My transistor gets very hot. Where should I be putting a resistor?

Audioguru

Joined Dec 20, 2007
11,248
3) The DC current gain (hFE from the datasheet) is around 100 for the BC548 so you only need about 200uA of base current to saturate. Not sure where you got a gain of 20 but 1mA of base current won't hurt the transistor. It will just saturate the it a bit harder and make Vce smaller.
No.
HFE is used only for a linear transistor that has a lot of collector to emitter voltage (5V for the BC548). The datasheet for every transistor shows the Max Saturation Voltage Loss only when the base current is 1/10th or 1/20th of the collector current when the transistor must saturate.
The datasheet for the BC548 doesn't even show its minimum HFE at 100mA, just the typical HFE of 110.

Maybe you can measure hundreds of transistors to find one that saturates well with a low base current. The guaranteed spec's in the datasheet are for all transistors, not just for the best ones.
 

Wendy

Joined Mar 24, 2008
23,415
A general rule of thumb (that I picked up at this site), base current should be 1/10 collector current, no matter what the beta. This way the transistor is guarenteed to saturate.

A lot of my earlier confusion is because you showed one thing and described another. The 0.6V BE voltage is pretty hard core, if you don't have it either you're doing something wrong or the transistor is blown.



Either one of these will work.

One proviso to what I said about the 0.6V BE voltage, this is if the transistor is on. Less voltage is always OK.
 

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SgtWookie

Joined Jul 17, 2007
22,230
a nice little circuit diag.. seems easier to draw than me using visio.. what did you use may I ask..?
Since Bill's not around, here's a link to his blog on what he calls (PaintCAD); basically a library of component images that you can copy/paste using MS Paint to create circuit diagrams.

It's a handy set of items to have if you don't want to (or can't) install any more software on the computer you're using.
 

Ratch

Joined Mar 20, 2007
1,070
samoz,

After I leave my transistor connected for a while, I find that it gets very hot. Why is this and how can I fix it? Should I put a resistor on the base pin?
Because your transistor has voltage across it and current existing through it. That means power, which is dissipated as heat. A good way around this problem is to shunt the current thru a resistor when it becomes excessive. A neat way to do this is the circuit is shown in the next paragraph.

This circuit is an oldie, but I have not seen it published anywhere other than the reference to be given. I used it in a power supply I built in 1973, and it works great. You can see it physically implemented at message number 49 http://forum.allaboutcircuits.com/showthread.php?t=16487&highlight=project&page=5 . Notice the thick wiring going to the resistor. If those transistors had to take that current at a moderate voltage, they would fry in no time. But by connecting one transister in series with the resistor and another transistor in parallel with the resistor, the resistor takes most of the heat.

The circuit came from a book called the DC Power Supply Handbook published in 1970 by Hewlett Packard. It contains a treasure load of power supply information. Unfortunately it is out of print. I scaned the circuit for you in the file attachment.

Ratch
 

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Wendy

Joined Mar 24, 2008
23,415
Thanks Wook.

The 2 minimum power dissipation modes a transistor has is either fully on, or fully off. This is because power = volts X amps. If either volts or amps is close to zero then the power number will be very low. This also describes digital logic, which is pretty convenient.
 
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