Transistor selection help

highlandsun · Jul 3, 2003

Showing off my ignorance here... The transistors I'm currently using for my LightWasher upgrade are in plastic TO92 cases. It seems to me I'm abusing them pretty badly; in one case I burned my fingertip when I touched the case.

The deal is this - I want a current source that varies from 0-350mA (or 0-700mA) where Vc is +12V, but the load may only need 3V. E.g., if I have only one Luxeon connected. In this case, the transistor is dissipating the other 9V @ 350mA, which naturally makes it heat up a lot. If I have 3 Luxeons connected in series, then of course they soak up most of the power and the transistor is dissipating less.

But I want something safe for the 1-Luxeon situation, and I'm not sure what I'm looking for. I think this calls for a TO126 or TO220 case, but what specs should I be checking to tell me that this is in the safe zone?

I'm also thinking about using a MOSFET instead of a BJT; since I want to have the option of controlling the current source using my 10K slider pot. Since the slider basically acts as a voltage divider, that would seem to be better (a voltage-controlled device) overall. Make sense?

Ray_of_Light · Jul 3, 2003

TO be on the safe side, you should use something like a BD743B, a power TO220 transitor, with a 15 A max current. Of course, the transistor has to be mounted on heatsink of about 3 °C/watt.

By using a trimpot driving the base of the transistor, I suppose in common-collector configuration (otherwise called "emitter follower"), you are creating a voltage controlled source. Altough is possible to make a current source using transistors and a current sensing resistor, I strongly suggest you to use an LM317T is current-controlled configuration. Check the National or Texas site for diagrams. If you have any problem, post it here, I'll draw the schematic on a sheet of paper, scan it and email to you.

LM317 will use 2.5 volts for itself, so with 12 V you can use max 2 Luxeon in series.

A more sophisticate circuit with LM334 and a boost transistor will limit the drop to less than 1 volt.

Hope it is of help

Anthony

Doug Owen · Jul 3, 2003

[ QUOTE ]
TheProphet said:

A more sophisticate circuit with LM334 and a boost transistor will limit the drop to less than 1 volt.

Anthony

[/ QUOTE ]

For my money, this is 'the ticket'. Check out:

LM 334 data from Jameco

Note the "Higher output current" circuit on page 8. Just use a suitable TO-220 (or similar) PNP and you're jake. You might consider the 2N6042, TIP-106, 107, 125 or 127 all around half a buck from Jameco (who has the LM 334 under a buck). They're 3 Amp PNP Darlingtons (two series transistors in the same package) and have gains over 1000 at 3 Amps (plenty for our use). If it were me, I'd add an additional 1000 Ohm or so resistor between the + pin on the 334 and the base of the transistor (to protect the 334's output in case of problems).

You can use 'regular PNPs' (not Darlingtons) but you'll need to watch the gains. The 334 can only drive 10 mA and you want nearly an Amp for the LED (meaning you need a transistor gain of about a hundred, or more). Gains this high don't often happen at these sorts of currents in bipolars. I'd stick with Darlingtons.

Doug Owen

highlandsun · Jul 3, 2003

Thanks for the suggestions. I'll have to study the data sheet some more, I'm not sure I understand it yet.

For now I've ordered 3 2SA0963 transistors from DigiKey. I figure I have up to 20mA to play with coming out of the LightWasher, and with a minimum gain of 80 that's plenty to drive a single string or two parallel strings...

I haven't got the manual slider control working well yet; have to figure out how to bias things better so that the range of adjustment isn't all bunched up.

Ray_of_Light · Jul 4, 2003

Highlandsun,

the current flowing in the base of the 2SA0923 has to be Ic/HFE, where Ic is the collector current and HFE the static gain of the transistor. So, if your load is 1 Amp max and the gain of your transistor of 80 times, the base current has to be 12.5 mA. So your 10K slider pot is not going to work! You need a 1 K slider pot. Put also a 100 ohm resistor in series with the base of the transistor, so you don't blow the pot when it is near to full power.

Anthony

highlandsun · Jul 4, 2003

Thanks, that makes sense.

The load through each transistor is currently only 350mA max. There are 3 transistors, one each for red, green, and blue. If I add a second string in parallel then I'll want 700mA max. My present setup is working fine with one string, 3 series Luxeons in each color. The current actually peaks at around 380mA right now, but it spends so little time there that it hasn't been a problem.

I looked at the LM317 and LM117 and decided against them, partly due to price. I already have 80% of a good setup, I just want to replace the Darlington transistors that I'm using right now with units that can handle the heat. The LM334 approach looks good, but at this point it would mean discarding a bunch of my parts and starting over.

In fact, I don't expect that I would use just one set of LEDs, but I'd like to know that it won't burn up or melt if I try it. Right now the circuit can only run safely on 12V if I have all 3 series modules in use.

Anyway, I have to go find some 1K sliders. Most of the pots I find online are dials/knobs...