How to limit current

[alexalex]

What is your 5V signal source? Is it still 5V when it supplies 30mA? Is it able to supply a current as high as 30mA?

It would be the Digital Pin of the Arduino board which I believe is 40mA.

 

[alexalex]

Also I might just add that I have a general NPN transistor connected to a 6.2k resistor for the base and the C and E go to a motor controller to connect the two wires together which forms the connection that controls the motor.
The motor does spin the same amount if I connect the two wires together directory or put them to the transistor. The motor controller and motors are powered by 4 x AA batteries.

Is there something special going on here that by allowing 0.7mA into the transistor base (assume 10 hFE = 7mA) that its spins the motor just fine?

Edit: Hmm strange, I just put the multimeter to measure amps and it says 0.00, so maybe it's just a link connection though even those should have some current flowing through it?

Edit 2: I touch both wires and the motor spins very slowly, so I'm thinking this senses small resistance to some degree? Seems like when I measure resistance when connected to it it's about 800K to make the motor spin a little.

 

[ericgibbs]

hi Alex,
What is the specification of the motor.?
Its resistance, etc..
Its pointless talking about transistors until we know what the load is.

 

[alexalex]

Hi Eric,

I'm not too sure because when I measured resistance I just got 0, I will need to try it again tonight but I think it looks similar to this one (but probably not as powerful): **broken link removed**

Please find below a picture of the motor controller. I'm interesting in how this controller has no resistors and it looks like it has 3 transistors for each for each motor's direction (2 motors x 2 directions x 3 transistors = 12 transistors).

As you see I connect Pin 1 and 5 together and one motor spins, I touch both Pin 1 and 5 and it slowly spins the motor. Some how they have wired the 3 transistors together along with the capacitor (one for each motor) so that the transistor doesn't blow up if you connect Pin 1 to 5 directly. Pin 7 is ground (but isn't used at all).

**broken link removed**

 

[ericgibbs]

hi Alex,
If the motor you are working with has the same specification as this one, then

Voltage: 6V

NO LOAD,,Speed (RPM): 13500,Current (AMP): 0.5
The motor/transistor will require a current of 0.5Amps just to run with No Load.

AT STALL,Current (AMP): 5.25
The motor/transistor will carry a current of 5.25Amps if you Load the motor and stop it from turning.

AT MAX EFFICIENCY, Current (AMP): 2
The motor/transistor will carry a current of 2Amps a its optimum speed and load.

AT MAX POWER, Current (AMP): 2.6
The motor/transistor will carry a current of 2.6Amps a its optimum speed and load.

If you study these figures you will see that the driving transistor must be capable of
carrying 5.25A under the worst case conditions.
The power supply must be able supply a steady 2.6A and a peak of 5.25A.

You should consider using a suitably rated Power N MOSFET device to drive this motor.
or
get a smaller, lower power motor.

 

[alexalex]

Ok thanks to both of your for your help with all of this

Now I need to research about motor back EMF and how to handle it. Maybe I should make a new post for this, but here goes.

**broken link removed**

Assume the inductor is a 6v motor that spins both ways, so we have 2 x Zener diodes rated at 5.6v @ 1W, the most current the diode pass is 170mA. If we want to allow 100mA of current we use a 60Ohm resistor. In this circuit, is it right to use a resistor?
My reasoning for the 2 x Zeners is that if the motor spins both ways, the current that goes through the Zener will be the same in both directions, if we only had a diode then spinning one way would let the current through and spinning the other way it wouldn't.

 

[ericgibbs]

hi,
Thats not the best way to apply back emf clamping.
Look at the LM293 datasheet, it shows 4 diodes for bi-directional

 

[alexalex]

Thanks again Eric!

 

[alexalex]

Hmm, that design routes the motor's current back to Vc and not to itself, isn't it dangerous to route current of the motor back to the battery? Instead of routing back to the Vc and ground why not replace both of those with a resistor (seems to work ok in the circuit simulator).

 

[ericgibbs]

Hmm, that design routes the motor's current back to Vc and not to itself, isn't it dangerous to route current of the motor back to the battery? Instead of routing back to the Vc and ground why not replace both of those with a resistor (seems to work ok in the circuit simulator).

hi Alex,
No, it not dangerous using clamp diodes this way.
A resistor will not give the same protection a diode will, in clamping the back emf 'spikes' generated the motors inductance.

 

[Vizier87]

hi alex,
I saw yout PCB picture and I saw the word Eaglemoss on it. Makes me wonder: Are you buying modules to build a Cybot? Just guessing...

