How can i calculate Power Resistor value , Help me Please ??

[wini-1]

Hi there




I have a stepper motor that appear in the picture below (it labeled with 1.8A ), and it's coil resistance is : r = 3 [ohm].
I want to use a transistors to drive this motor like the schematic in picture 2. [ the transistors are ( MJE3055T ), data sheet is appear below ] .

my questions are :

1. If i want to run this motor at [ 12v & Ic = 1.5A ] , How can i calculate the correct value of Rc and Power Dissipation of it [watt] ?.
and how can i calculate the value of Rb ?

2. what is the difference if i use 24v in place of 12v ?.

3. What is the role of (Rc) here ? Is it for limit the current at the value which we needed (Ic = 1.5A) OR to pull the needed value of current (Ic = 1.5A) from the power supply??

Can any one explain the way to achieve that?



THANX FOR ALL.

 

[AdvaRes]

Re: How can i calculate Power Resistor value , Help me Pleas

Hi,

I found the compleate datasheet. For IC=1.5A VBe≈0.83Volt.
Since Ic=10 Ib you got:

Rb=(5-0.83)/0.15=27.8Ω

Use the curve of the datasheet to calculate the rest.

 

[karesz]

Hi,
If we must, as worstcase assume, that a (current limiting) resistor is neeeded, their is maybe between standig "on swithched" transistor and motor:
You must konw what is motor coils resistance!
If Rcoil=3Ohm,Ures=1.5Ax3Ohm=4.5V,
Transistors
Udsat, I say= 0.5V, you have a rest of voltage 12-5V=7V over resistor, with 1.5A =>>7/1.5=4.7Ohm
But remember pls, I have assumed the saturation voltage of your transistor by 1.5A!
Pls dont forget: YOU must apply COOLINGS!!
Good luck!
K.

Added after 1 minutes:

Sorry I have overssed the Res= 3 Ohm, & corrected the calculation...

 

[wini-1]

Re: How can i calculate Power Resistor value , Help me Pleas

Dear AdvaRes and karesz,

thanx for your helps.


Mr AdvaRes

you say : Rb=(5-0.83)/0.15=27.8

Why did you assume Ib = 0.15 A ???

Does this value for any microcontroller and LPT port in any PC ???

Does we use [ (hfe) value] from data sheet in this procedure ???
i mean : Ic=(hfe).Ib ?


_________________________________

Mr karesz

In data sheet there are 2 value for [Vce(sat)].
Which of them can i consider (for Ic = 1.5 A )???

AND what abuot the [watt] of Rc ??

My motor has coils resistance = 3 ohm ,[with Vce(sat)=1.1] ==>> Rc = 4.3 ohm
Is it right ??



Thank you.

 

[svhb]

Re: How can i calculate Power Resistor value , Help me Pleas

The transistor you want to use has a rather limited hFE, lets use 20.
We need the transistor in saturation FOR SURE. Otherwise there will be too much voltage across the CE pins and the transistor burns out (Ptransistor = Vce*Ic)

This means the base current must be at least 1.5/20 = 75mA, and to be sure take 100mA.

For delivering this current, a printer port or microcontroller will not be suitable, so you need an extra transistor to supplu the base current. Or use a darlington power transistor. (see for example BD679... (https://www.st.com/stonline/products/literature/ds/4201/bd680.pdf)) This transistor has a combined hFE of 750. So the minimum base current reduces a lot, and your printer port will be happy.

Remark, if you calculate the base resistor for a darlington, the Vbe to use is more!

Still, don't forget the heastsinks, you can easely burn your fingers

 

[wini-1]

Re: How can i calculate Power Resistor value , Help me Pleas

All my fingers and my test board have been burned before i post this topic .
this is the reason of why did i asked about this subject

 

[karesz]

Yes, t
his is a little bit dangerous circut!
You can become molded thin too (solered resistor leads)_or bourning (tree/plastic coated)table_if you didnt applyed proper cooling:-((...

Usat: sorry, I had no checked your datasheet (was to hurry)...
K.

Added after 2 minutes:

Other question/proposing;
if you hve the resistor on GND(in Emitter) = you can meausre simpler a current proportional voltage too...
I searched yet for a detailed datasheet: Ucesat is at 1.5A so ca. 0.8V...
You can calulate with them exacter.

 

[AdvaRes]

Re: How can i calculate Power Resistor value , Help me Pleas

Hi,
From the datasheet take the value of Vbe and Vce from the curve when Ic=1.5A. It is marked that Ic=10 Ib so you can do your calculation of the passive components.

Concerning the microcontroller current of the port you can use a source driver darlington like the UDN2981 or the m5456p to have sufficient current output.

 

[FvM]

Additional remarks not directly related to the original question:
- Did you realize, that you need 4 drivers for the stepper (assuming an unipolar winding scheme, as the 6 wires suggest)
- for a unipolar stepper driver, you need only two current limiting resistors, connected to the winding center taps
(you should verify the windings scheme with a multimeter, if you don't have a motor datasheet)
- a freewheeling diode connected directly across the stepper winding will considerably reduce the available motor
speed and also cause a short circuit current when powering the opposite winding
- using MOSFETs for the output stage would allow to omit freewheeling diodes (the internal zener clamp is sufficient), achieve
optimal efficiency and also solve the drive current problem

 

[wini-1]

Re: How can i calculate Power Resistor value , Help me Pleas

Thanx a lot for all.

Dear FvM,
you say :
(- for a unipolar stepper driver, you need only two current limiting resistors, connected to the winding center taps )



below is the scheme that i use.
does that wrong if i use one current limiting resistors for each transistor ??
because you mean that the value of each resistor will be decreased to allow to bass 3A if we need to run the motor with Half-step mode.

what is the better 4 resistors or 2 resistors?



Do you know what is the number of MOSFETs that suitable for this motor ?


thank you for your reply.

 

[wini-1]

Re: How can i calculate Power Resistor value , Help me Pleas

Thanx a lot for all.

Dear FvM,
you say :
(- for a unipolar stepper driver, you need only two current limiting resistors, connected to the winding center taps )



below is the scheme that i use.
does that wrong if i use one current limiting resistors for each transistor ??
because you mean that the value of each resistor will be decreased to allow to bass 3A if we need to run the motor with Half-step mode.

what is the better 4 resistors or 2 resistors?



Do you know what is the number of MOSFETs that suitable for this motor ?


thank you for your reply.

 

[FvM]

The drive sequence is such, that the windings with a common (center tap) connection won't be energized simultaneously.
In so far, they can share a common resistor. But you can use individual resistors if you prefer.

I think, any MOSFET with a several A current rating and Vds of 30 V or above can be used do drive the motor.

 

[svhb]

Re: How can i calculate Power Resistor value , Help me Pleas

You best select a MOSFET with 'logic level' Vgs specification. For example the IRF3708PBF. This device has an Rdson that is so low that you don't need any heatsink on the transistor.

Since a MOSFET draws no current on its gate, you need to provide a resistor between gate and source to prevent switch on when power is applied to the motor system and not to the logic system.

If you want a faster turn off (faster decay of the current in the winding), you can put a resistor in series with the freewheel diode (for example 5 Ohms). I think this is safer than dissipate the energy in the MOSFet like FvM suggests.

If you revise your schematic, post it (with component values) for comments.