Generating DC microamps  - Page 1
Forum section: electronics / projects

author: abrasumente
author_rank: Contributor
timestamp: March 23, 2025, 01:21:19 pm
content: So I am working on building a low voltage training board to help teach HVAC diagnostics to some technicians.In a furnace, we use a short metal rod with a ceramic insulated base, mounted to the metal casing of the chamber, that extends out into the flame cone and (from what I understand) through flame ionization generates 2-6 microamps DC from 120vac line voltage. This proves the presence of flame back to the furnace control board.Obviously it would be poor form to have a torch blazing on the flame rod while I'm trying to explain how the circuit works, so what would be an option to generate this signal? My only requirements are that however this signal is generated, that I can do it with a source voltage of 120vac or 24vac, it can be mounted to plywood without catching on fire, relatively compact, and doesn't cost a stupid amount of money. I can build what's necessary, I just lack the knowledge on how to do this with small electronic components.Thanks for any help you can provide.

author: SuzyC
author_rank: Frequent Contributor
timestamp: March 23, 2025, 01:59:43 pm
content: Twist some bare copper wire with another common household-found bare wire..like aluminum or steel wire. This incompetent thermocouple will generate a big pile of microamps when heated..or wrap bare copper wire around a zinc plated iron/steel nail and wrap tinned copper wire to the metal nail's end and this will be the second conductor of the thermocouple.This, of course, is not a true flame rod but flame detection can be accomplished also with a thermocouple, no 120 VAC or 24VAC needed, just some heat. A flame rod can just be any piece of metal inserted into a furnace where the flame can reach it.

author: Ian.M
author_rank: Super Contributor
timestamp: March 23, 2025, 03:50:29 pm
content: The flame, when active, + sensing rod is equivalent to a weak leaky diode, cathode to ground (via the burner). Try a 1N4007 diode with a 6.8 megaohm resistor directly across it + a 3.3 megaohm resistor in series with the combo, going to the sensor wire.However the problem is going to be sequencing this, as the furnace control board will expect an open circuit before ignition, and may fault out if it detects a signal from the flame sensor before the gas valve opens, or before it triggers ignition.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 23, 2025, 05:13:01 pm
content: Do you actually need to do any sensing for your demo, or just generate the signal? If it's just generating the signal, you need a switch, a few MOhm level resistors, and Ohms law to tell you how much resistance given your source voltage. You want to know if it's measuring from the high or low side so your push button can source or sink, but making the current source should be quite simple if it's just for manually actuating.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 23, 2025, 05:22:40 pm
content: Quote from: abrasumente on March 23, 2025, 01:21:19 pmTry for a demo a candle flame, or even better, a cigarette lighter flame, because Butane gas doesn't make smoke (like a candle does). My guess is the detection should work just as well as for the furnace flame, but I didn't try.

author: Gyro
author_rank: Super Contributor
timestamp: March 23, 2025, 07:22:51 pm
content: Doesn't the flame rod normally double as the spark ignition electrode, or is that just on domestic boilers?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2025, 07:54:00 pm
content: Some battery (e.g. 9 V bock or CR2032) and large resistor in the 0.5 to 2 M range is likely the easiest way.

author: abrasumente
author_rank: Contributor
timestamp: March 23, 2025, 08:56:17 pm
content: Quote from: Gyro on March 23, 2025, 07:22:51 pmSome furnaces have a combo igniter/flame rod but 90% of what I've seen in the last 5 years uses a hot surface igniter and a separate flame rod.Quote from: Ian.M on March 23, 2025, 03:50:29 pmI can probably use a delay on make timer to address that. I already have the back of the board set up with a couple rotary switches to simulate fault conditions, another doodad on the back won't hurt.Appreciate the responses so far!!

author: Ian.M
author_rank: Super Contributor
timestamp: March 23, 2025, 09:06:41 pm
content: This may help with the background theory

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 23, 2025, 09:08:56 pm
content: Quote from: abrasumente on March 23, 2025, 01:21:19 pmA bare flame would get their attentionStill, I can appreciate real flame might be off the table, but what about a simulated flame with a flickering blue & orange LEDs ?There are led candles in novelty stores, they look quite realistic - correct colour and flickering and people register it as a candle, not a LED.Quote from: abrasumente on March 23, 2025, 08:56:17 pmIf you have delays yes/no options, a visual indication of flame present would be useful anyway.

author: abrasumente
author_rank: Contributor
timestamp: March 23, 2025, 10:50:54 pm
content: Quote from: PCB.Wiz on March 23, 2025, 09:08:56 pmRight on the money.We have some folks who don't speak very good English, so I've plopped LED's into every part of the circuit, that way there is a visual indication on whether or not the particular component is actually receiving power. A light on transcends language barriers was my thoughtIt would require a microcontroller to simulate the flickering flame though, no?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 23, 2025, 11:03:39 pm
content: Quote from: abrasumente on March 23, 2025, 10:50:54 pmA MCU would do it, but there are also thesehttps://www.aliexpress.com/w/wholesale-flickering-flameless-candles.htmlor this for 5V operation, claims flickeringhttps://www.aliexpress.com/item/1005008125555639.html

author: Grandchuck
author_rank: Super Contributor
timestamp: March 25, 2025, 05:49:02 pm
content: This post piqued my curiosity. A quick experiment. A dozen wraps of #22 solid, tinned wire on a galvanized nail heated by a soldering iron.

author: abrasumente
author_rank: Contributor
timestamp: March 29, 2025, 01:23:42 am
content: Quote from: Grandchuck on March 25, 2025, 05:49:02 pmYa know, me too. I've had to unlearn a lot of stuff about HVAC over my career. This is another one of those things. Nobody could ever really explain to me WHY this happened, just "if it's not between 2-5uA DC it's bad"

author: MathWizard
author_rank: Super Contributor
timestamp: March 29, 2025, 03:24:13 pm
content: IIRC, it will be related to the different energy levels of the valance electron and conduction bands, of the different metals. And when they are different, and the temperatures are right, there's a net electric field formed between the metals, and you have a voltage difference as you move across it, and you get a closed circuit when you wire up the wire I guess.And the hotter it gets, the more electrons escape their atoms, and go roaming around the metals for a while.But yeah, this is just more stuff, I wish I had the time to experiment with. I want to try and turn on a BJT with it.Hmmm flame rectification, yeah I was thinking more of thermocouples, which don't need a separate voltage source, and generate power.

author: abrasumente
author_rank: Contributor
timestamp: April 09, 2025, 01:30:13 am
content: The method Ian.M suggested worked great! Generates a stable 2.3 uA DC from 24vac. It also led me to finding LTspice, which I am sure to waste 100s of hours on.Excuse my lack of a proper test bench. I am currently in the middle of reorganizing my desk/fighting with my wife on divvying up some of her crafting space

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2025, 06:38:18 am
content: Quote from: SuzyC on March 23, 2025, 01:59:43 pmwow I had no idea that these kinds of things could actually make a sensitive detector. I thought it would be some hard to measure useless demo