# Transistor needed to drive a solenoid actuator

#### Vincenzo1309

Joined Dec 28, 2008
57
Dear all,

I am working on a project where I have to actuate a solenoid actuator which can be operated in the range of 6V - 12V DC. I am now using a microcontroller to control this actuator. The logic supply out from the pins is only 5V, which is insufficient to actuate the solenoid actuator. So I would like to know what kind of transistor is suitable. The power supply used is a 6V battery pack which contains 4 AA batteries. what are the criteria in choosing a transistor? The resistance of the solenoid actuator is 8 Ohms. So can I say that the current needed to actuate this actuator is 6V / 8 Ohms = 0.75A?
Then I willl have to choose a transistor which is able to supply me 0.75A of minimum collector current. Am I right in this case?

One funny thing that I observed while sourcing for a solenoid actuator is a small actuator needs 12VDC to be operated, while the one I purchased which is bigger than the small one only need the range 6V - 12V. Is this got to do with the number of coils on the solenoid?

Regards,
Vincent

#### mik3

Joined Feb 4, 2008
4,846
Yes it has to do with the number of coils.

As for the transistor choose a darlington transistor which can handle more than 2 Amps through its collector. Make sure that the transistor fully conducts as not to overheat it. Also, you will need a diode across the coil to absorb the back EMF when you switch off the transistor.

#### Vincenzo1309

Joined Dec 28, 2008
57

So does that mean that with greater number of coils in the solenoid, it would take lesser voltage for it to be activated?

I have tried using the BD679 which can handle 4A, but it can't seems to work in activating the solenoid actuator. My supply is a 6V battery pack with 4 AA batteries. Any advice?

Regards,

#### Vincenzo1309

Joined Dec 28, 2008
57
Hi, sorry again

The way how I calculated the current that is needed to activate the solenoid actuator which is 0.75A, am I right in that?

Regards,

#### mik3

Joined Feb 4, 2008
4,846
Greater number of coils does not always means less operating voltage. It depends on the resistance of the coil and the force needed.

What is the maximum output current of the microcontroller pin?

Post a diagram of what have you done if possible.

#### Vincenzo1309

Joined Dec 28, 2008
57
Hi, I am currently using Basic Stamp 2. From what I can find is this:
Source/Sink Current per I/O: 20 mA / 25 mA
Is this what you want?

I have attached a picture, I need to find a suitable transistor.
Previously I was using a BD679 and the supply is a 9V battery pack, it works perfectly. However now I would like to change the supply to a 6V battery pack, but this BD679 don't seems to work.

Regards

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#### mik3

Joined Feb 4, 2008
4,846
Hi, sorry again

The way how I calculated the current that is needed to activate the solenoid actuator which is 0.75A, am I right in that?

Regards,
It is if the coil has reached its steady state of operation.

#### mik3

Joined Feb 4, 2008
4,846
It should work if the coil can work on 6V.

Are you sure the batteries are full?

#### Vincenzo1309

Joined Dec 28, 2008
57
Yup, the batteries are full!
This battery pack is also used to power up 2 DC motors, will it affect the performance of the solenoid actuator?
So do you mean that this BD679 can be applied in this case?
Or are there any other transistor available?

#### mik3

Joined Feb 4, 2008
4,846
The voltage across the coil is not 6V because of the voltage drop across the transistor which is about 1V. Use a MOSFET which is designed for logic level operation (5V Vgs to fully turn on). MOSFETs have less voltage drop across them when conducting. Also, the voltage of the battery drops across its internal resistance because the 2 motors draw significant current.

Power the solenoid directly form the batteries to see if it works.

#### Vincenzo1309

Joined Dec 28, 2008
57
Hi, the solenoid works when powered directly from the batteries.
So what kind of MOSFET do you reccomend? So in this case, I can't use BD679?
If I am using the MOSFET, do I still need to include in the 1N4004 diode?
Also, I would like to know why is 1N4004 suitable in the circuit? I got this circuit from the internet. Wy not other diode? how do they select it?

Regards,

#### mik3

Joined Feb 4, 2008
4,846
Search for logic levels MOS in Digikey, Mouser etc to find one.

Of course you need a diode when using a MOS.

This diodes is suitable for you because it can handle 1A of continuous current and thus it won't burn when the MOS switches off and the 0.75A flowing through the coil will flow through the diode for a few us due to the back EMF generated by the coil.

#### Vincenzo1309

Joined Dec 28, 2008
57
Hi, just want to find out.

Which criteria do you look out for to deem it suitable?

Is transistor different from MOSFET?

Regards,

#### mik3

Joined Feb 4, 2008
4,846
Both BJTs and MOSFETs are transistors!!!

They differ in the way the control current through them.

To choose one the basic criteria are maximum current they can handle, maximum power dissipation, break down voltages and gain.

#### Vincenzo1309

Joined Dec 28, 2008
57
Hi, I am sorry..
For transistor, there are 2 types, namely BJT and MOSFET
BJT has base, collector and emitter
MOSFET has gate, drain and source.
Base = Gate
Collector = Drain
Emitter = Source
Am I right in this case?

#### mik3

Joined Feb 4, 2008
4,846
Hi, I am sorry..
For transistor, there are 2 types, namely BJT and MOSFET
BJT has base, collector and emitter
MOSFET has gate, drain and source.
Base = Gate
Collector = Drain
Emitter = Source
Am I right in this case?

You are right but they work in a different way.

#### bertus

Joined Apr 5, 2008
19,939
Hello,

Here is a little drawing I made for switching BJT versus Mosfet.

Greetings,
Bertus

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#### Vincenzo1309

Joined Dec 28, 2008
57
Hi, so if I use a MOSFET, the way how I connected up the circuit is not the same as how I connected it using a BJT?

There are 2 resistors for the MOSFET circuits, the 100 Ohm is connected to the pin out of microcontroller, the 10K is connected to the ground. May I know why are there 2 resistors?

Can I use a MOSFET but I connected the circuit as seen on the left (which has only 1K resistor)?

For my case, can I use a BJT transistor instead?