Help understanding transistors

Hi, I am fairly new to Arduino and electronics in general so I'm a little unsure about certain things. In the past I built a robot arm from a kit and I thought it might be an interesting project to control it using my arduino. I found out I will need to use transistors although I am a little uncertain on certain details. As I am using arduino I would like it so that there is minimum resistance between the collector and emitter when 5v is applied to the base and I don't know what to look for in the specification or how to effectively modify this if I can't find any of that specification.

Can anyone answer this?

You want to use transistors to sink current from a pin to Gnd? Use N-channel MOSFET. AOI516 or AOI518 from digikey.com, 55 cents each. Resistance is very low, so voltage drop across the part is very small. With 1A of current Vds will be just 0.008V. http://www.digikey.com/product-search/en?vendor=0&keywords=aoi518 http://www.digikey.com/product-search/en?vendor=0&keywords=aoi516

With BJT transistor, will be 0.5 to 0.7V.

I have a set of batteries and a set of motors (in the robot arm). I want to connect the set of batteries to the collectors of the transistors, the arduino to the bases of the transistors and the motors to the emitters of the transistors. I just want to know what to look for in the specification so I know that the resistance will be minimal when 5v is applied to the base.

That description requires P-channel MOSFETs. Fewer choices, pricier.
Are you sure you can’t connect battery + to motor “+”, motor “-” to MOSFET drain, MOSFET source to battery -/Gnd, and drive transistor gate from arduino?

You keep saying resistance - resistance, Rds, is property of MOSFETs.
If you want low resistance, and thus low voltage loss as V = IR, then you want a MOSFET.

BJT transistors have a voltage from collector that is pretty steady, independent of current. If you want to give up 0.7V of your 3V (4.5V? 6V?) in the transistor, and thus have a slower motor, go with a BJT transistor. PNP per your description, NPN per mine. Look for a low Vce at saturation, then search for the Ic current the motor needs.
http://www.digikey.com/product-search/en/discrete-semiconductor-products/transistors-bjt-single/1376376?k=pnp%20transistor&stock=1

I just want it so the motor turns when the arduino output is HIGH and doesn't turn when the arduino is LOW.

Then you want N-channel MOSFET, or NPN transistor, connecting motor "-" to Gnd. This is a low side switch.

With P-channel/PNP set up as a high side switch, between battery+ and motor, you need a Low on the gate/base to turn the device on; or an addtional NPN in front of the P-channel/PNP device.

But what about the 5 volt bit? Surely it would be different spec for one that had minimal resistance at 10 volts.

I'm sorry I no very little about this stuff. I just want to know what to ask for at Maplin :|

colonelhomer815: But what about the 5 volt bit? Surely it would be different spec for one that had minimal resistance at 10 volts.

That will best be handled by using a 'logic level' N-channel MOSFET. They can reach their best Ron spec with just +5vdc gate voltage applied.

What is the name of the spec that describes base voltage for minimum resistance between the collector and emitter? That's all I want to know.

colonelhomer815: What is the name of the spec that describes base voltage for minimum resistance between the collector and emitter? That's all I want to know.

In a FET this is the Rds - Resistance drain and source. Along side it will be a gate voltage to achieve this. In a transistor you want the gain sometimes called the Hfe, this tells you how many more times the collector to emitter current is than the base current. Typical gains are from 30 to 300 depending on the transistor.

People have found this link useful:- http://electronicsclub.info/transistorcircuits.htm

Thanks but I don’t know what any of the variables mean so it doesn’t really make sense.

So if you don't know ask!

colonelhomer815: What is the name of the spec that describes base voltage for minimum resistance between the collector and emitter? That's all I want to know.

Perhaps you missed it but your question was precisely answered in reply #8. You want a "logic level" N-channel FET; that IS the spec that you are looking for. That means the transistor is turned "fully on" with minimal resistance between drain and source with 5volt to the gate . If you look at the general specs for the FET's linked in reply #1, you will see that they are indeed labeled as "logic level".

As recommended in previous replies, FET's are generally preferable to Bipolar Junction Transistors for your application. But if you are unfamiliar with N-channel FETs, simply consider the Drain of the FET similar to the Collector of your NPN BJT, the Gate of FETs the same as the Base of your BJT, and the Source of the FET similar to the emitter of your BJT.

So typically your circuit would be something like this: connect your Arduino output pin to the gate of the fet. connect the battery " + " to the " + " of the motor. connect the motor "-" to the drain of the fet. connect the source of the fet to both the battery "-" and the Arduino ground. just like the diagram here: http://www.hobbytronics.co.uk/arduino-tutorial9-power

But for your application I am concerned that you will need to reverse the motor in order to move the arm in two directions, the above circuit will only allow the motor to turn one direction. Forward and reverse will require more complicated circuitry, usually involving(you will need to do a search) H-bridges. You also need to search about "flyback" diodes, which are typically added to the above circuit in parallel with the motor to prevent damaging voltage spikes.

edited......here is another good link I just found that touches on the "flyback diode" and seems well put together http://bildr.org/2012/03/rfp30n06le-arduino/

good luck!

Cheers, I just assumed whatever it was would have 5 volt in the name, and your right it will need to go two ways although I believe there might be something to help with that in the PCB that belongs to the robot arm, I am unable to take a look at the moment but I will check in the future.

What is the name of the spec that describes base voltage for minimum resistance between the collector and emitter?

There isn’t one. The Base to Emitter voltage is relatively constant at about 0.7V.

I don’t know quite where to start.