Are these current limiting resistors and why 2 in series? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 12, 2018, 01:43:48 pm
content: 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

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2018, 02:41:41 pm
content: 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.

author: eb4fbz
author_rank: Regular Contributor
timestamp: July 12, 2018, 03:08:00 pm
content: Maybe 100R is not used in that design, but yes 33 and 68R.

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 12, 2018, 03:41:22 pm
content: Quote from: Hero999 on July 12, 2018, 02:41:41 pmSurface 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 0.125WThanks Hero999 and eb4fbz!

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2018, 03:59:54 pm
content: Quote from: Chris Wilson on July 12, 2018, 03:41:22 pmIf 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.

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 12, 2018, 04:08:33 pm
content: Quote from: Hero999 on July 12, 2018, 03:59:54 pmOK, thanks for that, but can you suggest why, having measured them all with Kelvin clips on an in spec Philips / Fluke LCR meter, the 68 Ohm ones are the ones in the 3 inoperative bands, that have gone high resistance, and not the lesser resistance, lesser Wattage 33 Ohm ones please? I would have thought the lesser Wattage lower resistance value ones (and physically smaller) ones would be the ones taking the strain of any glitches? Thanks again, just trying to learn form this as I fumble along

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2018, 04:31:28 pm
content: Quote from: Chris Wilson on July 12, 2018, 04:08:33 pmI'm not sure if I fully understand the question.What do you mean by 3 inoperative bands?I thought they were both the same power rating, since they're both the same package size, unless one is made from a material, with a higher temperature rating, but that's not easy to tell, just by looking at the PCB.Of course, for two resistors connected in series, the one with the higher resistance, 68R in this case, will dissipate more than the lower resistance: 33R, so if the 68R has failed, than that would be why. Resistors generally also fail higher resistance, than lower resistance, as some of the element burns away. The only exception is high value, say above 1M, resistors which may arc over, causing some charring, which would reduce the resistance.

author: thm_w
author_rank: Super Contributor
timestamp: July 12, 2018, 08:38:03 pm
content: He said they are different packages.If 68 ohm is dissipating 0.25W, would be 61mA through. Which gives 0.123W on the 33 ohm.So both are very closely matched for power dissipation, 68 ohm just a tiny bit higher.

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2018, 09:01:14 pm
content: I can't see any logical reason for it. The value doesn't look critical and it can't be parasitic inductance or capacitance because there are decoupling caps to ground right next to them. If it's to increase thermal capacity then it would be much more logical to use a pair of 47 or 51R 1206, rather than two different values and packages.Maybe the designer got bored one afternoon and decided to play with tweaking the dissipations of different packages just for the hell of it. Maybe the bigger one is used as a jumper over more tracks on the PCB (it's probably single sided).Edit: Or yes, possibly to utilize existing BOM component values.

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2018, 10:25:50 pm
content: Quote from: thm_w on July 12, 2018, 08:38:03 pmYes, you're right. I missed the different sized packages part.

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 13, 2018, 02:03:59 pm
content: OK, thanks, understood re the resistors and loading. I have now changed any that had changed value, and where one only in the series pair had changed, I changed the other one, too, "just in case". I now have a fully functioning TS-590 once again! Sincere thanks for help here and in the RF section!Total repair cost £35 UK Sterling and a few more grey hairs.