Transistor as low sattured as more heat?

Other Hardware General Electronics
1

Hi, Folks. I'm just trying to learn more about how transistores heats so I've seen something about it.

If a Transistor is 500mA / 100 Hfe and I apply 0,005mA on Base you'll have full saturation, and less heat, right?

What If I Apply only 0,001mA on Base will it have more heat?

Am I Right?

I'm sorry it it is a stupid question, but I'm still learning

Tks a lot

2

A bipolar transistor with a base resistor, emitter to rail and collector to load, can be in 3 states:

  • closed
  • open (saturated)
  • in between.
    If closed, no base current, no collector current, voltage across CE high : Power dissipation = zero current * Voltage = 0
    If open, the base current allows for as much current as available by load, there's a very little VCE
    Power dissipation VCE * IC = e.g. 0.2 = 500mA = 100mW
    In between, the current is throttled, so there's a rather high resistance between C and E, and a considerable voltage.
    Power dissipation e.g. 3V * 200 mA = 600 mW . This is when the transistor gets hot.

In digital switching mode, we avoid have transistors in the "in between" state.
This is for analog amplifiers and linear regulators.

see datasheets for the relation between IC and VCE

3

Sorry, in English "closed" means connected, shorted together, "open" means "no connection",
which is different from the terms used for water taps. If you want to avoid confusion always
use the terms "on" and "off".

To saturate a transistor usually needs between 2 and 10 times as much base current as
the DC-gain would suggest, the requirement becomes higher for larger collector currents.

Its best to check the graph in the datasheet for the particular collector current you are using,
they tend to give curves for several levels of base current (or ratio of collector to base current).

You never get saturation without overdriving the base, typically the transistor
will operate in a linear region down to a volt or two above the emitter voltage, then
tail off.

"Superbeta" transistors are the best behaved with less overdrive needed and lower
saturation voltages. My favorite example being the ZTX851 - its instructive to compare
the datasheets of this device with the old favorite the 2N2222A.

4

Sorry, in English "closed" means connected, shorted together, "open" means "no connection",
which is different from the terms used for water taps. If you want to avoid confusion always
use the terms "on" and "off".

Thanks. My fault. Sure a closed switch is conducting, and an open switch interrupts.
And we use transistors as switches.

Luckily, Cabecinhas does not seem to be native English neither :wink:

5

It is confusing as English says "closed" for a water tap or valve meaning "no flow",
but "closed" for an electrical switch is "flow". However I only realised its confusing
after answering questions on this forum as you never see the illogicalities of your
own language.

Its originally due to knife switches where the mechanics of the switch are like
a box lid opening and closing:

6

michael_x:
In digital switching mode, we avoid have transistors in the "in between" state.
This is for analog amplifiers and linear regulators.

Could I be sure if the transistor if sully satured by just check the voltage between Base em Emiter until it returns abour 0,7 of difference between pins?

7

No, it will have 0.7 V in the linear and saturated region. It is saturated when the collector is only a few tenths of a volt above the emitter.

What you want to look for is as low a Collector voltage as you can get.