What is the forward voltage of this dot-matrix display - Page 1
Forum section: electronics / projects

author: expertmax
author_rank: Regular Contributor
timestamp: May 30, 2013, 08:11:19 pm
content: Hi,I'm currently building a huge 80x8 dot-matrix led display using 10x 8x8 displays and 10x MAX2719 IC's.However, I would like to know the forward voltage of this item I bought from ebay :http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=150706532260The seller told me : 2V-3V input and do not exceed 10mA.So now I am working out the Ri (resistor value for brightness adjustment on the MAX2719) and I just bought 10x 22 ohm resistors for another project and I would like to know if it's safe to use them.Thank you VERY much.Have a nice day,Max.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: May 30, 2013, 08:36:00 pm
content: Hi,Just put 10mA through it and measure VfI knocked up a simple two transistor current source with switched in resistors to give me 1mA, 5mA, 10mA, 20mA and 30mA. I just connect my led(s) start at 1mA, select a brightness i like, measure Vf and select Rled from there, quick and easyMr Smiley

author: SeanB
author_rank: Super Contributor
timestamp: May 30, 2013, 08:36:36 pm
content: Depending on how you multiplex them you might find that you need a higher current, but you have to keep the average current below 10mA. If you are doing a driver that does a row at a time you can eg set the current to 80mA, which will make the display brighter, but will still keep the current per diode at 10mA.So if you are using red LED's which typically have a Vf of around 1V5 and want 10mA then use Rset of 68k on the driver. That will be a safe value for most red displays, though it will not be terribly bright, it may wash out in very bright light.

author: expertmax1
author_rank: Contributor
timestamp: May 31, 2013, 02:04:17 am
content: Thanks for the replies !I have no way of limiting the load on the led because I have very basic equipment. I will then need to buy some 66k resistors and some 52k-ish resistors and see if it draws more than 10mA ?Is there a "normal" version on the market that has the same specs available on the market in order to download its datasheet ?Thanks.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 31, 2013, 05:13:41 am
content: I suggest you read the data sheet on the 7219 as it tells you the default ri value. You can also address the brightness within the chip. By sending appropriate code

author: Strada916
author_rank: Frequent Contributor
timestamp: May 31, 2013, 05:16:26 am
content: Quote from: SeanB on May 30, 2013, 08:36:36 pmThe max7221 and 7219 takes care of the multiplexing. He just needs to set ri and send it data.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 31, 2013, 05:43:17 am
content: All you need is a couple resistors a meter and a power source. Start off by connecting one LED from the display to a 470 ohms resistor and your power source. Since the chip runs off 5v suggest power source of 5v this will give just over 1mA. Now read the LED forward voltage. Next use a 68 ohms resistor should give about 10 mA well less. The max 7219 will only sink 40mA per output leg. Hope that helps.

author: expertmax
author_rank: Regular Contributor
timestamp: May 31, 2013, 04:12:25 pm
content: It doesn't really answer my question, I just want to know the forward voltage of this dot-matrix display, AKA a datasheet !

author: Bored@Work
author_rank: Super Contributor
timestamp: May 31, 2013, 04:22:55 pm
content: Quote from: expertmax on May 31, 2013, 04:12:25 pmApparently there is non. So really, if you care, get your ass up, and do the simple measurements.By the way, you bought common-anode displays, but want to drive them with common cathode drivers. Luckily for you you can fix that by swapping rows and columns. You would be screwed if you had 7 segment displays.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: May 31, 2013, 04:59:19 pm
content: If he is using a MAX7219 and not a MAX2719 as stated (which doesn't exist), this is essentially the same part as the AS1106 minus a few commands. The section of the data sheet for the AS1106 regarding selection of Rset is much clearer than the MAX7219 data sheet.The value for Rset will range between 5kand 20kas I recall depending on Vf of the LED and the desired current. So one of the original posters questions....will 22ohmresistors work for Rset is no.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 31, 2013, 11:11:38 pm
content: Quote from: expertmax on May 31, 2013, 04:12:25 pmLooks like you are going to have to find out on your own like I mentioned earlier.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 31, 2013, 11:14:52 pm
content: Quote from: Jon Chandler on May 31, 2013, 04:59:19 pmmax7219 and max7221 share the same data sheets as they are the same apart from 2 things as mentioned in the data sheet.Since there no data sheets for 8x8 you'll need to work it out. As suggested.

