Transistor selection help,1 transistor step-up circuit

I have built the circuit mentioned at

http://edusite10.tripod.com/led3/onetran/single_tran.html

I found any old NPN transistor and used it. I am only getting the 5mm white led to draw about 7mA. I took a look at the specs of the 2SC3279 it is a power transistor with a 2A collector current rating, 750mW power and an Hfe of 160/600. The device I used was only about 500mA,500mW and lower gain.
I would assume the gain is the most important factor here as it is unlikely to reach the 2A and 750mW in this application. Probably the reason that I am not getting more out of the circuit.
Is this assumption correct?

I initially only got 5mA draw but after lowering the valie of R1 to 500 ohms it went up to about 7mA. Should I continue to lower R1 to drive the transistor harder or am I wasting my time unless I can get a higher gain transistor?

Voltage measured at the input and output is the same however the LED barely lights when connected across the input and is much brighter across the output. How can one measure the efficiency or gain of the circuit correctly?

By the way I found a yellow core which I used to make the coil. Not the usual grey ferrite colour, could the characteristics of this core make a difference?

TIA
Streak

My Landrover Horror Story

well, 150Mhz should be no problem then

I would put a pot in place of the resister and see if you can tune it up a bit. If that doesn't help, and if you can't find a sweet spot then the problem is probably the inductor.

I can say that the versions of this I have built dont get terribly bright. But it's certainly usable.

I think the key factors here are the gain of the transistor and the resistance of the transistor at saturation.

Probably the two things you should try are:

a) Reduce the resistor even more. (In case the problem is gain)

b) Try seeing if two transistors in parallel works.

c) If you're not using the schottky diode and filter cap, there's a good chance your current readings will be inaccurate. Typically ammeters (esp. digital ones) are only accurate for DC or 60 Hz AC. With the diode/cap, the current through the LED will be relatively constant, compared to pulsing once per oscillation.

Thanks for the tips.
I would expect that with the correct setup of this circuit, you should easily be able to overdrive a single LED with 1.5 volts.
Nowhere nearly as elegant as the ARC AAA but look what is achieved there!!
I'll fiddle more today and report back.

Hi Streak,

I've build a bunch of these out of various things, but I have mostly used the cheap, box of 100 generic npn from radio shack.

The frequency that the circuit runs on will be mostly determined by the characteristics of the coil (at least mine were) I measure it in the range of 40khz in one sample! If the transistor you're using is designed for power and not for speed then it may be very inefficient at these frequencies.

If you think that the inductor you're using doesn't store enough flux then try fewer turns of wire around it. I find that one kind of toroid I used works considerably better at 10 turns than 20. The frequency will be higher, but you wont be saturating the coil as quickly.

Finally, you should probably experiment with a variable resister in place of the fixed one to start with. For each circuit I built there is a sweet spot where the LED jumps to a noticeably brighter state than anywhere else and a long range of values where it is just mediocre.

Did you also add the cap and the schottsky diode? If you're using any other type of diode in there then it will drop a volt of power or so right there. It has to be a low drop or most of your power will get eaten up right there.

When you do the tuning with the variable pot make sure you don't have a multi meter in the circuit measuring the output. I just can't get mine to do this accurately as it adds way too much resistance and any output is considerably less than when it's out of the loop.

Then measure the value of the pot and put a fixed on in thats as close to that value as possible.

Good Luck,
James

Thanks James, useful information.
I did not use the diode or cap, wanted to keep things simple to start with.
By speed as opposed to power do you mean the frequency rating of the transistor?
The 2SC3279 is rated at 150Mhz.

Thanks again

Joel

With my experience in doing single-transistor step-ups, I have found the Ic and Vsce rating of the transistor are key factors that affect the over-all current (and thus brightness) of the circuit.

When I used FMMT495CT-ND (1A, 0.25V), the maximum I could get was about 60mA; with ZXT11N150DFCT-ND (3A, 0.15V), I could get close to 200mA; with ZXTD6717E6TACT-ND (4A, 54mV), I could push it to over 250mA.

The base resistor can not be too small, otherwise the circuit will stop oscillating, and you will have very low efficiency just before that. Using a trimpot to find the sweet sopt is a good idea, and I suggest using a resistor slightly larger (a few tens of ohms) than the "sweet spot" value.

Also try using different ferrite cores and different number of turns in the inductor. These factors need to be tried out on a case-by-case basis.

If you want a super-bright step-up circuit, you will have more luck using those IC-driven ones. Single-transistor units simply do not have the efficiency (consider it a good design if you can get it over 50%), and should only be used in low-power situations (like 50mA or so).

Happy modding!

BTW, it might be possible to get better results with a Darlington-style 2-transistor setup. Simpler and smaller than an IC-based setup (esp. if you want to avoid SMT parts, at least in the "simpler" arena.), but likely to be more efficient. (Note: I haven't seen anyone do this yet. The only 2-transistor setups I've seen were multivibrator-based, which require a number of extra resistors/capacitors, although oscillate quite reliably and are probably reasonably efficient.)

With such a setup, you would connect the inductor (plus resistor) to the base of one transistor. Doesn't have to be too beefy, it won't take that much current. Use this transistor to pull up the base of your switching transistor. (I'll try to give a diagram of what I'm talking about tonight or tomorrow.) This will greatly increase the gain of the setup, and excessive gain is the simplest road to oscillation in most cases. In addition, the switching transistor will be more easily driven into saturation.

Vsce will be very important for efficiency, essentially Vsat*the inductor current = waste heat.

Interesting link:

http://users.cableaz.com/~cappels/dproj/ledpage/leddrv.htm

Pretty much the exact same circuit.

Thanks Scoper for the link to the other circuit.

I changed the input voltage from 1.5 alkaline to 2xNiCd's. Now I am getting serious current through the circuit. Using a variable resistor I can adjust the LED from off to a few 100mA!

Still experimenting and learning more.

Pretty much the exact same circuit.

Click to expand...

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True. But some interesting points.
Also the alkaline battery data sheets.

Entropy:

Yes I agree that the efficiency of the 1 trans solution is pretty bad. I build many of them with big transistors (~2A) and found that most of the power has gone in heating up the transistor. (but true, it mostly depends on the wireing of the inductor (tip: for best results it should be around 25 - 10 uH). right now i am no longer using one transistor booster. much more reliable and also very easy to build, besides of better "fine tuning", is good old two transistor astable multi vibrator circuit. even one more advantage is that you can buy the inductor right out of the box. this circuit gives excellent results on 1.5 (1.2) and 3 V (2.4). I drive up to 10 LEDs with excellent brightness and long burn time. you might want to check:

http://www.b-kainka.de/bastel36.htm

(but its in geman only!)

-TS

Excellent page! And Google does a decent job of translating it

Google Translation

EDIT: fixed the long url

A few things are lost in translation though!

"The emptier the battery will, the smaller becomes the river."

Hey TS,
That's brilliant thank you. Will try and build the circuit in the morning.

hi streak,

did you succeed?

I will add some images for you. (i am still looking for a better case for it. maybe i'll do a nice mod...)



and it burns quite bright:



Tom

That look great Tom.
I have a whole lot of old analogue satellite reciver PCB's with many inductors. The problem is that they all range in the 10 to 100 micro Henry region which is much too small. I dont have any ferrite beads the right size to roll my own so my search for the inductor continues!!
I'll post a picture as soon as I am finished.

By the way your case looks like the perfect size.

I have many old IR television remote controls. I plan on building the light into one of those, replacing the IR Leds with white ones. They run off 2xAAA batteries so I will not have to boost too much.

Thanks again

Joel

Streak:

Well, I use 10uH coil for my booster! Thats perfect! Go On

well, mine is not as pretty as yours These picts were taken before I potted the thing so the componants are kind of hanging out.

This circuit is the bare bones single transister circuit. It does not have the second diode and capacitor. So the LED's are reasonably bright, but not spectacular.

I did not use a PC board, just made solder art

The whole thing fits quite well into a 2AAA battery holder from radio shack. Some of the radio shack battery holders like this have a switch built in, but the 2AAA one doesn't.

There is enough room at the top, above the battery connectors to hold the wireing for the LED's. With a very small smd circuit you might be able to fit the whole thing in there and use 2AAA batts.





I made this quite a while ago, the coil is 10 turns around a tiny toroid from allelectronics. I forget the actual part number but you can find them on their website. It works better at 10 turns than at 20. I think it does not hold as much flux as the larger beads, but it works OK.

EDIT: Ok I think the images are woking now...

James S:

Yeah, looks nice!
btw. you should use an extra cap & diode that gives extra brightnes!