I was reading some of the thread over at OU about transistor behavior. The discussion was about the BC337-25 and the 13003, which is often found in CFL bulbs. One poster said the 13003 was rated at 1 amp, and the BC337-25 was rated at 0.8 amps. But in the Joule Thief the BC337-25 drew more current than the 13003.
In the device’s datasheet, the manufacturer gives some maximum values, but these are more like physical limits, and they don’t explain the behavior of the device. For a better insight into what is going on inside of the transistor, one has to look at the graphs, and compare them to other transistors. But first off, the graphs are based on a “typical” transistor, which may or may not be close to the one you are working with. However if one thinks of the lines on the graph as more of the middle of a wider band, within which all of the transistor should fall within, then one might not get stuck on minor differences when comparing one transistor’s graph with another’s.
BTW, you can search on the device name and add datasheet to the end and often come up with the datasheet for that device. Also, the 13003 number is a shortened version of the originalk, which used to be MJE13003. But MJE was given to it by Motorola, and most of those transistor are no longer made by Motorola or its successor, On Semi. So they’re just called 13003.
Comparing the two transistors, one can see that the BC337.25 was made for lower voltages, below 40 volts. The 13003 was made to handle higher voltages, a few hundred volts. It doesn’t have to perform as well at low voltages.
Then the datasheets show that the BC337.25 has a current gain of about 250, whereas the 13003 has a current gain of about 25, only about ten percent of the BC337-25. No wonder the 13003 doesn’t do very well in a JT.
Since the 13003 is going to be used at high voltages, it doesn’t have to have good beta (current gain) holdup at low voltages. The BC337-25 on the other hand has a lot better beta holdup at low voltages. This can be seen in the Vce(sat) figures given in the datasheet and in graphs (not all datasheets have graphs, but the better ones do).
So the 13003 might be a good choice when the supply V is a few hundred volts, it’s a poor choice for a JT with a supply V of only 1.5V.
And if you want to find out why I’m not posting this to OU, read the bottom of this blog, just before the update.