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Topic: resistors on transistor base (Read 1 time) previous topic - next topic

dcheckoway

I was looking at the schematic of SparkFun's "Beefcake Relay Control" circuit, and something is puzzling me.  I'm trying to understand the purpose of "R1" here:


I'm assuming R1 and R2 form a voltage divider, allowing the voltage at the base of the transistor to be controlled.  I can't get my head around why that would matter, if the intent is to saturate the transistor.  I'm probably missing something simple (daily occurrence).

In an effort to learn more, I found essentially the same circuit on http://www.electronics-tutorials.ws/transistor/tran_4.html, i.e.:

...but they don't explain what "R" is for, or how its value might be calculated.

Pardon my ignorance, but what purpose does that resistor serve?  What could happen if it's omitted?

Thanks in advance!!!
-- Dan Checkoway

fungus

It stops the transistor turning on while the Arduino is powering up (pin is floating).

Once the Arduino is powered up it serves no real purpose.
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pito

Except defining the off state the resistor R1 in the base is important when you require faster on/off speed. It discharges the parasitic and Miller capacitance to the ground.. Not so important with a relay, though..  :)

fungus


Except defining the off state the resistor R1 in the base is important when you require faster on/off speed. It discharges the parasitic and Miller capacitance to the ground.. Not so important with a relay, though..  :)


If you require faster on/off speed, a 10K resistor isn't going to help much. Better to design a proper circuit...

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pito

#4
Feb 17, 2013, 04:12 pm Last Edit: Feb 17, 2013, 04:21 pm by pito Reason: 1
..of course we do not discuss a proper design with transistors.. that would discourage the most of arduino fans/users, I would say.. :)
BTW, when muxing for example an LED 7 segm display you may see a few kohms in the base makes a difference..

dc42

R1 is redundant in that circuit, unless the input is wire is routed in such a way that it is likely to pick up noise while the Arduino feeding it is powering up, or the input is left unconnected AND likely to pick up noise. Since Sparkfun don't know how you will use the relay board, they have played safe and included the resistor.
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Docedison

If one looks carefully one notices a connector in the base circuit of the transistor in question. The resistor R1 in the base circuit isn't redundant, it is good engineering. If the connector is disconnected without the resistor the base bias is undefined. with it the base bias is defined for all conditions.

Bob
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MarkT


If one looks carefully one notices a connector in the base circuit of the transistor in question. The resistor R1 in the base circuit isn't redundant, it is good engineering. If the connector is disconnected without the resistor the base bias is undefined. with it the base bias is defined for all conditions.

Bob


Not really, an NPN with open base will only conduct a small leakage current of nA to uA or so - not enough to switch a relay,
so is defined enough I reckon.

I suspect the board was designed to be compatible with either an NPN transistor or an n-channel MOSFET, and the pull-down
resistor is really there for the latter case - it causes no harm or benefit for an NPN transistor.
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