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Another simple transistor question.

A

AJ

Jan 1, 1970
0
Hi all

I know that a PNP transistor will not work if the collector is higher than
the emitter but will it be damaged?

Regards,


AJ
 
E

Eeyore

Jan 1, 1970
0
AJ said:
Hi all

I know that a PNP transistor will not work if the collector is higher than
the emitter but will it be damaged?

May I ask why you want to know ?

Graham
 
J

John Popelish

Jan 1, 1970
0
AJ said:
Hi all

I know that a PNP transistor will not work if the collector is higher than
the emitter but will it be damaged?

If by higher, you mean more positive, then this just reverses the
roles of emitter and collector. As long as this doesn't break down
the base to emitter junction in the reverse biased direction, nor
exceed any current rating, no damage is done. Ordinarily, the reverse
beta (collector and emitter roles reversed) is only a fraction of the
forward (normal reverse biased collector to base junction and forward
biased base to emitter junction) current gain.
 
J

John Popelish

Jan 1, 1970
0
Eeyore said:
May I ask why you want to know ?

Of course you may ask. No one can stop you from asking. Ask. ;-)
 
A

AJ

Jan 1, 1970
0
John Popelish said:
Of course you may ask. No one can stop you from asking. Ask. ;-)

I have a 3.6V Lithium battery and a 5V supply that power a 3.3V regulator.
I have a diode on the input to the 3.3V regulator and I wanted the battery
to power this regulator when the 5V disappears. I don't want to have to put
a diode in between the battery and the 3.3V regulator because of the voltage
drop but I need something there to stop the battery receiving a charge from
the 5V rail (via the diode) instead of its proper charger circuit. This is
where I was thinking of using PNP transistor with tis base permanently tied
low. If the battery is dead flat, the collector could be higher than the
emitter, thus my original question. I have since thought that I could use a
zener/transistor to detect when the input to the 5V supply drops and switch
a MOSFET accordingly but my first idea would be better if it would work ok?

I will give it all a test in the next couple of days but its 2am here in
Australia and my eye's are the size of pea's, not sure I can cope right now.
Anyway, its always nice to hear what other people think first, and I always
get good answers, Thanks John.


Regards,

AJ
 
J

jasen

Jan 1, 1970
0
I have a 3.6V Lithium battery and a 5V supply that power a 3.3V regulator.
I have a diode on the input to the 3.3V regulator and I wanted the battery
to power this regulator when the 5V disappears. I don't want to have to put
a diode in between the battery and the 3.3V regulator because of the voltage
drop but I need something there to stop the battery receiving a charge from
the 5V rail (via the diode) instead of its proper charger circuit. This is
where I was thinking of using PNP transistor with tis base permanently tied
low. If the battery is dead flat, the collector could be higher than the
emitter, thus my original question. I have since thought that I could use a
zener/transistor to detect when the input to the 5V supply drops and switch
a MOSFET accordingly but my first idea would be better if it would work ok?


something like this maybe,

-------+5--+-->|----------------+---[reg]---------
| | |
+-/\/\----. | |
| | | |
| ----- | |
| 7| \ | |
| / \ | |
Vbat------|-----' `------' |
| |
| |
/ |
\ |
/ |
\ |
| |
-----------+--------------------------+-----


Bye.
Jasen
 
E

ehsjr

Jan 1, 1970
0
AJ said:
I have a 3.6V Lithium battery and a 5V supply that power a 3.3V regulator.
I have a diode on the input to the 3.3V regulator and I wanted the battery
to power this regulator when the 5V disappears. I don't want to have to put
a diode in between the battery and the 3.3V regulator because of the voltage
drop but I need something there to stop the battery receiving a charge from
the 5V rail (via the diode) instead of its proper charger circuit. This is
where I was thinking of using PNP transistor with tis base permanently tied
low. If the battery is dead flat, the collector could be higher than the
emitter, thus my original question. I have since thought that I could use a
zener/transistor to detect when the input to the 5V supply drops and switch
a MOSFET accordingly but my first idea would be better if it would work ok?

I will give it all a test in the next couple of days but its 2am here in
Australia and my eye's are the size of pea's, not sure I can cope right now.
Anyway, its always nice to hear what other people think first, and I always
get good answers, Thanks John.


Regards,

AJ

You could use a relay:

+ 3.6 -------------+
|
N/C V --------
-------|3.3V reg|----
+5 ----+-----+-----^ --------
| | N/0
[D1] [Ry1]
| |
Gnd----+-----+

Normally closed point connects the 3.6 volt
battery to the reg until +5 is present - then
the relay transfers disconnecting the normally
closed point and connecting the normally open
point.

Ed
 
C

Chris

Jan 1, 1970
0
AJ said:
I have a 3.6V Lithium battery and a 5V supply that power a 3.3V regulator.
I have a diode on the input to the 3.3V regulator and I wanted the battery
to power this regulator when the 5V disappears. I don't want to have to put
a diode in between the battery and the 3.3V regulator because of the voltage
drop but I need something there to stop the battery receiving a charge from
the 5V rail (via the diode) instead of its proper charger circuit. This is
where I was thinking of using PNP transistor with tis base permanently tied
low. If the battery is dead flat, the collector could be higher than the
emitter, thus my original question. I have since thought that I could use a
zener/transistor to detect when the input to the 5V supply drops and switch
a MOSFET accordingly but my first idea would be better if it would work ok?

I will give it all a test in the next couple of days but its 2am here in
Australia and my eye's are the size of pea's, not sure I can cope right now.
Anyway, its always nice to hear what other people think first, and I always
get good answers, Thanks John.


Regards,

AJ

Hi, AJ. I'm assuming you'd also want seamless transfer to avoid
glitching on your regulator.

Doing this well really isn't that simple, but it's obviously a common
problem with consumer electronics. So sometimes the best thing to do
is just cast around and see what the semi manufacturers who play with
these things have come up with.

I'd recommend taking a look at the Linear LTC4411 for your problem.
(Maxim makes some good product too, but their great ideas tend to
disappear after a couple of years). It's only available in a 5-pin
SOT-23 package, but it's otherwise just about perfect for what you're
doing. It's got an internal 140 milliohm Rds(on) P-channel MOSFET that
turns off if the output voltage is more than the input voltage, like
this (view in fixed font or M$ Notepad):

|
| Shottky .-----------.
| + Diode | 3.3V | +
| o-------->|-------------------o--| LDO |---o
| | | |
| | '-----------'
| .-------------. | |
| | LTC4411 | | |
| | | | |
| .-----oVin OUTo--' |
| | | | |
| 5V 3.6V| | | | 3.3V
| +| | | |
| --- .--oCTL Statuso |
| - | | | |
| | | | | |
| | o--oGND | |
| | | | | |
| | | '-------------' |
| - | | | -
| o------o--o----------------------------o---------o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1142,C1079,P2430
(This is the product page -- you can download the data sheet from
here).

If you don't mind a "store-bought" one IC solution, and you can live
with an SOT-23 package, it'll git r done.

Good luck
Chris
 
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