Joule Thief - still not working....

[fungus]

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?

 

[David Eather]

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.

 

[default]

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).
--

 

[default]

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


--

 

[Jon Kirwan]

On Thu, 23 Jul 2009 06:16:52 -0700 (PDT), fungus
<openglMYSOCKS@artlum.com> wrote:

On Jul 23, 2:26 pm, default <defa...@defaulter.net> wrote:

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)

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

 

[petrus bitbyter]

"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

 

[fungus]

On Jul 23, 2:03 pm, default <defa...@defaulter.net> wrote:

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.

If you get it wrong the LEDs don't light up at all.

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'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.

 

[fungus]

On Jul 23, 1:52 pm, 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.

 

[Jon Kirwan]

On Thu, 23 Jul 2009 06:49:21 -0700 (PDT), fungus
<openglMYSOCKS@artlum.com> wrote:

On Jul 23, 2:26 pm, 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.  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

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.

Called a 'speed-up' capacitor. For a short time, it acts as a short
circuit across the resistor. And in this case, it will dramatically
increase your BJT dissipation due to substantially increased base
current for a short time near the early on-time. If it is anything
near 1000's of pF, anyway. (22nF is way, way too much, I think.)

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."

The amount of delivered power to the LEDs goes down. In other words,
at some point it is a bad idea because of the transistor's increased
dissipation.

You made me curious enough to try a simulation with 200uH on each half
of the transformer, a 2N2222 BJT, a 1N5819 freewheeling diode, 10uF
output cap (across the 6 LEDs with Vfwd=3, Ron=12.6), and 3 fresh
batteries. The Spice results look like:

Battery C BJT LEDS
436mW 0pF 28mW 19.9mA
428mW 22pF 16mW 20.0mA
425mW 220pF 19mW 19.7mA
392mW 2.2nF 50mW 16.5mA
173mW 22nF 142mW 0.0mA

So battery power does go down. But BJT power goes up and they
eventually cross and the whole endevour is a waste. In other words,
the BJT starts eating up all the power and eventually eats up
everything there is.

If you look at the above closely, there does seem to be a small range
of values that work well. Say up to a few hundred pF in the example
case I let Spice run on. Everything depends, though. So mileage may
vary.

Jon

 

[fungus]

On Jul 23, 2:26 pm, default <defa...@defaulter.net> wrote:

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)

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.

 

[fungus]

On Jul 23, 2:26 pm, 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.  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

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."

....

 

[John Larkin]

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?

That's a horrible circuit. Too many conflicting parameters depend on
the value of R1. A proper blocking oscillator uses an RC time constant
to set the rep rate, and a separate resistor to limit the base
current.

ftp://jjlarkin.lmi.net/BlockOsc.JPG

John

 

[default]

On Thu, 23 Jul 2009 06:49:21 -0700 (PDT), fungus
<openglMYSOCKS@artlum.com> wrote:

On Jul 23, 2:26 pm, 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.  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


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."

...

Core material and number of turns will affect the frequency of

oscillation and that can affect power dissipation. As frequency goes
up (fewer turns or less permeable core) the transistor may get warmer
due to core losses and storage time (staying on when it should be off
for a few nano to microseconds - doing it more often).

Then there's D8. That should be a high speed diode not a run of the
mill 60 Hz rectifier. A cheap 1N4148 or 1N914 switching diode will be
good enough for LEDs - they switch very fast and can tolerate 100
milliamps and up to 100 Volts (manufacture specs may vary, but that's
a general idea of what they are rated at).

The cap across the resistor is, in my opinion, a good idea as long as
you don't get too carried away. (and .022 uf sounds a little high -
but a lot depends on how fast it is running too) It serves to drive
the transistor harder when the initial switching occurs and that is a
good thing. The base emitter dissipation will rise - but the emitter
- collector voltage should go down (and that usually accounts for more
dissipation since the current is much higher there). You want it to
switch fast and not dilly-dally in the linear region. (all other
things being equal . . . )
--

 

[Jon Kirwan]

On Thu, 23 Jul 2009 09:24:55 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

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?

That's a horrible circuit. Too many conflicting parameters depend on
the value of R1. A proper blocking oscillator uses an RC time constant
to set the rep rate, and a separate resistor to limit the base
current.

ftp://jjlarkin.lmi.net/BlockOsc.JPG

Would you care to provide some sample values and analyze that circuit
for us? I see the RC node moving towards a bias point, but not really
setting the frequency at which the BJT goes on and off. But I haven't
sat down more than to glance over it, yet.

Jon

 

[bw]

"fungus" <openglMYSOCKS@artlum.com> wrote in message
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

Start with the original circuit, and get it to work on one battery.
http://cappels.org/dproj/ledpage/leddrv.htm#Rusty_Nail_Night_Light

then go from there to what you want to do.

