Common Practice for Transistors to be used as fuses?

[AggieSawDust]

My buddy dropped his ReplayTV off at my place the other day for me to
take a look at. It turns out that one of the power supply output pins
had a blown transistor attached to it. After further inspection, I saw
the middle leg (Base or Gate, couldn't tell what kind of transistor it
was) was cut and not connected to anything. This was apparantly done
by design since none of the other two transistors attached to output
pins had their middle legs connected to anything either.

So anyway, best I can tell, these were designed to be used as fuses
(that must make these normally closed transistors JFETs, right?). Not
being too experienced, I've never run across this. Is this a common
practice? Wouldn't it make sense to use actual fuses to make servicing
easier?

Thanks,
-Patrick

 

[Ernie Werbel]

Sometimes transistors are used as diodes.

The middle leg, I have found, does not necessarily have to be the base. The
configuration can be EBC, but it can also be ECB or CEB, these I have seen
often among others.

Ernie

 

[Ernie Werbel]

Oh, one other thing that I remembered. It's a bit OT but in the 60's many
transistors were used as diodes (the emitter-base junction), or even fakes
were soldered onto the board (not actually going anywhere) to increase
transistor count. Sure an AM radio could be built with 5 or 6 transistors,
but if you got the 12-transistor model, it just HAD to be better, right?

Ernie

 

[default]

My buddy dropped his ReplayTV off at my place the other day for me to
take a look at. It turns out that one of the power supply output pins
had a blown transistor attached to it. After further inspection, I saw
the middle leg (Base or Gate, couldn't tell what kind of transistor it
was) was cut and not connected to anything. This was apparantly done
by design since none of the other two transistors attached to output
pins had their middle legs connected to anything either.

So anyway, best I can tell, these were designed to be used as fuses
(that must make these normally closed transistors JFETs, right?). Not
being too experienced, I've never run across this. Is this a common
practice? Wouldn't it make sense to use actual fuses to make servicing
easier?

Thanks,
-Patrick

Somewhat "common practice"

The reversed biased base-emitter junction on a silicon transistor
behaves as a low power zener diode with a breakdown voltage in the 6
to 8 volt range.

2N3904 is a good candidate

Not fuses . . .

 

[AggieSawDust]

Jasen,

I guess I'm not 100% sure it's a transistor, but it is in a TO-92
enclosure. The only thing I'm certain of is that I should be seeing no
voltage drop across this device and each leg should show 12V with
respect to ground (the other two behave like this).

So if this indeed a transistor, anyone have a favorite replacment?

-Patrick

 

[AggieSawDust]

By replacement, since I don't have a recognizable part number to share,
does anyone know of a good transistor that can be used as a 1.5 A fuse
(based on writing on the circuit board)?

 

[catguy]

By replacement, since I don't have a recognizable part number to share,
does anyone know of a good transistor that can be used as a 1.5 A fuse
(based on writing on the circuit board)?

It's an IC protector, Just a fuse in a TO-92 package. Read about them
here....Paul


http://www.repairfaq.org/REPAIR/F_audiofaq8.html#AUDIOFAQ_001

 

[GeorgeDubyaRulez!]

How can there be a fuse in a TO-92 package? You need a class tube otherwise
there will be no air for the wire to be in. A fuse can't work if it's solid

 

[ePerson]

its common to use transistors as a diode or diode(s) never seen them used
as fuse

base to collector on a power transistor makes a fine power diode....

DR EE

Sure not emitter-base junction? That give a definite 0.7V drop.

 

[AggieSawDust]

Based on the info from this link, I've determined that my part in
question is definitely an IC protector. I even looked at the board
again and saw the part's notation: PR102. Thanks for the assistance.
In all my education, I've never seen or even heard of these. I guess
nothing can take the place of good old-fashioned experience!

My next question is, who sells these? My local parts peddlers don't
carry them.

 

[catguy]

Based on the info from this link, I've determined that my part in
question is definitely an IC protector. I even looked at the board
again and saw the part's notation: PR102. Thanks for the assistance.
In all my education, I've never seen or even heard of these. I guess
nothing can take the place of good old-fashioned experience!

My next question is, who sells these? My local parts peddlers don't
carry them.

Mouser has a resettable protector.....Paul

http://www.mouser.com/catalog/626/443.pdf