Replacment for BCR148

[Scarr]

Hi,

I have had a PCB made and found I made a mistake! I have connected the E of a BCR148 to the C and vice versa, does anyone know off hand of a similar NPN transistor that has these switched around in a SOT-23 package?

Thanks

 

[mneary]

Can you use a common transistor, or must it have the 47k internal resistors?

MMBT3904 has the pin out you want, but no resistors.

 

[Scarr]

Well that might be a question for another guru

mneary,

I don't think so as base is connected directly to a MCU pin but having said this it is only used briefly <5m pulse (if that) and the pulse only occurs everyonce in a while in an extreme case (probably once a month if that). Basically the MCU is monitoring a LED, if LED does not flash for XX seconds it pulses PIN (BASE) this in turn drops power to entire circuit via ON/OFF of regulator, it's basically a watchdog

But maybe a guro can say "yes" this will work as our only doing it for XX and very infrequently, so it should last for years?

P.S. Power is brought back slowly via a RC giving a nice reset function, and whatever I use MUST be open collector.

 

[Mikebits]

Blue jumper wire and an exacto will fix you problem. Not pretty but cut and jump is a common way to solve these issues...

 

[mneary]

How many do you have to build? I would consider lifting the base and inserting a 0201 or 0402 resistor. Almost invisible, but your techs would need a steady hand.

Modify the paste mask so the base doesn't get soldered, then pry the pin up and insert the resistor. Solder carefully.

Better idea replace the BJT with a logic level MOSFET such as the 2N7002.

 

[Diver300]

Hi,

I have had a PCB made and found I made a mistake! I have connected the E of a BCR148 to the C and vice versa, does anyone know off hand of a similar NPN transistor that has these switched around in a SOT-23 package?

Thanks

I've looked up the data sheet for the BCR148, and it has the same layout as just about all SOT-23 transistors, with the collector on its own, with the base and emitter on the other side from the collector.

I am guessing that the layout that you show is what you are looking for not what the BCR148 is?

If so, I have two suggestions. One is mechanically fairly horrible and the other is electrically doubtful, but I think that either would work.

1) Bend the pins of the BCR148 up so that they are level or above the top surface of the package. Now turn the package over, and rotate it so that the base is where is was, and the emitter and collector have been swapped. Solder it by hand and hope no-one looks too closely.

2) As others have mentioned, the base resistors may not be needed. In that case fit a standard transistor like the MMBT3904 in the way that you have laid out. If the collector voltage is small, less than 6 V for that transistor, and you don't need much gain, the transistor will work the wrong way up, just badly, but that might be good enough for your application.

The NPN transistor has N regions for both collector and emitter, with the P region for the base. There are all sorts of clever features in the construction, such as uneven doping of the P base region to improve the gain, which reduces the gain when using it backwards. However, the NPN transistor with the collector and emitter swapped is still an NPN transistor, so it may well work.

 

[Scarr]

The MOSFET looks good

Thanks everyone,

The 2N7002 looks like it is the best option but being a newbie can someone confirm this will just drop in?


P.S. If had know a little more I would have picked the mosfet for the job in first place as it looks like the right device for the job.

 

[Hero999]

That's a power MOSFET which might work depending on the circuit.

 

[Scarr]

They dont' work DAM

Nope the Mosfet does not work as a drop in replacement

Had another thought, what about a TO-92 case as I can bend legs easy, maybe once it's led on board it might line up nicley. Anyone know of a TO-92 cased NPN with built in resistors?

Thanks

 

[Scarr]

Comments please

OK in desperation I'm going to try a normal NPN transistor like BC817 it's a SOT-23 with legs in right position, I'm going make board work this transistor a few thousand times, if it passes this it should be OK on my board.

If anyone has any comments about driving this with current limiting resistor via a AVR M128 pin please shout up


Thanks

 

[ericgibbs]

Blue jumper wire and an exacto will fix you problem. Not pretty but cut and jump is a common way to solve these issues...

hi,
I agree, solder the device on the pads, cut the track close and jumper the tracks across. Kynar wire ideal for the jumpers.

 

[mneary]

The BC817 has the same pin locations as the BCR148. No good.

Maybe you need to draw two pictures. One, what you think the BCR148 pin out is, two, what your board layout is.

Maybe you have a data sheet with errors.