TheBackShed.com

Forum Index : Microcontroller and PC projects : MAX7219CNG 7 seg Led driver

Author

Message

DaveC166
Regular Member

Joined: 13/09/2012
Location: New Zealand
Posts: 44

Posted: 11:57pm 07 May 2015

Copy link to clipboard

Print this post

Earlier this year i got 10 max7219 chips off ebay for $10:00. i am in the process of bulding a MK2 Umite clock using 4 Knightbright 25mm com cathode 7 segement green Led displays. these displays have a foward voltage drop of 4.2 volts per segment.
Four of the max7219 dont appear to work at all, but 6 of the MAX7219 chips will work from a supply voltage of between 4.20 and 4.40

volts, any higher voltage (up to 5.0 volts) and no segements will light on the displays. are their issues with using Max7219's with double led segment displays, or have i just bought 10 fake chips?

WhiteWizzard
Guru

Joined: 05/04/2013
Location: United Kingdom
Posts: 2794

Posted: 12:24am 08 May 2015

Copy link to clipboard

Print this post

Hi Dave,

I have built several MAX7219 LED Matrix clocks with a MicroMite controlling it all. People on this forum know my thoughts about 'cheap' components. They are great for testing and/or personal use but just be prepared to waste many hours head-scratching.

The genuine parts do cost several $s each so do you really expect to get 10 good ones for $10 (probably included shipping too!). Take the ones that work and then be prepared for them to not work as they should. They will also probably pull much more current than the genuine part (easy to check - simply read the official data sheet , look at current consumptions, then measure your chips).

Official chips will work from 4v up to 5v5. The other thing to bear in mind is also the LEDs being driven - are they pulling too much current due to being 'clones'.

Just out of interest, todays price at Farnell is �9.34 each!

I will put a link to a thread you may find useful (if you've not already seen it)
Here is the link

WWEdited by WhiteWizzard 2015-05-09

For everything Micromite visit micromite.org

Direct Email: whitewizzard@micromite.o

bigmik

Guru

Joined: 20/06/2011
Location: Australia
Posts: 2870

Posted: 02:11am 08 May 2015

Copy link to clipboard

Print this post

Hi Dave, (I sound like HAL from 2001 space odyssey)

Here is an interesting article on fake MAX7219 chips..

In his tests they all worked but he wasnt trying to drive LEDs with 4v2 forward voltage..

Link!!

Also the specs state that both GNDs (Pins 4 and 9) MUST be connected to GND

Also the Voltage out of each segment can be as low as 1v less than the supply voltage so using at 4.2v supply could give drive voltages as low as 3.2V

Regards,

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

DaveC166
Regular Member

Joined: 13/09/2012
Location: New Zealand
Posts: 44

Posted: 09:21am 08 May 2015

Copy link to clipboard

Print this post

Thanks for the replies. The seven seg leds were obtained from Element14 so should be genuine parts.I thought i had ordered just 1 max7219 from the ebay site, NZ$10.00 was $2.00 cheaper than Element14. There must be a lot of fake chips out there

bigmik

Guru

Joined: 20/06/2011
Location: Australia
Posts: 2870

Posted: 02:57pm 08 May 2015

Copy link to clipboard

Print this post

Hi Dave,

Just out of interest, how do your chips compare to the `fakes' in the link I posted?

I say `fakes' because they most likely are, but they could also be rejects (the silicon dies) may have been scooped up from the reject bin and taken elsewhere for plastic dip packaging.. So they may be genuine dies but fake casing.. As they seem to work and draw very similar currents to the known genuine I suspect that is what they have done.

Regards,

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

bigmik

Guru

Joined: 20/06/2011
Location: Australia
Posts: 2870

Posted: 03:00pm 08 May 2015

Copy link to clipboard

Print this post

Dave,

Re. those units that didnt work...

Did you try them with a standard LED with a typical 2.1v forward voltage drop?

It may be that they exhibit 1v lower (as is specified in the data sheet that they `may' do) than the V+ supply which is lower than the fwd voltage of the displays you wish to use.

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

DaveC166
Regular Member

Joined: 13/09/2012
Location: New Zealand
Posts: 44

Posted: 11:01am 10 May 2015

Copy link to clipboard

Print this post

Thanks bigmik, i will them with a single diode per segment display and see what happens
edit, after Googling "fake max7219 chips" it appears mine arnt genuine Maxim chips. A In4007 diode between +5V and The VCC pin does the trickEdited by DaveC166 2015-05-11

Print this page

To reply to this topic, you need to log in.