Transistor base resistor question - Page 1
Forum section: electronics / beginners

author: MikeCL
author_rank: Contributor
timestamp: November 23, 2013, 06:40:54 am
content: 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 .60AI want to say 330 Ohm but I might be wrong..

author: Psi
author_rank: Super Contributor
timestamp: November 23, 2013, 06:52:05 am
content: From the datasheet..QuoteAlso note the Electrical Characteristics section that states:Vout high is 2.4V when the chip runs from 5VVout high is 3.2V when the chip runs from 12VWhich 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.

author: MikeCL
author_rank: Contributor
timestamp: November 23, 2013, 07:01:38 am
content: 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

author: Psi
author_rank: Super Contributor
timestamp: November 23, 2013, 07:06:58 am
content: If it was me i'd probably use a mosfet instead.

author: MikeCL
author_rank: Contributor
timestamp: November 23, 2013, 07:09:38 am
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: November 23, 2013, 07:32:41 am
content: 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.

author: MikeCL
author_rank: Contributor
timestamp: November 23, 2013, 07:38:07 am
content: Quote from: SeanB on November 23, 2013, 07:32:41 amSo 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

author: SeanB
author_rank: Super Contributor
timestamp: November 23, 2013, 07:42:32 am
content: 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.

author: Psi
author_rank: Super Contributor
timestamp: November 23, 2013, 08:24:11 am
content: Quote from: SeanB on November 23, 2013, 07:32:41 amThere 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

author: MikeCL
author_rank: Contributor
timestamp: November 23, 2013, 08:29:55 am
content: 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..

author: Psi
author_rank: Super Contributor
timestamp: November 23, 2013, 09:09:11 am
content: Quote from: MikeCL on November 23, 2013, 08:29:55 amStrictly 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.

author: KJDS
author_rank: Super Contributor
timestamp: November 23, 2013, 10:15:20 am
content: Quote from: MikeCL on November 23, 2013, 08:29:55 amThis fan, is it a purely resistive load, or is it an inductive load with some stray capacitance hence forming a tuned circuit?

author: Udayan Sinha
author_rank: Regular Contributor
timestamp: November 23, 2013, 01:26:09 pm
content: 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.

author: MikeCL
author_rank: Contributor
timestamp: November 23, 2013, 07:33:42 pm
content: Quote from: Udayan Sinha on November 23, 2013, 01:26:09 pmI'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

author: MikeCL
author_rank: Contributor
timestamp: November 24, 2013, 12:06:39 am
content: How about a TIP31 NPN transistor instead

author: Psi
author_rank: Super Contributor
timestamp: November 24, 2013, 05:28:49 am
content: 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 600mABasically, 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.

author: Udayan Sinha
author_rank: Regular Contributor
timestamp: November 24, 2013, 07:27:17 am
content: Best go for MOSFETs

author: MikeCL
author_rank: Contributor
timestamp: November 24, 2013, 02:11:28 pm
content: 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.

author: trackman44
author_rank: Regular Contributor
timestamp: November 24, 2013, 05:02:24 pm
content: A Darlington transistor would make more sense. The output voltage of the MIC502 isn't high enough to turn on a MOSFET.Will

author: MikeCL
author_rank: Contributor
timestamp: November 24, 2013, 10:44:34 pm
content: 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

author: MikeCL
author_rank: Contributor
timestamp: November 25, 2013, 12:05:52 am
content: Speed control still not working :-/

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 25, 2013, 01:04:21 am
content: Here's a well done video on selecting resistor size for transistor switches:

author: MikeCL
author_rank: Contributor
timestamp: November 25, 2013, 01:06:04 am
content: I changed R2 to 47k I grounded VT2 and VSLP still no speed control.

author: trackman44
author_rank: Regular Contributor
timestamp: November 25, 2013, 05:50:05 am
content: 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

author: KJDS
author_rank: Super Contributor
timestamp: November 25, 2013, 06:36:06 am
content: Quote from: trackman44 on November 25, 2013, 05:50:05 amI 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.

author: trackman44
author_rank: Regular Contributor
timestamp: November 25, 2013, 12:54:51 pm
content: QuoteYes, 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