Simple transistor question

Other Hardware General Electronics
1

Why are two transistors used like that? Is there a reason?

2

Wrong choice of transistor. :roll_eyes:

3

Wrong circuit, wrong components, bad design!

Should be using a TPIC6A595.

4

1824×1861 345 KB

is this any good. even this has 2 transistors grouped.

5

They're just doubling the driver current capability. You could also use an IC or a single transistor per cathode line, that is actually rated for the job, instead.

6

aarg:
You could also use an IC or a single transistor per cathode line, that is actually rated for the job, instead.

As I pointed out in #2. :grinning:

7

Paul__B:
As I pointed out in #2. :grinning:

Sometimes the nail requires more than one hammer strike.

8

Paul__B:
As I pointed out in #2. :grinning:

Meow :smiley_cat:

9

larryd:
Meow :smiley_cat:

Hiss, spit!

10

;D

11

karthyk:
http://www.pyroelectro.com/projects/8x8x8_led_cube/img/8x8x8_schematic.png

is this any good. even this has 2 transistors grouped.

At least this circuit has base resistors. However the idea of trying to double the current from a BJT
driven from an Arduino like this is fatally flawed:

The base current needs to be about 5% to 10% of the collector current in a standard switching BJT,
and as the 2N2222 already has 600mA rating, it requires at least 30mA base current to deliver that
and the Arduino can't do much more than that without reaching the absolute-maximum-never-exceed
pin current rating.

The best approach is to switch to using MOSFETs, or perhaps one of the super-beta devices that can
work well with as little as 2% base-to-collector ratio like the ZTX851.

12

MarkT:
The best approach is to switch to using MOSFETs, or perhaps one of the super-beta devices that can
work well with as little as 2% base-to-collector ratio like the ZTX851.

Eight FETs built into the TPIC6A595 as per #2. :sunglasses:

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