LED Matrix - 12V

A "new fashioned" televisor, using an Arduino to drive the motor and display.

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Re: LED Matrix - 12V

Postby Andrew Davie » Mon Jun 12, 2017 8:04 pm

Steve Anderson wrote:One thing I would like to give a 'heads-up' on... you need to make sure the 12V supply is truly 12V.


Considering an adjustable DC-DC converter for this job.
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Re: LED Matrix - 12V

Postby Klaas Robers » Mon Jun 12, 2017 8:13 pm

Indeed, Steve is right. In some way you have to stabilize the light output of the LEDs. In the example of the current source, this source circuit is the stabilizer. In this case the "top" of the LED-chains can be connected to an unstabilized voltage that only has to be and stay "high enough".

But if you plan to run the LEDs PWM switched on a voltage with a rather low ohmic resistor in series, you need a stabilized voltage for it. Otherwise you will see all variations in the motor current also as brightness variations of your picture.

Measure the voltage of an adaptor. If a 12V DC adaptor gives an unloaded voltage of 12 volt, it will be stabilized. If the voltage is much higher, and only 12 volt when loaded with the indicated current, it isn't. Most heavy adapters are unstabilized, the lighter electronic adapters are stabilized. The heavy adpters contain a (heavy) transformer. That is old fashioned technology.

There is another way to see that a power adapter is stabilized. If the input voltage is defined as 100-240V AC, it is definately stabilized.
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Re: LED Matrix - 12V

Postby Andrew Davie » Mon Jun 12, 2017 8:28 pm

Klaas Robers wrote:Indeed, Steve is right. In some way you have to stabilize the light output of the LEDs. In the example of the current source, this source circuit is the stabilizer. In this case the "top" of the LED-chains can be connected to an unstabilized voltage that only has to be and stay "high enough".

But if you plan to run the LEDs PWM switched on a voltage with a rather low ohmic resistor in series, you need a stabilized voltage for it. Otherwise you will see all variations in the motor current also as brightness variations of your picture.

Measure the voltage of an adaptor. If a 12V DC adaptor gives an unloaded voltage of 12 volt, it will be stabilized. If the voltage is much higher, and only 12 volt when loaded with the indicated current, it isn't. Most heavy adapters are unstabilized, the lighter electronic adapters are stabilized. The heavy adpters contain a (heavy) transformer. That is old fashioned technology.

There is another way to see that a power adapter is stabilized. If the input voltage is defined as 100-240V AC, it is definately stabilized.


Thanks. Yes, understood. I already see this motor/brightness issue.
The solution seems to me to move towards a higher voltage input (say, 19V as already suggested to me) and use DC-DC regulator on the higher voltage and supply the LED array with a higher reugulated voltage, and 12V to the motor and maybe even 5V to the Arduino (although it is happy with 12). It's taken me a while to understand why this is better, but at least I now understand.
Interesting info on the adaptors, thanks.
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Re: LED Matrix - 12V

Postby Steve Anderson » Mon Jun 12, 2017 9:21 pm

Andrew Davie wrote:The difference being that the voltage changes don't affect the current nearly as much.

Exactly, in a circuit like this I prefer to reserve around 30% of the available voltage for the dropper resistor, or whatever you want to call it. Some may say less, others may say more. A 7812 regulator has tolerances, it's actual output voltage at 25 Celcius, is usually +/-0.25V, but mainly much better than this. Temperature effects, ambient as well as its own due to dissipation, and input voltage. In this case I'm being somewhat pedantic, you don't really have to worry about it. But it is there.

But if you are using a 12V supply on the LEDs as proposed, simply make sure 12V is 12V and stays 12V. A +/-0.25V difference won't upset things unduly.

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Re: LED Matrix - 12V

Postby Steve Anderson » Mon Jun 12, 2017 9:32 pm

Andrew Davie wrote:Considering an adjustable DC-DC converter for this job.

The output is 5V, is that what you want in the context that we're talking about a 12V supply?

Also note that most supplies of this nature have a minimum load current too, though not all. But you can deal with this by adding a resistor on the output to provide a minimum load should it be needed.

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Re: LED Matrix - 12V

Postby Andrew Davie » Mon Jun 12, 2017 9:36 pm

Steve Anderson wrote:
Andrew Davie wrote:Considering an adjustable DC-DC converter for this job.

The output is 5V, is that what you want in the context that we're talking about a 12V supply?


Not so sure about that; when reading the title that's what I thought too. But when you look at the pictures and see selectable voltages, and read the dot-points, I think it's actually configurable for a number of output voltages...

Module Properties: non-isolated step-down module (BUCK) synchronous rectifier
Input voltage: DC 4.5-24V
Output voltage: integrated adjustable and fixed output, the back can choose a fixed output voltage
Adjustable range (0.8-17V), fixed voltage (1.8V 2.5V 3.3V 5V 9V 12V)
Output Current: 3A (Please note that full cooling), the actual test input 12V 1.5A output less heat without special treatment.
Conversion efficiency: up to 97.5% (6.5 turn 5V 0.7A)
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Re: LED Matrix - 12V

Postby Steve Anderson » Mon Jun 12, 2017 9:51 pm

Yep, I should have read the whole blurb on it. But as it's a 'buck' converter it probably needs a higher input voltage than what it outputs. Yes, 6.5V in you can get 5V out, but probably not the reverse, 9V in, 12V out. But again I could well be wrong, it's not unknown!

