I just created a simple JT that you see around the net using my own wound bobbin (4t:~40t) using a S8055. As soon as I put in a BC337 along with the other transitor the LED will get brighter. It doesn't matter if it is a 20ma or a 700ma one and frankly I forgot what that means.
Anyone re-enlighten me please.
Quote from: Legalizeshemp420 on October 24, 2013, 03:04:05 AM
I just created a simple JT that you see around the net using my own wound bobbin (4t:~40t) using a S8055. As soon as I put in a BC337 along with the other transitor the LED will get brighter. It doesn't matter if it is a 20ma or a 700ma one and frankly I forgot what that means.
Anyone re-enlighten me please.
The S8055 has been an unknown transistor type for me. If you did not make a typo, my search resulted as it is a thyristor, see here:
http://www.littelfuse.com/products/switching-thyristors/scr/sxx55x/s8055n.aspx
So what you think?
By the way, if the S8055 is indeed a normal bipolar transistor and you parallel a BC337 with it, then the two work together as an improved transistor for instance with less saturation voltage between collector and emitter, increased Beta (dI
C /dI
B) etc.
And if the S8055 is a thyristor, then the BC337 takes the "role of the boss" for sure i.e. mainly the BC337 parameters dominate and the thyristor has a "second hand job". 8)
Sorry that was a typo I meant S8050 not S8055 and when you go to look up the datasheet they have this part wrong it is a NPN transistor NOT a PNP.
For fun, here is a mini treatise on the behaviour of a Joule Thief:
-----------------------------------------------------
MECHANICAL EQUIVALENT CIRCUIT OF A JOULE THIEF
Imagine you go to the gym and you find an old-style exercise bicycle. The type with a seat and pedals and a chain link to a big flywheel, like a regular bicycle. There is a friction belt that goes around the circumference of the flywheel. You set the tension on the friction belt to adjust the difficulty level.
Imagine the belt is completely loose. You pedal for a few seconds and get the flywheel spinning and then you stop pedaling. Then you add tension to the belt and the flywheel spins down and stops. Then you loosen the belt and repeat the whole process all over again.
Even when you are completely exhausted, it's still possible for you to pedal and get the flywheel spinning if you pedal slowly and take your time to build up the speed. Don't forget that the friction strap is loose when you pedal.
That's a Joule Thief. You are the battery. When you are exhausted it's like you are a dead battery. The flywheel is the coil. The friction belt is the LED.
The torque that you put on the flywheel from pedaling is the battery voltage. The torque that the flywheel puts on the belt during the braking is the coil voltage when it's de-energizing. The rotational speed of the flywheel is the current through the coil.
You pedal for a few seconds and get the flywheel spinning - in the alternate universe the battery did the work required to get current flowing through the coil. You stop pedaling and engage the brake and the flywheel spins down and heat is produced - in the alternate universe the Joule Thief circuit snaps off and the inductor discharges through the LED and light and heat are produced.
-----------------------------------------------------
MileHigh
So, what does it mean when you add another transistor in parallel with the other one and the LED gets brighter to the naked eye?
Quote from: Legalizeshemp420 on October 24, 2013, 04:40:27 PM
So, what does it mean when you add another transistor in parallel with the other one and the LED gets brighter to the naked eye?
I answered your question in my above post.
Quote from: gyulasun on October 24, 2013, 05:07:20 PM
I answered your question in my above post.
It isn't a Thyristor though and has way better stats than the BC337-25. The thing is I have done this before and did not achieve these results. How many can I parallel before diminishing returns sets in?
Quote from: Legalizeshemp420 on October 24, 2013, 04:40:27 PM
So, what does it mean when you add another transistor in parallel with the other one and the LED gets brighter to the naked eye?
Normally two transistors are paralleled to generate the equivalent of one transistor that has a lower collector-emitter resistance so that more current will flow through the circuit. As the current is increased in the collector coil the output voltage and power will also increase. However, if the transistors are not well matched, one of the transistors may end up hogging most of the current and thus reducing the benefit of having a second transistor.
Quote from: xee2 on October 25, 2013, 07:24:12 PM
Normally two transistors are paralleled to generate the equivalent of one transistor that has a lower collector-emitter resistance so that more current will flow through the circuit. As the current is increased in the collector coil the output voltage and power will also increase. However, if the transistors are not well matched, one of the transistors may end up hogging most of the current and thus reducing the benefit of having a second transistor.
