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Old 16th October 2015, 08:26 AM   #1
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Default Best Way To Install Parallel Resistors

I will be paralleling two 8G16A 500r Rhopoint resistors to make a 250r value to set the current on my Unbalancer's output tubes (R13/14) and I need some advice, please.

Obviously, when paralleling resistors, tightly stacking them on top of one another will likely build heat, which sort of defeats the purpose and possibly shortens resistor life, I would suspect. And it may cause them to vibrate together, as well. Please correct me if I am wrong here, though.

What is the best way to install parallel resistors where there is only one set of holes in the PCB, space is tight, and the resistors will need to be vertically arrayed?

How much distance should be kept between them to allow heat dissipation?

And how do you combine leads? Twist them together or simply solder the top resistor's lead to the one underneath, keeping all leads as short as possible? (With the latter approach being what I would strongly suspect would be best, but maybe not. So I am asking.)

My approach would be to vertically stack them, with about .06"/1.5mm of clearance between them, and then (gently) bend the top resistor's leads downward to touch the bottom resistor's leads and solder them together where they touch, trimming away any excess lead. This approach seems logical and simple, but maybe I am spacing them too far apart, or maybe not far enough, or maybe I am simply missing something obvious, so I figured I would ask.

My second point is whether any damping material should be employed to keep the resistors from vibrating? Again, heat build-up is my concern here, but maybe that's something I shouldn't be too concerned about. I could put very tiny amounts of some type of damping material (either EAR C-1002 or Deflex) between the UB pc board and the bottom Rhopoint, and/or between the resistors themselves. But maybe using damping material is a bad idea, heat-wise. (I would think so.) But maybe not. That's why I am asking.

Sorry for the dumb questions, and thank you for your patience. But this is my first time paralleling resistors, so I figured I would, "ask the experts".

Any suggestions will be greatly appreciated. TIA!

WS
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Old 16th October 2015, 08:29 AM   #2
kaputt is offline kaputt  Germany
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Forget about that "vibrating" stuff.
How hot do they actually get? Have you touched or measured them?
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Old 16th October 2015, 08:44 AM   #3
DF96 is offline DF96  England
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250R is low enough that for audio purposes you can ignore stray capacitance.

250R is high enough that for audio purposes you can ignore stray inductance.

Hence the only thing you need to think about is heat. How much current are you expecting? What is their power rating?
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Old 16th October 2015, 08:46 AM   #4
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I don't touch live parts in tight areas where high voltages exist. No idea of how hot they actually get. Sorry. But supposedly they don't have much current going through them, so my guess would would be not very, but again, I speak from ignorance here.(or why would I be asking). This is a general best practice Q. If that means you cannot provide meaningful input, again, sorry. Thanks, though.
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Old 16th October 2015, 08:50 AM   #5
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DF96-

My first response was to kaputt, not you.

I believe the Rhos are .3W each. I can look that up for you. I don't recall off the top of my head though. Sorry. But .3W is what I recall. And I was told by Bob Prangnell that these will have negligible current, but again, that was months ago, so I will go back and look.
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Old 16th October 2015, 08:56 AM   #6
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Okay, I just looked up the old e-mail from Bob and he says: "R13, r14 and all other resistors take negligible power." Not exactly sure how much is 'negligible' is, but Bob definitely knows his stuff. However, these are being stacked right on top of one another, and I don't want to take any chances. Thus, my query. Plus, I don't want to bug Bob, as he is really busy kicking audio butt with his MSA company, so I posted my request for help here. Hope this helps. Thanks! :-)
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Old 16th October 2015, 08:58 AM   #7
kaputt is offline kaputt  Germany
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I had a look at the "unbalancer". Don't worry abot heat. Bend the legs of one resistor around the legs of the other (one turn is enough), solder and stick the unbent legs through the PCB holes and solder. It is good practice to leave a bit of space between the resistors and the board. Like 5mm.
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Old 16th October 2015, 08:59 AM   #8
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Follow-up: yes, the Rhos are .33W at +85C and 0.25W at +110C, according to their data sheet.
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Old 16th October 2015, 09:01 AM   #9
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Hi kaputt-

Thank you for your prompt and thoughtful response. 5mm gives a lot of breathing room. I like margins of safety. Sounds good. Same for soldering advice, where we agree on approach. Cool. Thanks again. :-)
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Old 16th October 2015, 09:13 AM   #10
kaputt is offline kaputt  Germany
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In that case you can also stack the resistors without open space between them.

All the best,
Daniel
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