Joule Thief - still not working....

fungus wrote:

> ie. Lower frequencies mean the transistor will be switched on for less
> percentage of the time and more electrons will go through the load
> instead of being dumped to ground via the transistor.

No, the point is that the transistor should be either
fully on or fully off for as great a proportion of
time as possible.

The time that the transistor takes to switch on or
off (given a sharp driving waveform to the base) is
fairly constant and depends on the characteristics
of the transistor.

So increasing the duration of the on-time makes the
proportion of in-between time smaller.

-- 
Greg

John Larkin wrote:

> In general, "on" pulse width is set by the volt-second saturation of
> the inductor

In the circuit you gave, it seems to me that if the
inductor saturates, rather than terminating the
on-pulse, the collector current is just going to
shoot sky-high.

-- 
Greg

fungus wrote:

> To drive six LEDs at 20mA with one battery you'd
> have to get the frequency up into the mHz (which
> isn't going to happen).
> 
> The solution seems to be to raise the input voltage

That's not the only way -- you can also raise
the peak collector current, so that you draw
power from the battery at a low voltage and
high current, and deliver it to the LEDs at a
higher voltage and lower current.

To get a higher peak collector current, you need
to increase the base current. You can do that
either by lowering the base resistor, or increasing
the number of base winding turns relative to the
collector winding.

Be careful, though -- keep an eye on the average
LED current and make sure it doesn't go over
20mA.

Also keep the maximum current rating of the
transistor in mind. If the voltage drop across
the LEDs is N times the battery voltage, and the
average LED current is 20mA, then the average
transistor current will be N times 20mA.

However, the transistor will be on for only
1/N of the time, so the *peak* transistor current
will be N *squared* times 20mA.

-- 
Greg

whit3rd wrote:

> That kind of oscillator (blocking oscillator) depends on saturation
> of the core

Actually, no. That's what I thought at first, but
Jon pointed out that the Joule Thief most likely
works by a different mechanism.

The collector current rises until it reaches the
maximum supportable by the base current, which
depends on the induced voltage in the base winding
and the base resistor. Then the collector voltage
begins to rise, whereupon positive feedback via
the base winding causes the transistor to turn
off sharply.

So the on-time depends on the feedback ratio, the
base resistor and the beta of the transistor. The
latter is rather unpredictable, so you have to
adjust the base resistor by experiment to get the
result you want.

I've speculated that a version of the Joule Thief
circuit could be designed to work by core saturation,
and that the results would be more predictable.
But I don't know of anyone who's actually built one
that way yet.

-- 
Greg

Jon Kirwan wrote:

> Are the saturation of cores more predictable than BJT beta -- keeping
> in mind that we are talking about the same part number AND
> manufacturer in both cases?

I can't say for sure, but I'd be surprised if it wasn't.
Transistor beta is a spectacularly crappy parameter to
rely on -- it would be quite hard to do any worse!

Also I know for a fact that pulse-generating circuits
using saturating inductors were quite common at one
time, so they must have been reasonably predictable.

BTW, I've had another thought about the use of saturation
in a Joule Thief type circuit. Using it for pulse
generation is one thing, but in that case you're only
interested in the output voltage. The inductor doesn't
have to store much energy, so you can use a high
permeability material that is easy to saturate and
has a fairly sharp saturation characteristic.

But in our case we need energy storage, so we need
lower permeability. That makes the core harder to
saturate, and may also make it saturate more "softly"
with a less sharply-defined saturation point. So
it might not work so predictably after all.

But that's pretty much guesswork on my part at the
moment.

-- 
Greg

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.  You are likely to be disappointed by the joule thief,
for what you are trying to do, unless you just want to experiment
with it.  You are also likely to be confused by all the hand
waving and arguing going on in replies in this thread.

I can see it now - you finally get your joule thief running
without killing transistors - and your next questions will be:
"why does it last for only a day?"
"how can I keeep the brightness up? It gets dim over time"
"can I get an  Obama bailout for the cost of all these batteries?"

