pwm to control a dc motor

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mariomoskis

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hello,i am trying to control a dc motor (1.5-3) with a transistor 2n2222a, at the 555output i have (supply voltage-1)=7-1=6V and the circuit is the next:



my transistor has gain=40 and the maximum load of y motor will be 0.3A,so i need at my base a current at least I=0.3/40=7.5mA to get saturation, and the resistir at the base must be 6/7.5=800 maximum to get this, so i use a resistor=270ohm, all this is correct?

but the problem is that when i make the circuit, the transistor get so hot, why am i having this problem? how could i solve this problem?
pd: the diode on parallel with my motor is in correct way!

could somebody help me with this, i am really loss alot time with this but i don´t get a solution

thanks
 

Use heat sink for the transistor or change it to 2n1711 look for transistor that has at least 1amp collector current . the pulse from from the 555 should be pure square wave from zero to almost vcc.. if its not the transistor will not work in the switching mode but may work partially in the linear region .

JUst found out u are sourceing the lm555 output and the data sheet show its a few milliamps 2-3 ma. so you have a problem . u can use it in the sink mode that means usind pnp 2n 2907 transistor with emmiter to vcc and collector to motor + and motor - to grd
 
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it is a NI CE555,not a LM555

i don´t have a heat sink and another kind of transistor,is there another solution?
how can i know that the transistor is really in saturation,because with a oscilloscope i see this wave
 

Adjust the scope on dc input and check the scope output on pin 3 and print the readings on scope i can see the voltage in pic

---------- Post added at 18:28 ---------- Previous post was at 18:26 ----------

Adjust the scope on dc input and check the scope output on pin 3 make sure u using pin 3 for output and print the readings on scope i can see the voltage in pic
most circuits using 4.7k resistors at pin 3 output
 
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You need a chop-rate high enough that the motor winding
inductance prevents current beyond what you expect. At
locked-rotor or starting from zero RPM there will be no back
EMF, and the current may be well greater than running at
some RPM.

That waveform is pretty creepy, indicates to me that you
have no catch-diode. As a result you might be pulling the
collector and thus the base (since its out-looking impedance
is limited by your IC and board base resistor network) more
negative than BVebo rating, likely damaging the device.
Can't do more than speculate, with the V/div and t/div
text turned off on the 'scope.
 



i have my oscillocope on dc and i can see at the pin 3 the pwm signal well, if i try to see the voltage on another diode,i can see it weel too,the problem is on the diode parallel with the motor

i see too that when my motor is working in its more value of voltage 3V, (when i have 6V at the 555 output with 50% duty cycle) the motor consumes a little more than its maxium current 0.3A

---------- Post added at 17:54 ---------- Previous post was at 17:53 ----------
could you explain me it more clear please? my native language isn´t english and it is little difficult to understand some sentences

thanks
 

change r 2.2 k min preferable 4.7k and add another transistor for darling-ton .
sorry !!! but if u can get parts u are in the wrong business .
 

i just did a darling-ton,with two transsitors and r=2.2 but i had the problem that when the supply voltage is more than 5V the motor consumes more tan 0.3A,and i can´t keep increasing the supply voltage more to get the 6V at the 555 output

when i have two transistor,which voltage should i use to calculate the duty cycle to get 3V on my motor?is it the same that with one transistor?
 

"two transsitors and r=2.2 : " u mean 2.2 k ???
more u need to add resistor to emitter of the last transistor
post the new circuit and send the output voltage on scope in dc volt and give the value.
take a metal plate 10 by 10 mm and stick with heat compound very thin coat on last transistor u can use grease or toothpaste remember thin coat .
 
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this is the circuit:



when i have a supply voltage higher than 4.5V,my motor consumes its maximum current possible 0.3A
and i can see too that el motor changes its current consumes sometimes and its speed changes too

output 555 for Vcc=6V


 

very gooood now all u need to do is add a resistor that will take out 2 volts and 0.3 amps off the motor thats 7-15 ohms or 2 in4001 series diodes (try adding them series of the emmiter or motor or collector and see what happens.
good luck
 

sorry two questions,
why i have to take out 2V?
and when you said: addding them series of the emisor,which transistor in? or which colector?
 

u said your motor is 5v or 4.5 volts and your supply is 7 volts.... simple vcc supply-max motor volt allowed
both collectors junction to motor... and maybe u will need three or four diodes try combination of diodes and resistor remember each diode will take 0.7 volts do the math .. u can do it.
u can also try adding resistor/diode in series of vcc and the motor junction.
 
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no,my motor work between 1.5 and 3V

so you refer about this:
 

mariomoskis said:
no,my motor work between 1.5 and 3V

so you refer about this:
View attachment 71104
Click to expand...
OK dont put resistor on emmiter put it in the collectors it should hold 4v and 300ma thats 15 ohm 1.5watt use resistor with diodes in serial so the diodes will take part of the voltage and resistor will be smaller and less wattage
 

and why we didn´t do this when we had one transistor? which is the different?
 

"i just did a darling-ton,with two transsitors" u answered your own question.
so a)u got hfe=40*40=1600 !!! u need for the proper working switching range of the transisators.

more...555 will work in 5vdc...b) your motor is 1.5-3v and supply is 5-7 volts so u need to add parts in series to motor that will take the differ between motor and supply range
 

but for example the darling-ton:

is there a voltage drop across the darlington? maybe 1.4V, but will it affect only in the saturation zone or in the cut-off zone?

''your motor is 1.5-3v and supply is 5-7 volts so u need to add parts in series to motor that will take the differ between motor and supply range''
but can it depend of the duty cycle too? if the transistor is saturated, i have Vcc=7V and the duty cycle is 3/7=42% to get 3V across my motor, and is 1.5/7=21% to get 1.5V across my motor, is it correct? so should i need add parts in series to motor?
 
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NO connection between duty cycle and max ratings .
at 99% duty cycle max speed your motor will get 3v and vce sat=0.1-0.2volts so your resistor will take 4 volts.
when duty.. 50 % your motor will stil get 3v but its average voltage will be 1.5v because u on/off the motor 50% and so speed control.
at this point use ltspice or another spice and see what u get...or build the circuit and operate .remember at any duty.. the voltage at collectors will be between 0v and supply .
its time to jump in the pool and see what happens and if transistor heats.
a lot of 555 circuits here..
**broken link removed**
 

ok,and one think that i don´t have still clear:

when the 555 output is high,the transistor is saturated
when the 555 output is low,the transistor is in cut-off

but which voltage and current are across the motor in each moment, sorry about this question but i have a mess in my head now about this
 

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