 

[alexalex]

Thanks Eric I'll take your word for it. So below is my design with these changes. Is there anything wrong with it as far as you can see? (Ignore the green lines and the resistor values)

**broken link removed**

So when I put the first low to high, it circulates the current as normal, then when switching back to low, it circulates the current through the diodes to the Vc of the battery and then comes out from the Gnd of the battery back to the diodes. Also what's happening is about 1/5 of the current goes through the first transistor up the top.

Thanks again for all of your help.

Vizier87: Good eye, it is indeed a Cycbot but it's only got the motor controller, motors, gears, etc, I think I got up to issue 6 or 7 before I gave up with it. Now that I've got the Arduino I thought I'd build a robot and remembered I had this Cycbot, so I thought I'd build on this robot and learn more about electronics that way . As you can tell I am thinking about making my own motor controller to replace the one they have to learn even more and maybe make it into a PCB with different configurations.

 

[ericgibbs]

hi Alex,
This is your circuit redrawn in a more conventional way for this type of circuit.
You have not indicated any voltage levels, so I have added what they might be.

Look at this circuit and tell me what are the problems and possible failures of your circuit.

BTW: get a better schematic drawing program, there a number of free ones on the web

 

[Vizier87]

I see.... I bought up to issue 30-something.. we had it up to the IR remote control and games on the remote as well.
**broken link removed**

This is my initiation into robotics..

 

[alexalex]

Thanks Eric, that looks much better (I always keep drawing things going left to right ). From what I can see a problem would be if both inputs were on it would short the battery. Maybe if the motor has been run and then the input stops there could be some damage to the transistors (is this right?). Is there anything else I might be missing?

 

[ericgibbs]

Thanks Eric, that looks much better (I always keep drawing things going left to right ). From what I can see a problem would be if both inputs were on it would short the battery. Maybe if the motor has been run and then the input stops there could be some damage to the transistors (is this right?). Is there anything else I might be missing?

hi Alex,
Consider the switching ON, of the 2 top transistors, when using a 5V logic drive and a 12V motor supply.???

BTW: the short you mentioned would be via the transistors.!!!

 

[alexalex]

Hmm, so if Q1 and Q3 are both on at the same time, something would happen? If they are both on, there is no connection to ground so no current would flow? Or are you saying that because the supply voltages are different that something else would happen? I'm not too sure :S

In normal operation, Input A is on, so Q1 and Q4 are on, so current would flow from 12v to Q1, through the motor to Q4, then to ground. Input A current would go through Q1, the motor, to Q4 (and also come from Q4) then to ground. Would everything work properly?

 

[ericgibbs]

Hmm, so if Q1 and Q3 are both on at the same time, something would happen? If they are both on, there is no connection to ground so no current would flow? Or are you saying that because the supply voltages are different that something else would happen? I'm not too sure :S

In normal operation, Input A is on, so Q1 and Q4 are on, so current would flow from 12v to Q1, through the motor to Q4, then to ground. Input A current would go through Q1, the motor, to Q4 (and also come from Q4) then to ground. Would everything work properly?

hi,
Consider that Q1 is switched ON by the +5V to the base from 'A' what is the voltage you would expect at Q1 emitter.??

 

[alexalex]

I would have expected 12 volts at Q1 emitter, but I've tried it in circuit simulator and it says 5V no matter what voltage V1 (12v) is. It seems to only pass through the current of V1 and not matter about the voltage. In a way it's kind of like a voltage regulator? Will it damage the transistor passing 12v at collector and then 5v at emitter?

Is there a way to make it pass through 12 volts using the transistor as a switch or would we have to configure the transistor in a different manner or just use one power source?

 

[ericgibbs]

I would have expected 12 volts at Q1 emitter, but I've tried it in circuit simulator and it says 5V no matter what voltage V1 (12v) is. It seems to only pass through the current of V1 and not matter about the voltage. In a way it's kind of like a voltage regulator? Will it damage the transistor passing 12v at collector and then 5v at emitter?

Is there a way to make it pass through 12 volts using the transistor as a switch or would we have to configure the transistor in a different manner or just use one power source?

hi,
This is the reason I asked about the +5V on the base, remember for the transistor to be ON and conducting the base has to be about +0.7V greater than the emitter.
You will have to consider driving the top section in a different way, try to avoid having the motor in the emitter circuit, its not efficient.

Draw up a new circuit and post it.