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Joule Thief - still not working....

Started by fungus July 23, 2009
I just got some proper parts to start making joule thieves but I'm
still
having problems.

The circuit is this: http://www.artlum.com/jt/joulethief.gif

Except I have R1 and L1 one the other way around (as in the original
web page at http://www.emanator.demon.co.uk/bigclive/joule.htm )

The problem is that my transistors keep on overheating and dying.
Why should this be? I'm using a 2N2222 in metal can (as shown here
http://en.wikipedia.org/wiki/2N2222 ). These can switch at hundreds
of megahertz so I don't think it's because of slow switching.

I measured the current at point X and it seems high - over 100mA.
Could this be the cause of the overheating? Even if it isn't the
problem
it seems wasteful. I tried putting in a resistor there but the circuit
shuts down.
.
I also tried a honking big "high speed switching" transistor pulled
out of a PSU but it made the LEDs go very dim.

Any ideas?
fungus wrote:
> I just got some proper parts to start making joule thieves but I'm > still > having problems. > > The circuit is this: http://www.artlum.com/jt/joulethief.gif > > Except I have R1 and L1 one the other way around (as in the original > web page at http://www.emanator.demon.co.uk/bigclive/joule.htm ) > > The problem is that my transistors keep on overheating and dying. > Why should this be? I'm using a 2N2222 in metal can (as shown here > http://en.wikipedia.org/wiki/2N2222 ). These can switch at hundreds > of megahertz so I don't think it's because of slow switching. > > I measured the current at point X and it seems high - over 100mA. > Could this be the cause of the overheating? Even if it isn't the > problem > it seems wasteful. I tried putting in a resistor there but the circuit > shuts down. > . > I also tried a honking big "high speed switching" transistor pulled > out of a PSU but it made the LEDs go very dim. > > Any ideas?
Yes. Figure out what you want to do and state it explicitly and exactly. Then work to that goal in steps you understand.
On Thu, 23 Jul 2009 04:20:21 -0700 (PDT), fungus
<openglMYSOCKS@artlum.com> wrote:

>I just got some proper parts to start making joule thieves but I'm >still >having problems. > >The circuit is this: http://www.artlum.com/jt/joulethief.gif > >Except I have R1 and L1 one the other way around (as in the original >web page at http://www.emanator.demon.co.uk/bigclive/joule.htm ) > >The problem is that my transistors keep on overheating and dying. >Why should this be? I'm using a 2N2222 in metal can (as shown here >http://en.wikipedia.org/wiki/2N2222 ). These can switch at hundreds >of megahertz so I don't think it's because of slow switching. > >I measured the current at point X and it seems high - over 100mA. >Could this be the cause of the overheating? Even if it isn't the >problem >it seems wasteful. I tried putting in a resistor there but the circuit >shuts down. >. >I also tried a honking big "high speed switching" transistor pulled >out of a PSU but it made the LEDs go very dim. > >Any ideas?
Check the phasing of the feedback coil versus the collector coil -switch ONE only pair of leads around . The coil can be wound with a tap or separately. If you use a tap and keep winding in the same direction it will be phased right. OR the "start" of one winding must be connected to the "finish" of the other and wound in the same direction. Phased wrong and it won't oscillate, but will pull current. Once you get it working. Keep a load on the output. It is possible to get voltage spikes over 80 volts with no load and that may be enough to eat your transistor. I haven't fried a 2N2222A due to over voltage, but some 2N4401's I had died almost instantly. BTW this circuit (and some variations like cap across the resister or electrolytic cap in series with the resistor and variable resistors) is rightly called a "blocking oscillator." It dates back from the days of vacuum toobs. I put one on my bicycle for a horn - it appeared in Radio Electronics under the heading of "Build the Sonic Shake Table" - basically driving a speaker voice coil with variable frequency (you could put mercury or flour, or other fine particles - or liquid with a plastic liner - in the upturned woofer speaker cone and watch the standing waves form in the material as you tuned through its resonant frequency). --
On Thu, 23 Jul 2009 04:20:21 -0700 (PDT), fungus
<openglMYSOCKS@artlum.com> wrote:

