I came across this guy’s EMP blog describing his experiments with various Joule Thiefs, and the results he got. I couldn’t find an email address or how to contact him, so right now I just have to say it in my blog. He has been methodical and taken measurements while he has experimented and he shows the graphs of his results.
He has done some research, and viewed some Joule Thief info online, one of them was my blog. I’m proud to say that he thought highly of my Supercharged Joule Thief. But I was concerned that he might damage something when he said that the transistor was overheating when he used three cells: the battery voltage was higher than the LED voltage.
When experimenting with Joule Thiefs, there are a few things that are important to remember. One is that the LED is connected almost directly to the battery. The resistance of the coil’s primary winding is very low, less than 1 ohm typically. One end is connected to the battery, the other end to the LED, so it is almost the same as a direct connection to the battery. The blue or white LED’s forward voltage is about 3.2 volts. If the battery voltage is increased to 3V, the LED will start conducting, even with the transistor disconnected. This means the battery voltage is too high.
Two fresh alkaline cells in series will give 3.1 to 3.2 volts, which will light the LED very brightly. Two used alkaline cells at 1.4V each will give about 2.8V, which should light the LED dimly, and may work okay. Or two NiMH or NiCd rechargeable cells at 1.4V maximum each will give 2.8V or less, and should work okay. If the LED still lights when the base of the transistor is disconnected, then the voltage is probably too high.
Another item that is important is the maximum negative voltage on the base, which is 5V or 6V for most silicon transistors. The emitter to base junction of most silicon transistors will start to break down at about 7 to 10V, and acts like a Zener diode. But this permanently damages the transistor, the current gain will be permanently lowered.
Also, as the higher voltage causes current to flow in the LED, the LED’s resistance goes down, which loads the rest of the circuit, and prevents the transistor from switching on and off. Instead, the current through the1k resistor just turns the transistor on and it gets hot from excessive current.
Emitter To Base Breakdown
The voltage across the feedback (base) winding is the same as the voltage across the primary (collector) winding, but it is negative, and the battery voltage, typically 1.5V, is subtracted from it. So if there are two LEDs in series, the total peak voltage across them is typically 4 to 4.5 volts each, or a total of 8 to 9V. Subtract 1.5V, and it’s 6.5 to 7.5 volts. This is more than the 5 to 6V maximum rating, and may damage the transistor.
This indicates that it’s a bad idea to put two or more LED in series; instead the LEDs should be put in parallel. If the LEDs must be put in series, then the number of turns on the feedback winding should be reduced. Half as many turns means half the voltage.
Hi Watson, I’m Eric Pierce, owner of the EMP blog you have kindly linked. Thank you for sharing your thoughts on my circuits and experiments. I’m still pretty new to the field of electronics, and really appreciate your feedback and in-depth explanations.
I did not know about the emitter-to-base breakdown of transistors; I’ve seen that figure in the datasheet (6V, for my SS8050) but wasn’t sure what it meant. I also hadn’t considered the ramifications of LEDs in series.
I would agree that parallel LEDs give better results (and that’s what I’ve used for the final soldered-together version of all my JTs), though I have read about possible problems with parallel LEDs–specifically, that slight manufacturing variations can result in different forward voltages (say, anywhere from 3.0V to 3.4V) causing some LEDs to pass more current than others, failing sooner and resulting in cascade failure of the others. I think I understand the theory here, but in practice parallel LEDs have worked just fine for me, while series LEDs have been somewhat unpredictable. There was even one case, when I tried using a ~4.0V input with Quantsuff’s HPJT, where I got *more* current with 2 LEDs in series (~20mA) than I did with only 1 LED (~14mA). Your suggestion about the transistor staying on full-time might explain this anomaly.
Anyhow, the multi-cell power source seems fraught with problems, at least with the circuits I have tried so far. I did not really understand why, but your comments have definitely helped–thanks again!
It’s great to hear from you, Eric. I’m glad you found my blog. I suggest you add a way to contact you in your blog. I once used blogger/blogspot. but if you read my web page you will see why I recommend that people shouldn’t use it. I lost my blogspot blog almost two years ago, so I don’t know how bad it is with blogger, but on my blog, which uses WordPress, I was getting dozens of comment spams until I enabled a free plugin program. Once I enabled that, almost all of the comment spams were put into a spam directory. Right now there are 135 spams after about a week, and I have to empty it. If you use your email address, make sure you don’t use the at sign, because the spammers search for email addresses with at signs. I just tell people that my yahoo.com email address is acmefixer, and they have to put the two together and add the at sign.
The issue of parallel LEDs without individual current limiting resistors is important with direct current power, like from a battery. When the Joule Thief puts out a pulse to the LED, the peak current is so high that the voltage drop across the LED is 4 to 4.5 volts, due to the internal resistance of the LED. I can see this high peak voltage on an oscilloscope. The LED’s internal resistance helps spread the current out more evenly over 2 or more parallel LEDs. Thus the differing currents in parallel LEDs isn’t so important with the Joule Thief. Also, mounting all of the parallel LEDs on the same heat sink or PC board helps keep the currents closer by keeping them all at the same temperature. If one hogs current and gets hotter, the others get hotter, too. I’ve written more in my blogs; if you want to read them you can search on certain words like forward voltage and parallel LED.
Best of success and I hope to hear from you again.