How to limit current

[alexalex]

Hi all,

I'm wondering how I can limit current from a battery power source that is 5V and 2500mAH which is connected to power a motor through a transistor?

Here is the transistor: https://www.electro-tech-online.com/custompdfs/2010/04/PN100PDF.pdf
Will it blow the transistor as it only accepts up to 500mA? or am I reading it wrong and it won't actually blow?

Should I place a resistor between the 5V 2500mAH and the motor, if so would a 1W one do or should I go with a 2.2 Ohm resistor 15W?
Is there a way to wire it so I don't need a resistor? I've seen some motor controllers that don't have any resistors at all.

Diagram below.
**broken link removed**

Thanks for your help,
Alex.

 

[Grossel]

Very easy. The circuit you're looking for is called current limiter.

 

[ericgibbs]

hi alex,
The motor will determine the current drawn from the battery, not the 2.5Ahr rating of the battery.
Whats the motor voltage and what is the rated current.?

example: if the motor is say, 5V and it requires a current of 200mA to run, then it will only draw 200mA from the battery, even if the battery was 100Ahr.!!

 

[audioguru]

A DC electric motor draws a very high current when it is started and when it is stalled. It should be listed on its datasheet or you can measure its resistance then use Ohm's Law to calculate its max current.

If you limit the current to the motor with a series resistor then it might not start running and its torque will be reduced.

You will blow up the transistor because it simply shorts the power supply instead of powering the motor.
You will also blow up the transistor because you are applying 5V from base to emitter without limiting the current.
Your transistor might be an "emitter-follower" instead of a switch. So it will have a voltage loss of up to 1.5V and it will get very hot.

 

[alexalex]

Ah, thanks for that ericgribbs, forgot that motors have ratings!

So lets say it was 200mA, according to the PDF I linked to, looking at the Vce, it says you need to let in 20mA to the base to let 200mA flow from the collector to emitter?
So you would probably be safe putting 30mA to the base so it lets 300mA flow so that's enough?
Or am I reading it wrong?

Thanks again

Edit: Oh sorry I forgot to add a resistor in the diagram for the base of the transistor of 10K. audioguru, if I did add a 10K resistor to the base, and left the circuit as it was (without adding any other resistors between the 5V and motor),would anything else blow up?

 

[ericgibbs]

hi,
Get an ohm meter and measure the resistance of the motor.
The 200mA example I gave is the running current, as pointed out, the start and stall currents are only limited by the resistance of the motor.

Putting 30mA into a transistor base dosnt mean you get 300mA out.

 

[audioguru]

A 10k series base resistor limits the base current to only (5V - 0.7V)/10k= 0.43mA which is much too low.
If your 5V supplies are connected together or are one supply then your transistor is simply shorting it.

Why don't you use the transistor as a switch instead of as an emitter-follower that wastes voltage and makes heat?

 

[alexalex]

Ok, I thought it was already acting as a switch? Both power supplies are separate from each other. How many mA should I be looking into, 10mA, 20mA? If I do increase the mA, this will allow more current to flow between the collector and emitter, correct?

 

[alexalex]

Here is an example circuit I've done in a circuit simulator. It seems to follow what you say about 0.43mA however with the hFE at 100 it lets 43mA pass through the collector to emitter, does that seem right to you?
In the datasheet of the PN100, where exactly does it say, xxx mA of base = xxx mA of flow between collector to emitter?

**broken link removed**

 

[ericgibbs]

Here is an example circuit I've done in a circuit simulator. It seems to follow what you say about 0.43mA however with the hFE at 100 it lets 43mA pass through the collector to emitter, does that seem right to you?
In the datasheet of the PN100, where exactly does it say, xxx mA of base = xxx mA of flow between collector to emitter?

hi Alex,
You are looking at the transistor operation incorrectly.
Typically there are two modes.
1, when used as an amplifier, the transistor operates in a linear mode.
2, when used as a switch ie: on/off ,,, conducting to collector saturation current and no collector current.

Which mode are you asking about.??

 

[alexalex]

Hey Eric,

I'm asking about just the switch mode. So if there is 5V at the base, it connects us the collector and emitter to each other so the LED powers on. In this mode does it really matter the mA applied to the base? In the simulator it seems like it does?

