Author Topic: Transistor base resistor question  (Read 8762 times)

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Offline MikeCLTopic starter

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Transistor base resistor question
« on: November 23, 2013, 06:40:54 am »
I have a MIC502 which is a fan IC controller for it's output it needs a resistor for the transistor base (BD135) however I'm not sure what my resistor value should be... My input voltage is 12V and the fan is 12V .60A

I want to say 330 Ohm but I might be wrong..

 

Offline Psi

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Re: Transistor base resistor question
« Reply #1 on: November 23, 2013, 06:52:05 am »
From the datasheet..

Quote
Base-Drive Resistor Selection

To minimize on-chip power dissipation
in the MIC502, the value of RBASE should be determined
by the power supply voltage. The Electrical
Characteristics table specifies a minimum output current
of 10mA. However, different output voltage drops (VDD –
VOUT) exist for 5V vs.12V operation. The value RBASE
should be as high as possible for a given required
transistor base-drive current in order to reduce on-chip
power dissipation.
Referring to the “Typical Application” and to the
“Electrical Characteristics” table, the value for RBASE is
calculated as follows.

For VDD = 5V systems, IOH of OUT
(pin 7) is guaranteed to be a minimum of 10mA with a
VOH of 2.4V.
RBASE then equals (2.4V – VBE) ÷ 10mA = 170ohms.

For VDD = 12V systems, RBASE = (3.4 – 0.7) ÷ 0.01 =
250ohms

Also note the Electrical Characteristics section that states:
  Vout high is 2.4V when the chip runs from 5V
  Vout high is 3.2V when the chip runs from 12V

Which is where those figures come from.

Note: You may encounter gain problems trying to drive a 600mA load from <10mA base current but try it and see.
You can always use a darlington transistor if needed.
« Last Edit: November 23, 2013, 09:14:31 am by Psi »
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Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #2 on: November 23, 2013, 07:01:38 am »
I was thinking about using a 100 ohm resistor, my voltage input is 12V and the fan would have a max current rating of .60A but I can't forsee it running that hard unless the temp rises
 

Offline Psi

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Re: Transistor base resistor question
« Reply #3 on: November 23, 2013, 07:06:58 am »
If it was me i'd probably use a mosfet instead.
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Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #4 on: November 23, 2013, 07:09:38 am »
I did before but Not sure if it was my PCB or what but my voltage regulator would heat up over 40C in about 10 seconds..

I took the MOSFET out and it's fine..

Why would they call for a transistor instead of a MOSFET? I really don't have a lot of space.
 

Offline SeanB

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Re: Transistor base resistor question
« Reply #5 on: November 23, 2013, 07:32:41 am »
You will need a gate resistor on the MOSFET anyway, so there is no difference at all. Finding room for a 0.4W metal film resistor next to the TO220 power device is no problem if you mount the resistor on end. You probably did not have any gate stopper resistor on the FET and it was merrily oscillating at 100MHz.
 

Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #6 on: November 23, 2013, 07:38:07 am »
You will need a gate resistor on the MOSFET anyway, so there is no difference at all. Finding room for a 0.4W metal film resistor next to the TO220 power device is no problem if you mount the resistor on end. You probably did not have any gate stopper resistor on the FET and it was merrily oscillating at 100MHz.
So the whole resistor thing is just to prevent the MIC502 from dissipating too much?

all I have is a few IRF510's but that a bit overkill
 

Offline SeanB

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Re: Transistor base resistor question
« Reply #7 on: November 23, 2013, 07:42:32 am »
The resistor is there to keep the fet from oscillating due to the long lead lengths, it is typically a 100-220R resistor as close to the gate as possible. I dissipates almost no power but damps down the oscillation. The IRF devices will do, and will not need any heatsink.
 

Offline Psi

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Re: Transistor base resistor question
« Reply #8 on: November 23, 2013, 08:24:11 am »
You will need a gate resistor on the MOSFET anyway, so there is no difference at all. Finding room for a 0.4W metal film resistor next to the TO220 power device is no problem if you mount the resistor on end. You probably did not have any gate stopper resistor on the FET and it was merrily oscillating at 100MHz.

There is a big difference,  with the transistor you need to worry about the gain. <10mA from the IC may not be enough for a 600mA load.
With a mosfet you dont have that problem
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Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #9 on: November 23, 2013, 08:29:55 am »
Being as the MOSFET is right next to the output pin is the resistor still needed? wire length from the FET to the fan is only going to be about 4 inches at best..
 

Offline Psi

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Re: Transistor base resistor question
« Reply #10 on: November 23, 2013, 09:09:11 am »
Being as the MOSFET is right next to the output pin is the resistor still needed? wire length from the FET to the fan is only going to be about 4 inches at best..

Strictly speaking a mosfet will work fine without the gate resistor in most situations but it's always best practice to use one.
between 10 and 200 ohms is quite a common value to use for mosfet gates.
« Last Edit: November 23, 2013, 09:14:01 am by Psi »
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Offline KJDS

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Re: Transistor base resistor question
« Reply #11 on: November 23, 2013, 10:15:20 am »
Being as the MOSFET is right next to the output pin is the resistor still needed? wire length from the FET to the fan is only going to be about 4 inches at best..

