim-ic.com/app-notes/index.mvp/id/1196
The other thing I found is in the MAX7219 datasheet on page 12. It uses the MAX394 quad analogue switch IC to drive a MOSFET per digit. 2 problems with that solution are that a) it seems to be for higher current, not higher voltage (supply voltage still at 5V) and b) I can only find the MAX394 as SMD and I don't have SMD soldering equipment.
http://datasheets.maxim-ic.com/en/ds/MAX7219-MAX7221.pdf
I've been thinking about this for a while but just can't work it out. If this was logic outputs I'd probably find a way. But since both anode and cathode of each segment are connected to the MAX directly in a matrix kind of way I just can't get my head around this.
The best thing I could come up with is to use a matrix of 64 opto-isolators. But that would be slightly exaggerated, I hope.
Does anyone have any further ideas, please?
Chris