pwm to control a dc motor

[mariomoskis]

ah sorry,my transistor is 2n2222a!

then if i understand well(if something is wrong say me):

-the resistor at the base must be calculate with a colector current=0.3A,yes?
so 0.3/160=0,001875( minimum current to get saturation) and the resistor at the base must be maxium 6V(output 555)/ 0.001875=3.2K(maxium value to get saturation)

-and the cut-off zone the voltage across my motor is 0.7V (about the diode in parallel),and the saturation zone the voltage across my motor is (7-1.4)*duty cycle ?
so if all this is correct should it works well now??

 

[(*steve*)]

ah sorry,my transistor is 2n2222a!

I thought we had established that simply wasn't going to work and that now you were using a darlington (presumably either a discrete one with some higher power transistor, or a single device which is internally this structure)?

then

There is no THEN. You need to have a device that is capable of switching the current.

I would recommend a mosfet. And I would recommend a MTP3055

 

[mariomoskis]

ok,i just try this with crio and my digital module ni9401,i use darling(with 2n2222a) to get more gain and to have lees current at my base as i need,i gave a supply voltage to motor 3V

it looked works ok,
when i check the voltage across my motor with a voltmeter i have 1.5V with duty cycle 50%,well!

but when i do the same for duty cycle 75% i check 1.9V but i should see 3*0.75=2.25V or 3*sqrt(0.75)=2.6V ?? but both case aren´t well !
why i have this problem?

the formula for a pulse signal is Vacross motor=Vsupply motor*sqrt(duty cycle) or without sqrt?? i am checking it with a voltmeter

thanks

 

[jackorocko]

What happen to the voltage drop across the transistor?

 

[mariomoskis]

circuit:
http://imageshack.us/photo/my-images/42/darle.jpg/

3V supply voltage,so my motor should see 3V 100%,
2.25V with 75%(i see 1.9V at the multimeter,why?)
1.5V 50%(i see at the mutimeter1.5,well)
transistor down the motor get little hot)


V colector:
http://imageshack.us/photo/my-images/819/010voj.jpg/
on the oscilloscope i see a pulse signal with values between 1V and 2V, 1V/div,strange not?maybe it is caused about the motor,

i try too with supply voltage 4.2V and two diodes 1n4004 at the emisor of the transistor down the motor(transistor down the motor don´t get hot,why not in this case?)but i don´t have still the image,tomorrow i will take it

one question that i still don´t understand:
at the datasheet of the motor we see I(no load)=0.3A,and I(at maxium efficiency)=1.05A ,which is the different?and when i have each case?

 

[mariomoskis]

and the second transistor of darling ton as a said,get little hot too but less than when i had only one transistor

 

[mariomoskis]

so i have the problem again that i don´t know if my transistor is saturated or not?
is there a way to know it without a oscilloscope?

 

[(*steve*)]

Is your darlington made from two 2n2222s?

 

[mariomoskis]

yes,there are two 2n2222a

 

[(*steve*)]

yes,there are two 2n2222a

Well, that's not going to work.

 

[mariomoskis]

yes,i see it,because the transistor isn´t saturated, i have 1V between colector and emisor

the problem is about the transistor?

 

[mariomoskis]

at the transistor data sheet i can see:

0.8A max colector ,but we never have this value because we don´t have load on the motor,so the max current will be 0.3A,YES?so it won´t be a problem?

and i saw too that total dissipation for 2n2222a is 0.5W at Tamb<25ºC and 1.8W at Tcase<25ºC,
so when we have the worst case P=V*I=3*0.3=0.9W, are we in the range,yes?

 

[(*steve*)]

You are pulsing the current to the motor, so whilst the average current may only be 0.3A, the peak may be 0.6, 0.9, or higher. -- and that's while it's running.

When the motor is starting, the current can be several times higher.

With a load it could approach several amps, possibly higher if it gets stalled.

How many times do you need to be told that the 2222 is insufficient?

Then it says the dissipation is 1.8W with a case temperature of 25C -- clearly the case temperature is higher than that, so you need to de-rate it.

The thermal resistance junction to ambient is 200 degC per watt. To keep the junction temperature below 100C (a reasonable requirement for a small transistor) at an ambient temperature of 25C, the max power dissipation is (100-25)/200 = 0.375W, so clearly, even assuming the most favourable conditions (saturation and 0.3A), you're trying to dissipate three times too much power. even at this current the junction temperature is likely to rise to 25 + 0.9*200 = 205C -- and that's well above the absolute maximum temperature of 150C

 

[mariomoskis]

ok,i am gonna change the transistor
i have a BD135 which can have 1.5A at the colector
do you think that all will work well with this?

 

[mariomoskis]

i just try it with that transistor,and when the supply voltage fro 555 is higher than 3.5V the motor consumes more than 0.3A(max) ,why is it happening?

 

[twister]

The current gain of the 2n2222A is 40 @ 500mA Ic, refer to the datasheet here http://www.uib.es/depart/dfs/GTE/staff/jfont/InstrETT/2n2222.pdf

0.3A/40 = .0075A of base current needed. Double this number, because a little extra won't hurt.

so then you take the 555 output voltage, which you have it as 7V, using ohms law we get

7V/.015 = 466 ohms R1


I still don't understand why you need R. It will be in series with the motor so the overall resistance of that branch of the circuit will increase limiting current to the motor. Besides, using PWM will control how much voltage the motor sees.

An emitter resistor is used to prevent thermal runaway. The more current thru the resistor, the more voltage is across it, which is negative feedback to the base. The hotter a bipolar transistor gets, the more current it passes, until it shorts or burns out. It is usually a low value. I have seen .1 to 1 ohm used. A mosfet is just the opposite, so it doesn't need a resistor.

 

[mariomoskis]

but i am not using a resistor at the emisor,it is at the base

as i said i changed to a another transistor BD135 which has 1.5A at its colector,it has gain is 25,
and here is when the supply voltage for 555 is higher than 3.5V the motor consumes more than 0.3A(max) ,why is it happening?

 

[jackorocko]

which is negative feedback to the base

What? I guess I need a better explanation because I don't get it as usual.

 

[mariomoskis]

motor work(1.5-3V)
max current no load0.3 max current at max efficienty 1.05A

quickly resume:
first circuit was this:
http://imageshack.us/photo/my-images/705/sinttulo2mn.jpg/
didn´t work about the transistor had a colector current 0.8A and it get so hot when i try to run it. and the transistor didn´t get satured when i saw the Vc with a oscilloscope

i tried this too because later i will have a crio module ni9401 and i will need less current at the base
http://imageshack.us/photo/my-images/707/darl.jpg/
but same problem about transistor

so finally i tried this:
http://imageshack.us/photo/my-images/705/sinttulo2mn.jpg/

but with a transistorBD135 which has more current at its colector 1.5A instead the 2n2222a 0.8A
but i have the problem about that when i try to run it,when i am increasing the supply voltage more than 3.5V the motor consumes its maximum current no load 0.3A and i don´t why it is happening

 

[twister]

What? I guess I need a better explanation because I don't get it as usual.

A npn transistor has to have a positive voltage on the base and a negative voltage on the emitter in order to be on. What will happen if the emitter voltage goes up but the base voltage stays the same? This is called negative feedback because it is out of phase with respect to the base. You know that the voltage across a resistor increases when the current increases? That's why that emitter resistor is in that circuit. For thermal protection. A mosfet doesn't need that because they conduct less current when they get hot.
I don't know the math used to determine what size is needed, but a .1 ohm would probably work.
Negative feedback is also used in high gain circuits that tend to oscillate when you don't want them to.