BJT as electronics switch

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Hi,
I meaned, I don't know how many current should be used to run a DC motor or a relay, then how am I going to choose a suitable transistor for it?
 
bananasiong said:
Hi,
I meaned, I don't know how many current should be used to run a DC motor or a relay, then how am I going to choose a suitable transistor for it?
Click to expand...

Read the spec on the device, with a motor bear in mind that the current can vary over a HUGE range, with a stall current MUCH higher than running (even under load).

Relays are simple, their load is constant, and have the resistance value printed on them, or in the datasheet.
 
but nothing is written on the DC motor.. it is 'cheap' type, i think. and the relay as well, the amount of current to switch the lever is not printed on it, only voltage. how do i know? by testing it using power supply with different current but same voltage (as printed)?
 

Measure the resistance of the relay, along with it's voltage you can then use ohms law to calculate it's current.

For the motor, feed it from a high current supply (of the correct voltage) via an ammeter - measure the current with no load. Then apply a load to the motor (for a small motor use your fingers), measure the current under differing loads. Finally apply a load until the motor stalls - measure that current, expect this to be well in the amps - use a 10A or 20A range on your meter.

The reason for the high current supply is so the supply can actually feed enough current for the stall and load tests.

I did measurements on the Cybot (magazine robot) motors years back, using a PJ996 battery (6V with the springs on top) I got over 6A stalled, but that was a limit of the battery rather than the motor!.
 
Oh!! So I just use a power supply and adjust the current to the maximum with the voltage which is going to be used?

measure the current with no load
Click to expand...
why is this needed?

Finally apply a load until the motor stalls - measure that current, expect this to be well in the amps - use a 10A or 20A range on your meter.
Click to expand...
Stall means stop by overloading it?? The current when the motor stalls is the right amp for it to run?

thanks for replying.. XD
 
Stall is when the motor shaft is locked and cannot move. In the stall state the motor will draw it's maximum current and is what a system should be designed to handle at most (not neccessarily continuous since running any motor at stall for too long overheats it and damages it).

Measuring the current that the motor draws when you have no load applied gives you the minimum current the motor will draw.

Using the minimum and maximum current values will help you determine some of the graphs for the motor. Circuits always have a maximum value so you know what to design your circuit to. Also, some circuits/components won't function if the current in them is too low- and knowing the minimum current the motor draws will help you design a circuit that always works with the motor. For example, relays tend to have a "minimum contact load current" listed in their datasheets. I'm not sure why a relay might not be able to conduct a current that is too small, but it must be there for a reason.
 
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Be ware about the minumum, with any motor with carbon brushes (apart from shunt wound ones) the minimum current will be 0A. When the the commutator is rotating the power always becomes disconnected at some point as the brushes slip over the contacts.
 
bananasiong said:
Oh!! So I just use a power supply and adjust the current to the maximum with the voltage which is going to be used?
Click to expand...

No, while testing the motor your suply needs to be able to supply MORE current than the motor ever requires, otherwise you're testing the supply and NOT the motor.

Stall means stop by overloading it?? The current when the motor stalls is the right amp for it to run?
Click to expand...

No, run current will be FAR less, but you need to be aware of the massive stall current requirement, and either provide enough capacity for it, or limit the current in some way.

As already mentioned, you don't want a motor to sit stalled for very long, the motor will soon burn out.
 
Nigel Goodwin said:
No, while testing the motor your suply needs to be able to supply MORE current than the motor ever requires, otherwise you're testing the supply and NOT the motor.
Click to expand...
I don't know how much current does the motor need, that's why I want to test it out. Isn't this the method??? I'm getting confused now..


The run current will be less than stall current? 1/2 of it or what? Or should I take the average between max and min current?
 
Does connecting a load across the collector and the ground effect the Dc biasing conditions? I mean does it change the currents and also the collector, ground voltage?
 
bananasiong said:
I don't know how much current does the motor need, that's why I want to test it out. Isn't this the method??? I'm getting confused now..
Click to expand...

I've already explained how, what's confusing about it?.

The run current will be less than stall current? 1/2 of it or what? Or should I take the average between max and min current?
Click to expand...

Again, it all depends on the load - but stall current may well be ten or twenty times the normal running current.
 
Sorry about that, but i've already read the replies again, and still can't understand. You said that the current supplied should be more than the required current of the motor, but I don't know how much the motor requires.
Is it minimum when there is no load? Then the transistor i should use is more than the stall current?
 

It's minimum with no load, and maximum under stall conditions, normal running will be somewhere between the two.
 
Beenuseren,
Please start your own topic.
Mention if your transistor is an NPN or a PNP.
Attach a schematic.
 
Nigel Goodwin said:
normal running will be somewhere between the two.
Click to expand...
You mean any current between these two? As long as that current is supported by the transistor, right? Then I think their average will be good for me.



Another question, just to test my understanding, from the datasheet of tip41, this transistor can handle 3A, and 6A is the maximum, right? The how much should the base current be if I run the motor with 6V and 3A? Is is as mentioned, 1/10th of the collector current?
https://www.electro-tech-online.com/custompdfs/2006/09/TIP41A-DPDF.pdf
 
3000mA/10 is 300mA isn't it?
The motor's voltage doesn't have anything to do with it, except maybe the motor's supply is the source of the base current.
 

As Audioguru says, the voltage doesn't really matter - but to reduce dissipation in the transistor you really need to drive it hard ON - so plenty of base current is a good idea!.
 
Nigel Goodwin said:
so plenty of base current is a good idea!.
Click to expand...
As long as it is more than 0.7V, the current doesn't matter? So the base current is not necessary 1/10th of the collector current, right?


So, am I right with this: the tip41 can handle 3A current, 6A maximum.
 
bananasiong said:
As long as it is more than 0.7V
Click to expand...
No. Look at the datasheet. At a collector current of 3A and a base current of 300mA, the base-emitter voltage is typically about 1.0V and could be as high as 1.5V. At 6A it is higher.

the current doesn't matter?
So the base current is not necessary 1/10th of the collector current, right?
Click to expand...
Wrong. Of course the base current is extremely important. If it is more than 1/10th of the collector current then the base-emitter junction might fail. If it is less then the transistor might not turn on hard enough then it will have a voltage across it and a high current through it which makes a lot of heat in it.


So, am I right with this: the tip41 can handle 3A current, 6A maximum.
Click to expand...
Yes, through its collector-emitter. With 3A and a base current of 300mA, its saturation voltage could be as high as 1.0V, which makes 3W so it better have a heatsink. Its absolute max heat dissipation in 25 degrees air without a heatsink is 2W.
 
I don't see why you're messing around with BJTs, a MOSFET is probably more suited to your application.
 
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