# Driving LEDs with constant voltage

• posted

I have some 7-segment displays I'd like to drive from a uP output...the segments have a common cathode.

5 volt digital outputs from the uP are available, and the red LED segments have a forward voltage of around 2.something volts. Current-limiting resistor networks are annoying..so I was thinking of just connecting the display cathodes to one output per digit through a TL431 with its REF and anode connected to give 2.5, basically using it as a zener to drop the voltage, and switch those outputs to scan the multiplexed digits.
• posted

Den torsdag den 21. juli 2016 kl. 00.28.51 UTC+2 skrev bitrex:

constant current and scan the segments too?

• posted

Plain old LEDs don't like constant voltage -- if you find a detailed enough data sheet you'll see that the "something" in 2.something volts is very temperature sensitive, and that the current is very voltage sensitive. The "something" will also vary from diode to diode.

So if you don't mind something that won't light up at cold because there's not enough voltage, and will make the whole circuit severely unhappy at hot because there's way too much current -- sure! Go for it!

Personally, I'd just accept the eight resistors needed in a multiplexed LED display as the cost of doing business, and decide to be happy.

```--
Tim Wescott
Wescott Design Services ```
• posted

What would be the way to do constant current for a segment? Since all the segments are connected to a common cathode the required current is going to be different, depending on how many segments are illuminated...

• posted

But you can scan both the cathodes and the anodes so each LED is driven separately. Then a single current source does the job. Considering how many transistors it would take to switch the current it seems like it would be easier to have a current source for each digit or each segment (whichever is less).

```--

Rick C```
• posted

Ah, right, that's a good idea.

The TL431 + transistor makes a good current source, I could use a uC pin per digit connected to "Vin" in the following schematic to enable/disable it:

• posted

The current source solution will probably work since the digits are common cathode, but I think I'll try constant voltage just for fun...I wonder how cold/hot it'll have to get before the display starts malfunctioning.

• posted

LEDs are pretty resistive in the normal lit region. I've driven them pretty close to "voltage mode"[*] and it worked quite well.

[*] 3.something volt LEDs with a 10-ohm series resistor, mainly because I was chicken.

It's not that bad. Really. Light output won't be very stable but it works.

Resistors require more headroom, too.

• posted

It'll work with common-anode parts, as well. Just use the other sex. Try constant voltage. You might be surprised.

• posted

Since the uP knows what combination of segments are going to be lit, just use PWM to regulate the average current. If only "1" is displayed, use a low duty cycle, if "8" is displayed, the PWM duty cycle would be close to 100 %.

• posted

I did that once, but it was with 74LS on the breadboard. So, no big deal for output current; the I_OH is roughly a 100 ohm resistor to begin with.

(The ribbon leaving on the right side, wraps under and goes to the 8 digit matrix. It's just wired between a pair of 74LS374s. Or maybe they were

273s, I forget.)

A digit was pretty "hot" (over 20mA DC, I think) if the matrix scanning routine froze, and intensity varied somewhat with the number of segments-per-digit lit. Don't think I would recommend it for production.

I'd recommend it even less with CMOS. For an MCU, you may quite easily exceed the current-per-pin, or per-IO-bank, or per-supply-pin rating (usually something like 50 or 100mA, so not even a half dozen LEDs at rated current). It's really quite desirable, and easy, to plop in a proper driver (like a ULNx thingy, but also, you need a high-side one), or an LED driver proper, which can be SPI or I2C and offload a lot of activity (like doing the matrix scan automatically).

TL431 is kind of slow, so don't expect it to respond within a microsecond. You may see some "drool" or shadows between characters, unless you reserve enough dead time for things to settle first. Keep that in mind, if you want to use it like this. Even with the speed issue, it's probably better than a

2.5V zener (which won't be "2.5V" at most currents!).

Tim

```--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design ```
• posted

My concern there is that if the uP locks up due to a software error, the outputs could latch HIGH, and if a bootloader is being used to launch the firmware there's very likely enough time between the watchdog timer resetting the uP and the code restarting for the segments to be damaged...

• posted

The problem when I was testing it last night is that when connecting it up, the LED immediately starts drawing too much current, and the TL431 anode voltage doesn't ever rise high enough for it to drop into regulation...

• posted

CV led drive works reasonably well, but you've got to get the voltage about right. Most LEDs are fairly ohmic at reasonable operating currents. The ohmic and the junction drop components have opposite tempcos.

uP CMOS ports have impedances, too, approaching constant-current away from the rails.

```--
John Larkin         Highland Technology, Inc

lunatic fringe electronics```
• posted

Just connect some LEDs to power supplies and see.

People here theorize a lot and don't experiment enough.

```--
John Larkin         Highland Technology, Inc

lunatic fringe electronics```
• posted

A better gadget would be a negative voltage regulator, LM337 maybe, pulling the common cathode down towards ground. Adjust the cathode voltage until things look nice.

```--
John Larkin         Highland Technology, Inc

lunatic fringe electronics```
• posted

While this might be an issue during system development, I do not understand why a product delivered to the customer would have such software or hardware bugs. _Period_.

• posted

Yes, development was mainly my concern. LED displays aren't that expensive but I'm not made of money (and time) here...

• posted

Interesting. I think I have a few of those in TO-92, will give it a try

• posted

Sadly, there are a couple pieces of music production hardware I've used that run embedded Linux internally, and still have showstopper bugs in them, multiple known ways you can crash the hardware from the UI, and still haven't been fixed ~5 years and three or four software revisions later.

It's kind of criminal that products with bugs like this were even made available for sale...

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