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.Also what is the resistance value of R1? In the interim try this: 1 - increase R1 by a factor of 10 and next by a factor of 100, does that reduce the heating of the 2n2222? (if you are over-driving the base of the transistor and the inductance of the transformer is too small this would cause the problem you are seeing - I think JK is also thinking along these lines) 2 - increase the number of turns in both l1 and l2 by the same ratio e.g . double the number of turns on both or triple the number of turns on both. This should make the transformer more efficient (and you probably want to do that anyway)
Joule Thief - still not working....
Started by ●July 23, 2009
Reply by ●July 23, 20092009-07-23
Reply by ●July 23, 20092009-07-23
On Jul 24, 1:18=A0am, David Eather <eat...@tpg.com.au> wrote:> > What voltage are you running the joule thief on?Right now? I just measured 3.8V.
Reply by ●July 23, 20092009-07-23
David Eather wrote:> 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. > > Also what is the resistance value of R1? > > In the interim try this: >forget about point 1 - I made a wrong assumption about your circuit. I assumed you had a resistor connected from the base of the transistor to the junction l1 and d7. If you want to try it put a 1k resistor in -you may need to change this value upwards if the transistor still runs too hot or downwards if the circuit stops working.> 1 - increase R1 by a factor of 10 and next by a factor of 100, does that > reduce the heating of the 2n2222? (if you are over-driving the base of > the transistor and the inductance of the transformer is too small this > would cause the problem you are seeing - I think JK is also thinking > along these lines) > > 2 - increase the number of turns in both l1 and l2 by the same ratio e.g > . double the number of turns on both or triple the number of turns on > both. This should make the transformer more efficient (and you probably > want to do that anyway)
Reply by ●July 23, 20092009-07-23
On Jul 24, 2:06=A0am, David Eather <eat...@tpg.com.au> wrote:> 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 exactl=y.> >> 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. > > Also what is the resistance value of R1? >1k> In the interim try this: > > 1 - increase R1 by a factor of 10 ... does that > reduce the heating of the 2n2222? >Yes, but the current going through the LEDs dropped from 18mA to 5mA.
Reply by ●July 24, 20092009-07-24
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) and also sets OFF time, as part of the L/R decay. 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. 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.> >>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. > >I think someone posted some of that, some time back, in >sci.electronics.design. I'll see if I can track any of that down. >sadly, other than that possibility, I don't have ready access. > >>Some texts referred to rf squegging circuits as blocking oscillators. > >I'll look to see if I can find a lucid description.Terman's "Radio Engineering" texts refer to squeggers as blocking oscillators. But Terman was a little weird. Millman&Taub's classic "Pulse and Digital Circuits" has a whole chapter "Pulse Transformers and Blocking Oscillators", with a bunch of analysis. All tubes. John
Reply by ●July 24, 20092009-07-24
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.>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 apologize for my ignorance on this, but that's where I'm at right now.>>>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. >> >>I think someone posted some of that, some time back, in >>sci.electronics.design. I'll see if I can track any of that down. >>sadly, other than that possibility, I don't have ready access. >> >>>Some texts referred to rf squegging circuits as blocking oscillators. >> >>I'll look to see if I can find a lucid description. > >Terman's "Radio Engineering" texts refer to squeggers as blocking >oscillators. But Terman was a little weird. > >Millman&Taub's classic "Pulse and Digital Circuits" has a whole >chapter "Pulse Transformers and Blocking Oscillators", with a bunch of >analysis. All tubes.I'm modestly familiar with vacuum tubes, load lines, grid leak resistors (what a pain I had with those), etc. So I may somehow hope to be able to follow along. Jon
Reply by ●July 24, 20092009-07-24
fungus wrote:> On Jul 24, 2:06 am, David Eather <eat...@tpg.com.au> wrote: >> 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. >> Also what is the resistance value of R1? >> > > 1k > >> In the interim try this: >> >> 1 - increase R1 by a factor of 10 ... does that >> reduce the heating of the 2n2222? >> > > Yes, but the current going through the LEDs > dropped from 18mA to 5mA.Iff (if and only if) you are in the mood for experimenting, more turns on L2 might increase this current through the LED while R1 is 10k. Also the range of currents 18ma to 5ma (4:1) vs the range of resistance 1k to 10k (1:10) suggests that some value between 1k and 10k will reduce power consumption of the transistor with a much smaller decrease in current to the LED. Perhaps there is a suitable compromise.>A different subject - I am seeking information. How long does this thing have to run on one set of batteries? and if you can how much current is coming out of the batteries when the LED's are getting their 18ma? (if the 2n2222 is getting hot then this is a missing piece of information.)
Reply by ●July 24, 20092009-07-24
fungus wrote:> On Jul 23, 10:18 pm, "bw" <bweg...@hotmail.com> wrote: >> Start with the original circuit, and get it to work on one battery. >> >> then go from there to what you want to do. > > That's what I'm doing... > > The original circuit lights up a LED but the current > is very low - about 5mA. > > 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).No, the frequency thing is not correct. You can increase the output voltage by putting more turns on L2. It is the ratio of turns between L1 and L2 that mostly determines the output voltage. To go from 1 LED to 6 and upping the voltage by 3 times try doubling the turns on L2 and increasing R1 to about 3k (2.7k or 3.3 would both be fine) If you really wanted to nail it, triple the turns on L1 and put 6 times the number of turns you already have on L2 and change R1, but just L2 and R1 will get you closer to where you want to go.> > The solution seems to be to raise the input voltage > so that's what I'm trying to get working. Unfortunately > I never studied electronics beyond what they told me > in high-school physics class so I'm at a disadvantage. >
Reply by ●July 24, 20092009-07-24
fungus wrote:> The problem is that my transistors keep on overheating and dying. > Why should this be?My first suspicion would be switching losses in the transistor. What does the voltage waveform across the emitter and collector of the transistor look like? Does it have nice sharp vertical edges? If not, then you need to lower the operating frequency until the switching time is negligible compared to the on and off times. Increasing the number of turns on the inductor should do that.> I'm using a 2N2222 in metal can (as shown here > http://en.wikipedia.org/wiki/2N2222 ). These can switch at hundreds > of megahertzLike Jon said, f_T is NOT the relevant parameter here, it's only about amplifying small signals. You can't expect the transistor to switch between full-on and full-off at anywhere near that speed.> I measured the current at point X and it seems high - over 100mA.From the Wikipedia article, the 2N2222 should be able to handle that, so I don't think it's a concern, provided the transistor is switching efficiently.> I also tried a honking big "high speed switching" transistor pulled > out of a PSU but it made the LEDs go very dim.If it's designed to handle very high currents, it may not work very efficiently at the modest power levels you're concerned with here. From what Jon said, it sounds like the 2N2222 should be capable of the job as long as you don't try to run it at more than 100kHz or so. You really need to look at things with a scope and find out what's going on. Just fiddling randomly with the parameters is likely to lead to a lot of frustration and dead transistors. -- Greg
Reply by ●July 24, 20092009-07-24
fungus wrote:> I don't see > how the number of turns would be related to the transistor > temperature.Increasing the number of turns increases the inductance, so the current rises at a lower rate when the transistor is on. Assuming the base current remains the same (which it will be if you increase the number of base winding turns by the same amount and keep the base resistor the same) then more time will be taken for the collector current to reach its maximum value and trigger a turn-off. This means that the operating frequency will be lower, and the switching time of the transistor will be a smaller fraction of the on-time, leading to less power dissipation and a cooler transistor. That's the theory, anyway. -- Greg
