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Topic: Super Big 6.5' 12V Segment Display (Read 5288 times) previous topic - next topic

PaulRB

I'm using a 750 Ohm resistor to get around 16.9 mA drawn across the led strips in each segment.
Sorry, I missed that in your first post.

A 750R resistor with 16.9mA flowing will drop 12.7V, leaving nothing to light your led segments! So what did you mean by that?

stevenplanet

#16
Jul 25, 2015, 08:51 pm Last Edit: Jul 25, 2015, 08:55 pm by stevenplanet
Wow I have a lot to learn! There's so many options to choose from haha. I was planning on driving 6 displays, like HH:MM:SS. I would probably need a bigger power supply right?

About the 750Ohm.... in the beginning I wasn't sure what the max current was so I used a random resistor in fear of burning out my display. But the segments seem to run fine with no resistor at 12 volts.

EDIT: so I actually was digging around my arduino kit and I found an NXP SAA1064 DIP! I'm going to read up on this a little bit. 12V should be able to handle one display right? I'll just lose a little brightness

PaulRB

#17
Jul 26, 2015, 01:01 pm Last Edit: Jul 26, 2015, 02:04 pm by PaulRB
I was planning on driving 6 displays, like HH:MM:SS. I would probably need a bigger power supply right?
Those displays require 20mA per segment, or 30mA if multiplexing.

If not multiplexing, that's 20 x 8 x 6 = 960mA.

If multiplexing, you would need 30 x 8 x 6 x n where n is your multiplexing ratio. So with a multiplex ratio of 1 in 2 (=0.5) that would be 720mA, or with a multiplex ratio of 1 in 6 (=0.167) it would be only 240mA. But the higher the multiplex ratio (lower value of n), the lower the brightness of the displays.

My main concern is not the current output of your supply (although 960mA is a little too close for comfort), but the voltage. As I said before, you always need more than the forward voltage of the display, for switching/current limiting etc.

About the 750Ohm.... in the beginning I wasn't sure what the max current was so I used a random resistor in fear of burning out my display. But the segments seem to run fine with no resistor at 12 volts.
You should never connect leds to power without some kind of current limiting. Even if the led forward voltage matches the power supply. if you want to know why, I think Grumpy_Mike gives an explanation on his web site.

There are two ways to limit the current. One is to use resistors, the other is to use a chip who's outputs have "constant current circuits". Most chips (e.g. Arduinos, shift registers) don't have those. But some chips like max7219 and saa1064 do, so you don't need current limiting resistors.

EDIT: so I actually was digging around my arduino kit and I found an NXP SAA1064 DIP! I'm going to read up on this a little bit. 12V should be able to handle one display right? I'll just lose a little brightness
Definitely worth a go. I've used this chip before to drive 1.5" blue 4-digit 7-seg displays with a forward voltage of 6.5V from a 9V supply, so I had an extra 2.5V than I needed. You have virtually no extra voltage, so I don't know, try it. I will draw you up a diagram. What kind of Arduino are you using?



SAA1064 Data Sheet

Arduino Wire Library

Test sketch:
Code: [Select]
#include <Wire.h>
#define SAA1064ADDRESS 0x70

void setup() {
  Wire.begin();
  Wire.beginTransmission(SAA1064ADDRESS);
  Wire.write(0); //Update control register
  Wire.write(0b01110110); // No multiplexing, max segment current
  Wire.endTransmission();
}

void loop() {
  for (int i=0; i<=255; i++) {
    Wire.beginTransmission(SAA1064ADDRESS);
    Wire.write(1); //Update digit 1
    Wire.write(i);
    Wire.endTransmission();
    delay(100);
    }
}

stevenplanet

#18
Jul 26, 2015, 07:59 pm Last Edit: Jul 26, 2015, 08:07 pm by stevenplanet
Those displays require 20mA per segment, or 30mA if multiplexing.

If not multiplexing, that's 20 x 8 x 6 = 960mA.

If multiplexing, you would need 30 x 8 x 6 x n where n is your multiplexing ratio. So with a multiplex ratio of 1 in 2 (=0.5) that would be 720mA, or with a multiplex ratio of 1 in 6 (=0.167) it would be only 240mA. But the higher the multiplex ratio (lower value of n), the lower the brightness of the displays.

My main concern is not the current output of your supply (although 960mA is a little too close for comfort), but the voltage. As I said before, you always need more than the forward voltage of the display, for switching/current limiting etc.
You should never connect leds to power without some kind of current limiting. Even if the led forward voltage matches the power supply. if you want to know why, I think Grumpy_Mike gives an explanation on his web site.