_Limited run time_

You want to run 6 LEDs at 20 mA. Let's assume that each LED has a
1.8 volt Vf. That means you need to boost your supply to 10.8 volts.
The power needed by the LEDs is 10.8 * .02 or 216  miliwatts.  A
typical new AAA cell is rated at 1.5 volts, 1250 mAh.  To produce
10.8 volts at 20 mA from that cell, you must draw 144 mA assuming a 
perfect conversion circuit. (Your joule thief is far from perfect.)
If you use three cells, you can get (mathematically) only 26 hours
run time.
Solution: larger batteries and/or more of them. To overcome losses
in the joule thief, use a better circuit, but you are in all cases
limited by the power available from the batteries vs the power used
by the LEDs.

_LED brightness will decrease over time_

The joule thief will not deliver constant current to the LEDs, so
brightness will decrease as battery voltage drops.
Solution: a better (constant current or PWM) circuit.

_Cost_

The joule thief will "chew up" batteries quickly.  Imagine the
cost of replacing 3 AAA's every day or 3 D's every three weeks.
Solution:  mains power.  Mains power solves the other issues,
as well.

If you _must_ use battery power, there are one chip solutions
better than the joule thief.

Hopefully, you are in this more for the experimentation than
anything else.  In that case, the joule thief is a wonderful
circuit to play with, and learn from.

Ed

On Fri, 24 Jul 2009 07:50:41 GMT, Jon Kirwan
<jonk@infinitefactors.org> wrote:

>On Thu, 23 Jul 2009 20:25:03 -0700, John Larkin
><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Thu, 23 Jul 2009 23:34:04 GMT, Jon Kirwan
>><jonk@infinitefactors.org> wrote:
>>
>>>On Thu, 23 Jul 2009 16:06:41 -0700, John Larkin
>>><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>>>
>>>>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."
>>>
>>>Are the saturation of cores more predictable than BJT beta -- keeping
>>>in mind that we are talking about the same part number AND
>>>manufacturer in both cases?
>>>
>>>>>>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.
>>>
>>>But the existing schematic (the joule thief thing) does that, within
>>>bounds.  Assuming fixed battery voltage and fixed winding ratio of the
>>>transformer, the base resistor sets the Ib.  The beta then establishes
>>>the peak Ic.  I'm not sure of any advantages in the new arrangement
>>>you suggest, yet.  (And I suspect it's behavior is harder to analyze,
>>>besides.)
>>
>>In the OP's ascii-art circuit, R1 determines ON base current (and
>>perhaps ON time, if the inductor doesn't saturate first)
>
>Yes.  R1 determines the ON base drive, together of course with the
>battery voltage, less something for the Vbe.  The base drive
>determines and BJT beta determines the peak sustainable Ic (leaving
>out core satuation considerations), which then determines the ON
>_time_.
>
>>and also sets
>>OFF time, as part of the L/R decay.
>
>This, I do NOT see.  Off time is determined by the required voltage
>across the collector winding when the BJT goes OFF and the field
>collapses, reversing the polarity.  I really do NOT see how R1 plays
>into that calculation, at all.  The base winding really isn't tapping
>much field energy -- most of which is being either driving out through
>the LEDs or else via the freewheeling diode (1N5819?) and cap on the
>output I suggested elsewhere.  (I suggested also a diode to protect
>the BJT base, but that's a separate issue.)
>
>Can you explain how R1 plays into an L/R decay time?  I'd like to hear
>about it.

No need to get snippy about things.

When the transistor turns off, the flyback voltage gets dumped into
the LEDs, guaranteeing part of the OFF interval. When the transformer
runs out of stored energy, Il=0 and the secondary essentially
disappears from the circuit. Now we have the feedback winding in
series with R into the base. Base current will build up through that
L/R until the transistor turns back on and we get another positive
feedback cycle. It's complicated by the ringing in the transformer
(Millman and Taub caution about the complications of ringing!) which
can restart a cycle, so the turn-on situation is very messy. At any
rate, the value of R influences both the ON time and the OFF time.

Adding a cap across R enforces a more predictable OFF time. Adding a
parallel RC in series with R is even better, and is a variant on the
circuit I posted. That also allows independent base drive and OFF time
tuning.

I wasn't kidding when I said blocking oscillators are complex.
Impressive for so simple a circuit.

