Thanks for a very informative answer Russell, I'm driving 12V LED load using Arduino PWM. While looking for transistor suggestions and before reading your post, I used MPS2222A (Motorola TO92 2222) from by parts bin. The current draw from the LEDs (with PWM) was 135mA @12v. I determined the wattage to be 1.62W. Ta < 25C. I used a 1K base resistor and at 100% duty cycle, the base current was 3.8mA (hFE ~100) At 100% duty cycle the collector current was 136mA At 50% duty cycle the collector current was 67mA http://i.imgur.com/E8IjHhq.jpg Although MPS2222A in a TO92 package can only sustain 625mW dissipation, it was not running hot. I ran it >24 hours and I saw no heat issues. So you are right that small transistors (TO92) like BC337 can be used. Now my questions: 1. In a similar case (load, transistor, supply, etc) how much can I expect from a TO92 case? 2. How to calculate the tipping point to go to TO126 or TO220 package? 3. Is it safe to assume that using a MOSFET can even be thermally more effective due to low Rds(on) Thanks, Chetan Bhargava On Mon, Oct 21, 2013 at 12:49 AM, RussellMc wrote: > > Whether current gain matters depends on the current drive capability of t= he > source and the frequency. > At beta=3D30 (as specified in one suggested device) Ib =3D 350/30 ~=3D 1= 2 mA. > That's beyond the dive capabilities of some microcontrollers. > A microcontroller's max pin drive spec may be 10 or 5 or even 2 mA max in > some cases. > > If you are using on/off PWM you do not need a "5 Watt" transistor. > IF you can get Vsat =3D say 0.3V you dissipate about 0.1 Watt worst case. > Lower Vsat =3D less dissipation. > > An eg BC337/327 or BC817/807 (NPN/PNP TH/SMD TO92/SOT23 ) would work > with good base drive. > Get the -40 version with beta >=3D 250 (250-600 with mean =3D 400) and ba= se > drive can be well above the notional 350 mA / 250 ~+ 1.5 mA needed giving > good saturation voltage and low dissipation. > The SOT23 parts would need thermal calculations and Vsat checked but shou= ld > be able to be designed to wpork well. > The through hole parts would handle this easily. > > Lower than higher base drive will make switching faster, all else being > equal. > > What is the drive source? > What drive current capability. > What is PWM frame rate and min/max on % required (apart from 0 and 100) > eg a 10 kHz PWM frame rate with a 1% step size gives 100 uS frame length > with 1 uS pulse width at 1% on. > IF you want a 1% pulse width at a 10 kHz rate you need a switch that will > operate at some MHz. > "Easy enough", but not trivial. > > > > Russell McMahon > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .