Go Down

### Topic: Choosing the right Capacitor for right Applications (Read 2023 times)previous topic - next topic

#### taz3m

##### Apr 01, 2013, 08:27 am
Hello,

My question is how to choose the right capacitor, for the right application. I admit its kinda of very basic question here, i a bit confused with some of my search i did until now.

Im controlling a pump with my arduino, driving a transistor to supply a 24V power to the pump , i know i should be connecting a capacitor in parallel to the pump.

I did navigate on how to choose a proper capacitor, whereby i did learn that the capacitor voltage to be chosen must be equal to the voltage supply or higher, in terms of charging voltage. I also came across, that a resistor in series is also a good practise as to reduce the high current initially to the capacitor. Can anyone confirm if im right?

How do i still choose the capacitance value for my circuit. From ohms law, we should be using Q = CV.
Bearing in mind that 1 amp is a charge of 1 coulomb past a point in 1 second, with the data i have in hand how should i calculate my farads value?

Voltage supply = 24V
Pump current rating = 0.35A

I attached my circuits here, for a view on what im doing.

Thanks!
taz

tz
.....

#### Grumpy_Mike

#1
##### Apr 01, 2013, 09:09 am
Quote
I did navigate on how to choose a proper capacitor, whereby i did learn that the capacitor voltage to be chosen must be equal to the voltage supply or higher, in terms of charging voltage. I also came across, that a resistor in series is also a good practise as to reduce the high current initially to the capacitor. Can anyone confirm if im right?

Basically right.
As your motors are DC the you can have a polerised capacitor and you don't need the resistor that is generally used with AC motors to correct the phase angle. In this case the capacitor is for interference suppression so the value is not critical and there is nothing much to calculate. A value of 0.1uF is normall used.
A capacitor shorts out AC interference due to the fact that the capacitave reactance is a frequency dependant value given by 1/2Pi FC

#### Erdin

#2
##### Apr 01, 2013, 09:19 am
Thanks for the schematics.
You have to make some changes.

The led (with the 330 ohm resistor) can be connected to a pin of the Arduino.
You don't need a transistor driver for that.

The 1N4749 has no use at that point, please remove it.
I don't know the HJ A42, what is that ?
The IRF540N is not a logic level power mosfet. Please get a "logic level" power mosfet.
Most mosfets need about 9V at the gate to turn fully on. The Arduino output pin is only 5V (or 3.3V for some boards), so you need a mosfet that is fully on at 5V.
I buy my "logic level" mosfet at Ebay, regardless of the type.
Sparkfun has selected a good and cheap "logic level" mosfet, https://www.sparkfun.com/products/10213

You need a flyback diode over the fan and motor. They are motors, so they are inductive.
Scroll down for the picture "Connect a DC motor" in this ABC guide, http://arduino.cc/forum/index.php/topic,154549.0.html

A capacitor of 100nF is often placed over the motor without resistor.
That ABC guide uses 1uF without resistor. That is also valid.
I prefer 47 or 100 ohm and 100nF, it is called a snubber circuit.
You don't have to use a snubber circuit, but it could prevent RF noise.

The capacitor can perhaps be 500pF to 10uF. It is only to reduce the very short voltage spikes.

#### taz3m

#3
##### Apr 01, 2013, 10:40 am
Ok i did get it now, thanks for the understanding.

Quote

The led (with the 330 ohm resistor) can be connected to a pin of the Arduino.
You don't need a transistor driver for that.

I know it can be connected directly, as for now it it connected like this, im simulating my output with leds only up to now.
Since im having a power supply with 5.1 V and 24 V both, im driving the led voltage through the transistor, for i to reduce the current consumption to the max on my Uno board. Is that not neccessary ?
I do also have 3 inputs in total, my LDR, Temp sensor and Soil Moisture sensor. Im using the external 5.1 V to power all my input and leds, reducing the current consumption on the Arduino, any suggestion if im right?
Quote

The 1N4749 has no use at that point, please remove it.

Should i be using the flyback diode in that position instead.

Quote

I don't know the HJ A42, what is that ?

Can i use the same transistor, A 42 to drive my both fans? as you right about the IRF 540N, i dont know how i came up to here ..

I shall be looking over the links you posted here, and grasp a good understanding of things.

Thanks

taz
tz
.....

#### Erdin

#4
##### Apr 01, 2013, 11:38 am
That transistor is a high voltage, low current transistor.
It is for a current of 5mA (!), with higher currents the Hfe drops significantly.

The easiest way is to use the same "logic level" power mosfet for all fans and pumps.

You can drive leds, sensors and so on with the Arduino.
If you have 10 leds, you might use a seperate power supply.

This is how to test it: if you can not keep your finger on the voltage regulator of the Arduino, it is too hot and you need a seperate power supply.

The flyback diode needs to be over the fans and pump. See that ABC guide.

#### taz3m

#5
##### Apr 01, 2013, 04:17 pm
Thanks for the clarifications .. that will surely help.

Do you have any links, where i can read and understand the basic specifications to look upon, when choosing a transistor for an application.

Thanks for the help ..

Regards
taz..
tz
.....

#### Erdin

#6
##### Apr 01, 2013, 05:24 pm
It is hard to tell what a good transistor is... too many options.

BC547B or 2N2222 for small signals.
Darlington transistors like the TIP122 for more power.
Darlington transistors have a voltage drop, that is why "logic" level power mosfets are used.
If a few things are controlled, some ICs have 8 darlingtons, or complete modules with mosfets.
But also mechanical and solid state relays are often used.
And then there are H-bridge drivers for motors and so on.

#### taz3m

#7
##### Apr 01, 2013, 05:35 pm
Thats rightly said,

I just ran onto a web, where it explains rightly the use of transistors, but not the choice, along with ICs and H-Bridge.

The link may help someone like me further : http://lizarum.com/assignments/physical_computing/2008/motors.html
tz
.....

#### Erdin

#8
##### Apr 01, 2013, 05:56 pm
In the middle on that page is a led connected to pin 13 without resistor. That's not good.

Go Up

Please enter a valid email to subscribe