There are two ways to limit the current. One is to use resistors, the other is to use a chip who's outputs have "constant current circuits". Most chips (e.g. Arduinos, shift registers) don't have those. But some chips like max7219 and saa1064 do, so you don't need current limiting resistors.

Definitely worth a go. I've used this chip before to drive 1.5" blue 4-digit 7-seg displays with a forward voltage of 6.5V from a 9V supply, so I had an extra 2.5V than I needed. You have virtually no extra voltage, so I don't know, try it. I will draw you up a diagram. What kind of Arduino are you using?



SAA1064 Data Sheet

Arduino Wire Library

Test sketch:
Code: [Select]
#include <Wire.h>
#define SAA1064ADDRESS 0x70

void setup() {
  Wire.begin();
  Wire.beginTransmission(SAA1064ADDRESS);
  Wire.write(0); //Update control register
  Wire.write(0b01110110); // No multiplexing, max segment current
  Wire.endTransmission();
}

void loop() {
  for (int i=0; i<=255; i++) {
    Wire.beginTransmission(SAA1064ADDRESS);
    Wire.write(1); //Update digit 1
    Wire.write(i);
    Wire.endTransmission();
    delay(100);
    }
}

Thank you SO MUCH! You are a scholar and a gentleman! I'll try my best to pick up a power supply later at a store. I might have to read up a little more on multiplexing later again. I believe I've done multiplexing before but I kind of did it my own way with 4 digit segment displays. Heres what I did if you're interested.

https://github.com/stevenan93/arduinoTimer/blob/master/sketch_jul14a.ino

Every time I switch numbers, I added a delay of around 100ms. The program was just a timer that incremented in seconds. So every 1000ms, I updated the numbers. I just summed up the delays in the loop and every time it reached 1000ms I updated each value.

I will read up more on multiplexing and multiplexing ratios. My current power supply has 1A and 12V but like you said perhaps it would be best if I had some more voltage. I think I have a 15V charger lying around somewhere if I'm too lazy to run to Frys Electronics.

I'm using an Arduino UNO R3. Is that enough information? I'm still new to this and arduino so I'm not sure if theres any other specifications you need. Thank you again so much for the code and the sketch and all the help. I'll give it a go with the 12V Power supply I have if not I'll try something a little bit higher. What would you recommend if I wanted to power six of these? a 60V power supply?

Can I ask what program you used to draw the circuit connections? I really really appreciate it also its really pretty.

So I was looking at your schematic....shouldnt there be current limiting resistors anywhere? Or is this case fine since I'm using the SAA1064 chip like you said?

P.S - OH And you were right back a few posts, I did blow the fuses on my multimeter. But I have had them replaced so the mA and microA ammeters work good now :)

PaulRB

#19
Jul 26, 2015, 08:14 pm Last Edit: Jul 26, 2015, 08:33 pm by PaulRB
I'm using an Arduino UNO R3. Is that enough information? I'm still new to this and arduino so I'm not sure if theres any other specifications you need.
No, that's OK. The SDA and SCL pins are marked on the circuit board on Unos, I think. If not, they are analog pins A4 & A5.
I'll give it a go with the 12V Power supply I have if not I'll try something a little bit higher. What would you recommend if I wanted to power six of these? a 60V power supply?
60V? No! You're getting your volts & amps mixed up. You still only need 12V (if that works) or 15V, but for 6 digits, perhaps 1.5A current.
Can I ask what program you used to draw the circuit connections? I really really appreciate it also its really pretty.
Cadsoft Eagle, v7.3. It can be a little awkward to use until you get the hang of it, but its free for designing smaller circuits.

Here's how you would connect up to 8 digits using 2 chips:

That circuit uses the multiplexing functions of the chips, so the brightness will be less. You need to decide how bright you need them to be. For outdoor use you need as much brightness as you can get, in which case multiplexing is not great. With these chips, you could drive 6 digits with 3 chips and no multiplexing.

stevenplanet

#20
Jul 26, 2015, 08:36 pm Last Edit: Jul 26, 2015, 09:10 pm by stevenplanet
Hmmmmm I see. I noticed that your schematic shows two connections between the SAA1064 and the arduino ATMEGA is PC4 and PC5. I looked up the pinout for my ATMEGA 328P and its analog pins 4 and 5. Did I do the right thing? I hooked up everything and nothing shows up on the display. I triple checked my connections. Are you sure I don't need any capacitors/resistors/transistors? Other tutorials showed we needed those, especially for multiplexing. I measured the output of the power supply and it was 12.57 Volts. Perhaps this is not enough to power the display. Also really stupid question. But Vcc on the arduino is taken care of if I plug it into USB right? Like when I upload the code and plug it in to my computer, that is enough to power the ATMEGA?

