Phrazemaster

Resistor Power Rating
« on: September 13, 2019, 10:45:14 PM »
Hey all,

I just want to make sure I'm correctly interpreting this snippet of a schematic. Specifically I'm referring to the resistors R43 and R44.

Starting with R43:

There's a voltage drop of 310-108V, which is 202V.

Using I=V/R, that means there is a current in that resistor of 202V/10000R = 0.02A, or 20mA.

That puts power rating of P=VA at P=202V *.02A = 4.1W.

Doubling that for safety, I could use an 8W+ resistor.

Do I have that right?

Same for R44:

There's a voltage drop of 365-310V, which is 55V.

Using I=V/R, that means there is a current in that resistor of 55V/1500R = 0.037A, or 37mA.

That puts power rating of P=VA at P=55V *.037A  = 2.02W.

Doubling that for safety, I could use an 4W+ resistor.

I'm asking because the BOM for this project specifies 20W resistor for the 1.5K resistor R44, and a 12W resistor for 10K resistor R43. Seems way overkill, although I know higher wattage is fine.

It's easier to find resistors a bit less wattage rated, as I want to use NOS Koolohm resistors. And I just want to make sure I'm not going to fry something. I found a 9W 10K Koolohm resistor for R43 and if I have the calculations correct, it should be more than adequate.

Thanks in advance for your thoughts.

Mike

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** * Kablooie!


PRR

Re: Resistor Power Rating
« Reply #1 on: September 14, 2019, 01:21:52 AM »
Don't need current. You can simply square the voltage  and divide by resistance.

202V^2 is 40,804. 40,804/10k is 4.0804 (4.1) Watts.

> P=VA at P=202V *.02A = 4.1W.
> Doubling that for safety, I could use an 8W+ resistor.


"Double" and then round UP. I see twice 4.1 as 10 Watts. I have cried (I was very young) when a marginal resistor in "my" record-player blew-up a month out of warranty. I only sniff a bit when I throw 25W resistors at a 5 Watt job, knowing they will last forever.

> the BOM ...specifies 20W resistor for the 1.5K

If it was a resistor after the rectifier and before the first-cap, over-over-size that. The spike waveform has much higher RMS power than you'd guess from DC meter readings.

BUT this one is after a C-L-C filter. Voltage smooth like a Corgi's back. Either the Designer fumbled the math, or thought a 20W "has smoother sound", or just had 20W coming out his drawers and used them cuz he had them. I'd use >=5W here.

Phrazemaster

Re: Resistor Power Rating
« Reply #2 on: September 14, 2019, 01:27:34 AM »
As always, PRR, thank-you. It makes sense.

You can't get wisdom like this from just anywhere. You're a treasure.

Thanks again,

Mike
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** * Kablooie!

Phrazemaster

Re: Resistor Power Rating
« Reply #3 on: September 14, 2019, 01:39:13 AM »
Perhaps a stupid question, but should I be worried about inductance if using wire wound resistors? The schematic doesn't specify, but I think original units used non inductive.

Non inductive are harder to find.

Thanks,

Mike
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** * Kablooie!

PRR

Re: Resistor Power Rating
« Reply #4 on: September 14, 2019, 11:54:34 AM »
Inductance, why??

Inductance improves filtering. But you can NOT get enough inductance in a wire-wound resistor to make a bit of difference. For that you need large iron cores.

Phrazemaster

Re: Resistor Power Rating
« Reply #5 on: September 14, 2019, 05:30:23 PM »
I understand. I didn't state my question well.

I read that back-emf spikes from the inductance of wire wound resistors could cause damage to equipment. I noticed the original equipment I'm cloning uses non-inductive power resistors, so I was wondering if this is something I should adhere to. When I designed a little relay board I added a flyback diode to prevent reverse voltage spikes from damaging things.

Many thanks PRR.

Mike
***********************
*********************
******************
***************
************
********
*****
** * Kablooie!

PRR

Re: Resistor Power Rating
« Reply #6 on: September 14, 2019, 07:54:53 PM »
There's either a capacitor or a voltage-clamp at both ends of these resistors. How can it spike?

And surely you are not running relays on this rail.

Re: Resistor Power Rating
« Reply #7 on: September 14, 2019, 09:09:54 PM »
180 ohm cathode resistor on a quad II o/p stage , dissipates around 3 watts ,3 watt (old style)rated component ,runs a little hot in close proximity to the cathode cap , long leads and turret tag mounting help and it still  gives a lifetimes usage ,
Could we compromise the safety of the O/P transformer primary  by increasing  resistor wattage in this  instance ?
 


 

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