Resistor wattage question (Schematic example posted)

J

JoeyPee

Guest
Hi,

I have a question about the resistor power capability for this small
schematic that I posted as a GIF here:

http://mysite.verizon.net/cowman136/resistors.gif

All resistors are 10-watt. According to the documentation, "20 values
of power resistors at 20-watts are available in 20 different steps."

Can anyone explain how this is possible?

As I look at the schematic, the power rating at each position would
be:
- R25 position would be clearly only 10-watt (one 2.5 ohm 10-watt
resistor)
- R26 position would be 20-watt (two 2.5 ohm 10-watt resistors in
series)
- R27 position would be 30-watt (three 2.5 ohm 10-watt resistors in
series)
etc

Likewise, R38 power would be same situation as R25.

Is this analysis correct, or is there something that I am not
considering?
 
JoeyPee <rpesq@my-deja.com> wrote in news:e6c03f8c-8e38-49fa-b294-
47e9eace68d2@f38g2000pra.googlegroups.com:

Hi,

I have a question about the resistor power capability for this small
schematic that I posted as a GIF here:

http://mysite.verizon.net/cowman136/resistors.gif

All resistors are 10-watt. According to the documentation, "20 values
of power resistors at 20-watts are available in 20 different steps."

Can anyone explain how this is possible?

As I look at the schematic, the power rating at each position would
be:
- R25 position would be clearly only 10-watt (one 2.5 ohm 10-watt
resistor)
- R26 position would be 20-watt (two 2.5 ohm 10-watt resistors in
series)
- R27 position would be 30-watt (three 2.5 ohm 10-watt resistors in
series)
etc

Likewise, R38 power would be same situation as R25.

Is this analysis correct, or is there something that I am not
considering?
Click to expand...
Either it's impossible, or it depends on other parts in unseen context, so
what's the context?

The only way I can make sense of the claim without extra info is that the
switch appears to a single ganged pair, so in any position there is a minimum
of ten watts safe dissipation per circuit, total 20, but if that's what they
mean it sounds like bad specsmanship.
 
On Sun, 14 Jun 2009 12:24:42 -0700 (PDT), JoeyPee <rpesq@my-deja.com>
wrote:

There is no other context. It is a component substitution box, and
that is the full schematic for this section. What you see is the
entire section.

My only issue is that the R25 and R38 positions cannot dissipate 20
watts because each is only a 10-watt resistor and no other resistors
are in series with them. Hence, the "20-watt" rating claim is not
accurate for those positions.

I was just asking for another opinion in case I was missing something
obvious. As I see it, the 20-w claim is only accurate for the 2nd
position on each top and bottom section. Higher positions would
actually be rated above 20w, while the lowest positions are clearly
10w.

Disagree?
Click to expand...
The single resistor position certainly can't be better than 10W,
unless there are some heroic means to cool the resistors. There may
be case cooling issues that would limit the box to 20W total, no
matter how many resistors are switched in.
 
There is no other context. It is a component substitution box, and
that is the full schematic for this section. What you see is the
entire section.

My only issue is that the R25 and R38 positions cannot dissipate 20
watts because each is only a 10-watt resistor and no other resistors
are in series with them. Hence, the "20-watt" rating claim is not
accurate for those positions.

I was just asking for another opinion in case I was missing something
obvious. As I see it, the 20-w claim is only accurate for the 2nd
position on each top and bottom section. Higher positions would
actually be rated above 20w, while the lowest positions are clearly
10w.

Disagree?
 
On Jun 14, 11:44 am, JoeyPee <rp...@my-deja.com> wrote:
Hi,

I have a question about the resistor power capability for this small
schematic that I posted as a GIF here:

http://mysite.verizon.net/cowman136/resistors.gif

All resistors are 10-watt.  According to the documentation, "20 values
of power resistors at 20-watts are available in 20 different steps."

Can anyone explain how this is possible?

As I look at the schematic, the power rating at each position would
be:
- R25 position would be clearly only 10-watt (one 2.5 ohm 10-watt
resistor)
- R26 position would be 20-watt (two 2.5 ohm 10-watt resistors in
series)
- R27 position would be 30-watt (three 2.5 ohm 10-watt resistors in
series)
etc

Likewise, R38 power would be same situation as R25.

Is this analysis correct, or is there something that I am not
considering?
Click to expand...
I think you're correct.

I'm not sure where the original verbiage came from but it sounds like,
and the schematic looks like, something from a Popular Electronics
article 30 or 40 years ago. Not exactly peer-reviewed or anything :).

It's interesting how slowly the standard E12/E24 values have been
accepted in the power resistor world. Many of the values on that
schematic are still commercially produced today.

Tim.
 
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