EDIT:
I found a 24 V 300mA power supply! Do you think its okay to plug it in?? The current isn't the same as the other but its got a lot more voltage!

PaulRB

No! Do not connect that 24V supply, you will fry something.

stevenplanet

#22
Jul 27, 2015, 12:55 am Last Edit: Jul 27, 2015, 12:56 am by stevenplanet
No! Do not connect that 24V supply, you will fry something.
lol good thing I was patient. This is an incredibly stupid question but could I create a voltage divider to drop the voltage down to 15 V using the 24 V power supply? It might drop the current a lot but perhaps its enough? The supply lists a 300mA current output. I don't know why the display isn't running :(

EDIT:
Is there a way to test if my SAA1064 is functioning properly? It actually suffered a pretty steep fall off the edge of my table. The pins are alright but hopefully nothing broke on the inside :(

Chagrin

lol good thing I was patient. This is an incredibly stupid question but could I create a voltage divider to drop the voltage down to 15 V using the 24 V power supply?
That would require very large/pricy resistors, and it would still be incredibly wasteful of power. Use a switching regulator; modules based on the LM2596 are very inexpensive.

Regarding breaking the chip by dropping it, that's nearly impossible. You would see physical damage to the chip (a large crack or such).

PaulRB

#24
Jul 30, 2015, 12:09 am Last Edit: Jul 30, 2015, 12:13 am by PaulRB
lol good thing I was patient. This is an incredibly stupid question but could I create a voltage divider to drop the voltage down to 15 V using the 24 V power supply?
No, voltage dividers only work if you draw almost no current from them. As soon as you draw any significant current, the voltage changes.

It might drop the current a lot but perhaps its enough? The supply lists a 300mA current output.
As Chargin suggested, you could get a DC-DC converter like this. Much more cool and efficient than a regulator.

Is there a way to test if my SAA1064 is functioning properly? It actually suffered a pretty steep fall off the edge of my table. The pins are alright but hopefully nothing broke on the inside :(
Chips are physically tough inside, if you haven't broken the legs off I'm sure it will be fine. To test it, just connect a few ordinary leds with their anodes to 5V and their cathodes to the P1-P8 pins on the chip. Power the saa1064 from the 5V supply also when you do this.

When you were using the 12.5V supply, did you connect the grounds together? Meaning the Arduino/USB ground and the ground from the 12V supply?

PaulRB

#25
Jul 30, 2015, 12:28 am Last Edit: Jul 30, 2015, 12:32 am by PaulRB
I noticed that your schematic shows two connections between the SAA1064 and the arduino ATMEGA is PC4 and PC5. I looked up the pinout for my ATMEGA 328P and its analog pins 4 and 5. Did I do the right thing?
Yes, that's correct. But check you got them the right way around. These two pins are the "I2C Bus". Busses allow several devices to be connected using shared lines. You could also try 4K7 pull-up resistors on each of those two lines, to pull them up to 5V (not 12V!). Pull-ups are often not needed with ATMega chips as long as there is only one device connected to the i2c bus, and the connections are short (a few cm).

I hooked up everything and nothing shows up on the display. I triple checked my connections. Are you sure I don't need any capacitors/resistors/transistors? Other tutorials showed we needed those, especially for multiplexing.
The saa1064 data sheet does not show any other components in the suggested circuit. But a 0.1uF across and close to the chip's power pins is always a good idea. We are not multiplexing yet. This chip can run 2 digits without multiplexing, or 4 digits with multiplexing.

I measured the output of the power supply and it was 12.57 Volts. Perhaps this is not enough to power the display.
Yes, that's one possibility why its not working, for the reasons I explained before.

Also really stupid question. But Vcc on the arduino is taken care of if I plug it into USB right? Like when I upload the code and plug it in to my computer, that is enough to power the ATMEGA?
Yes.

Paul__B

Just a quick comment.

Am I wrong?  TLDR.  A "12V" LED display would have internal current limiting resistors.  Have we been given the essential weblink where we can check how it is configured and thus, how to use it?  Otherwise, it's just pi$$ing in the wind.

To use a digital multimeter to measure current, you either connect your test leads to the left hand and centre jacks on the meter and use the "10A" range, or you connect to the right and centre jacks and use any other current range (in this case, the mA).

PaulRB

Am I wrong?  TLDR.  A "12V" LED display would have internal current limiting resistors.  Have we been given the essential weblink where we can check how it is configured and thus, how to use it?  Otherwise, it's just pi$$ing in the wind.
The OP gave us the link to the data sheet back in reply #13. It shows no internal series resistors.

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