>
>>It may be hard to pick one value
>>that does both right, namely produces an efficient duty cycle, as
>>witnessed by the many blown up transistors.
>
>Well, until I gather your point about the OFF time's L/R, I have to
>withhold further comment.
>
>>The big advantage of the circuit I posted is that rep-rate can be set
>>independent of pulse width... two knobs to turn. That allows low duty
>>cycles which won't fry transistors. And brightness control, if you
>>want it.
>
>I need to first fathom your L/R point and then I need to spend more
>time with the suggested circuit you gave before I can agree.

I don't see why. The circuit I posted enforces OFF time deliberately
and un-complicates things.

John

"ehsjr" <ehsjr@NOSPAMverizon.net> wrote in message 
news:1ojam.1040$MA3.967@nwrddc02.gnilink.net...
> 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.  You are likely to be disappointed by the joule thief,
> for what you are trying to do, unless you just want to experiment
> with it.  You are also likely to be confused by all the hand
> waving and arguing going on in replies in this thread.
>
> I can see it now - you finally get your joule thief running
> without killing transistors - and your next questions will be:
> "why does it last for only a day?"
> "how can I keeep the brightness up? It gets dim over time"
> "can I get an  Obama bailout for the cost of all these batteries?"
>
> _Limited run time_
>
> You want to run 6 LEDs at 20 mA. Let's assume that each LED has a
> 1.8 volt Vf. That means you need to boost your supply to 10.8 volts.
> The power needed by the LEDs is 10.8 * .02 or 216  miliwatts.  A
> typical new AAA cell is rated at 1.5 volts, 1250 mAh.  To produce
> 10.8 volts at 20 mA from that cell, you must draw 144 mA assuming a 
> perfect conversion circuit. (Your joule thief is far from perfect.)
> If you use three cells, you can get (mathematically) only 26 hours
> run time.
> Solution: larger batteries and/or more of them. To overcome losses
> in the joule thief, use a better circuit, but you are in all cases
> limited by the power available from the batteries vs the power used
> by the LEDs.
>
> _LED brightness will decrease over time_
>
> The joule thief will not deliver constant current to the LEDs, so
> brightness will decrease as battery voltage drops.
> Solution: a better (constant current or PWM) circuit.
>
> _Cost_
>
> The joule thief will "chew up" batteries quickly.  Imagine the
> cost of replacing 3 AAA's every day or 3 D's every three weeks.
> Solution:  mains power.  Mains power solves the other issues,
> as well.
>
> If you _must_ use battery power, there are one chip solutions
> better than the joule thief.
>
> Hopefully, you are in this more for the experimentation than
> anything else.  In that case, the joule thief is a wonderful
> circuit to play with, and learn from.
>
> Ed

My experiments show that the efficiency is dependent on minor adjustments to 
the windings due to ugly waveform. I'll check my notes, about 50 percent was 
the best I found, at 50 to 100 kHz, using a ferrite core, with 10 percent 
duty.

On Fri, 24 Jul 2009 23:05:56 +1200, greg <greg@cosc.canterbury.ac.nz>
wrote:

>John Larkin wrote:
>
>> In general, "on" pulse width is set by the volt-second saturation of
>> the inductor
>
>In the circuit you gave, it seems to me that if the
>inductor saturates, rather than terminating the
>on-pulse, the collector current is just going to
>shoot sky-high.

Yes, very briefly. The resulting rise in collector voltage is coupled
into the base circuit such as to turn the transistor off, in a
positive feedback loop (just as the turn-on was positive feedback.)
The turnoff snap is usually very fast, or at least should be if
everything is done right. A capacitor across the resistor helps here.

The cycle can also terminate if the transistor runs out of beta
(against the current ramping up in the inductor) or if the capacitor
runs out of charge driving the base. So the inductor need not
saturate, although that is the most efficient way to use the
magnetics.

I think the origial joule-thief (or whatever) worked well because the
1.5 volt battery was about half of Vbe_on, which made the duty cycle
work about right, ballpark 50%. Higher battery voltages push the ON
duty cycle up and toast transistors. Something like that.