Heyas, Xee :)
Well, my next question is the Imaginary transistor that is a combination of all of my transistors in parallel how do I determine what its stats are? I would like to figure out what my stats are and see if a single transistor solution is possible, or close enough. Is there a formula for figuring out the stats of the iTransistor?
In my recent video I have 5 transistors in parallel and it got brighter and brighter with each one but by #5 it was so small of an improvement that #6 would have been redundant. Diminishing returns and all of that.
Quote from: Legalizeshemp420 on October 26, 2013, 12:13:26 AM
Heyas, Xee :)
Well, my next question is the Imaginary transistor that is a combination of all of my transistors in parallel how do I determine what its stats are? I would like to figure out what my stats are and see if a single transistor solution is possible, or close enough. Is there a formula for figuring out the stats of the iTransistor?
In my recent video I have 5 transistors in parallel and it got brighter and brighter with each one but by #5 it was so small of an improvement that #6 would have been redundant. Diminishing returns and all of that.
Paralleling transistors is like paralleling resistors. The more you put in parallel, the lower the e effective resistance will be. However, paralleling transistors can be tricky. I recommend only paralleling identical transistors.
Quote from: xee2 on October 26, 2013, 03:47:15 AM
Paralleling transistors is like paralleling resistors. The more you put in parallel, the lower the e effective resistance will be. However, paralleling transistors can be tricky. I recommend only paralleling identical transistors.
So, there is no way to know what numbers are changing so you can go out and look for a transistor that has the same specs as the multiple ones in parallel end up having?
Quote from: Legalizeshemp420 on October 26, 2013, 03:59:33 AM
So, there is no way to know what numbers are changing so you can go out and look for a transistor that has the same specs as the multiple ones in parallel end up having?
When using identical transistors, every time you double the number of paralleled transistors the effective "on resistance" is cut in half. However, paralleling transistors is usually done to allow switching a current that exceeds the maximum collector current spec.
Quote from: xee2 on October 26, 2013, 09:38:28 AM
When using identical transistors, every time you double the number of paralleled transistors the effective "on resistance" is cut in half. However, paralleling transistors is usually done to allow switching a current that exceeds the maximum collector current spec.
Which value is that on a typical transistor datasheet?
Something isn't making sense. You show the collector current of 1.5 amps so if I put two of those in parallel you said that would be cut in half so that would make it 750ma which i not good.
Quote from: Legalizeshemp420 on October 26, 2013, 04:56:51 PM
Something isn't making sense. You show the collector current of 1.5 amps so if I put two of those in parallel you said that would be cut in half so that would make it 750ma which i not good.
I said each one you add reduces the equivalent collector-emitter resistance (thus more current for a specific Vce). Each additional transistor is another current path and thus each time you add a parallel transistor you increase the current going into the junction of the collectors of all the transistors. The higher the collector current is, the higher the current in the coil will be, and the larger the voltage spike will be when the transistors turn off.
Ahhh, yes, to use Ohms Law and since it is less resistance more electrons will flow.
Now I understand I think. So, in your 1.5Adc ifI had two of those in parallel it would be as if I had 1 transistor with 3.0Adc which makes sense as the current capacity would double but is that really what I am after when my light becomes brighter the more I add?
Thank you.
Quote from: Legalizeshemp420 on October 26, 2013, 07:23:35 PM
Ahhh, yes, to use Ohms Law and since it is less resistance more electrons will flow.
Now I understand I think. So, in your 1.5Adc ifI had two of those in parallel it would be as if I had 1 transistor with 3.0Adc which makes sense as the current capacity would double but is that really what I am after when my light becomes brighter the more I add?
Thank you.
The higher the collector current is, the higher the current in the coil will be, and the larger the voltage spike will be when the transistors turn off. Bigger voltage spike = brighter LEDs.
Quote from: xee2 on October 26, 2013, 08:04:36 PM
The higher the collector current is, the higher the current in the coil will be, and the larger the voltage spike will be when the transistors turn off. Bigger voltage spike = brighter LEDs.
That makes sense, of course, but I read that as 1.5A dc max before POOF not that is what is making it go brighter as opposed to simply allowing it to go brighter.
Take for instance 13003 vs BC337-25. Collector current is 1A for the 13003 and 800ma for the BC337-25 YET the 13003 is dimmer than the BC337-25 so there is more to it than just this.
Quote from: Legalizeshemp420 on October 26, 2013, 08:27:36 PM
That makes sense, of course, but I read that as 1.5A dc max before POOF not that is what is making it go brighter as opposed to simply allowing it to go brighter.
Take for instance 13003 vs BC337-25. Collector current is 1A for the 13003 and 800ma for the BC337-25 YET the 13003 is dimmer than the BC337-25 so there is more to it than just this.
??? Ic max is the maximum current the transistor will take before burning up. Just having a transistor with a large Ic max does not mean you are going to magically get that current when you use it. Yes, there is a lot more to it. I think you need to review how transistors work. The collector current is dependent on many variables. Primarily Ic equals base current times the transistors gain. Thus you can change Ic by increasing and decreasing the base current.
Quote from: xee2 on October 26, 2013, 09:42:12 PM
??? Ic max is the maximum current the transistor will take before burning up. Just having a transistor with a large Ic max does not mean you are going to magically get that current when you use it. Yes, there is a lot more to it. I think you need to review how transistors work. The collector current is dependent on many variables.
I know there are a lot of things and my question was a simple question "is there anyway to determine what 2 transistors are doing to create the new imaginary third transistor" and I was just told about the collector current which is not what would be what I asked for. All it would do is allow more current to flow before the transistor would POP/FRY/EXPLODE/BURN UP, etc...
So, I know what the stats on a transistor means I just never played with magically creating a new imaginary transistor with the stats of the 2 I would put together. That is what my question was about not about adding to the current capacity of the circuit since even one transistor was enough for my purposes but by adding more and more the LED grew brighter.
Quote from: Legalizeshemp420 on October 26, 2013, 09:51:02 PM
I know there are a lot of things and my question was a simple question "is there anyway to determine what 2 transistors are doing to create the new imaginary third transistor" and I was just told about the collector current which is not what would be what I asked for. All it would do is allow more current to flow before the transistor would POP/FRY/EXPLODE/BURN UP, etc...
So, I know what the stats on a transistor means I just never played with magically creating a new imaginary transistor with the stats of the 2 I would put together. That is what my question was about not about adding to the current capacity of the circuit since even one transistor was enough for my purposes but by adding more and more the LED grew brighter.
If you parallel two identical transistors the result will usually be to double the collector current. If the transistors are not identical there is no easy way to tell what will happen.
Quote from: xee2 on October 26, 2013, 10:07:48 PM
If you parallel two identical transistors the result will usually be to double the collector current. If the transistors are not identical there is no easy way to tell what will happen.
That can't be all that happens though can it? I would think the hfe would be effected as well as the other things not just the current.
In my circuit 175ma is being drained from the a 1.23V battery and giving 48ma to the LED so pretty much any transistor would work but by adding the transistors together the LED grew brighter and brighter to a point of 5 transistors and #6 would have been so small of an improvement you might as well not add it.
Quote from: Legalizeshemp420 on October 26, 2013, 10:11:02 PM
That can't be all that happens though can it? I would think the hfe would be effected as well as the other things not just the current.
In my circuit 175ma is being drained from the a 1.23V battery and giving 48ma to the LED so pretty much any transistor would work but by adding the transistors together the LED grew brighter and brighter to a point of 5 transistors and #6 would have been so small of an improvement you might as well not add it.
No that is not all that is happening. Transistors can get very complicated if you want to understand everything. Perhaps someone else can do a better job of helping you. I do not seem to be making much progress.
Quote from: Legalizeshemp420 on October 26, 2013, 10:11:02 PM
That can't be all that happens though can it? I would think the hfe would be effected as well as the other things not just the current.
In my circuit 175ma is being drained from the a 1.23V battery and giving 48ma to the LED so pretty much any transistor would work but by adding the transistors together the LED grew brighter and brighter to a point of 5 transistors and #6 would have been so small of an improvement you might as well not add it.