This fan, is it a purely resistive load, or is it an inductive load with some stray capacitance hence forming a tuned circuit?

Offline Udayan Sinha

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Re: Transistor base resistor question
« Reply #12 on: November 23, 2013, 01:26:09 pm »
Since you need 0.60A, you need to determine what base current is required to get a collector current of 0.60A. For this use the hFe value given in the datasheet(Ic/Ib=hFe). Now you have the base current. Now find out what voltage your IC is feeding into the transistor. Required base resistor value = V/Ib.
 

Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #13 on: November 23, 2013, 07:33:42 pm »
Since you need 0.60A, you need to determine what base current is required to get a collector current of 0.60A. For this use the hFe value given in the datasheet(Ic/Ib=hFe). Now you have the base current. Now find out what voltage your IC is feeding into the transistor. Required base resistor value = V/Ib.

I'm not sure what it is, but looking at the datasheet for the MIC502 it has a Iout current of just 10mA which is lower then the 0.50mA that the BD135 needs to turn on, and the MOSFET wont fully turn on unless the voltage is 10V+

the MIC502 shows 8.6V for the output when at 10mA
 

Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #14 on: November 24, 2013, 12:06:39 am »
How about a TIP31 NPN transistor instead
 

Offline Psi

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Re: Transistor base resistor question
« Reply #15 on: November 24, 2013, 05:28:49 am »
The output is PWM so it's switching the transistor on into saturation which means the gain will be lower than the transistors hfe in the datasheet.
Probably around the 10-20 range which isnt enough to get 10mA switching 600mA

Basically, the MIC502 isn't designed to drive a transistor that's switching 600mA loads.

Use a darlington transistor or a mosfet instead and it will be fine.
« Last Edit: November 24, 2013, 05:31:36 am by Psi »
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Offline Udayan Sinha

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Re: Transistor base resistor question
« Reply #16 on: November 24, 2013, 07:27:17 am »
Best go for MOSFETs
 

Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #17 on: November 24, 2013, 02:11:28 pm »
You see those resistors R1 & R2 I tried a 100K and a 3.9K and the fan just runs at full speed if I put the power leads after the 3.9K nothing turns on at all (using a darlington pair) but if I put it before the 3.9K resistor it runs full speed.
 

Offline trackman44

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Re: Transistor base resistor question
« Reply #18 on: November 24, 2013, 05:02:24 pm »
A Darlington transistor would make more sense. The output voltage of the MIC502 isn't high enough to turn on a MOSFET.

Will
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Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #19 on: November 24, 2013, 10:44:34 pm »
Ok now time to find out how to get it to control the fan speed..

I'm looking at the typical 12V circuit in the datasheet, I'm not using VSLP or VT2, OTF VSLP is grounded to ground, here is where I'm a little confused So T1 is between R1 & R2 the other end goes to ground?

To me it seems like one leg of T1 goes to the supply and the other between R1 and R2 in which that wire is connected to VT1
 

Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #20 on: November 25, 2013, 12:05:52 am »
Speed control still not working :-/
 

Offline JohnnyGringo

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Re: Transistor base resistor question
« Reply #21 on: November 25, 2013, 01:04:21 am »
Here's a well done video on selecting resistor size for transistor switches:
"Anyone who has never made a mistake has never tried anything new." - Albert Einstein
 

Offline MikeCLTopic starter

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Re: Transistor base resistor question
« Reply #22 on: November 25, 2013, 01:06:04 am »
I changed R2 to 47k I grounded VT2 and VSLP still no speed control.
 

Offline trackman44

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Re: Transistor base resistor question
« Reply #23 on: November 25, 2013, 05:50:05 am »
Which darlington transistor are you using and what are the specs for it? Remember, darlingtons have a gain of 10000 or so, Ib is around 60uA, so the base resistor is (Vin-Vbe)/Ib and you get (8.6-1.4)/0.00006=120Kohm. Use a 100K resistor to be sure the darlington will turn on.

Will
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Offline KJDS

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Re: Transistor base resistor question
« Reply #24 on: November 25, 2013, 06:36:06 am »
Which darlington transistor are you using and what are the specs for it? Remember, darlingtons have a gain of 10000 or so, Ib is around 60uA, so the base resistor is (Vin-Vbe)/Ib and you get (8.6-1.4)/0.00006=120Kohm. Use a 100K resistor to be sure the darlington will turn on.

Will

I think your darlington gain may be out by an order of magnitude. I also prefer that to switch a transistor on then it should be driven properly on, not just tickled in the right direction.

Offline trackman44

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Re: Transistor base resistor question
« Reply #25 on: November 25, 2013, 12:54:51 pm »
Quote
I think your darlington gain may be out by an order of magnitude. I also prefer that to switch a transistor on then it should be driven properly on, not just tickled in the right direction.

Yes, i maybe wrong in the gain, that's why I stated "what are the specs". You don't want the base resistor to be too low to the point where you might damage the darlington. It would be a good idea to first test the circuit in LTSpice. Another thing, does the MIC502's output stay on if it only draws 60uA, or 0.6mA, or 6mA? Output is 8.6V at 10mA, you might need to draw off some of that current through a resistor to ground before the base resistor.

Will
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