John

On Fri, 24 Jul 2009 08:51:39 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Fri, 24 Jul 2009 07:50:41 GMT, Jon Kirwan
><jonk@infinitefactors.org> wrote:
>
>>On Thu, 23 Jul 2009 20:25:03 -0700, John Larkin
>><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>>
>>>On Thu, 23 Jul 2009 23:34:04 GMT, Jon Kirwan
>>><jonk@infinitefactors.org> wrote:
>>>
>>>>On Thu, 23 Jul 2009 16:06:41 -0700, John Larkin
>>>><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>>>>
>>>>>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."
>>>>
>>>>Are the saturation of cores more predictable than BJT beta -- keeping
>>>>in mind that we are talking about the same part number AND
>>>>manufacturer in both cases?
>>>>
>>>>>>>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.
>>>>
>>>>But the existing schematic (the joule thief thing) does that, within
>>>>bounds.  Assuming fixed battery voltage and fixed winding ratio of the
>>>>transformer, the base resistor sets the Ib.  The beta then establishes
>>>>the peak Ic.  I'm not sure of any advantages in the new arrangement
>>>>you suggest, yet.  (And I suspect it's behavior is harder to analyze,
>>>>besides.)
>>>
>>>In the OP's ascii-art circuit, R1 determines ON base current (and
>>>perhaps ON time, if the inductor doesn't saturate first)
>>
>>Yes.  R1 determines the ON base drive, together of course with the
>>battery voltage, less something for the Vbe.  The base drive
>>determines and BJT beta determines the peak sustainable Ic (leaving
>>out core satuation considerations), which then determines the ON
>>_time_.
>>
>>>and also sets
>>>OFF time, as part of the L/R decay.
>>
>>This, I do NOT see.  Off time is determined by the required voltage
>>across the collector winding when the BJT goes OFF and the field
>>collapses, reversing the polarity.  I really do NOT see how R1 plays
>>into that calculation, at all.  The base winding really isn't tapping
>>much field energy -- most of which is being either driving out through
>>the LEDs or else via the freewheeling diode (1N5819?) and cap on the
>>output I suggested elsewhere.  (I suggested also a diode to protect
>>the BJT base, but that's a separate issue.)
>>
>>Can you explain how R1 plays into an L/R decay time?  I'd like to hear
>>about it.
>
>No need to get snippy about things.

I didn't think I was being.  I was directly telling you what I see and
asking for an explanation why I am wrong.  And yes, I really would
like to hear about it.  I meant it.  What else did you read into this?

>When the transistor turns off, the flyback voltage gets dumped into
>the LEDs, guaranteeing part of the OFF interval.

I'm with you, here.  My first assumption would be that it determines
fully the OFF interval, without additional consideration (which I'm
not brushing off, as it is important to do.)

>When the transformer
>runs out of stored energy, Il=0 and the secondary essentially
>disappears from the circuit.

Agreed.  That's always as I saw it, too.

>Now we have the feedback winding in
>series with R into the base.

The secondary (base) winding which, initially, will have zero voltage
across it and thus there will only be one battery voltage to work
with, at first.

>Base current will build up through that L/R until the transistor
>turns back on and we get another positive
>feedback cycle. It's complicated by the ringing in the transformer
>(Millman and Taub caution about the complications of ringing!) which
>can restart a cycle, so the turn-on situation is very messy. At any
>rate, the value of R influences both the ON time and the OFF time.
><snip>

This is where I put my feet down and say I cannot follow you, John.

Yes.  I agree that there is an L/R time constant here.  Completely
agree.  But it simply isn't important in the larger picture.  In most
cases (those along the lines of discussion here, anyway), we are
talking about perhaps a hundred nanoseconds for tau.  And well before
even that first tau is exhausted, the BJT is already ON.  It happens
so fast it just 'doesn't count.'  That's how I see it.

What I see happen instead, brushing away that unimportant detail, is
that the BJT turns on from the push of a single battery voltage at
first and then, once it turns on, there is another battery voltage
added to it by the secondary (base) winding to goose up the base
current to about twice (not quite) what it starts out as.

The few nanoseconds part of L/R before the BJT goes back ON just
aren't something to worry over.  As I see it.  And they certainly
don't contribute meaningfully to the OFF time, __as I see it.__

So this is where I'm stuck and cannot find agreement with you, yet.
The OFF time is NOT determined by R1, so far as I can tell.  And your
explanation doesn't in any way suggest that it should be.

Can you address yourself squarely here?  I'm seriously trying to learn
this.

Jon