Pages: [1] 2   Go Down
Author Topic: Transistor question  (Read 1721 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Newbie
*
Karma: 0
Posts: 36
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Sorry if my question is silly, but I'm not really good with electronics.

I have to power a device with 3V and 200 mA. So the 3.3V output pin of the arduino seems great, and in fact I can power my external device which runs fine.

However, I have to switch the power to that device via software, so I thought a transistor would help me. I connected the 3.3V to the emitter, one of the arduino pins to the base, and the collector goes to my device.

The fact is with this configuration I got too much voltage or current to my device, which doesn't work well. How can I switch the 3.3V from on to off? Do I have to use the transistor? If yes, how can I use it to make it pass only 3.3V and 200 mA?

Thank you in advance
Logged

0
Offline Offline
Sr. Member
****
Karma: 7
Posts: 476
what?
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

which transistor is it?

for the safety of your arduino always use a current limiting resistor to drive the base of the transistor
Logged

Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17294
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Sorry if my question is silly, but I'm not really good with electronics.

I have to power a device with 3V and 200 mA. So the 3.3V output pin of the arduino seems great, and in fact I can power my external device which runs fine.

The arduino product page for Uno still only rates the 3.3v pin as having 50ma capacity. This probably a legacy spec when the FTDI was supplying the 3.3v pin on older arduino boards. The Uno uses a dedicated 3.3vdc voltage regulator to supply that pin, but I would think it's safe ratings is around 100ma or so, so you may be really stretching it drawing 200ma. But if it works it works I guess.

However, I have to switch the power to that device via software, so I thought a transistor would help me. I connected the 3.3V to the emitter, one of the arduino pins to the base, and the collector goes to my device.

NPN or PNP transistor? No resistor on the base is a good way to damage both the transistor and the output pin.

The fact is with this configuration I got too much voltage or current to my device, which doesn't work well. How can I switch the 3.3V from on to off? Do I have to use the transistor? If yes, how can I use it to make it pass only 3.3V and 200 mA?

Use a relay?

Thank you in advance
Logged

Offline Offline
Edison Member
*
Karma: 116
Posts: 2205
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
I got too much voltage or current to my device

If you put a load to its rated voltage, you will get its rated current, at max. If you get more than its rated current, something is seriously wrong.
Logged

Valencia, Spain
Offline Offline
Faraday Member
**
Karma: 146
Posts: 5483
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

which transistor is it?

for the safety of your arduino always use a current limiting resistor to drive the base of the transistor

Yep. Put something like a 330 ohm resistor between the Arduino and the base of the transistor.

Logged

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Valencia, Spain
Offline Offline
Faraday Member
**
Karma: 146
Posts: 5483
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Sorry if my question is silly, but I'm not really good with electronics.

I have to power a device with 3V and 200 mA. So the 3.3V output pin of the arduino seems great, and in fact I can power my external device which runs fine.


What device? A transistor will eat up 0.7V so you'll really only be getting 2.6V across the device if you use the 3.3V pin. It might be better to use 5V and add a resistor to eat up the excess 1.3V @ 200mA (use Ohm's Law to find the resistance needed for that...)


Logged

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 625
Posts: 34099
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
However, I have to switch the power to that device via software, so I thought a transistor would help me. I connected the 3.3V to the emitter, one of the arduino pins to the base, and the collector goes to my device.
That is the way to configure a PNP transistor only. These are not the most common type. You always need a resistor between the base and the arduino pin.
Please say what number the transistor is so we can check if you have it wired correctly.
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 36
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

I have a BC337 transistor. I understood that I have to connect a resistor between the arduino and the base.

So as someone is saying it would be better for me to use a PNP transistor, which will pass the emitter to the base when no input is given on the base. Did I understood this correctly?

Thanks
Logged

Offline Offline
Edison Member
*
Karma: 116
Posts: 2205
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
it would be better for me to use a PNP transistor

It depends. You can switch from the high side with a pnp or the low side with a npn. Which is better depends on your requirements. Either can be made to work.
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 36
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
it would be better for me to use a PNP transistor

It depends. You can switch from the high side with a pnp or the low side with a npn. Which is better depends on your requirements. Either can be made to work.


Ok, but the problem is that with an NPN, doesn't the base add itself to the emitter into the collector? Because this is what seems to me, so using a PNP could leave this problem out.
Logged

Germany
Offline Offline
Faraday Member
**
Karma: 57
Posts: 3026
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

You need a base resistor anyway, see Grumpy_Mike's comment earlier.

If you have an NPN type transistor (  BC337 ), and need to switch a 200mA 3.3V device,
you connect emitter to GND, collector to the - side of your device, + side of the device to 3.3V
and a resistor of <=1k between Arduino pin and base.

Hope there's no need to have your device - directly connected to GND. (What is it ?)

BTW1: If load is an inductance (coil / solenoid / motor) provide a protective diode across your device.

BTW2: the 3.3V regulator of a 5V Arduino UNO is too weak for 200 mA
http://arduino.cc/en/Main/ArduinoBoardUno : DC Current for 3.3V Pin -- 50 mA
Logged

Offline Offline
Edison Member
*
Karma: 116
Posts: 2205
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
but the problem is that with an NPN, doesn't the base add itself to the emitter into the collector?

What does that mean?
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 36
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
but the problem is that with an NPN, doesn't the base add itself to the emitter into the collector?

What does that mean?


Doesn't the voltage and the current coming from the base are added to the emitter? Maybe I got this wrong -_-
Logged

Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17294
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
but the problem is that with an NPN, doesn't the base add itself to the emitter into the collector?

What does that mean?


Doesn't the voltage and the current coming from the base are added to the emitter? Maybe I got this wrong -_-

Well the total emitter current will be the sum of the base + collector current, but the collector current will remain base current X device beta.

Lefty
Logged

Offline Offline
Edison Member
*
Karma: 116
Posts: 2205
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
Doesn't the voltage and the current coming from the base are added to the emitter?

Yes. But why does that matter to you?
Logged

Pages: [1] 2   Go Up
Jump to: