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Topic: 24V in but less than 5V out on 2N4401 (Read 530 times) previous topic - next topic

mOskit

Dec 12, 2017, 03:09 pm Last Edit: Dec 12, 2017, 03:09 pm by mOskit
Hello Everybody

Trying to work out why am i having such low voltage on the emitter on the 2N4401

24V on collector, 4.7V on emmiter. Arduino 5V

Vin 24V

I`m require to see ~24V on emmiter.

I`ll appreciate any help

terryking228

Hi,

You have made an "Emitter follower" and that's what it is doing, following the base voltage.

You need to look at transistors and amplifiers in some book..  You will need to add a transistor to have the base voltage of the 2N4401  (A small NPN transistor) approach 24V.  You will still have a drop of about 0.7V
Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

mOskit

#2
Dec 12, 2017, 03:31 pm Last Edit: Dec 12, 2017, 03:34 pm by mOskit
Hi.
Can  i do anything with the current design? I`ve got the board made already hence the issue.
http://www.rason.org/Projects/transwit/transwit.htm -> figure 2 will sort that out?

jremington

#3
Dec 12, 2017, 04:49 pm Last Edit: Dec 12, 2017, 04:57 pm by jremington
You can't use an NPN transistor as a high side switch if Vin > 4.3 V. The schematic shown in figure 2 on (http://www.rason.org/Projects/transwit/transwit.htm) won't work in your case because you can't turn the PNP transistor off completely with a 5 V Arduino.

You need something like the diagram below.


Grumpy_Mike

#4
Dec 12, 2017, 05:41 pm Last Edit: Dec 12, 2017, 05:44 pm by Grumpy_Mike
I`ve got the board made already hence the issue.
Time to get your scalpel out and fixe it with mod wire.

Did you miss the bit on that page that said
Quote
However, in the NPN transistor, R1 must be shorted to the positive end of the supply to turn the switch on.

mOskit

#5
Dec 13, 2017, 09:19 am Last Edit: Dec 13, 2017, 09:43 am by mOskit
Got it.  so basically i can set output pin to LOW and use PNP 2N3906 instead of NPN 2N4401 to switch High side right?

Grumpy_Mike

#6
Dec 13, 2017, 10:01 am Last Edit: Dec 13, 2017, 10:09 am by Grumpy_Mike
Got it.  so basically i can set output pin to LOW and use PNP 2N3906 instead of NPN 2N4401 to switch High side right?
That will turn on the transistor but in order to turn it off you have to supply at least 11.4V to the emitter's base resistor.

mOskit

That will turn on the transistor but in order to turn it off you have to supply at least 11.4V to the emitter's resistor.
So thats fine as the emmiter will have 24V constantly on and the " load is on the collector side. right?

Grumpy_Mike

#8
Dec 13, 2017, 10:08 am Last Edit: Dec 13, 2017, 10:11 am by Grumpy_Mike
Sorry I meant to say the base's resistor.
Origional post now corrected.

That circuit will also fry your Arduino pin as it will apply 12V to the output pin when you try and turn the transistor off.

mOskit

So this is the only way right? (see attached)
Not sure about the resistor values. I think 2K for R19, R20 and 100k for R25, R26 followed by 2K2 for R23 and R24?

Grumpy_Mike

They are not too critical if the current is not high, what is the current you want to switch.

mOskit

It`s just a signal to another micro controller (or relay) on the machine. I guess few mA but still waiting to get this confirmed.

jremington

#12
Dec 13, 2017, 05:37 pm Last Edit: Dec 13, 2017, 05:37 pm by jremington
Quote
I guess few mA but still waiting to get this confirmed.
It would be a good idea to actually know details like this, and design and test appropriate circuitry, before you have a custom board made.

MarkT

If its a few mA an opto-isolator might be simpler.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

TomGeorge

Hi,
Sounds like you are switching to some industrial devices/PLC.
Opto would have been easier and provide isolation from the other system.
Pity you did not prototype first.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

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