I may not be able to help you either but it took me a little over a year to understand semiconductors when it comes to transistors or mosfets when you look at the datasheets for each transistors you also need to look at the graphs and the gains cause there all different.. at different frequency .. and voltages so for a example is if you had 3 different transistors paralleled you may have 3 different frequencys running into you coil.. some transistors act different with different wave forms to the base and may draw more or less current ... also your coil inductance plays a big role in how much current passes and how much goes out.. .. it also helps to see about how much resistance and capacitance the transistor has.. hope I could help some how..
robbie
Quote from: kooler on October 27, 2013, 01:13:12 AM
I may not be able to help you either but it took me a little over a year to understand semiconductors when it comes to transistors or mosfets when you look at the datasheets for each transistors you also need to look at the graphs and the gains cause there all different.. at different frequency .. and voltages so for a example is if you had 3 different transistors paralleled you may have 3 different frequencys running into you coil.. some transistors act different with different wave forms to the base and may draw more or less current ... also your coil inductance plays a big role in how much current passes and how much goes out.. .. it also helps to see about how much resistance and capacitance the transistor has.. hope I could help some how..
robbie
I can't add anything extra either, both Xee2 and kooler have given good explanations. When youv'e played about with these circuits as much as we have you will come across many strange occurrences which are not easily explained mainly due to the JT's circuit altering parameters as you add extra devices.
OK.... here's another one.
I always test random transistors in my JT TestBed. This is a basic JT circuit that uses a 13-13 turn toroid, a 1 K base resistor, and has a load bank of 24 white LEDs in a 2S12P arrangement. It has a switch that can put a 70 nF capacitor across (in parallel) with the base resistor. It typically runs on a AAA battery. You all have seen this JT of mine, I'm sure. I even use it as sort of a transistor tester.
It runs on all kinds of NPN transistors, with slight differences or great differences in brightness and the response to switching in the base capacitor. I call the performance with a 2n2222a the "baseline standard".
Now that I have some BC337-25 transistors, I decided to try one in there. It works fine, about like a 2n2222, when it's hooked up properly, and it dims a bit when the base cap is switched in, and is silent. BUT... when I flip it over, so that C and E connections are swapped, it works with Much Brighter LEDs in both modes and it makes an audible whine. Interesting, yes? Some other transistors will also work "flipped", but by no means all.
I also tried a 2sc4508... and it is _even brighter_ than the BC337, and hardly any dimming with the base cap. But it does not work at all in the "flipped" orientation.
MJE3055: in the standard orientation, very bright, with audible whine. Switching in the base cap causes no change in brightness but a change in pitch of the whine. Does not work "flipped".
2n2369a: moderate brightness in standard orientation. Switching in the base cap is very interesting. The lights are very dim, but lit, and slowly brighten a bit. No sound. Does not work at all "flipped".
The monster 2sd1877... does not work at all in this JT but I will be trying it in Gnino's dual-incandescent-battery-charge circuit next.
So I'm liking the BC337-25, "flipped", and the 2sc4508. I have not yet scoped these different configurations.
Yes, I stumbled onto the flipped situation with the BC337-25 about 2 weeks ago now. As I was removing my transistors I noticed it was reversed yet it still worked which is just odd.
Hi all
I'm a JT noob, and have read through the JT 101 thread, and several others. All this information is very interesting. I have not been able to pull myself away from this stuff for hours. :P I have very basic electronics knowledge from some classes I took in college many years ago. So I know what this circuit is doing and how, but am amazed at the voltage output. My first JT worked perfectly the first time. However. I do have a couple of questions of my own. hopefully one of you experts can educate me on this.
So my first JT I did with only one LED (3.3v 28ma) using an old AAA battery putting out 1.25v. I thought the JT was providing close to the 3.3V for the LED. But then I found a post on how to measure the output using a diode and capacitor. With the LED in place, I measured the output at 1.8v. With the LED removed, I was astonished to fine it measure at about 25v.
Now I understand that with the LED in place, it can act as almost a dead short. Which would account for the difference in my voltage measurements. But am I really dumping 22+ volts into my single LED?
In addition, I have added up to 80 LEDs in parallel, and they are BRIGHT. But that would mean I'm drawing 2.24 amps from the circuit. I'm using an NTE293 transistor that's rated at 1A continuous and 1.5A peak. How is that possible?
Also, can someone tell me if there is a way to measure the amp output of the circuit? My meter just reads 0.0. I'm assuming its because it cant keep up with the oscillation speed of the circuit.
ddredar (http://www.overunity.com/profile/ddredar.79940/)
The Joule thief only lights the LED for a short time each cycle, most of the time the LED is off. You do not see this because your eyes can not respond fast enough. Thus it is only using power for short periods of time, not continuously. As a result he average power is very low. An LED is a diode and will always have a fixed forward voltage drop when it is conducting, no matter how much voltage is applied.
Quote from: ddredar on November 12, 2013, 03:34:31 AM
Also, can someone tell me if there is a way to measure the amp output of the circuit? My meter just reads 0.0. I'm assuming its because it cant keep up with the oscillation speed of the circuit.
Hi, ddredar and welcome.
Xee2 has answered your question well, if you wish to experiment further with these or similar circuits, I would strongly suggest you invest in an oscilloscope. You should be able to pick up a decent one reasonably cheaply which will give you the opportunity to analyse the waveforms and measure the signal amplitudes etc. Digital meters don't perform well when inputing these type of signals.
Regards Crow
That's right.
It appears that there is still some confusion about the LED ratings. If your package says "3.3 v, 28 mA" that means that the LED will produce its rated light output at a DC current draw of 28 mA. The LED is never a "direct short", in fact it is a non-linear load to the power supply. When it is supplied with a voltage below its "forward voltage" then it looks like an open circuit and no current flows. At the "fwd voltage" a current begins to flow, and at voltages slightly above the fwd voltage the LED begins to look like a short but with that fwd voltage drop included. To calculate the current through the LED you do an Ohm's Law calculation. I = V/R, for example. Around a circuit, the whole supply voltage is "dropped" (Kirchoff's circuit rules). But the LED drops the voltage some already, so your series resistance drops the rest.
(Supply voltage - LED fwd voltage)/series resistor = current in the circuit.
So if you have a LED with a 3.3 volt fwd voltage, and a 12 volt supply, and you want 28 mA in the LED (to get the rated brightness) you do R = V/I :
(12 -3.3)/0.028 = about 310 Ohms. 330 Ohms is a standard value for a resistor, so use that and accept a slightly lower current in the circuit.
The above is for the DC case.
Pulsed LEDs are the same... but different. The LED can withstand a lot more voltage if the pulse is kept very short. For example I have made some LED strobes to freeze motion of my MHOP pulse motor, and they are getting a full 12 volts pulse with no series resistance, but this is very short, so the LED doesn't fail. It also doesn't get very bright!
So the JT works like this: it makes high voltage pulses when not loaded. The capacitor-diode arrangement will charge the cap to the maximum output voltage of the JT, given enough time. When you hook up the LED, though, the voltage will only go as high as the fwd voltage of the LED, because then the LED looks like a "short" (clipping the voltage rise of the JT), and the light comes on. It might be very bright, but only for a short time, then the JT shuts off until it makes another pulse. The eye "integrates" the bright flashes into what looks like a constant light at some brightness less than the maximum. The eye is a very non-linear "brightness" measurement and what you perceive is a complex function of the true pulse brightness, the duration of the pulse and the time between pulses.
To measure the current during these very short pulses, the proper way is to use an oscilloscope to monitor the voltage drop across an inline resistor in series with the LED. A one-ohm resistor will show a voltage drop across it that can be directly converted to the current by Ohm's law. I = V/R, where the V is the voltage drop read on the scope, and the R is 1 ohm... so the volts indicated = amps. A 0.1 ohm resistor will disturb the circuit less, but then you need to be able to divide by 0.1 instead of 1 !
But this current only flows for a short time out of the total process. Say it flows for 1/3 of the whole cycle as shown on the scope. The _average_ current for the whole cycle then is 1/3 of the value you have calculated based on the voltage drop across the "current sense resistor" CSR, sometimes called CVR (current viewing resistor, the term I prefer).
Nowadays, even cheap DMMs can be surprisingly accurate at "average" current and voltage measurements even in pulsed circuits. There are videos and documents available that show this to be true. Poynt99 has published a very good demonstration/test of this, but I can't find the link at the moment.
Thank you all for the information. Even though I don't completely understand this circuit, I do have a much better idea of what is going on. Would any of you have a recommendation for an oscilloscope? I will probably order it online due to the horribly over priced local electronics store. Also are there any documents or threads you would recommend for reading?
Quote from: ddredar on November 13, 2013, 01:38:43 AM
Thank you all for the information. Even though I don't completely understand this circuit, I do have a much better idea of what is going on. Would any of you have a recommendation for an oscilloscope? I will probably order it online due to the horribly over priced local electronics store. Also are there any documents or threads you would recommend for reading?
Analog or digital? I personally would recommend a used analog 60mhz, or above, for 50-100 dollars. Get a 100mhz analog scope for 75-100 and that is all you would need for most JT work.
Quote from: Legalizeshemp420 on November 13, 2013, 02:46:07 AM
Analog or digital? I personally would recommend a used analog 60mhz, or above, for 50-100 dollars. Get a 100mhz analog scope for 75-100 and that is all you would need for most JT work.
Oh my, please tell me where you are finding those prices. When I look online, most of them are in the $600 to $5000 range. I found a few in the $250 area, but not too many and they are not good for 60mhz or higher.
Quote from: ddredar on November 13, 2013, 12:05:13 PM
Oh my, please tell me where you are finding those prices. When I look online, most of them are in the $600 to $5000 range. I found a few in the $250 area, but not too many and they are not good for 60mhz or higher.
Ebay and I grabbed my 20mhz analog for 34 dollars plus 25 shipping (kinda bulky with a little weight to it) and it had all the original items + manual + original bags and box. 20mhz is fine but to really home in on some of the JTs I was working on I would need 60-100mhz and those can be had for 50-150. Mind you it took me 3 weeks of a lot of bids and I finally grabbed mine via a snipe bid 3 seconds before the end.
Quote from: Legalizeshemp420 on November 13, 2013, 12:23:36 PM
Ebay and I grabbed my 20mhz analog for 34 dollars plus 25 shipping (kinda bulky with a little weight to it) and it had all the original items + manual + original bags and box. 20mhz is fine but to really home in on some of the JTs I was working on I would need 60-100mhz and those can be had for 50-150. Mind you it took me 3 weeks of a lot of bids and I finally grabbed mine via a snipe bid 3 seconds before the end.
Looks like I will have to dust off the old ebay account login ans start searching.
Think I'll go play some Planetside2 before I dive into that search. :P
Quote from: Legalizeshemp420 on November 13, 2013, 02:46:07 AM
Analog or digital? I personally would recommend a used analog 60mhz, or above, for 50-100 dollars. Get a 100mhz analog scope for 75-100 and that is all you would need for most JT work.
So, why do you recommend analog over digital? I'm old enough to know that in many cases analog is better than digital. I just wanted to know your reasoning for this recommendation.
And if anyone has a different opinion, I'd like to know the reasoning behind that too.
Quote from: ddredar on November 14, 2013, 05:38:07 PM
So, why do you recommend analog over digital? I'm old enough to know that in many cases analog is better than digital. I just wanted to know your reasoning for this recommendation.
And if anyone has a different opinion, I'd like to know the reasoning behind that too.
For quirky circuits like this I just think it handles it better for the price range we mere mortals can afford. Have US $2k+? If so then go digital but be sure to get a persistence vision model.
I'm an Analog type guy to begin with as I prefer analog for certain things since we aren't on quantum computers just yet. 0/1 and that is it but for an analog it isn't that simple since it can be off, on, and all sorts of states in between. Some will argue in favor of digital and the one thing I will give digital is all of the extras thrown in it. You get a frequency counter, a FFT, and math functions out the wazoo that the poor PURE analog loses against. We also have the digital sampling that can get in the way and give you headaches on something like a RIGOL 1102E.
For me, personally, I have my 20mhz analog scope and love it but it has its limits due to being 20mhz but I want one more scope before I die and that should do it. About 300mhz 5G/s persistence vision digital with quad channel. Know the price of such a beast? Last time I looked around 10K-15k-20K US dollars. I think I will take my 36 dollar Analog for now. :)
I prefer analog kit myself, but if I was going to buy a "first scope" today on a limited budget, and I had the computer already, I would probably get this one:
Hantek 6022BE
http://www.theoscilloscopeshop.com/item-hantek-6022be-pc-based-usb-digital-storag-oscilloscope-2channels-20mhz-48msa-s_221270085136_US_Hantek.html (http://www.theoscilloscopeshop.com/item-hantek-6022be-pc-based-usb-digital-storag-oscilloscope-2channels-20mhz-48msa-s_221270085136_US_Hantek.html)
Comes with probes and software for under 100 USD.
They also have a 40MHz version for a little more money.
Somebody gave me an old Link DSO that is very similar, and I had the perfect laptop for it, a ThinkPad 600e with parallel port. It is very useful around my lab, because I mostly work with low frequency stuff-- and I had no real use for the laptop any more, so it's a double bargain. I have three analog scopes too; one very popular and inexpensive analog scope is the Tektronix 2213a, a 60MHz scope with some nice delayed timebase features.
Watch out for Atten scopes... I don't trust them.
Almost any scope at all is better than no scope at all, though.
Oops. Sorry, wrong thread.