PS

Another possibility is the choice of a poor core material.  You may
have something that has too low permeability for this application.  If
that's the case, a few more turns of wire may make a difference.
(like 30-40 turns instead of 10-20)

Check out this site, he's got some better drawings and circuit
variations:

http://cappels.org/dproj/ledpage/leddrv.htm#Rusty_Nail_Night_Light


-- 
On Jul 23, 2:03=A0pm, default <defa...@defaulter.net> wrote:
> > Check the phasing of the feedback coil versus the collector coil > -switch ONE only =A0pair of leads around . > > The coil can be wound with a tap or separately. =A0If you use a tap and > keep winding in the same direction it will be phased right. >
If you get it wrong the LEDs don't light up at all.
> Once you get it working. =A0Keep a load on the output. =A0It is possible > to get voltage spikes over 80 volts with no load and that may be > enough to eat your transistor. =A0 >
I'm doing that. The LEDs are lighting up nicely but the transistor gets hotter and hotter until eventually it gives up. I thought it was because the voltage at the transistor base was going to negative 10V. Adding D7 this morning cured that but the transistor is still getting hot.
On Jul 23, 1:52=A0pm, David Eather <eat...@tpg.com.au> wrote:
> fungus wrote: > > > > Any ideas? > > Yes. Figure out what you want to do and state it explicitly and exactly. > Then work to that goal in steps you understand.
a) I want to light up some LEDs (eg. six of them) using batteries, eg. three AAAs. Circuit is decorative and has to be small because I want to hide it. b) I want them to be as bright as possible - the full 20mA or as close to it as I can get. c) It's a battery ... so voltage is going to drop over time (from 4.6V to 3.3V), this makes part (b) problematic. I accept that current will drop a bit, but if it can stay in the range 15-20mA then that's Ok. I've figured out that a Joule Thief is much closer to these characteristics than a simple resistor circuit doesn't. See the graph I plotted here: http://www.artlum.com/jt/jt_vs_res.gif But ... at the moment it's eating up transistors.
On Jul 23, 2:26=A0pm, default <defa...@defaulter.net> wrote:
> > Another possibility is the choice of a poor core material. =A0You may > have something that has too low permeability for this application. =A0If > that's the case, a few more turns of wire may make a difference. > (like 30-40 turns instead of 10-20) >
I've just got my reel of magnet wire in the post so the next step is to play with the number of turns to get the right output current. OTOH I'd be raising the current, not lowering it, and I don't see how the number of turns would be related to the transistor temperature.
On Jul 23, 2:26=A0pm, default <defa...@defaulter.net> wrote:
> On Thu, 23 Jul 2009 04:20:21 -0700 (PDT), fungus > > <openglMYSO...@artlum.com> wrote: > > PS > > Another possibility is the choice of a poor core material. =A0You may > have something that has too low permeability for this application. =A0If > that's the case, a few more turns of wire may make a difference. > (like 30-40 turns instead of 10-20) > > Check out this site, he's got some better drawings and circuit > variations: > > http://cappels.org/dproj/ledpage/leddrv.htm#Rusty_Nail_Night_Light >
There's an interesting bit at the bottom which says: "I was intrigued with getting the BC107 to run and added a small capacitor (22nF) across the base resistor to 'kick-start' the oscillations. It worked so well - with various transistors and coils that I was further intrigued to see how much I could increase the resistor value - hence the 20k trimpot. (The 22nF also got over the problem of the oscillator failing when I tried to add an ammeter in the battery circuit). I found it would continue to oscillate right up to 20k ohms and this also had the effect of reducing the supply current (osc only) from 90mA to 800microamps - very important if using batteries." ...
On Thu, 23 Jul 2009 06:16:52 -0700 (PDT), fungus
<openglMYSOCKS@artlum.com> wrote:

>On Jul 23, 2:26&#2013266080;pm, default <defa...@defaulter.net> wrote: >> >> Another possibility is the choice of a poor core material. &#2013266080;You may >> have something that has too low permeability for this application. &#2013266080;If >> that's the case, a few more turns of wire may make a difference. >> (like 30-40 turns instead of 10-20) > >I've just got my reel of magnet wire in the post so the next step is >to play with the number of turns to get the right output current. >OTOH I'd be raising the current, not lowering it, and I don't see >how the number of turns would be related to the transistor >temperature.
BJT temperature is related (obviously) to its power dissipation. That power dissipation comes from a variety of possible corners from my hobby viewpoint: (1) The transistor has been damaged (diode put in later, after it was already ruined perhaps?) and isn't operating well anymore; or, (2) the base-emitter junction current; or, (3) collector-emitter current times collector-emitter voltage; or, (4) frequency of operation is too high for the reverse transit time of the BJT. Your comment about it being able to operate at 100's of MHz is mostly wrong, by the way... what you saw in the Wiki article is the f_t, which is NOT what you can operate it at in this circuit. That is the frequency where the short circuit signal current gain drops to 1 and it varies with Ic... quite a bit, if I recall. I think the spec you are reading assumes Ic=20mA, or so. You don't operate BJTs anywhere near their ft for this. Divide by a factor of 20, at least. Also, capacitances can become increasingly important as your frequencies rise. I think these are in the 10's of pF for the 2N2222, so I don't think power dissipation due to capacitances will be the problem in this case. For higher frequencies, I think the reverse transit time will be a problem. Item (2) has Vbe which remains high during the BJT on cycle (0.9V?) and the current remains fairly fixed. Item (3) has a linear rise in current that gets pretty high near the end, but the Vce voltage remains relatively low (under 0.2V) for most of the time. Hand waving, I'd guess that power dissipation in item (3) should be about 4-5 times higher than in item (2), so I'd focus on Ic*Vce as more likely the problem if your frequency of operation is under 100kHz or so. But as the frequency rises much above that (and few turns/low inductances in your transformer will do that to you), the power dissipation (for the same output current) goes way up as you start encroaching on the reverse transit time. With the 2N2222, this figure is about 1/10th of a microsecond. So by the time you get up to 1Mhz or so (reverse transit is about 10% of your total time in this case), you are in serious trouble with item (3). This means that there is a very large Vce on your 'off' transistor __but__ there is still a significant Ic while charges are swept out of the BJT -- which means power dissipation. So wind more turns and get the frequency near or under 100kHz, or so, where the reverse transit will be only be a few percent or so and won't be wasting a lot of power. The
"fungus" <openglMYSOCKS@artlum.com> schreef in bericht 
news:ddeb7f42-6adb-4135-8544-a913ef763879@e27g2000yqm.googlegroups.com...
>I just got some proper parts to start making joule thieves but I'm > still > having problems. > > The circuit is this: http://www.artlum.com/jt/joulethief.gif > > Except I have R1 and L1 one the other way around (as in the original > web page at http://www.emanator.demon.co.uk/bigclive/joule.htm ) > > The problem is that my transistors keep on overheating and dying. > Why should this be? I'm using a 2N2222 in metal can (as shown here > http://en.wikipedia.org/wiki/2N2222 ). These can switch at hundreds > of megahertz so I don't think it's because of slow switching. > > I measured the current at point X and it seems high - over 100mA. > Could this be the cause of the overheating? Even if it isn't the > problem > it seems wasteful. I tried putting in a resistor there but the circuit > shuts down. > . > I also tried a honking big "high speed switching" transistor pulled > out of a PSU but it made the LEDs go very dim. > > Any ideas?
Well, you bluntly got a circuit designed to run on <=1.5V and powered it with 4.5V. Suppose the coil worked fine in the original circuit (did you try?) two important things will happen: - The frequency will raise. It easyly raises beyond the maximum frequency the coils core can handle. As a result, the transistor will stay long in the lineair region and get hot. Too hot. - The base-emitter voltage of the transistor will fall below -5V, which will make the transistor malfunction and may even damage it. The diode you added makes things worse as the off-time of the transistor will be shortened. So you need to move the diode. Place it between R1 and the base of the transistor. Alternatively you can place it between the emitter and gnd but make sure it can handle the current. In both cases make in point the same direction as the be-junction. Next step is adding more turns to the coil as others stated already. Guess you will need 1.5-2 times the original number of turns. petrus bitbyter