Edit: I found this example: **broken link removed** Example 2, it gives a formula I think this is what I need?

 

[ericgibbs]

Hey Eric,

I'm asking about just the switch mode. So if there is 5V at the base, it connects us the collector and emitter to each other so the LED powers on. In this mode does it really matter the mA applied to the base? In the simulator it seems like it does?

Edit: I found this example: **broken link removed** Example 2, it gives a formula I think this is what I need?

hi,
That link looks OK as a starting point example.
Look up the term Vce saturation.
The HFE [large current gain] of a transitor typically falls as the collector current rises.

 

[audioguru]

Your transistor is used as a switch. But it is very unusual to have an input power supply that is completely separate from the output power supply.

A transistor has its hFE spec'd when it has plenty of collector to emitter voltage so it is a linear amplifier. Your transistor is spec'd with a collector to emitter voltage of 1V and 5V.

A transistor is saturated when it is used as a switch. Then its maximum saturation voltage loss is spec'd when its base current is 1/10th the collector current. hFE is not used because it is low when a transistor is saturated.

The simulator operates a "typical" transistor that has lots of gain. But the transistor that you buy might have minimum spec's so it needs maximum base current as per the datasheet.

 

[alexalex]

Eric: Great, thanks. So from the PDF, if I wanted to allow 200ma through C and E, the graph says the hFE will be about 180, so the Ib needed is 200/180 = 1.1 mA. Then 5v - 0.7v / 1.1 = 3.9k resistor is needed.

It seems to say that Vce is mostly just used for amplifying circuits and for switching purposes Vce should be 0 (I'm guessing you don't need to do anything to make it 0 Vce?) which allows the maximum current to be obtained when you apply the current to Ib.

 

[audioguru]

The graph of current gain is only for a "typical" transistor at room temperature and only when the collector to emitter voltage is 5V so it is not a switch.
You don't know how much gain (hFE) your transistor has unless you measure it but every transistor is different so it is a poor way to limit current.

As a switch you want the transistor to have as low a voltage loss as is possible. The max saturated voltage loss is listed on the datasheet when the base current is 1/10th the collector current. For a collector current of 200mA the base current should be 20mA. The load and its power supply voltage determines the collector current, not the transistor.

 

[ericgibbs]

Eric: Great, thanks. So from the PDF, if I wanted to allow 200ma through C and E, the graph says the hFE will be about 180, so the Ib needed is 200/180 = 1.1 mA. Then 5v - 0.7v / 1.1 = 3.9k resistor is needed.

It seems to say that Vce is mostly just used for amplifying circuits and for switching purposes Vce should be 0 (I'm guessing you don't need to do anything to make it 0 Vce?) which allows the maximum current to be obtained when you apply the current to Ib.

hi,
To get a Vce of zero is not possible for real transistors.
Looking at this extract from your datasheet, note the Beta 'β' value on the graph, tell me what you think about your base current calculation.??

 

[alexalex]

Oh audioguru just saw your post, so who is right? So you are saying I need to put in Ib as 20mA to let it do 200mA between Ic and Ie if it's just used as a switch? :S Why does the example 2 in that link I showed only calculate it using hFE? This is confusing :\

Edit: Just read your new post, hmm

 

[audioguru]

Oh audioguru just saw your post, so who is right? So you are saying I need to put in Ib as 20mA to let it do 200mA between Ic and Ie if it's just used as a switch? :S Why does the example 2 in that link I showed only calculate it using hFE? This is confusing :\

The tutorial is WRONG!
The datasheet for every transistor shows a very high base current so that a transistor saturates well.

 

[alexalex]

audioguru: Ah, makes sense.

audioguru + Eric: Ok, so I should ignore the graph that says Typical Pulsed Current Gain vs Collector Current because that is calculated by applying 5v to C and E which uses the transistor in a different way (not as a switch as I want). The graph which Eric shows B (another term for hFE) is very low at 10. Using the formula, B = Ic / Ib, it would be Ib = Ic / B which is Ib = 200 / 10 (so this is where you got the 10 from!) = 20mA. So for 300mA Ic I need 30mA Ib and so on.

So that's right?

 

[audioguru]

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?