# Do I need resistors with my transistor?

Discussion in 'General Electronics Discussion' started by Mat17, Oct 21, 2015.

1. ### Mat17

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Oct 13, 2015
I'm going to to use a 12V battery (with Solar Panel) to power an Arduino AND a 12V solenoid. I will use a transistor to switch the solenoid on after a digital output is sent from the Arduino.

The circuit will be similar to this raw design here, (minus the relay, and the motor is a solenoid).

Also, I have no idea about the 1K resistor or what value it should be. It was just a raw schematic from something else.

P.S. I also know I will need capacitors for the 7805l

2. ### hevans1944Hop - AC8NS

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Jun 21, 2012
If you intended to upload a schematic diagram or image, try again. It didn't work.

Arouse1973 likes this.
3. ### Minder

3,066
657
Apr 24, 2015
If driving a solenoid from an Arduino look at a Fet such as 2n7000.
If over 200ma load then a logic level power mosfet will work.
M.

Last edited: Oct 22, 2015
hevans1944 likes this.

23
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Oct 13, 2015
5. ### BobK

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Jan 5, 2010
Mat17 and hevans1944 like this.
6. ### Minder

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Apr 24, 2015
That is the advantage of a Fet, It is voltage and not current driven.
M.

23
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Oct 13, 2015
8. ### Arouse1973Adam

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Dec 18, 2013
If you tell us the spec of the solenoid and what transistor your using I am sure we can work something out.
Cheers
Adam

9. ### BobK

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1,688
Jan 5, 2010
For a saturated switch, my rule of thumb is base current 10% of collector current.

Collector current is what the solenoid draws. Call it Is.

Then it is simply Ohms law from there. The voltage across the base resistor is the voltage you apply to it -0.6V, the approximate Vbe.

So:

Ib = Is / 10

V = I * R

(Vin - 0.6) / Ib = R

Bob

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