 

[John Larkin]

On Thu, 23 Jul 2009 19:04:43 GMT, Jon Kirwan
<jonk@infinitefactors.org> wrote:

On Thu, 23 Jul 2009 09:24:55 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

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?

That's a horrible circuit. Too many conflicting parameters depend on
the value of R1. A proper blocking oscillator uses an RC time constant
to set the rep rate, and a separate resistor to limit the base
current.

ftp://jjlarkin.lmi.net/BlockOsc.JPG

Would you care to provide some sample values and analyze that circuit
for us?

No, too much work.

I see the RC node moving towards a bias point, but not really

setting the frequency at which the BJT goes on and off. But I haven't
sat down more than to glance over it, yet.

In general, "on" pulse width is set by the volt-second saturation of
the inductor (although a small value of C can make it shorter.) Base
current is limited by R2 (the one connected to the base.) While the
transistor's on, the base current charges up the cap, and that charge
will back-bias the transistor until R1 recharges the cap back up to
+0.7 volts, at which it fires again.

Something like that.

Try R1=1K, R2=100 C=100nF as very rough starting points. A lot depends
on the inductor. It won't Spice unless the model includes inductor
saturation.

It's probebly easier to use a Tiny Logic schmitt-trigger oscillator to
drive the transistor, and just use a single-winding inductor. Blocking
oscillators are tricky.


John

 

[Jon Kirwan]

On Thu, 23 Jul 2009 13:23:15 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 23 Jul 2009 19:04:43 GMT, Jon Kirwan
jonk@infinitefactors.org> wrote:

On Thu, 23 Jul 2009 09:24:55 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

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?

That's a horrible circuit. Too many conflicting parameters depend on
the value of R1. A proper blocking oscillator uses an RC time constant
to set the rep rate, and a separate resistor to limit the base
current.

ftp://jjlarkin.lmi.net/BlockOsc.JPG

Would you care to provide some sample values and analyze that circuit
for us?

No, too much work.

Hmm.

Just to goose things along, for the joule thief circuit I get
something like this for the frequency:

(Vbattery - Vsat) * (Vout + Vfreewheeldiode - Vbattery)
f = -------------------------------------------------------
Ic_peak * L_collector * (Vout + Vfreewheeldiode - Vsat)

Ic_peak may require an iteration or two with a datasheet to
approximate. I just go in with an assumed Ic, look up a beta estimate
for that on one curve and then grab the Vbe estimate from another
curve, and apply them into:

Ic_peak = beta*(Nratio*(Vbattery - Vsat) + Vbattery - Vbe))/Rbase

That Ic_peak is then used to repeat the process. When it settles,
I've usually got a reasonable figure that I can use to compute 'f'.
(Nratio is the turns ratio, usually just 1.) I tend to use Vsat=0.2V.

If your suggestion is so nicely designable, can't you at least provide
an approximate equation?

I see the RC node moving towards a bias point, but not really
setting the frequency at which the BJT goes on and off. But I haven't
sat down more than to glance over it, yet.

In general, "on" pulse width is set by the volt-second saturation of
the inductor (although a small value of C can make it shorter.)

So in your circuit case, it does depend on saturation of the core.
What would happen in an air core case?

Base
current is limited by R2 (the one connected to the base.) While the
transistor's on, the base current charges up the cap, and that charge
will back-bias the transistor until R1 recharges the cap back up to
+0.7 volts, at which it fires again.

Something like that.

Try R1=1K, R2=100 C=100nF as very rough starting points. A lot depends
on the inductor. It won't Spice unless the model includes inductor
saturation.

Yes. I gather.

It's probebly easier to use a Tiny Logic schmitt-trigger oscillator to
drive the transistor, and just use a single-winding inductor. Blocking
oscillators are tricky.

Single BJTs are cheap and, if you saw one of the web sites mentioned
some time back in the related thread, you'd have seen that the whole
thing is tiny enough to place inside a small flashlight bulb base.

....

Since you write, "That's a horrible circuit. Too many conflicting
parameters depend on the value of R1. A proper blocking oscillator
uses an RC time constant to set the rep rate, and a separate resistor
to limit the base current," shouldn't it be the case that you can tell
me how to compute the frequency with ease? Isn't that the entire
point of saying all that? Or did I miss your point, here?

Jon

 

[default]

On Thu, 23 Jul 2009 14:30:02 -0700 (PDT), fungus
<openglMYSOCKS@artlum.com> wrote:

On Jul 23, 2:03 pm, default <defa...@defaulter.net> wrote:

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.


