Does reducing the resistance affect the wattage rating
of a power rheostat? I'd imagine it does, but I'm not
sure how it would be derated..

Pete

Uncle Peter wrote:
Does reducing the resistance affect the wattage rating
of a power rheostat? I'd imagine it does, but I'm not
sure how it would be derated..
============================
The power rating of a rheostat is based upon its total resistance R.
allowing for a max current I with P(max) equals I*I*R

If the adjustable resistance is reduced to say 1/5 of the total
resistance that reduced resistance can accept the same current I .
This means the power rating has been reduced by a factor 5 ,in other
words the power rating is proportional to the set resistance.

Frank GM0CSZ / KN6WH

On Apr 17, 2:35 pm, Highland Ham
wrote:
Uncle Peter wrote:
Does reducing the resistance affect the wattage rating
of a power rheostat? I'd imagine it does, but I'm not
sure how it would be derated..

============================
The power rating of a rheostat is based upon its total resistance R.
allowing for a max current I with P(max) equals I*I*R

If the adjustable resistance is reduced to say 1/5 of the total
resistance that reduced resistance can accept the same current I .
This means the power rating has been reduced by a factor 5 ,in other
words the power rating is proportional to the set resistance.

Frank GM0CSZ / KN6WH

That's logical.
But just to nit-pick! :-)
Got one of those ceramic tubular adjustable slide resistors. Rated at
300 watts IIRC.
If I put the slider half way along I will have half the resistance
wire in the circuit and dissipating heat at full current rating;
correct?
But still the same thermal mass of ceramic and metal. So could one
perhaps argue that it might be 'slightly' (very slightly perhaps) a
little more than half the power rating. Probably nothing to have any
effect whatsoever in practice because one would not run a component at
maximum rating at any setting anyway?

"terry" wrote in message
oups.com...
On Apr 17, 2:35 pm, Highland Ham
wrote:
Uncle Peter wrote:
Does reducing the resistance affect the wattage rating
of a power rheostat? I'd imagine it does, but I'm not
sure how it would be derated..

That's logical.
But just to nit-pick! :-)
Got one of those ceramic tubular adjustable slide resistors. Rated at
300 watts IIRC.
If I put the slider half way along I will have half the resistance
wire in the circuit and dissipating heat at full current rating;
correct?
But still the same thermal mass of ceramic and metal. So could one
perhaps argue that it might be 'slightly' (very slightly perhaps) a
little more than half the power rating. Probably nothing to have any
effect whatsoever in practice because one would not run a component at
maximum rating at any setting anyway?


Hi Terry

That's what I was wondering, the mass of the heatsink
the windings are on should have a bearing on
the wattage... Lacking manufacturer data I guess it
is a moot point and I'll take a conservative tack..

Pete

On Apr 20, 1:22 pm, "Uncle Peter" wrote:
"terry" wrote in message

oups.com...



On Apr 17, 2:35 pm, Highland Ham
wrote:
Uncle Peter wrote:
Does reducing the resistance affect the wattage rating
of a power rheostat? I'd imagine it does, but I'm not
sure how it would be derated..

That's logical.
But just to nit-pick! :-)
Got one of those ceramic tubular adjustable slide resistors. Rated at
300 watts IIRC.
If I put the slider half way along I will have half the resistance
wire in the circuit and dissipating heat at full current rating;
correct?
But still the same thermal mass of ceramic and metal. So could one
perhaps argue that it might be 'slightly' (very slightly perhaps) a
little more than half the power rating. Probably nothing to have any
effect whatsoever in practice because one would not run a component at
maximum rating at any setting anyway?

Hi Terry

That's what I was wondering, the mass of the heatsink
the windings are on should have a bearing on
the wattage... Lacking manufacturer data I guess it
is a moot point and I'll take a conservative tack..

Pete


The manufacturers, all I've ever seen, take that conservative tack and
just put a maximum current rating on them. In the case of a rheostat,
you need to consider the rating of the slider, and I'll bet you get
longer slider life if you adjust it when the current is zero than when
it's the full rated value.

What Terry wrote has a lot of merit. In fact, with the tubular
ceramics with a hollow core, you could quite reasonably get more
dissipation with a given temperature rise if you orient the resistor
vertically in free air, as opposed to horizontally, since it acts like
a stack and will get considerable air flowing by virtue of the heat.
If you REALLY wanted to optimize such a resistor, you'd space the
turns further apart on the end that was going to be on top! Also, I
can tell you that little 0603 surface mount resistors, nominally rated
at 1/16 watt, can dissipate a whole lot more (1/4 watt!) than that if
you keep other heat dissipators away from them and provide wide copper
paths to lead the heat away from them; but on the othe hand if you put
a whole mess of them as close together as your design rules let you
and ask each to dissipate the same power, you'll find you may not be
able to go above 1/100 of a watt each without getting them too hot.
So not only does the amount of the rheostat in actual use make a
difference, it makes a difference how you cool it. Turn a fan on it
and don't adjust it while it's got current through it, and you MAY be
able to safely add 40% over its current rating. YMMV, though. You
know what to do to be conservative.

Cheers,
Tom

On Apr 21, 2:55 am, K7ITM wrote:
On Apr 20, 1:22 pm, "Uncle Peter" wrote:





"terry" wrote in message

roups.com...

On Apr 17, 2:35 pm, Highland Ham
wrote:
Uncle Peter wrote:
Does reducing the resistance affect the wattage rating
of a power rheostat? I'd imagine it does, but I'm not
sure how it would be derated..

That's logical.
But just to nit-pick! :-)
Got one of those ceramic tubular adjustable slide resistors. Rated at
300 watts IIRC.
If I put the slider half way along I will have half the resistance
wire in the circuit and dissipating heat at full current rating;
correct?
But still the same thermal mass of ceramic and metal. So could one
perhaps argue that it might be 'slightly' (very slightly perhaps) a
little more than half the power rating. Probably nothing to have any
effect whatsoever in practice because one would not run a component at
maximum rating at any setting anyway?

Hi Terry

That's what I was wondering, the mass of the heatsink
the windings are on should have a bearing on
the wattage... Lacking manufacturer data I guess it
is a moot point and I'll take a conservative tack..

Pete

The manufacturers, all I've ever seen, take that conservative tack and
just put a maximum current rating on them. In the case of a rheostat,
you need to consider the rating of the slider, and I'll bet you get
longer slider life if you adjust it when the current is zero than when
it's the full rated value.

What Terry wrote has a lot of merit. In fact, with the tubular
ceramics with a hollow core, you could quite reasonably get more
dissipation with a given temperature rise if you orient the resistor
vertically in free air, as opposed to horizontally, since it acts like
a stack and will get considerable air flowing by virtue of the heat.
If you REALLY wanted to optimize such a resistor, you'd space the
turns further apart on the end that was going to be on top! Also, I
can tell you that little 0603 surface mount resistors, nominally rated
at 1/16 watt, can dissipate a whole lot more (1/4 watt!) than that if
you keep other heat dissipators away from them and provide wide copper
paths to lead the heat away from them; but on the othe hand if you put
a whole mess of them as close together as your design rules let you
and ask each to dissipate the same power, you'll find you may not be
able to go above 1/100 of a watt each without getting them too hot.
So not only does the amount of the rheostat in actual use make a
difference, it makes a difference how you cool it. Turn a fan on it
and don't adjust it while it's got current through it, and you MAY be
able to safely add 40% over its current rating. YMMV, though. You
know what to do to be conservative.

Cheers,
Tom- Hide quoted text -

- Show quoted text -

Hadn't thought of the vertical Tom. But good point. I do recall years
ago, seeing vertically mounted ones used as stage light dimmers, now
that you mention. Fans good idea too. While all of this was a reaction
to the original basic question; interesting how one idea leads to
another, eh?
Regards.