Oops - emitter and collector transposed.  Circuit works anyway? - Page 1
Forum section: electronics / projects

author: Clear as mud
author_rank: Regular Contributor
timestamp: June 18, 2014, 06:27:00 pm
content: I made this little circuit for auto-switching a USB-powered device with another external power supply, so that it defaults to the non-USB power supply, but otherwise it will draw power from the USB if that is the only power available. The PNP transistor (2SB815) is switched off when power is available from the LM317, and switched on when it is not, allowing VDD to get power from Vbus. Vbus comes from the USB port via a filter bead.We laid out a PCB and had ten boards made using Surface-mount components, then I realized that C and E somehow got switched around on the transistor. But the circuit seems to work fine anyway. Is there any problem with using it like this? Here is adatasheetfor the transistor. Note that I have not tested this under load. The only load that is on VDD is the microcontroller and the power indicator LED.I just noticed that Vbe(max) is specified as 5 volts. I think this is not exceeded whenever the USB cable is connected, but when it is disconnected, the emitter is essentially floating. Is this OK, or should I provide some high-resistance voltage divider to keep the emitter within a couple of volts of the base?

author: dannyf
author_rank: Super Contributor
timestamp: June 18, 2014, 07:22:35 pm
content: Yes, it does, with much lower (current) gain, however.

author: mikerj
author_rank: Super Contributor
timestamp: June 18, 2014, 08:50:56 pm
content: Have you actually checked that you are getting 5v? Even if the transistor wasn't conducting, you'd still have a sneak path through the C-B junction, D2 and the 300R resistor.

author: dannyf
author_rank: Super Contributor
timestamp: June 18, 2014, 10:40:50 pm
content: QuoteThe Vceo breakdown voltage is much lower this way, usually 12 - 18v.

author: SirNick
author_rank: Frequent Contributor
timestamp: June 18, 2014, 10:56:27 pm
content: Why not just use a diode instead of Q? That way, whichever input has higher voltage wins. Since IME Vbus is often a little on the low side, your slightly overdriven regulator should prevail any time it's plugged in, unless it's dropping out. That might even be considered a feature.Many (if not most) "5v" components run perfectly well from 4v to 6v, so you shouldn't have to worry too much about diode losses or anything like that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 19, 2014, 02:24:49 am
content: 2SB815 datasheet says Vebo = 5V, so near 5V, it's probably pretty leaky. No inverted data. I have no reason to guess the inverted hFE should be high, but maybe it's not terrible anyway. It's not a 2N3906 (which always has poor inverted hFE), but it's a long way from one of those high current, low-Vce(sat) types (which typically do have good inverted hFE).Tim

author: Clear as mud
author_rank: Regular Contributor
timestamp: July 05, 2014, 01:54:46 am
content: Quote from: mikerj on June 18, 2014, 08:50:56 pmI haven't followed up on this for a while, but today I tested the output voltage. It's very close to the input voltage with only the LED and microprocessor load, but when I put a 100-ohm resistor load on it, it drops to 3.7 volts. I think the transistor must be conducting a little bit, because if the current was only going through the "sneak" path, it would drop more than that. But it is not able to conduct as much current as I had hoped. So, I think I am going to search and see if I can find a different SOT-23 transistor with C and E reversed to use on these boards. I don't know if they are made that way, but I guess it is worth searching.To answer another question, the reason I used the transistor instead of a diode was to avoid having the diode voltage drop. I think Vce(sat) is something like 0.2 volts for this transistor, which is a lower drop than I could get with a diode.

author: Rerouter
author_rank: Super Contributor
timestamp: July 05, 2014, 02:45:06 am
content: sot 23's are easy to flip to suit, if the outer 2 pins are transposed, just flip him on his head,