fillament power supply for dht tube - Page 1
Forum section: electronics / projects

author: scsi
author_rank: Contributor
timestamp: June 21, 2020, 09:29:53 am
content: Hey all! So I want to create simple, clean, regulated power supply for dht tube to implement fillament bias.Idea is verry simple: to use a 6.3V rms 3A transformer winding rectify it, than clean it with capacitance multiplier and fed it to tl431 current shunt regulator to get a stabilised 4.2V 650mA output. Could this work?

author: Zero999
author_rank: Super Contributor
timestamp: June 21, 2020, 07:12:36 pm
content: The circuit you posted won't work, because R5 will drop too much voltage, if any appreciable current is drawn.Why not use the LM317? It's cheap and easy to use.

author: james_s
author_rank: Super Contributor
timestamp: June 21, 2020, 07:14:52 pm
content: Or at least a classic voltage regulator circuit with a pass transistor, that circuit is bizarre, I'm not quite sure what the designer was trying to do.

author: TimFox
author_rank: Super Contributor
timestamp: June 21, 2020, 07:29:35 pm
content: The 1k value must be a typo: at 650 mA, it would drop 650 V! Perhaps 1 ohm was intended?The above suggestion of an LM317 is reasonable, and there are app notes that show how to include current limiting to three-terminal regulators to give a softer start to the filament heating.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2020, 07:30:38 pm
content: The dropout voltage of a 317 might make operation marginal without a lot of input capacitance however there are several low dropout equivalents to the 317 which would work fine in this application, or a discrete low dropout regulator could be implemented in several different ways.

author: TimFox
author_rank: Super Contributor
timestamp: June 21, 2020, 07:38:30 pm
content: For a similar heater load (not DHT), I used an LM350 in a TO-3 case on a reasonable heat sink. The total current load was 810 mA.The TO-3 package has a thermal resistance (junction to case) of 1.5 K/W (max), while the TO-220 is 4 K/W.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2020, 08:05:34 pm
content: I did it for 6.3 volt heaters using a 317 decades ago and used schottky rectifiers to get the input voltage high enough under all conditions.

author: TimFox
author_rank: Super Contributor
timestamp: June 21, 2020, 08:20:16 pm
content: I was caught short once with an 7824 heater voltage regulator when the bottom of the ripple waveform on the filter capacitor was just below dropout--an easy mistake to make, and not diagnosable with a DC voltmeter.

author: scsi
author_rank: Contributor
timestamp: June 22, 2020, 11:06:50 am
content: Thanks for the answers! I could use something very simple like lm1085-adj LDO voltage regulator.Still I am a bit concerned about ripple noise. Few questions : what rectifiers are best to use in this situation, should I do something about ripple filtration and how big C1 and C3 should be?Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: June 22, 2020, 01:13:04 pm
content: An excellent tool for designing conventional power supply rectifiers is PSUD2, from Duncan Amplification (freeware).Download link:http://www.duncanamps.com/psud2/download.htmlNotes:The transformer needs to be specified as no-load output voltage with series resistance, although the manufacturer specifies voltage at specified load.There is a set of rectifier models, including Schottky diodes. Choose one that is close to your choice.The computation will show the inrush performance, output ripple, transformer input current, and the ripple current through the filter capacitors.Check that the voltage waveform into the regulator with your load current does not dip below the dropout point.

author: David Hess
author_rank: Super Contributor
timestamp: June 22, 2020, 02:05:02 pm
content: Quote from: scsi on June 22, 2020, 11:06:50 amUse a C3 as recommended in the LT1085 application notes. Typically it will be something like a 10 microfarad solid tantalum or 47 microfarad aluminum electrolytic.The size of C1 depends on the allowable input ripple voltage and output current. I usually start with a ripple voltage equal to the dropout voltage and the rule of thumb if 8200 microfarads per amp/volt which comes out to 3500 microfarads so a standard value of 3300 or 4700 microfarads.Ripple should be about 250 microvolts.