Can a constant current boost converter be made out of a MAX1771? - Page 1
Forum section: electronics / beginners

author: WarFreak131
author_rank: Contributor
timestamp: June 22, 2020, 10:23:35 pm
content: Hello, after a roughly 10 year hiatus, I am deciding to build a new nixie clock. My question pertains to powering the tubes themselves. From reading up on them on Wikipedia, they exhibit negative resistance. Once turned on, they can be sustained with a lower voltage. The IN-17 tube in particular (according to the datasheet), is turned on at 170 V and can be sustained down to 105 V, with a current draw of 1.5 mA.My prior method of powering the nixies was the simplest, use a boost converter to boost up to 170 V, plop a resistor in series to deliver 1.5 mA, and call it a day. But with the negative resistance, once it turns on, the resistance drops, and I'll end up delivering more than 1.5 mA. I could use a high voltage linear regulator as a constant current source. A drop of 65 volts at 1.5 mA would be be less than 100 mW, which a TO-92 package should be able to sustain, but then again, I enjoy over-engineering things.The MAX1771 seems to be a popular choice for powering the tubes since they can boost to very high voltages, which leads me to the question at hand, could a constant current regulator be created out of a MAX1771 such that it will dynamically alter its boost so that it delivers 1.5 mA; automatically accounting for the initial negative resistance?Usually when creating a constant current regulator, I use an LM317, and tie the output pin to the 1.25 V reference pin through a resistor such that Current = 1.25/R. The only thing I can find on the MAX1771 is a reference pin that is 1.5 V (I think), nor does it have a Vout pin.https://datasheets.maximintegrated.com/en/ds/MAX1771.pdfI couldn't glean from the datasheet exactly how the FB pin or the reference pin worked. There is a supplemental design document that shows how to build a constant current regulator out of it, but I think that was only for sources that were less than the input voltage. I was hoping someone here could help.Thanks

author: aheid
author_rank: Regular Contributor
timestamp: February 21, 2021, 10:23:25 am
content: I assume you've figured something out, but in case not it seems to me the FB pin works like on others.That is, the controller will try to adjust the output so that the FB pin is kept at VREF level. If FB goes lower it increases output and vice versa.It should be trivial to use it as a constant current source with a low-side sense resistor, however there are two issues that makes it less than ideal I think.First is that low-side sensing is not ideal with nixie tubes, given you'll have to sense after the digit driving transistors. This raises the effective ground point (emitter voltage) of the transistors by 1.5V as well. If you use an integrated HV driver chip then its ground would have to be tied to the sense resistor an thus 1.5V high as well which can make it tedious to drive it.The other is that due to the blocking diode in the boost section, it can never sink current. Thus the load (tubes) will not be current limited once the effective resistance drops, and the output capacitor will be discharged as quickly as the load permits.As I understand it once started the tubes essentially acts as diodes (with very high forward voltage) and thus can pass a high current, which might not be ideal for the longevity of the tube AFAIK (sputtering and arcing).However given the drop from 170V starting voltage to about 130V or so once started, a small 1k series resistance would limit that issue.The main issue then is the output current sensing. Ideally it would be done high-side, but not quite sure how to do that with 170V up there.

author: mvs
author_rank: Frequent Contributor
timestamp: February 21, 2021, 01:00:58 pm
content: Quote from: WarFreak131 on June 22, 2020, 10:23:35 pmSeries resistor should be chosen for continuous operation, not for turn-on phase.Say, you wish to have 10V margin on ignition and operation voltages, so you set 180V at output of your boost converter and calculate with 115V at nixie tube. (180V-115V)/1.5mA=43KQuoteProbably yes, but do you wish to make separate boost converter for each nixie tube in your clock?--PS If you wish to lower power dissipation on series resistors, you can make output voltage of boost converter adjustable. During ignition phase to say 180V, and during normal operation to 140V. Short operation with higher current should not harm tubes a lot.

author: aheid
author_rank: Regular Contributor
timestamp: February 21, 2021, 03:05:30 pm
content: So reason I dragged up this old thread was I'm looking into driving some tubes myself.I had initially thought about just using the series resistor like a regular LED, but I also considered a two-transistor constant current circuit ala this one.The current regulation circuit could then be part of the driver for each tube.But perhaps this is overkill?:https://www.planetanalog.com/seemingly-simple-circuits-user-proof-external-supplies-circuit-3-the-two-bjt-current-limited-supply/