Robotics help for a beginner

[AceOfHearts]

Hi there,

I am commencing the design of my first ever robot. I need some beginner type help.

I will be using 5V DC motor to drive the wheels. I need help to chose the correct transistor to provide sufficient current.

I know BJTs use a small base current to provide a large collecter current, and FETs provide large drain current by means of a small input base voltage.

since the microcontroller output is at 5V (or the optoisolator chip), and the only problem is we need large enough current for the motor, it seems like a BJT is the ideal transistor.

Does anyone have recommendations for a particular BJT transistor, I will need the model name/number so I can purchase it online. The problem is that there are so many varieties with so many different specification, how do I chose the right one for me?

 

[dknguyen]

Just making sure about something because a bunch of beginners seem to think like this:
You aren't trying to amplify the 5V signal from your uC to drive the motor right? It seems you realize the current drive capability of the uC is not enough to drive a motor though.

It's better to think of the uC 5V signal as a switching signal (like a finger pushing a button) that tells the transistor to turn on and off a much larger current (a dam). This means that you can drive the motor at something other than 5V with a 5V uC as long as you connect a different voltage source to the source/drain or collector/emitter of the transistor. Similar to how you pushing a button with your finger can open up a dam- you aren't amplifying the force of your finger. Rather your finger is giving a signal to tell something else that is larger and more capable what to do.

With that thinking (rather than current amplifier thinking), FETs are a better choice but everyone has their preference. BJTs have a constant base-emitter voltage drop of 0.7V or so when being used as a switch. A MOSFET is more like a varirable resistor that goes really low or high. At low votlages the MOSFET's voltage drop is much much less than a BJT so the FET is bette. But at high currents the MOSFET voltage drop (since it's like a resistance) can be come quite large compared to the BJT's BE voltage drop (which remains at a cosntant 0.7V). So in this case the BJT is better. That's why FETs are are generally used for "low" power switching and BJT (or IBGTs which have the same characteristic) are used for high power switching. The definition of low and high is heavy industry's definition- so it is highly unlikely that you will ever build anything that even gets close to the upper limits of what industry considers low.

That said, you need to know how much current your motor will draw. Connect it to a battery of the same voltage you plan to power it with in series with an ammeter clamp the shaft and quickly switch on and off the motor just to see what the stall current is (the max current possible). Don't leave it on for too long or you will damage the motor.

The 5V that you speak of powering the motor with, is this voltage the same voltage coming out of the regulator that also powers the PIC? It's not a good idea to regulate the voltage to a motor especially with a linear regulator since the load is often too large for a regulator and a lot of heat is generated and it's not really needed. Just connect it straight to the battery (through the transistor C/E or S/D of course).

Don't forget to put a fast switching or schotky diode in anti-parallel with the transistor to protect it from the motor's inductive spikes!

 

[AceOfHearts]

Just making sure about something because a bunch of beginners seem to think like this:
You aren't trying to amplify the 5V signal from your uC to drive the motor right? It seems you realize the current drive capability of the uC is not enough to drive a motor though.

It's better to think of the uC 5V signal as a switching signal (like a finger pushing a button) that tells the transistor to turn on and off a much larger current (a dam). This means that you can drive the motor at something other than 5V with a 5V uC as long as you connect a different voltage source to the source/drain or collector/emitter of the transistor. Similar to how you pushing a button with your finger can open up a dam- you aren't amplifying the force of your finger. Rather your finger is giving a signal to tell something else that is larger and more capable what to do.

With that thinking (rather than current amplifier thinking), FETs are a better choice but everyone has their preference.

That said, you need to know how much current your motor will draw. Connect it to a battery of the same voltage you plan to power it with in series with an ammeter clamp the shaft and quickly switch on and off the motor just to see what the stall current is (the max current possible). Don't leave it on for too long or you will damage the motor.

The 5V that you speak of powering the motor with, is this voltage the same voltage coming out of the regulator that also powers the PIC? It's not a good idea to regulate the voltage to a motor especially with a linear regulator since the load is often too large for a regulator and a lot of heat is generated and it's not really needed. Just connect it straight to the battery (through the transistor C/E or S/D of course).

Hi there,

Thanks for your response.

I do realise that the microcontroller is not capable of providing the necessary current demand of a motor, and hence the transistor requirement. The motor itself is 5V and will be connected to 5V via the transister just like you say.

An FET will work, now I have to chose the right one...sorry, what do you mean by a 'regulator' in this context? Are you refering to the regulator connected to the power supply which provides a fixed 5V?

Thanks for your advise.

 

[dknguyen]

The regulator as in the device that sits between the battery and uC to make sure the voltage is a nice clean steady 5V. Sometimes power supply means battery (power source) other times people mean a voltage booster or reducer which may or may not regulate the voltage (keep it within tight bounds). The reaspn I ask is because I have never heard of a 5V battery.

Just choose a FET that can block the max voltage of your battery, can be triggered by a logic level voltage (a gate voltage considerably less than 5V like 0.7V or 2.5V or 3.3V for your application) and can carry the current you want. We'll ignore switching times and on-resistance for now since you don't seem to be using h-bridges or PWM.