Filament driver IC? - Page 1
Forum section: electronics / projects

author: MrAureliusR
author_rank: Unknown
timestamp: October 18, 2015, 02:59:40 am
content: Hey everyone.I'm trying to find a driver IC that will drive the filaments in a Numitron tube. At 5V, they draw about 20mA. I've tried to use a MAX6959 LED driver, but it didn't work. I could also try a MAX7219, but both of these chips are LED drivers. I didn't think that would make a difference, but it seems that even though the MAX6959 sources 22mA at full brightness, it still doesn't drive the filaments.Currently, I'm using a 74HC595 with a TLC59211. The TLC59211 pairs perfectly with the 595, it basically lets it drive a lot more current. It's basically a buffer driving an N-channel MOSFET. This works perfectly, but for each digit in the display (I will have four) it needs two 20-pin chips, which is a bit excessive.Other options are using transistors in an SMD package, but that still requires a lot of board space. Does anyone know of a driver IC that can drive filaments? I know there are nixie driver chips, but those are designed for higher voltage, and they're difficult to get my hands on. I'd rather find an in-production chip.Any help is much appreciated! Thanks!

author: unitedatoms
author_rank: Frequent Contributor
timestamp: October 18, 2015, 03:07:54 am
content: If you have 5V already available then any 20mA digital open collector/open drain or normal CMOS output will work. Looks like each numitron segment is just an incandescent lamp (resistor). Why is there special driver needed ? Is it because your board 3V powered ?

author: edavid
author_rank: Super Contributor
timestamp: October 18, 2015, 03:13:12 am
content: Quote from: MrAureliusR on October 18, 2015, 02:59:40 amIt's not that they are LED drivers, it's that they are multiplexed. The average current isn't high enough to light up the Numitrons.QuoteTPIC6C595 would replace both of those, and is not too expensive.

author: MrAureliusR
author_rank: Unknown
timestamp: October 18, 2015, 03:23:23 am
content: Right, but with four digits, seven segments each, there's not enough pins on my microcontroller, and I'd still like to avoid using 4 chips (ie, 74HC595). Plus, with seven segments per chip, that's 140mA per chip, which might be above the package limits? I'll have to check. The MAX7219, for example, can multiplex 8 7-segment displays, and it's a 20-pin slim DIP. [EDIT: The 74HC595 can only sink/source 70mA per chip, +-35mA per Qn pin maximum.]I was planning on running the tubes at 3.3V anyway, to increase their lifetime. Plus, at the reduced voltage, they have more of that lovely 'glow' as opposed to being bright and crisp.Quote from: unitedatoms on October 18, 2015, 03:07:54 am

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 18, 2015, 04:33:15 am
content: Quote from: MrAureliusR on October 18, 2015, 03:23:23 amULN2803

author: MrAureliusR
author_rank: Unknown
timestamp: October 18, 2015, 06:22:07 am
content: Right, but that's almost identical to the TLC59211 setup I have -- you still need a pin for every filament unless you use an HC595. What I would love is a multiplexing chip that can handle filament driving.Also, can someone explain why an LED driver wouldn't work? If it's a constant current driver, then the resistance shouldn't matter, right? Or would a filament be lower impedance than a filament... But at 5V, 20mA... that's 250 ohms. Forward voltage of a white LED is around 3V. Same current appears as 150 ohms on an LED. You'd think that wouldn't make a huge difference?Quote from: Alex Eisenhut on October 18, 2015, 04:33:15 am

author: DanielS
author_rank: Frequent Contributor
timestamp: October 18, 2015, 02:03:39 pm
content: Quote from: MrAureliusR on October 18, 2015, 06:22:07 amAs pointed out by someone else earlier, the reason why the filament is failing to light up is not because the LED driver is unable to drive them, it is because of the multiplexing: if you drive four multiplexed digits, each digit is receiving 1/4 of the power on average. This works fine with LEDs which respond instantaneously to current passing through them but is not enough to warm filaments up enough to produce usable light, so the filaments remain in their cold state.Do keep in mind that until filaments warm up, they have very low resistance: while the lamps may have a calculated nominal resistance of 250 ohms, measured cold resistance might be closer to 10. If you captured current draw on an oscilloscope when applying the lamp's rated voltage to a cold lamp, you may find that your 20mA nominal has a 500mA initial peak before settling around 20mA once the filament has reached its nominal operating temperature.If you looked at your multiplexed filaments with an IR camera, you would likely find out that the circuit is working fine.

author: edavid
author_rank: Super Contributor
timestamp: October 18, 2015, 03:32:46 pm
content: Another problem with multiplexing Numitrons is that you need an isolation diode for each segment.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 18, 2015, 04:24:53 pm
content: Quote from: edavid on October 18, 2015, 03:32:46 pmGood point. Without those, every filament of the currently selected multiplex lamp is basically in parallel with a pair of series filaments from every other digit, so you lose a large chunk of available drive current there too.

author: MrAureliusR
author_rank: Unknown
timestamp: October 19, 2015, 01:33:14 am
content: Quote from: DanielS on October 18, 2015, 04:24:53 pmQuote from: DanielS on October 18, 2015, 02:03:39 pmThanks for pointing out why it wouldn't work -- I had a hard time wrapping my head around that. It looks like using shift registers will be the way to go!Thanks for your input, guys. Love getting help and info here!

author: 0b01010011
author_rank: Regular Contributor
timestamp: October 19, 2015, 02:05:58 am
content: I just used the TPIC6C595 on a numitron clock and that worked fine - cascaded 6 of them in all and no problems.There is also a TPIC6B595 which has a higher current capacity and a higher Vds. The 6c595 should be fine though for numitrons.They are a useful IC as they have a high VDS on the output making them suitable for more than just logic level stuff. I think Mike used this IC for his LED filament clock.

author: calexanian
author_rank: Super Contributor
timestamp: October 19, 2015, 02:17:24 am
content: +1 for ULN2803. Its an open collector driver that can easily handle that current level. Cheap as, and commonly available.

author: Photon939
author_rank: Regular Contributor
timestamp: October 19, 2015, 01:46:12 pm
content: Quote from: MrAureliusR on October 18, 2015, 02:59:40 amI'm also currently in the process of building a Numitron clock and ran into the same issues. I think I found the perfect solution - check out the ST Micro STP16CPC26It has 16 channels of current regulated sink outputs connected to a latched shift register, so you can drive two 7 segment numitrons per chip. It's available in a hand soldering friendly SO-24 and costs less than a buck if you grab 10 on Mouser. The current regulated outputs have the bonus effect of soft-starting the filaments which should increase tube lifetime