I am trying to light up a LED dot matrix. I am using a Microchip uC, and extending the outputs with the 74HCT595. What I find though, is that the LEDS are very dim. (7 LEDS are driven by roughly 20mA).
Can someone please recommend how to get the LED's to glow brightly?
I cant source the high current 595 where I am.
I am trying to light up a LED dot matrix. I am using a Microchip uC, and extending the outputs with the 74HCT595. What I find though, is that the LEDS are very dim. (7 LEDS are driven by roughly 20mA).
Can someone please recommend how to get the LED's to glow brightly?
I cant source the high current 595 where I am.
Assuming you're multiplexing the LED's?, 20mA shared between 7 leds is less than 3mA each! - you need to provide 140mA for the 7 leds to give an average of 20mA to each one.
If you're using the 'HC595 to 'sink' current to a column (common cathode), you might consider switching to a Micrel MIC5841 IC instead... It's a serial to parallel 8-bit 500-ma sinking driver (example below)...
It might help if you tell us about your configuration... Is it several 7-segment displays or 5x7 or 8x8 matrices? Common cathode or command anode columns? Are you 'scanning' columns or rows? LED duty cycle?
As I said above, you've got to substantially increase the current, I would suggest driver transistors on both rows and columns (PNP to source, and NPN to sink) - a PIC won't handle the currents required either.
I suspect you will want to 'scan' the seven columns for an overall 14.3% LED duty cycle... Overdrive the LEDs for adaquate brightness... I suggest using seven N-Channel MOSFET row drivers (or NPN Darlington, or....) capable of sinking the 100 LEDs in each row and P-Channel MOSFET (or PNP transistor, or UDN2981A) 'source' drivers for the columns...
I suspect you realize you'll need a 3 or 4 amp power supply to drive the 100 LED rows in your twenty 5x7 displays...
If you need 100 drivers you need to think about segmenting the driver system. You will not have enough time to generate a flickerfree display with enough light.
I didn't make it a recommendation, it is just a thought. It was something I thought of to make the LED's brighter.
I hope no one used my thought as written.
If you do intend to use it, make sure there is a resistor in series with the arrangement or the LED(s) will malfunction.
and why can't you treat them like resistors?
Isn't that why two LED's in parallel light brighter than two LED's in series?
I mean, think about it. Let's pretend that each LED has one ohm of resistance. Two in series makes 2 ohms, which means current is in half. Two LED's in parallel will be about 1/2 an ohm. this means current is doubled.
the current is porportional to light intensity. That's why LED's can be treated like resistors. Just make sure you don't make the assumption that an LED has a LARGE resistance. In fact, it is safer to make the assumption that an LED has a SMALL resistance (under 1 ohm).
They don't, and you SHOULDN'T place LED's in parallel.
I mean, think about it. Let's pretend that each LED has one ohm of resistance. Two in series makes 2 ohms, which means current is in half. Two LED's in parallel will be about 1/2 an ohm. this means current is doubled.
the current is porportional to light intensity. That's why LED's can be treated like resistors. Just make sure you don't make the assumption that an LED has a LARGE resistance. In fact, it is safer to make the assumption that an LED has a SMALL resistance (under 1 ohm).
IT'S SAFER TO TREAT AN LED AS AN LED, UNDER NO CIRCUMSTANCE EVER TREAT THEM AS A RESISTOR - THE ONLY RESEMBLANCE BETWEEN THEM IS THE NUMBER OF WIRES!.
Doubling the current WILL increase the brightness of an LED (obviously, and is what I've said all along), but you do that by decreasing the value of it's current limiting resistor. Adding another LED in parallel (which you SHOULDN'T DO!) would split the available current between the two LED's, giving the same brightness split (unevenly, which is why you shouldn't do it!) across the two LED's.
with the uln2803, what do I do with the COM connection?
the one example I found, was using the 2803 to drive relays, and they had the COM connected to V+ via a 30v zener... I'm planning on driving LEDs, like the original poster... so do I need to work about the kick-back protection (that COM and the internal diodes) are providing?
I have connected the ULN2803 into the circuit: each output of the 74HCT595 I connect to the input of the 2803. I connect the GRND of the 2803 but not the COM.
I put in a +5V on the 595, and measure what is comming off the 2803. I see about 500mV on the output of the 2803. (Clearly I cannot drive a single LED using this).
What am I doing wrong? How should I be connectin the 2803 to get the 500mA current I need to drive the LEDs?