Measuring I or V of Joule Thief? - Page 1
Forum section: electronics / beginners

author: kalel
author_rank: Frequent Contributor
timestamp: August 26, 2017, 12:44:42 am
content: Not having an oscilloscope available, is there a simple way to measure current and voltage of things like a Joule Thief?In the simple form, it doesn't even have an output capacitor, and the LED is flashing at a high frequency. The exact frequency is not always clear either, depending on the method of production.With that, I'm not sure if I can use a multimeter to get any useful data. I don't have any quality brand name units.For instance, solar lights use similar circuits. How would I measure how much current the LEDs are receiving and at which voltage they are working? Will any multimeter work, or is an additional circuit required? If so, I'd be interested in the simplest solution.

author: Vtile
author_rank: Super Contributor
timestamp: August 26, 2017, 02:37:22 am
content: Unfortunately most multimeters have bandwith only up to 10 to 100 kHz. There is no simple accurate solution as far as I know if the frequency is above the multimeter bandwith. Some form of signal conversion would be required. First comes to my mind a rectifying bridge and low leakage capacitor to get peak voltage, but even it could be way off with mystery waveform. Also some form of frequency detector could be build to give an idea if the output is something that can be measured with multimeter. Scope is pretty much the way to go.That said if ie. joule thief is oscillating lets say 5 kHz (=5000 timed per second = 5000Hz .. you can see flicker up to 400 Hz iirc) and you have half decent true rms DMM with AC up to say 10kHz you can just measure I and V in AC mode with reasonably expectations of accuracy.

author: danadak
author_rank: Super Contributor
timestamp: August 26, 2017, 10:32:21 am
content: You could measure average value by using a RC filter to filter the pulse,then read with DMM.Attached some ap notes on filter design.Regards, Dana.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: August 26, 2017, 10:14:53 pm
content: Most JT circuits make very brief spikes to light up the LED(s) used as loads. The voltage of these spikes peaks at some value above the forward voltage of the LED, then drops and plateaus at around the Vf, then drops to zero, and the cycle repeats. Or even stranger waveforms. The frequency of these spikes also changes as the supply voltage changes (battery running down). These factors make the DMM averaging, or filter methods, less than reliable. If you are interested in accurate measurements you really do need an oscilloscope that can calculate waveform averages and areas accurately.QuoteMaybe you can, but most people cannot. Typical flicker fusion frequency is 30-40 Hz, with some people able to see as high as 60Hz. A typical flat-screen monitor running at 72 or 100 Hz looks rock-solid to most people. Oddly enough, if you are fatigued or ill, the FFF goes _up_ rather than down.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2017, 12:43:51 am
content: Put a rectifier and filter on the output, then the LED:(Note the coupled inductor is the active coil. The schottky diode and 10uF are added. Don't mind the other stuff.)You need a shunt resistor or current sensor to measure the LED and battery currents, if you want to measure everything and calculate efficiency.FYI, I typically get ~70% efficiency with my builds. This is a ~1W model:Tim

author: kalel
author_rank: Frequent Contributor
timestamp: August 27, 2017, 01:05:04 am
content: Quote from: T3sl4co1l on August 27, 2017, 12:43:51 amLooks good. But if you're running it at 1w, does the LED get hot? They also sell versions with little heatsinks.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: August 27, 2017, 01:06:51 am
content: This isn't:

author: kalel
author_rank: Frequent Contributor
timestamp: August 27, 2017, 01:08:52 am
content: Quote from: alsetalokin4017 on August 27, 2017, 01:06:51 amI like the battery holder.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2017, 04:49:58 am
content: Quote from: kalel on August 27, 2017, 01:05:04 amNah, it's got plenty of meat on there -- copper clad construction.This one gets a little toasty though...10W from a 18650 cell.Tim

author: kalel
author_rank: Frequent Contributor
timestamp: August 27, 2017, 05:01:04 am
content: Quote from: T3sl4co1l on August 27, 2017, 04:49:58 amNice, those 10W are not easy to deal with - without the right knowledge I guess.I found that distributing multiple 3W with mini built in heatsinks can produce a less hot 5-8W or so range without any special heatsinking added (obviously running each at much less than 3W).However, I never ran anything larger than an RGB diode with a Joule Thief (after adding a cap and a diode). How do you provision for the higher currents, do you use a power transistor and thicker coil wire? The one I built was without doing any accurate measurements. Just a simple coil wound from "some" wire (horrible multi-core wire from something that attracts a magnet with copper coating in fact), some resistor, and some NPN. It still worked well though for a small LED or two.That works to simply get an LED to work from a low voltage battery, but if working on a better version in the future, some measurements would be in order.

