Electronics > Beginners

Picking a transistor.

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magic:
They have an appnote which shows the internal construction and lists some part numbers with even more beta.
https://www.diodes.com/assets/App-Note-Files/zetex/an21.pdf

Base spreading resistance is clearly a factor given that their broadband noise is less than 10Ω resistor's. OTOH, dedicated low noise types still beat them at 1/f. Somebody posted a bunch of measurements here:
http://www.dicks-website.eu/low_noise_amp_part3/part3.html

edavid:

--- Quote from: David Hess on August 11, 2019, 06:56:13 pm ---The Zetex parts should be ideal for this.

--- End quote ---

Except that Diodes has jacked up the price - $1 each for a TO-92 part :-//


--- Quote ---Lots of companies make "Super E-Line" transistors now.  The KSD5401 that I mentioned is one of them.

--- End quote ---

KSD5041 is more reasonable, but you could still find TO-202 or TO-220 parts for less.

P.S. The trouble with some of the beefier MOSFETs mentioned is that OP said he wants to use PWM, so gate capacitance is an issue.

David Hess:

--- Quote from: edavid on August 11, 2019, 08:04:46 pm ---
--- Quote from: David Hess on August 11, 2019, 06:56:13 pm ---The Zetex parts should be ideal for this.
--- End quote ---

Except that Diodes has jacked up the price - $1 each for a TO-92 part :-//
--- End quote ---

Counting inflation that is still less expensive than they used to be.  But they have always been more expensive than jelly bean parts like the 2N4401.


--- Quote ---
--- Quote ---Lots of companies make "Super E-Line" transistors now.  The KSD5401 that I mentioned is one of them.
--- End quote ---

KSD5041 is more reasonable, but you could still find TO-202 or TO-220 parts for less.
--- End quote ---

Or a TO-126/TO-225 part but these packages are not used for newer bipolar transistors so tend to have lower hfe.  For a TO-220, the D44H11 might work with a forced beta of 100 so 10 milliamps of drive current.


--- Quote ---P.S. The trouble with some of the beefier MOSFETs mentioned is that OP said he wants to use PWM, so gate capacitance is an issue.
--- End quote ---

And if you add a gate driver for the capacitance, the advantage of simplicity is lost.

paulca:
So I tested 1 meter of the LED strips on the brightest white with the silly controller they came with and the whole set pulled 1.2A, 14W.  So that's only 400mA per colour, I think the green pulled 420mA the rest 390mA.

So rather than running 2x1meter strips with the option of going to 4x1meter strips from one set of transistors, I could always create a little array of small transistors.  For 4 strips it would be 12 transistors, but each would only need to handle 500mA.

As a bonus I could control each strip individually.  Although that would require 12 PWM channels, which I would need to check.  As it's for a fish tank I don't need much in the way of control, but I do need a way to shift the colour a little bit, probably dropping the green light a bit to favour the plants preferred spectrum, or favouring the blue to make the tank look prettier.  Then again fading out rather than cutting hard would be better for the fish too, which requires PWM on all colour channels.

Without 12 PWM pins I could of course just run 2 transistors in parallel off one PWM pin to run 2 strings of LEDs from 3 PWM pins and 6 transistors.  Meaning I only need 6 PWM pins which I think is available.

MagicSmoker:

--- Quote from: edavid on August 11, 2019, 08:04:46 pm ---...
P.S. The trouble with some of the beefier MOSFETs mentioned is that OP said he wants to use PWM, so gate capacitance is an issue.

--- End quote ---

The OP didn't specifically say the LEDs would be PWM'ed, but it's a fair (even likely) assumption. If the OP does intend to PWM the LEDs then there really is no avoiding either using a MOSFET driver or a smart low/high side switch here. You simply can't PWM any MOSFET bigger than a 2n7000 at more than a few hundred Hz with the typical 10mA available from an MCU I/O pin; not without said MOSFET spending most of the time in its linear region, anyway.



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