# NPN transistor CE or EC?

hi for NPN transistor normally Collector will be connected with Vcc, resistor then LED. and Emitter will be conencted to Vss. So whenever voltage applied to base LED illuminates.

however when Vcc connected to Emitter and Vss to Collector (the rest remains, LED polarity changed) LED illuminates too when there is a base voltage.

any idea? as far i know NPN conducts from C to E only.

thanks.

• Minor correction to first para : whenever current applied to base, LED illuminates. When voltage > 0.7 applied to base, base-emitter junction takes all the current available from voltage source and transistor burns out. The difference is; for this LED driver app, you need a current limiting resistor, usually in the base circuit. But get used to thinking of bipolar transistors as current amplifiers; it's the resistors around them that convert from voltage to current and vice-versa. – Brian Drummond Dec 24 '12 at 10:08
• Vcc is a bipolar transistor term (the "c" comes from "collector"). Vss is a field-effect transistor term (the "s" comes from "source"). Vee is used for bipolars. – Kaz Dec 24 '12 at 22:04

There are devices specifically designed to be used this way, commonly called reverse active region operation. Such devices typically have high forward active region gain ($h_{fe}$ of 500 or so), and a higher than normal rated $V_{be}$ reverse voltage (say 15V). When operated in the reverse mode would have $h_{fe}$ of around 5, but would also have very low $V_{ce}$ saturation approaching 100 or 200 $\text{\mu V}$. Here is an example part ... from MicroSemi (2n2432a: http://www.microsemi.com/existing-parts/parts/44905#docs-specs), or from Tohiba (2SC2878: http://www.toshiba.com/taec/components2/Datasheet_Sync/50/6479.pdf). These used to be used more for low drop switching, before MOS technology became so good.