Electronics > Beginners

Are these current limiting resistors and why 2 in series?

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Chris Wilson:
I got excellent help in the RF section here fixing up my Kenwood TS-590S transceiver. But it has left me with a very novice like question. In the circuit attached an linked tow, are the 68 and 33 Ohm resistors in series (one highlit in blue) in the collectors of the digital switching transistors, for current limiting? Why did Kenwood use two low values in series and not a single resistor? Thanks!

http://www.chriswilson.tv/question.jpg

Zero999:
To make a 101R resistor or because it's easier to dissipate the power in two smaller resistors, rather than a larger one.

What's the physical size of the resistors? Are they surface mount? I wonder if transmission lines are a factor.

eb4fbz:
Maybe 100R is not used in that design, but yes 33 and 68R.

Chris Wilson:

--- Quote from: Hero999 on July 12, 2018, 02:41:41 pm ---To make a 101R resistor or because it's easier to dissipate the power in two smaller resistors, rather than a larger one.

What's the physical size of the resistors? Are they surface mount? I wonder if transmission lines are a factor.

--- End quote ---

Surface mount right next to one another, the 68 Ohm are sized at 1206 and the 33 Ohm at 0805 5% tolerance. 68 Ohm is 0.25W and 33 Ohm 0.125W

Thanks Hero999 and eb4fbz!

Zero999:

--- Quote from: Chris Wilson on July 12, 2018, 03:41:22 pm ---
--- Quote from: Hero999 on July 12, 2018, 02:41:41 pm ---To make a 101R resistor or because it's easier to dissipate the power in two smaller resistors, rather than a larger one.

What's the physical size of the resistors? Are they surface mount? I wonder if transmission lines are a factor.

--- End quote ---

Surface mount right next to one another, the 68 Ohm are sized at 1206 and the 33 Ohm at 0805 5% tolerance. 68 Ohm is 0.25W and 33 Ohm 0.125W

Thanks Hero999 and eb4fbz!

--- End quote ---
If they're 5% tolerance, then there would be no sense in having 101R, rather than 100R, so sharing the power dissipation and eb4fbz's comment about minimising the number of different value resistors on the BoM make more sense.

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