author: expertmax
author_rank: Regular Contributor
timestamp: June 02, 2013, 10:50:42 pm
content: Quote from: Bored@Work on May 31, 2013, 04:22:55 pmDid not meant to sound rude there ! And yeah I know about the common-anode displays I bought, I was supposed to use them in a different project but I decided to use them in this particular project.Quote from: Strada916 on May 31, 2013, 11:14:52 pmThe problem is I need 10 displays working so I can't just mock around and burn some LEDs... I have no spare. So I'll use 68k as suggested and go lower each time until I get around 10mA. Should work right? And I'll keep you posted with my project with pictures and some screenshots from Atmel Studio.

author: c4757p
author_rank: Super Contributor
timestamp: June 02, 2013, 11:00:48 pm
content: Huh? Connect it to a variable power supply through a 470-ish resistor and stick a voltmeter across the resistor. Start at zero and slowly wind it up until you get desired brightness or the correct voltage for the desired current across the resistor. Then measure the voltage across the LED for VF.And 68k? WTF? You'll need 680V to put 10mA through that even without an LED! Edit: I may have misunderstood that part. Still, measuring VFis really easy...

author: expertmax
author_rank: Regular Contributor
timestamp: June 02, 2013, 11:28:38 pm
content: Quote from: c4757p on June 02, 2013, 11:00:48 pmI think that you misunderstand my situation. The voltage and the amperage are controlled by the MAX7912. I Just want to be sure that I set a decent value for Iset(Rset).

author: c4757p
author_rank: Super Contributor
timestamp: June 02, 2013, 11:29:43 pm
content: Rsetis dictated by I and VF. You said you want 10mA, measure VF, and use the datasheet, Luke!And yes, I finally went and read the datasheet so I don't make an ass of myself.

author: Strada916
author_rank: Frequent Contributor
timestamp: June 02, 2013, 11:31:46 pm
content: According to data sheets it just less than 10k for rset. 10k will do though.

author: Strada916
author_rank: Frequent Contributor
timestamp: June 02, 2013, 11:37:03 pm
content: You will also need to change the brightness on the chip. From memory it's not at full bright setting.

author: expertmax
author_rank: Regular Contributor
timestamp: June 02, 2013, 11:38:36 pm
content: I don't know why but the MAX7219 lacks so much information ! Or is it just me? Could be!

author: c4757p
author_rank: Super Contributor
timestamp: June 02, 2013, 11:40:01 pm
content: Looks fully specified to me. What are you looking for?

author: expertmax
author_rank: Regular Contributor
timestamp: June 02, 2013, 11:43:41 pm
content: Quote from: Strada916 on June 02, 2013, 11:31:46 pmOh thanks, even if the dot-matrix led display "datasheets" specifies not to exceed 10mA ?

author: mariush
author_rank: Super Contributor
timestamp: June 02, 2013, 11:52:45 pm
content: Hey man, I've checked your ebay link and it seems the 8x8 displays are common anode displays.I'm looking at the MAX7219 and it seems they'recommon cathodedrivers :http://datasheets.maximintegrated.com/en/ds/MAX7219-MAX7221.pdfAre you sure these two can be used together? I don't think they're suitable. Are you actually typing the correct max chip?

author: expertmax
author_rank: Regular Contributor
timestamp: June 02, 2013, 11:54:16 pm
content: Quote from: mariush on June 02, 2013, 11:52:45 pmYeah I just need to invert the Rows with the Columns and it will work just fine !

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: June 03, 2013, 05:39:45 pm
content: 30 seconds looking at the data sheetYou need table 11 and Vf of the led.Do you have to be told which page table 11 is on ?It EVEN GIVES a value for 10mAspoon fed or whatLife is so hard