# Help calculate Resistor to Saturate BD139.

Discussion in 'The Projects Forum' started by pixelsnpings, Feb 3, 2010.

1. ### pixelsnpings Thread Starter New Member

Feb 3, 2010
13
0
Good afternoon folks,

I am trying to build a circuit to control a very small DC motor for a hobby project im working on.

I am looking to calculate the base resistor(R1) value.
My search online found this equation.

R = Vc / (MMC / HFE * 1.3)

VC = Input Collector Voltage.
MMC = Maximum Current needed to drive the Motor.
HFE = Minimum HFE/Beta (DC Current Gain)

This equation allows for 30% more heat exchange for safety purposes.

The values I am using are as follows.
VC = +12V
MMC = 1000ma (1 Amp)
HFE = 63

The resistor value returned by my calculations is 0.58 ohms.
And this seems very odd.

Please note too that the logic level voltage used to control the base of the transistor will not excead +5V as it is connected to my MCU.

I am using group 10 version of this transistor.
Here is the link to the datasheet.
http://www.datasheetcatalog.org/data...fa3reo8uky.pdf

I have included a diagram attachment of the circuit I'll be using.

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2. ### ifixit Distinguished Member

Nov 20, 2008
639
110
1. To just barely turn the BD115 fully on would require a base current of 63 times less than the collector current. 1000/63=15.9mA.
2. Therefore R1= (5-Vbe)/15.9mA = 276 ohm.
3. However, to ensure good saturation, 10 times more base current is recomended. i.e. 159mA.
I doubt your 5V CPU drive can supply this. If not, just drive as much as you can over the minimum. Measure Vce to see that it is at least less than Vbe.

A MOSFET would do better here, or add another transistor to get more gain.

Good Luck

P.S. The diode should be reversed

Last edited: Feb 3, 2010
3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Not quite right. Vce(sat) on most transistors is specified with Ib=Ic/10, not Ib=10*Ic/β). That would make the resistor value ≈110Ω.

Pixelsnpings, you said BD115, but apparently you meant BD135. The transistor is only spec'ed to 0.5A, so it may not have good saturation characteristcis at 1A. A MOSFET would be a much better choice.

4. ### pixelsnpings Thread Starter New Member

Feb 3, 2010
13
0
Ron H and ifixit thank you for responding.

I wondering if you guys can offer any suggestions on a mosfet that will fit my needs.

My max 1 amp spec is a safty measure. The motor I'm running consumes roughly 0.65 amps in a constant on state. I was hoping that using this transistor with a pwm of 1kHz will hold a good max without over powering the motor. I placed the 1N4001 diode to handle any kick back spikes along with the cap to extend the life of the motor.

The motor normally runs between 3 and 7 volts and can handle a max of 1.5 amps. so even if the BD139 was just under saturated I believe I should get a good through put for motor control.

Please correct me if my thought here are in error.

If there is a MOSFET choice that would better suit my needs I would gladly grab them.

Thanks again for the help and info.

5. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Where would you shop for a MOSFET?

6. ### hgmjr Moderator

Jan 28, 2005
9,030
214
D1 in your diagram needs to be reversed.

hgmjr

7. ### ifixit Distinguished Member

Nov 20, 2008
639
110
Go to http://www.vishay.com. A IRFZ14 or a 100 others would do the trick for you.

Requirements...
1. Needs to have a low enough Rds(on) (on resistance) to minimize power loss. <0.3Ω would be equivelent to a 0.3V drop at 1 Amp. Same as a xstr. MOSFETs can usually do much better.
2. Handle a voltage of Vd=20V. Choose 60V, buy a few, there handy for other things.
3. The gate switch on threshold, Vgs(th), should be 2-4V so you can turn it on with 5V logic levels. The Vgs goes up, >4V, as the Id increases.
4. Handle 1 amp (0.65A?) drain current.
5. Choose a package. TO220 work for you?
6. Fast switching to handle 10 times the PWM frequency you need.
7. Gate capacitance could be an issue so don't go too far over size for the application. Power MOSFETs have a lot of C (several 1000pf) so you may need a logic buffer/driver if you need fast PWM edges.
Have Fun.

8. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
An IRFZ14 would not work; as it is a standard-level MOSFET requiring 10v on the gate.
An IRLZ14 would work; as it is a logic-level MOSFET.

However, you might look at an IRLD014 or IRLD024; these little MOSFETs come in a 4 pin DIP package and can sink 1.7A or 2.5A respectively.

9. ### Audioguru New Member

Dec 20, 2007
9,411
896
No. The threshold voltage is when it barely conducts only 0.25mA.
A threshold voltage of 2V to 4V is for an ordinary Mosfet that needs 10V on its gate to fully turn on. Most logic-level Mosfets have a threshold voltage of 1.55V to 2.45V and are spec'd to conduct very well with 4.5V on the gate.

10. ### ifixit Distinguished Member

Nov 20, 2008
639
110
The info supplied by the OP indicated the MOSFET is to be driven from 5V logic, which I took to mean 5V High and 0V low. The IRFZ14 data sheet indicates that this is more than enough to supply an Id of 1 amp.

However, I like the IRLD014 much better, it is more suited for this application.

I have a question about Qg (gate charge). It is spec'd in units of nC. I though it was input capacitance in nanofarad, but it isn't.

What is an nC and how do I use it?

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11. ### Audioguru New Member

Dec 20, 2007
9,411
896
Maybe yes or maybe no.
Go to your electronic parts store and ask for that Mosfet that is tested to be typical. They will laugh at you. The graph is only for a typical Mosfet.
The Mosfet has a range of spec's. Some of them are very good, some are typical and some are not very good. You won't know if yours is typical or not.
You should look at the minimum spec's or at the guaranteed spec's (10V on the gate).

12. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
If an IRFZ14 were used, at Vgs=4.5v with Id=1A, Vds would be about 1v.
They might as well use a saturated transistor; it would dissipate much less power as heat.

If an IRLZ14 were used instead, at Vgs=5v with Id=1A, Vds would be around 200mV, and would dissipate 1/5 the power the IRFZ14 would.

nC = Nanocoulomb.
See this Wikipedia entry: http://en.wikipedia.org/wiki/Coulomb