Resistor Power Rating

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

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.

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

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

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.

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

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.

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 ?