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Topic: 2.3" 16 segment common anode LEDs, HT16K33 and ULN2803 (Read 1 time) previous topic - next topic

MitchSF

I'm about to breadboard a circuit to drive six displays using these components. Cathodes will be connected to the ULN2803 and it will be driven by the HT16K33 in a 16x6 multiplex configuration.

I was researching the way to calculate the series resistor value for each multiplexed row segment when I realized that anywhere from 0-6 segments in each row will be on at the same time, so I'm stuck. Can you suggest a better way to do this? I want to stay with the HT16K33.

Thanks in advance.

MitchSF

I'll add that these displays use four LEDs for ten segments, each with two series LEDs in parallel, and five shorter segments use two LEDs in series.

I guess I could use 16 individual resistors on each of the displays segments for a total of 96 resistors, but I'm not sure how that would work with multiplexing and there must be a simpler way.

PaulRB

Post a clickable link to the data sheet or whatever technical detail you have on the displays please.

Why do you want to use uln2803?

MitchSF

Displays are common anode and I have some around, but if there is a better solution please let me know. I do want to stay with the HT16K33 because the software is working well. 

Grumpy_Mike

Quote
I was researching the way to calculate the series resistor value for each multiplexed row segment when I realized that anywhere from 0-6 segments in each row will be on at the same time, so I'm stuck.
Do don't multiplex it like that. Although how you plan to multiplex it is a bit of a mystery at the moment.

The resistors go in each cathode not in the anode.

MitchSF

Sorry, I see that I was not clear in my description.

The HT16K33 can't drive the big LED displays because they draw too much current. It handles the multiplexing and I have it working perfectly with smaller, .8" displays driving them directly.

I thought that using two, ULN2803s between the six, 16 segment displays and the HT16K33, would work. For that I'll need 96 (16 x 6 segments) resistors, one on each cathode. The other side of the resistors will be connected together for each segment to make the rows.

Will that work given that the displays are multiplexed? Is there a better way?

Thanks in advance for the help.

PaulRB

Do don't...
Did any of the bottles make it back from the supermarket unopened, Mike?

PaulRB

Quote
For that I'll need 96 (16 x 6 segments) resistors, one on each cathode.
No, just 16, I suspect.

Need to see your proposed schematic. And that link.

MitchSF


PaulRB

Is this similar? The below is common cathode, but your displays are common anode?

PaulRB

Quote
I have it working perfectly with smaller, .8" displays driving them directly.
Are those also common anode? I would have expected common cathode to be used with ht16k33.

MitchSF

The .8" displays I'm using are common cathode. They are connected directly to the HK16K33, and working perfectly.

Paul, the 2.3" displays are exactly as in the drawing you posted, but they are common anode. I'm working on a partial schematic now. I'll post it shortly. Thanks for the help.

PaulRB

I think you will need high-side drivers as well as the uln chips as the low-side drivers. Perhaps p-channel MOSFETS.

I am also concerned that the uln chip will drop too much voltage. If your supply is 5V (and maximum is 5.5V for the HT chip, so you can't go much higher anyway), the forward voltage of the segments will be at least 3.6V. The ULN chip could drop as much as 1.5V, leaving only 0.1V drop for your series resistors, which won't be enough, you need at least a volt.

Or...

Perhaps you could use a 6V supply for the displays, but still run the HT chip at 5V. Then you would have 1.1V to drop across the series resistors. But driving the p-channel MOSFETs would be tricky, you can't expose the COMM pins of the HT chip to >5V. But if you put another driver transistor in there, it would invert the signal...

MitchSF

I added a P-Channel MOSFET, but the connections are probably not correct. 12v is available and preferred because the blue displays require 6v. Please let me know if this is close to a working solution.  Thanks.

PaulRB



The MOSFET seems to be connecting the display anode to ground, should it not be 12V?

R2 should be removed.
R3 value is not critical, maybe 220R, just to limit current sunk by COM pin of the HT chip.
R1 will need to be different value for the segments with 1, 2 or 4 leds.

Pin 10 of the ULN chips does not need to be connected to 12V. I don't think it would do any harm, but is only needed for inductive loads, not for leds.

There is nothing to switch off the MOSFET. It needs its gate to be pulled up to 12V to switch off. But this can't be done with a simple resistor, I think, because that would expose the COM pin of the HT chip to 12V, way above its Vcc, which could damage it.

But I'm no expert on P-channel MOSFETs. Maybe Mike can advise (if he is no longer hung over).

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