author: danadak
author_rank: Super Contributor
timestamp: August 27, 2017, 09:26:23 am
content: Here is an interesting discussion of DC, averaging. as stated not precise butseems on the whole useful.http://web.mst.edu/~kosbar/test/ff/basicmeasures/DC.html#rcRegards, Dana.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2017, 08:29:08 pm
content: Quote from: kalel on August 27, 2017, 05:01:04 amFirst of all, this is your prototype, the blocking oscillator:As often happens in the world, the oft-repeated instructions (Cbb = 0, Rbias + Rb = 1k) are wrong. Not so wrong that they don't work at all, but very far from optimal. Typical efficiency is 30%, it's practically a class-A (linear) oscillator. Abysmal!Best performance (sharpest switching) is had when Rb = 0. Rbias supplies base bias, which is about hFE times less than the average input current. Cbb * Rbias is the base bias time constant: it should be near the inductor time constant, so that base bias runs out just as it's done with one switching cycle. If it's too large, the circuit runs for several cycles, in a burst -- this is called squegging. If it's too small, the pulse duration is too short, and you won't get much power output (or a high operating frequency, and more switching losses).This circuit is ancient: it has powered TVs since the invention of television! One advantage of the vacuum tube: the voltage ratings are massive, so they didn't have to worry about voltage, even at very high duty cycles (typical TV sweep signals are 80-90% duty cycle, so the peak voltages are quite high indeed).BJTs don't have this advantage. Consider when the transistor turns off: the collector voltage spikes up, and so the base voltage spikes down. If base voltage goes below about -5V, it begins to avalanche (i.e., the B-E junction is a zener diode too). Which pushes charge back into Cbb, which means the transistor is getting more bias than just Rbias supplies. The result? Too much output voltage, and the transistor latches on, burning itself like a runaway diesel engine!So this sets the maximum transformer ratio. If you need a 4V output from a 1.5V supply, that's a maximum turn-off collector "spike" of 2.5V. A 1:1 transformer is okay here. When the transistor turns on, it only needs a fraction of a volt (it turns on at about 0.6V, but that's already in Cbb when it begins to flip -- the extra drive from the transformer only kicks it on harder). So you might use an even lower ratio, because the base doesn't need much drive to force it on. When the transistor turns on, 1.5V drops across the transformer; you only need maybe 0.5 to 0.75V (or a 2:1 ratio).Then, Rbias sets power level. Rbias needs to supply base current, that was drawn out of Cbb when the transistor was on. And, note that the transistor sucks extra current out, because it drives current from Cbb into its own base, because of the transformer action. For this reason, the hFE we need to know, is not the linear-range value on the datasheet, but a lower value.For example, my 1W light has two ranges: Rbias = 1k and 100 ohm. (They're really not that different, visually, and probably 4.7k or 10k would be a better "low" setting. But the power is indeed about 1/10th.) The transistor is PBSS303NX, a high hFE, low-Vce(sat) part that works very nicely indeed here. I forget what the inductor is; even 1uH is a lot in this application, because the voltage is so small, and the frequency can be pretty high (this runs at 500kHz, on the 100 ohm setting).As for the 10W light, since the power level and supply voltage are higher, I created a semi-discrete SMPS controller for that, discussedhere. Its efficiency is still not very impressive, actually, probably because of switching and diode loss.Tim

author: kalel
author_rank: Frequent Contributor
timestamp: August 27, 2017, 08:55:04 pm
content: Thanks, I'll have to read that a few times and/or keep for reference.On the experiment I have, my "coil" has about 0.22 uH and a rough 1:1 ratio. Not sure about the frequency.At least it doesn't take a long time to wrap (can make it easier).Quote from: T3sl4co1l on August 27, 2017, 08:29:08 pm