I assume the winding after the tap has to go over the top of the
previous winding, right?

No. They can be side by side. There may be some slight advantage to
covering the whole toroid (ferrite bead) core (if that's what you
have) with the collector winding to keep magnetic leakage low.

I'd put the collector winding closer to the core or just wind them
side by side.

I like to wind bifilar when winding two windings with the same turns
count on a core. It is slightly more hassle to phase correctly, but
if my turns count is off, it will be off proportionately on both
windings (which is important when the winding has to be balanced -
which is not the case here)
--

 

[John Larkin]

On Thu, 23 Jul 2009 20:32:20 GMT, Jon Kirwan
<jonk@infinitefactors.org> wrote:

On Thu, 23 Jul 2009 13:23:15 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 23 Jul 2009 19:04:43 GMT, Jon Kirwan
jonk@infinitefactors.org> wrote:

On Thu, 23 Jul 2009 09:24:55 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

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?

That's a horrible circuit. Too many conflicting parameters depend on
the value of R1. A proper blocking oscillator uses an RC time constant
to set the rep rate, and a separate resistor to limit the base
current.

ftp://jjlarkin.lmi.net/BlockOsc.JPG

Would you care to provide some sample values and analyze that circuit
for us?

No, too much work.

Hmm.

Just to goose things along, for the joule thief circuit I get
something like this for the frequency:

(Vbattery - Vsat) * (Vout + Vfreewheeldiode - Vbattery)
f = -------------------------------------------------------
Ic_peak * L_collector * (Vout + Vfreewheeldiode - Vsat)

Ic_peak may require an iteration or two with a datasheet to
approximate. I just go in with an assumed Ic, look up a beta estimate
for that on one curve and then grab the Vbe estimate from another
curve, and apply them into:

Ic_peak = beta*(Nratio*(Vbattery - Vsat) + Vbattery - Vbe))/Rbase

That Ic_peak is then used to repeat the process. When it settles,
I've usually got a reasonable figure that I can use to compute 'f'.
(Nratio is the turns ratio, usually just 1.) I tend to use Vsat=0.2V.

If your suggestion is so nicely designable, can't you at least provide
an approximate equation?

I see the RC node moving towards a bias point, but not really
setting the frequency at which the BJT goes on and off. But I haven't
sat down more than to glance over it, yet.

In general, "on" pulse width is set by the volt-second saturation of
the inductor (although a small value of C can make it shorter.)

So in your circuit case, it does depend on saturation of the core.
What would happen in an air core case?

The classic tube "blocking oscillator" had its ON time determined by
inductor saturation. If it can't saturate, the ON interval ends when
the transistor runs out of beta (or the tube out of plate current), or
when C runs out of charge to drive the base/grid. The "blocking" part
was the negative swing on the tube grid from grid current charging the
cap; it fired again when R1 charged the grid the other way, back to
the turnon threshold.

Base
current is limited by R2 (the one connected to the base.) While the
transistor's on, the base current charges up the cap, and that charge
will back-bias the transistor until R1 recharges the cap back up to
+0.7 volts, at which it fires again.

Something like that.

Try R1=1K, R2=100 C=100nF as very rough starting points. A lot depends
on the inductor. It won't Spice unless the model includes inductor
saturation.

Yes. I gather.

Unless L can't saturate, of course. Then it's not an official
"blocking oscillator."

It's probebly easier to use a Tiny Logic schmitt-trigger oscillator to
drive the transistor, and just use a single-winding inductor. Blocking
oscillators are tricky.

Single BJTs are cheap and, if you saw one of the web sites mentioned
some time back in the related thread, you'd have seen that the whole
thing is tiny enough to place inside a small flashlight bulb base.

If you don't mind the 2-winding coil, and the additional futzing, the
blocking oscillator is potentially cheap.

...

Since you write, "That's a horrible circuit. Too many conflicting
parameters depend on the value of R1. A proper blocking oscillator
uses an RC time constant to set the rep rate, and a separate resistor
to limit the base current," shouldn't it be the case that you can tell
me how to compute the frequency with ease? Isn't that the entire
point of saying all that? Or did I miss your point, here?

As I said, a blocking oscillator is complex. I can't define the
frequency "with ease." But having separate control over base drive and
rep-rate helps orthogonalize things. Having one part control two
circuit parameters can get awkward. Three is a nightmare.

The MIT RadLab books are full of blocking oscillator theory and
circuits, especially vol 19. Tube radars were full of them, as
oscillators, comparators, pulse regenerators, and frequency dividers.

Some texts referred to rf squegging circuits as blocking oscillators.

John

 

[David Eather]

fungus wrote:

On Jul 23, 1:52 pm, 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.

What voltage are you running the joule thief on?