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LED drivers: Why are people not using transistor drivers?


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#1
keli

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There's alot of discussion here about LED drivers. Most of it seems centered around buckpucks. What I am wondering is why people aren't using the DIY transistor based drivers more? They are cheap ($1-3 in parts) and extremely easy to make (as easy as wiring 2-3 LEDs to a buckpuck) and can run 1-40 LEDs provided you're using the correct transistors and power supply.

The only real drawback I can see is that you may need to cool the transistor, and they are not as easily dimmable.

Am I missing something here that makes these drivers rubbish, or are people just afraid of the diagrams accompanied with these drivers?

#2
Catfishboy747

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mainly the latter. For me, with experience in building circuits, soldering and stuff like that, its easy, a few joints, wire, a pot and I have full dimmable controller.

another issue is efficency, While not exactly rubbish, they probably don't have the 95% efficency that pucks have.

There are other issues that evil is probably aware of that I don't.

#3
evilc66

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I use them, but they aren't as user friendly. As Catfishboy747 mentioned, they are not as efficient as a good buck driver (~60%-70% vs. 95%). The drivers I built had to be heatsinked above 500mA. Dimming them is also limited to pwm chopping the output, while a true buck driver will step the current. It's more noticable when you try to dim really low, as you start to see the flicker from the pwm.

Many people don't want to build circuits. For only a little more cost than the transistor based circuits, you can build a buck driver. For some/most, the extra $15 is worth it when you don't have to build and debug the circuit.

#4
keli

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Many people don't want to build circuits. For only a little more cost than the transistor based circuits, you can build a buck driver. For some/most, the extra $15 is worth it when you don't have to build and debug the circuit.


Building a buck driver, like using the stcs1? That one is already $10 per unit, so you're already up to the cost a buck, when including the pcb and other pieces of the build.
The main reason I'm contemplating using a transistor driver is because buckpucks aren't available here and would cost $30-40 total, including taxes and shipping if ordered from usa. For about $5 I can make a transistor driver here.

I'm a little twisted as how to pick mosfets for the driver - does it matter as long as it's rated big enough for the job and is a Nfet? And the secondary transistor (small one) - Can it be any NPN?


As for dimming, I imagine a switch could be constructed that switches between a f.e. 3 resistors.. 25%, 50%, 75% and 100%. For me that would be plenty, if it's as simple as it sounds (finding the correct resistors and connecting to a rotary switch f.e.)

Edited by keli, 03 March 2009 - 07:52 AM.


#5
evilc66

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The STCS1 is an option, but not the only one. You can also build those for cheaper than $10 if you are building a few of them, which most people would anyway. You don't have to get a professional pcb made to make it work. I built a few on smt prototyping boards that cost $1.49 for two. You would be surprised at how well you can etch your own board and still keep small detail.

When picking your N-FET, most specs won't matter to you. What you want to look for is the max operating voltage (higher than you are sourcing), max current (higher than you need), and RDS on resistance (as low as possible to reduce heat). All of these have to balance to a part that has a reasonable cost of course. Most any NPN transistor will work. It doesn't do the bulk of the leg work.

Careful with using a switch to select your currents. Many of these circuits don't like to be switched open while they are operating. It mainly applies to the load (LEDs), but I would be cautious about switching the currents live. 99% of switches are break before make, meaning they will lose contact before making contact in the new position, leaving an open circuit temporarily. You can switch currents while the power is off with no issues.

Anyway, where is "here"?

Edited by evilc66, 03 March 2009 - 08:22 AM.


#6
keli

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"here" is Iceland. There's only about 2 shops here that deal in electronic components, and they are not cheap. For example a single resistor, 5%, 0.25w will set you back about 20 cents. That's why I order most of my components abroad.

What other options are there than stcs1?

With the stcs1 I am not really concerned about the actual soldering of the legs or etching a breakout board, but rather the bottom pad that needs to be soldered, both for connecting to ground, and to keep it cool. I read up on stcs1 and it seems to heat up a bit when doing ~1A loads.

#7
evilc66

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The load isn't the problem. It's running 1A at high differential voltages (difference between source and total vf) that creates the heat.

Pretty much all the major manufacturers make LED drivers now. You will never be able to make a buck driver as cheap as a transistor based driver, but the extra cost might be worth it for the extra features (dimming, thermal shutdown, external on/off, higher efficiency, etc...)

#8
keli

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Ok, I think you've got me sold, seems like a diy buck could be a reasonable option. Do you know of a place where I can source a few ICs that are reasonably easy to work with? stcs1 would probably work, but I'm interested to hear what the resident LED guru recommends. :)

Thanks alot for your help by the way, it's greatly appreciated.

#9
evilc66

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Nationals LM3401 (external switched inductor) is pretty easy to use. It's pwm dimming only though (not too hard), and won't ficker due to the high output frequency.

Zetex ZXLD1360 is about as small and simple as it gets. Pwm or analog dimming input. It can be hand soldered too.