LED light . . . im stumped

zachtheterrible · Jul 13, 2005

So I made the DC-DC converter for my LED, and it doesn't seem to be working right.

If I turn the variable resistor one way, it gets dimmer, and I can see oscillations all over the circuit with my o-scope, but it is very dim, and the voltage feeding the LED is not any more than the supply voltage. If I turn the POT the other way, the circuit stops working, and the LED gets brighter because its just being direct-driven through the inductor.

The LED current is a measly 20 mA because the battery voltage is a little under 3 volts. i cant wait to see this thing @ 350mA!

Anyone have a clue whats going on??

Oznog · Jul 13, 2005

What did you use for an inductor?

audioguru · Jul 13, 2005

Hi Zach,
Don't forget that the leads on your 2N3906 are reversed from the BC559.
I hope you didn't use 33 ohms for R1, it is suposed to be 0.033 ohms. :lol:

zachtheterrible · Jul 13, 2005

i used a pre-made 22uh from farnell.

I'm pretty sure i connected the 2n3906 right, ill have to try that.

and the resistor is the right kind.

zachtheterrible · Jul 14, 2005

tried the 2n3906 both ways, and it still didn't work.

I just noticed that Q1 is getting very hot. This can't be normal. BTW, wuts the idea with Q1 connected straight from vcc to ground, all it would do is short out right? unless its not supposed to be driven hard. i am seeing .9v @ its base.

ive checked teh circuit over and over for shorts, and havent found any. what could be causing this!?!?

audioguru · Jul 15, 2005

Hi Zach,
I couldn't find your original thread about this flashlight to see the details of the IC manufacturer's circuit. I don't know the function of the PNP transistor, maybe it is Q1's driver.
The IC and Q1 are supposed to oscillate at about 65kHz. For each cycle, Q1 is turned on hard and allows the inductor to build up its current from the supply. When the current reaches a certain amount, it is sensed by the 33mohm resistor and the IC shuts-off Q1, and D1 passes the charge in the inductor to the output capacitor and LED. 'Scope it to see if it is oscillating.

zachtheterrible · Jul 15, 2005

hey audio, yeah it is oscillating, only its an extremely small oscillation. It never goes off, it is always on most of the way, leading me to think that Q1 is shorting vcc to ground.

heres the link to the other page: https://www.electro-tech-online.com...e-where-i-might-find-a-dtmf-decoder-ic.16663/

id look over it, but my monitor is crapping out on me. i replaced the capacitor that blew up, thinking that it would work, but i have to keep turning the monitor on and off because it gets way too white so that i cant read. and i see a small phosphor burn in the middle of my screen because the monitor is not shutting down properly, and concentrating the elctron beam right in the middle of the screen :roll:

ive had to turn the stupid thing on and off three times as ive written this message :evil:

williB · Jul 15, 2005

i guess almost everyone in California has an LCD screen allready ?,
cuz around here they are throwing out Monitors like used coffee grounds..

audioguru · Jul 15, 2005

I dunno what's wrong with your LED driver circuit. :cry:

zachtheterrible · Jul 15, 2005

williB said:
i guess almost everyone in California has an LCD screen allready ?,
cuz around here they are throwing out Monitors like used coffee grounds..

IM SPEECHLESS! throwing out LCD monitors!?!?! hey willi, i would be very willing to pay you S&H plus some extra money (name your price) to get one of those monitors and mail it to me if it works!

yeah, everyone but me has an LCD screen :cry: oh well, another circuitboard for the spare parts box :lol:

a friend of mine let me use her computer so i can take a look @ the datasheet for the DC-DC converter and be able to troubleshoot it better. Like i always say: it never works the first time

williB · Jul 16, 2005

umm no , no , no!
people are throwing out CRT monitors , cuz they are buying LCD monitors..
this is what i was really saying , without saying it...
If you look around , you may just find a CRT Monitor that somone threw out , because they bought a LCD Monitor....
ok ? :wink:

Someone Electro · Jul 16, 2005

Get a new monitor.

Im too on a CRT since i like an big screen so i can have a lot of windows open at once.And an 19" or 20" LCD is prety pricy.I have a big computer table so i have plenty of room for an big CRT.(i also like that you can put stuff on top of an CRT,I have all kinds of stuff on it)

What i hate about modern CRTs is that they have the brightnes and contrast in the settings.I like to thurn the birghtnrs down wen reading text (its easyer on the eys)and up wen plying games (so i can see more in 3D games)

Oznog · Jul 16, 2005

zachtheterrible said:
i used a pre-made 22uh from farnell.

Which one? What I'm getting at is its saturation current high enough?

3 amps requires some beef. And if you saturate, some bad things happen.

zachtheterrible · Jul 17, 2005

oh sorry about that willi, i jumped the gun i got so excited :lol:

yeah, its a 22uh 3 amp inductor. very large gauge wire.

ya know what i think is wrong? im using a 2n3906 instead of the bc559 that is specified. Thats the ONLY thing that i can think of that might be wrong. Something is saturating Q1. Its either the 2n3906 that im using, or pin 8.

tomorrow ill try disconnecting the transistor to see if it still saturates.

zachtheterrible · Jul 24, 2005

I just realized that my circuit is oscillating @ 370khz! Almost twice the nominal frequency that the datasheet recommends: 200khz.

Anyone know how to slow it down?

audioguru · Jul 24, 2005

Hi Zach,
Maybe the impedance of your battery is causing your problem. :?:
The IC circuit is designed to be used with a Ni-Cad or Ni-MH battery that has an extremely low impedance. I think you are using a disposable battery whose impedance is much higher. The datasheet says to add ten low ESR 1000uF input capacitors to it if you measure it with a power supply instead of the recommended low impedance rechargeable battery. :lol:

Oznog · Jul 24, 2005

audioguru said:
Hi Zach,
Maybe the impedance of your battery is causing your problem. :?:

If he used D cells, I don't think so. Even alkalines have a pretty impressive current capability. AA would probably do it for this current.

It wasn't designed for NiCd/NiMH- it says a "+3v" rail, not 2.4v. D cells would be an odd choice, rare in NiMH/NiCd on the consumer/surplus market.

audioguru · Jul 24, 2005

Look at what Zetex says, and look at the huge difference in internal resistance of different battery types:

Oznog · Jul 24, 2005

Wow that is a freaky design. I mean, 10x 1000uF is ludicrous, it takes up more space than another battery. If you had another battery you wouldn't even need a boost converter like this in the first place!

zachtheterrible · Jul 25, 2005

HOLY SMOKES!

I will try a rechargable battery with this circuit. If that is true, that REEALY sucks because I don't want to use rechargable batteries! They can't hold a charge over a long period of time.

LED light . . . im stumped

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