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Re: LED Matrix - 12V

Postby Klaas Robers » Tue Jun 13, 2017 6:18 pm

For the LED cluster: in case you chose for a 19 volt laptop power supply, I would make the strings of LEDs larger. Think of 5 LEDs per string, that is 17 volt and a resistor of 100 ohm per string. That gives you 6 strings (branches) and a total current of 120 mA. I would trust the 19 volt stabilized output enough.
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Re: LED Matrix - 12V

Postby Steve Anderson » Tue Jun 13, 2017 8:27 pm

If considering using an old laptop PSU/charger check its output voltage first, they are generally around 19V but I have seen them at 17.5V and (I think) 20.5V. It would be prudent to measure it anyway under a light load, say 100mA. Thy are a good source of power providing usually several amps, but at lighter loads it is worth checking the thing is still regulating.

The downside is they're switched-mode which may put some noise on the supply. It depends on the application, a high-gain mic pre-amp, no way - and you don't need several amps for that! Or at least you shouldn't do. For logic with a 12V, 5V 0r 3.3V linear post-regulator, perfectly OK.

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Re: LED Matrix - 12V

Postby Andrew Davie » Tue Jul 04, 2017 12:47 pm

My LED matrix boards arrived today. A bit disappointed, I guess. I think things are a bit smaller than I expected, particularly the power connector. I'm having a lot of stuff do that recently; oh well. Anyway, I'll wire it up tonight and see how it looks. I'll just have to use a different connector than I was planning. Perhaps a couple of header pins will do the job.

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Re: LED Matrix - 12V

Postby Andrew Davie » Tue Jul 04, 2017 5:19 pm

Populated two of the boards - working perfectly. Surprised that the LEDs remained lit until just under 8V (!) which indicates these LEDs require just 2.6V to actually light up. I used 68 ohm resistors on each chain of 3, and the total current used can be seen left-hand side of the power supply. That is, 270mA when running at 12V and I'm driving each LED at 27mA (rated 20mA). Comfortable pushing them a little bit - lifespan doesn't matter that much to me at this point. The nice thing with this array is that I'm definitely going to be able to run on my 12V televisor with no issues at all.

led12v.jpg
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led8v.jpg
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In the second picture, at 8V the LEDs are drawing only 10mA - that's 10 strings of 3 LEDs, drawing 1mA each string (?!). I'd reduce the resistor to increase brightness, but won't be running at this voltage - just an interesting point that the array does work at 8V.
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Re: LED Matrix - 12V

Postby Steve Anderson » Tue Jul 04, 2017 5:50 pm

LEDs, in fact all diodes, zeners, ordinary signal types, rectifiers, have a 'slope', that is the forward voltage might be 2.5V @ 10ma but 2.3V @ 5mA, likewise 2.7V @ 15mA. (A fictitious diode). It varies over quite a range depending on it's intended function. There are diodes, Gunn and Varactor, that have a negative voltage curve, as the current increases the voltage goes down. Useful as microwave oscillators in radar and other fields within a resonant circuit or cavity.

If you chart the voltage across the diodes (not the supply volts) against the current in Excel you'll see what I mean.

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Re: LED Matrix - 12V

Postby Klaas Robers » Tue Jul 04, 2017 7:40 pm

It is not that bad that you drive the LEDs to 27 mA for peak white. Unless on a full whit field the software gamma correction will see that they only now and then run at this peak current. But if you can, check the temperature of the LED cluster when it runs. You placed the LEDs very close and heat can almost not escape. If you did not clip off the wires, these wires can act as a kind of heat sink. When LEDs get hot their light output (efficiency) drops (and they get even hotter). May be you can include a small fan.
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Re: LED Matrix - 12V

Postby Andrew Davie » Wed Jul 05, 2017 12:24 am

Klaas Robers wrote:It is not that bad that you drive the LEDs to 27 mA for peak white. Unless on a full whit field the software gamma correction will see that they only now and then run at this peak current. But if you can, check the temperature of the LED cluster when it runs. You placed the LEDs very close and heat can almost not escape. If you did not clip off the wires, these wires can act as a kind of heat sink. When LEDs get hot their light output (efficiency) drops (and they get even hotter). May be you can include a small fan.


Thanks Klaas - understood about the gamma and LEDs being OK to peak at higher value.

As to the LED heat - well I did not really know this. I did clip the wires :) Since I'm taking the televisor to work to show&tell tomorrow, and there's a nice thermal imaging sensor at work, I should be able to get a good reading on the heat of the LED array after it's been running for a while. Another option, I assume, would be to have a "ground plane" covering the entire board (where possible) and thus acting as a heat sink too.
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Re: LED Matrix - 12V

Postby Andrew Davie » Wed Jul 05, 2017 8:17 pm

Klaas Robers wrote:It is not that bad that you drive the LEDs to 27 mA for peak white. Unless on a full whit field the software gamma correction will see that they only now and then run at this peak current. But if you can, check the temperature of the LED cluster when it runs. You placed the LEDs very close and heat can almost not escape. If you did not clip off the wires, these wires can act as a kind of heat sink. When LEDs get hot their light output (efficiency) drops (and they get even hotter). May be you can include a small fan.



LED matrix running for 5 minutes displaying Doctor Who video maxed out at 20 degrees C. Very cool. I don't think overheating will be an issue. Measured with thermal camera. Arduino however was at 97C running at 17v in. Gotta fix that!
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