Author Topic: CPLD replacement for 74141 decoder  (Read 1473 times)

0 Members and 1 Guest are viewing this topic.

Offline jmwTopic starter

  • Frequent Contributor
  • **
  • Posts: 296
  • Country: us
CPLD replacement for 74141 decoder
« on: October 18, 2019, 05:42:10 pm »
Russian clones of the 74141 are still cheap at the moment, but at times I have thought about building a BCD to decimal decoder in a PLD. The logic part is easy enough, but the high-voltage output driver is not. The 74141 is rated to withstand 60 V on its outputs, whereas no CPLD will tolerate that on a tri-stated pin. Is there a more cost-effective way to do this than 10 discrete driver transistors per digit?
 

Online Kleinstein

  • Super Contributor
  • ***
  • Posts: 15153
  • Country: de
Re: CPLD replacement for 74141 decoder
« Reply #1 on: October 18, 2019, 06:10:49 pm »
The main difficulty is the high voltage drive, not the logic.  Just 10 discrete transistors and resistors is probably the best bet. For the logic one may not even need an CPLD. A more normal 74 series chip (74HC42 should work) should be sufficient with NPN transistors in base circuit.

The transistor solution is just using quite a few parts and board space.
 

Offline SiliconWizard

  • Super Contributor
  • ***
  • Posts: 15797
  • Country: fr
Re: CPLD replacement for 74141 decoder
« Reply #2 on: October 18, 2019, 07:36:49 pm »
You're not saying anything about the end application. Do you just want a generic drop-in replacement, or do you have a specific use in mind (such as driving LEDs)?
If you're going to drive LEDs/LED displays, one suggestion would be to use an integrated LED driver as the output stage. You can find many with multiple outputs. Benefit is a lot of them are constant current drivers. Then the logic decoding can be done with pretty much anything you see fit (discrete logic, cpld, MCU, ...)
 

Offline jmwTopic starter

  • Frequent Contributor
  • **
  • Posts: 296
  • Country: us
Re: CPLD replacement for 74141 decoder
« Reply #3 on: October 18, 2019, 08:02:53 pm »
Application would be driving Nixie tubes, as was typical of the 74141 and hence the point about high voltage drive. The hoped for advantage of a CPLD would be the high pin count so you can cram 6-10 decoders in a single package without multiplexing. But if you need 10x(# of digits) in discrete low-power MOSFETs to protect those pins it's probably not saving money or space...
« Last Edit: October 18, 2019, 08:05:38 pm by jmw »
 

Offline SiliconWizard

  • Super Contributor
  • ***
  • Posts: 15797
  • Country: fr
Re: CPLD replacement for 74141 decoder
« Reply #4 on: October 18, 2019, 08:07:49 pm »
 

Offline jmwTopic starter

  • Frequent Contributor
  • **
  • Posts: 296
  • Country: us
Re: CPLD replacement for 74141 decoder
« Reply #5 on: October 18, 2019, 09:43:52 pm »
https://www.ti.com/product/SN75468

Hmm, this seems usable - 7 channel NPN Darlington array that can stand up to 100 V.
 

Online Kleinstein

  • Super Contributor
  • ***
  • Posts: 15153
  • Country: de
Re: CPLD replacement for 74141 decoder
« Reply #6 on: October 19, 2019, 07:12:36 am »
The leakage current could be a problem with the SN74468.  The transistor array also does not save that much space compare to small transistors (e.g using duals).  Normal transistors are probably easier to get in case of a later repair.

The maxim parts have the advantage of  having a kind of SPI input, so less input lines. It depends on where the signal comes from, if this is good or bad.
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 14117
  • Country: gb
    • Mike's Electric Stuff
Re: CPLD replacement for 74141 decoder
« Reply #7 on: October 19, 2019, 09:54:37 am »
Bear in mind that due to the voltage drop over the nixie, the driver doesn't have to stand off the full voltage.
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Online TurboTom

  • Super Contributor
  • ***
  • Posts: 1471
  • Country: de
Re: CPLD replacement for 74141 decoder
« Reply #8 on: October 19, 2019, 10:22:05 am »
Since nowadays usually one will drive the decoders from a microcontroller, the decode function isn't really necessary. For a pending mini-nixie wrist watch project, I had a closer look at the Supertex/Microchip HV5623. This should easily get the job done for a full four-digit clock with a few lines to spare (or three full decimal digits plus two decimal dots).
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 14117
  • Country: gb
    • Mike's Electric Stuff
Re: CPLD replacement for 74141 decoder
« Reply #9 on: October 19, 2019, 11:14:24 am »
Since nowadays usually one will drive the decoders from a microcontroller, the decode function isn't really necessary. For a pending mini-nixie wrist watch project, I had a closer look at the Supertex/Microchip HV5623. This should easily get the job done for a full four-digit clock with a few lines to spare (or three full decimal digits plus two decimal dots).
The Supertex chips are nice, but way more expensive than a bunch of HV transistors like MMBTA42
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Online TurboTom

  • Super Contributor
  • ***
  • Posts: 1471
  • Country: de
Re: CPLD replacement for 74141 decoder
« Reply #10 on: October 19, 2019, 11:57:02 am »
The Supertex chips are nice, but way more expensive than a bunch of HV transistors like MMBTA42

Yes that's correct but in a situation where PCB area and general compactness of a design matters more than price (i.e. "fun project" of a nixie wrist watch in my case, of which probably never more than ten pieces will be made), the supertex/microchip driver may still be the most reasonable choice.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf