| Author |
Message |
Gareth Magennis
Guest
|
 Posted:
Thu Sep 15, 2005 10:32 am Post subject:
Cheap Dummy Loads |
|
|
Hi,
is there any reason why you cannot use electric fire elements as cheap high
power dummy loads? e.g. is the resistance temperature dependent?
Cheers,
Gareth.
|
|
| Back to top |
|
 |
Gareth Magennis
Guest
|
| Posted:
Thu Sep 15, 2005 12:23 pm Post subject:
Re: Cheap Dummy Loads |
|
|
"Arny Krueger" <arnyk@hotpop.com> wrote in message
news:-4udnQLp87Lvx7TeRVn-2g@comcast.com...
| Quote: | "Gareth Magennis" <sound.service@btconnect.com> wrote in
message
news:dgbij1$13h$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com
Hi,
is there any reason why you cannot use electric fire
elements as cheap high power dummy loads? e.g. is the
resistance temperature dependent?
The resistance of just about everything, including most commercial
resistance elements, is temperature dependent.
I've tested a number of *cheap* non-inductive wirewould resistors sold by
Radio Shack, Parts Express, etc., and they are very temperature-dependent
as well. However, the 1% NI resistors made by precision resistor
manufactuers like Arco (see the Mouser catalog) are a picture of thermal
stability.
Ironically, cheap wirewound resistors aren't all that cheap. A 300 watt
Arco 1% 8 ohm NI part runs about $65, but is the equivalent of about 30 20
watt resistors that retail for about $2 each. Your apparent savings are
only about 33%, and you end up with 30 POS resistors instead of one good
one.
IME the temperature of a wirewound resistor operating in free air will
rise into the 300-400 degree range if dissipating its rated power, so the
thermal properties of resistance elements is of paramount importance.
I've observed close to 100% increase in resistance under these *normal*
operating conditions. Compare that to just about nil for a quality part.
Forced-air cooling is insufficient to reduce the temperature rise to
acceptable levels with so-called cheap resistors. An oil bath is better,
but still insufficient, and you still have all that heat to dissipate at a
relatively low temperature. All you gain is a messy oily pot of resistors
and a time delay before the inevitable.
|
Thanks Arny,
I've always used similar devices to the Arcol ones you mention as you can
easily wire through the eyes and bolt them to a heatsink. In the UK the
50Watt ones give you most power per dollar.
Trouble is this means it costs over £100 and the best part of a days work to
put together a 2kW box. You can get 1kW electric fire elements for pennies!
Ah well, time to dig deep.
Cheers,
Gareth. |
|
| Back to top |
|
|
Arny Krueger
Guest
|
| Posted:
Thu Sep 15, 2005 4:24 pm Post subject:
Re: Cheap Dummy Loads |
|
|
"Gareth Magennis" <sound.service@btconnect.com> wrote in
message
news:dgbij1$13h$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com
| Quote: | Hi,
is there any reason why you cannot use electric fire
elements as cheap high power dummy loads? e.g. is the
resistance temperature dependent?
|
The resistance of just about everything, including most
commercial resistance elements, is temperature dependent.
I've tested a number of *cheap* non-inductive wirewould
resistors sold by Radio Shack, Parts Express, etc., and they
are very temperature-dependent as well. However, the 1% NI
resistors made by precision resistor manufactuers like Arco
(see the Mouser catalog) are a picture of thermal stability.
Ironically, cheap wirewound resistors aren't all that cheap.
A 300 watt Arco 1% 8 ohm NI part runs about $65, but is the
equivalent of about 30 20 watt resistors that retail for
about $2 each. Your apparent savings are only about 33%, and
you end up with 30 POS resistors instead of one good one.
IME the temperature of a wirewound resistor operating in
free air will rise into the 300-400 degree range if
dissipating its rated power, so the thermal properties of
resistance elements is of paramount importance. I've
observed close to 100% increase in resistance under these
*normal* operating conditions. Compare that to just about
nil for a quality part.
Forced-air cooling is insufficient to reduce the temperature
rise to acceptable levels with so-called cheap resistors. An
oil bath is better, but still insufficient, and you still
have all that heat to dissipate at a relatively low
temperature. All you gain is a messy oily pot of resistors
and a time delay before the inevitable. |
|
| Back to top |
|
|
Ben Bradley
Guest
|
| Posted:
Thu Sep 15, 2005 8:51 pm Post subject:
Re: Cheap Dummy Loads |
|
|
On Thu, 15 Sep 2005 07:24:39 -0400, "Arny Krueger" <arnyk@hotpop.com>
wrote:
| Quote: | "Gareth Magennis" <sound.service@btconnect.com> wrote in
message
news:dgbij1$13h$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com
Hi,
is there any reason why you cannot use electric fire
elements as cheap high power dummy loads? e.g. is the
resistance temperature dependent?
The resistance of just about everything, including most
commercial resistance elements, is temperature dependent.
|
Bob Pease wrote a column on making high-current, low-resistance
shunts with plain copper wire. It was a squirrel-cage design with the
lengths of the wires basically in free air so the heat they generated
was easily transferred to the air, so they don't heat up much. Here's
the article:
http://www.elecdesign.com/Articles/ArticleID/2144/2144.html
Presumably the OP wants a resistive load for an audio power
amplifier, which would require 4 or 8 ohms. You can do this by putting
the wires in series rather than parallel.
OTOH, Pease claims his shunt dissipates 22 watts with "minor loss
in accuracy" so you'd have to make a quite large device (and/or put it
in oil) to have a reasonably stable resistance while dissipating the
several hundred watts an audio power amplifier may put out.
| Quote: |
I've tested a number of *cheap* non-inductive wirewould
resistors sold by Radio Shack, Parts Express, etc., and they
are very temperature-dependent as well. However, the 1% NI
resistors made by precision resistor manufactuers like Arco
(see the Mouser catalog) are a picture of thermal stability.
Ironically, cheap wirewound resistors aren't all that cheap.
A 300 watt Arco 1% 8 ohm NI part runs about $65, but is the
equivalent of about 30 20 watt resistors that retail for
about $2 each. Your apparent savings are only about 33%, and
you end up with 30 POS resistors instead of one good one.
IME the temperature of a wirewound resistor operating in
free air will rise into the 300-400 degree range if
dissipating its rated power, so the thermal properties of
resistance elements is of paramount importance. I've
observed close to 100% increase in resistance under these
*normal* operating conditions. Compare that to just about
nil for a quality part.
Forced-air cooling is insufficient to reduce the temperature
rise to acceptable levels with so-called cheap resistors. An
oil bath is better, but still insufficient, and you still
have all that heat to dissipate at a relatively low
temperature. All you gain is a messy oily pot of resistors
and a time delay before the inevitable.
|
|
|
| Back to top |
|
|
Ben Bradley
Guest
|
| Posted:
Thu Sep 15, 2005 8:58 pm Post subject:
Re: Cheap Dummy Loads |
|
|
On Thu, 15 Sep 2005 12:23:22 +0000 (UTC), "Gareth Magennis"
<sound.service@btconnect.com> wrote:
| Quote: | Thanks Arny,
I've always used similar devices to the Arcol ones you mention as you can
easily wire through the eyes and bolt them to a heatsink. In the UK the
50Watt ones give you most power per dollar.
Trouble is this means it costs over £100 and the best part of a days work to
put together a 2kW box. You can get 1kW electric fire elements for pennies!
|
Perhaps you can buy 10 or 20 of those "electric fire elements" (I
presume you mean what we call "electric heating elements"), and wire
them for the desired resistance, so that each one won't heat up much
and the resistance doesn't change much. Buy one, make measurements so
you can get a table of its power dissipation vs. resistance increase,
and from how much increase you can stand, buy enough of them to take
the power you need to give them. Be sure to space them apart rather
than have them all huddled together. :)
| Quote: |
Ah well, time to dig deep.
Cheers,
Gareth.
|
|
|
| Back to top |
|
|
GregS
Guest
|
| Posted:
Thu Sep 15, 2005 9:33 pm Post subject:
Re: Cheap Dummy Loads |
|
|
In article <dgbp3q$hua$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com>, "Gareth Magennis" <sound.service@btconnect.com> wrote:
| Quote: |
"Arny Krueger" <arnyk@hotpop.com> wrote in message
news:-4udnQLp87Lvx7TeRVn-2g@comcast.com...
"Gareth Magennis" <sound.service@btconnect.com> wrote in
message
news:dgbij1$13h$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com
Hi,
is there any reason why you cannot use electric fire
elements as cheap high power dummy loads? e.g. is the
resistance temperature dependent?
The resistance of just about everything, including most commercial
resistance elements, is temperature dependent.
I've tested a number of *cheap* non-inductive wirewould resistors sold by
Radio Shack, Parts Express, etc., and they are very temperature-dependent
as well. However, the 1% NI resistors made by precision resistor
manufactuers like Arco (see the Mouser catalog) are a picture of thermal
stability.
Ironically, cheap wirewound resistors aren't all that cheap. A 300 watt
Arco 1% 8 ohm NI part runs about $65, but is the equivalent of about 30 20
watt resistors that retail for about $2 each. Your apparent savings are
only about 33%, and you end up with 30 POS resistors instead of one good
one.
IME the temperature of a wirewound resistor operating in free air will
rise into the 300-400 degree range if dissipating its rated power, so the
thermal properties of resistance elements is of paramount importance.
I've observed close to 100% increase in resistance under these *normal*
operating conditions. Compare that to just about nil for a quality part.
Forced-air cooling is insufficient to reduce the temperature rise to
acceptable levels with so-called cheap resistors. An oil bath is better,
but still insufficient, and you still have all that heat to dissipate at a
relatively low temperature. All you gain is a messy oily pot of resistors
and a time delay before the inevitable.
Thanks Arny,
I've always used similar devices to the Arcol ones you mention as you can
easily wire through the eyes and bolt them to a heatsink. In the UK the
50Watt ones give you most power per dollar.
Trouble is this means it costs over £100 and the best part of a days work to
put together a 2kW box. You can get 1kW electric fire elements for pennies!
Ah well, time to dig deep.
|
I use the ARCOL or Dale equivalant types in multiples. I use "cheap" surplus units.
I can't remember the company I last bought those from, but more than likely
located in the state of Ohio. I mount these on heat sinks, and may
include fan cooling. I also found a Parts Express(one of the Ohio companies)
having special dummy loads at fairly decent prices.
greg |
|
| Back to top |
|
|
Gareth Magennis
Guest
|
| Posted:
Fri Sep 16, 2005 9:28 am Post subject:
Re: Cheap Dummy Loads |
|
|
"Ben Bradley" <ben_nospam_bradley@frontiernet.net> wrote in message
news:n36ji1tke78go40uvsquhdckcjsr03gt5h@4ax.com...
| Quote: | On Thu, 15 Sep 2005 12:23:22 +0000 (UTC), "Gareth Magennis"
sound.service@btconnect.com> wrote:
Thanks Arny,
I've always used similar devices to the Arcol ones you mention as you can
easily wire through the eyes and bolt them to a heatsink. In the UK the
50Watt ones give you most power per dollar.
Trouble is this means it costs over £100 and the best part of a days work
to
put together a 2kW box. You can get 1kW electric fire elements for
pennies!
Perhaps you can buy 10 or 20 of those "electric fire elements" (I
presume you mean what we call "electric heating elements"), and wire
them for the desired resistance, so that each one won't heat up much
and the resistance doesn't change much. Buy one, make measurements so
you can get a table of its power dissipation vs. resistance increase,
and from how much increase you can stand, buy enough of them to take
the power you need to give them. Be sure to space them apart rather
than have them all huddled together. :)
|
Yes, I'm going to buy some of the elements anyway, and mess around just for
the interest. Arny says he has seen up to a 100% increase in resistance for
cheap resistors, I guess these would behave similarly. We'll see.
For my purposes I don't always need stability and accuracy - I need a big
dummy load to test whether big amps are working OK and the fan switches in
etc. However, measuring power output would be rather difficult with a crap
load such as this.
Cheers,
Gareth. |
|
| Back to top |
|
|
Tim Martin
Guest
|
| Posted:
Fri Sep 16, 2005 8:39 pm Post subject:
Re: Cheap Dummy Loads |
|
|
"Gareth Magennis" <sound.service@btconnect.com> wrote in message
news:dge370$s5v$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com...
| Quote: | Yes, I'm going to buy some of the elements anyway, and mess around just
for
the interest. Arny says he has seen up to a 100% increase in resistance
for
cheap resistors, I guess these would behave similarly. We'll see.
|
The resistance of common metals used as conductors increases by about half a
percent per Celsius degree, so a 100% increase in resiistance corresponds to
a 200 Celsius degree rise in temperature.
I don't know what voltages you are going to drive these heater elements
with - a 500-watt amplifier delivers about 60 volts RMS into 8 ohms, so I
guess you couldn't deliver more than about 3 amperes into a cold 1K heating
element. So you'd need to run three in parallel to fully load the 500-watt
amplifier.
Tinkering and guessing with numbers:
Suppose the cold resistance is 20 ohms, and the hot resistance is 60 ohms.
With three in parallel dissipating 500 watts between them, each is
dissipating one-sixth of its maximum rating, and so its resistance will
increase by about one-sixth of the range from 20 ohms to 60 ohms. That is,
the resistance will increase from 20 ohms to 27 ohms, and the resistance of
the network will increase from 6.7 ohms to 9 ohms,.
OK. Now suppose instead we use twelve in a series-parallel network. (A set
of six in parallel, connected in series with another set of six in
parallel). Now we have twelve dissipiating 500 watts between them, each
element is diissipating only one-twenty-fourth of its maximum rating, so the
resistance of each will increase by only one twenty-fourth of the range from
20 ohms to 60 ohms. That is, the resistance of each will oncrease from 20
ohms to 21.7 ohms. and the resistance of the network will increase from 6.7
ohms to 7.2 ohms.
Tim
..
.. |
|
| Back to top |
|
|
Bob Urz
Guest
|
| Posted:
Sat Sep 17, 2005 4:44 am Post subject:
Re: Cheap Dummy Loads |
|
|
Gareth Magennis wrote:
| Quote: | Hi,
is there any reason why you cannot use electric fire elements as cheap high
power dummy loads? e.g. is the resistance temperature dependent?
Cheers,
Gareth.
Do a google groups search on AAPLS for dummy loads if you want to amuse |
yourself.....
I wanted to try water heater elements. Put the right ones in parallel
liquid cooled in a 1 to 5 gallon can or so. The big discussion on the
thread was cooling methods. Hold on to your shorts if you read it.
Bob
----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
|
| Back to top |
|
|
Gareth Magennis
Guest
|
| Posted:
Mon Sep 19, 2005 2:25 pm Post subject:
Re: Cheap Dummy Loads |
|
|
"Bob Urz" <sound@inetnebr.com> wrote in message
news:1126917467_1715@spool6-east.superfeed.net...
| Quote: |
Gareth Magennis wrote:
Hi,
is there any reason why you cannot use electric fire elements as cheap
high power dummy loads? e.g. is the resistance temperature dependent?
Cheers,
Gareth.
Do a google groups search on AAPLS for dummy loads if you want to amuse
yourself.....
I wanted to try water heater elements. Put the right ones in parallel
liquid cooled in a 1 to 5 gallon can or so. The big discussion on the
thread was cooling methods. Hold on to your shorts if you read it.
Bob
|
Thanks Bob, that was quite a read. I think I might be going for the ceramic
resistors in a bucket of water.
I thought I'd had a great idea on the weekend - convert a fan heater to a
dummy load. Took mine apart this morning to find a 100 ohm element that you
could easily bodge to 12 ohms - not low enough unfortunately.
Gareth. |
|
| Back to top |
|
|
GregS
Guest
|
| Posted:
Mon Sep 19, 2005 7:28 pm Post subject:
Re: Cheap Dummy Loads |
|
|
In article <dgmhou$h8d$1@nwrdmz03.dmz.ncs.ea.ibs-infra.bt.com>, "Gareth Magennis" <sound.service@btconnect.com> wrote:
| Quote: |
"Bob Urz" <sound@inetnebr.com> wrote in message
news:1126917467_1715@spool6-east.superfeed.net...
Gareth Magennis wrote:
Hi,
is there any reason why you cannot use electric fire elements as cheap
high power dummy loads? e.g. is the resistance temperature dependent?
Cheers,
Gareth.
Do a google groups search on AAPLS for dummy loads if you want to amuse
yourself.....
I wanted to try water heater elements. Put the right ones in parallel
liquid cooled in a 1 to 5 gallon can or so. The big discussion on the
thread was cooling methods. Hold on to your shorts if you read it.
Bob
Thanks Bob, that was quite a read. I think I might be going for the ceramic
resistors in a bucket of water.
I thought I'd had a great idea on the weekend - convert a fan heater to a
dummy load. Took mine apart this morning to find a 100 ohm element that you
could easily bodge to 12 ohms - not low enough unfortunately.
Gareth.
|
If damp ceramic does get hot, it might explode.
greg |
|
| Back to top |
|
|
Diamondcutter
Guest
|
| Posted:
Sun Sep 25, 2005 10:07 pm Post subject:
Re: Cheap Dummy Loads |
|
|
Coat the resistors with a thin film of RTV silicone sealant(DOW Chemical)
good to 600*F
"GregS" <szekeres@pitt.edu> wrote in message
news:dgmhur$f1v$2@usenet01.srv.cis.pitt.edu...
| Quote: | In article <dgmhou$h8d$1@nwrdmz03.dmz.ncs.ea.ibs-infra.bt.com>, "Gareth
Magennis" <sound.service@btconnect.com> wrote:
"Bob Urz" <sound@inetnebr.com> wrote in message
news:1126917467_1715@spool6-east.superfeed.net...
Gareth Magennis wrote:
Hi,
is there any reason why you cannot use electric fire elements as cheap
high power dummy loads? e.g. is the resistance temperature dependent?
Cheers,
Gareth.
Do a google groups search on AAPLS for dummy loads if you want to amuse
yourself.....
I wanted to try water heater elements. Put the right ones in parallel
liquid cooled in a 1 to 5 gallon can or so. The big discussion on the
thread was cooling methods. Hold on to your shorts if you read it.
Bob
Thanks Bob, that was quite a read. I think I might be going for the
ceramic
resistors in a bucket of water.
I thought I'd had a great idea on the weekend - convert a fan heater to a
dummy load. Took mine apart this morning to find a 100 ohm element that
you
could easily bodge to 12 ohms - not low enough unfortunately.
Gareth.
If damp ceramic does get hot, it might explode.
greg |
|
|
| Back to top |
|
|
Peter Larsen
Guest
|
| Posted:
Sat Oct 22, 2005 5:28 pm Post subject:
Re: Cheap Dummy Loads |
|
|
Arny Krueger wrote:
| Quote: | Forced-air cooling is insufficient to reduce the temperature
rise to acceptable levels with so-called cheap resistors. An
oil bath is better, but still insufficient,
|
Why not a water bath? - in a glass jar if electrical insulation is an
issue.
Kind regards
Peter Larsen
--
*******************************************
* My site is at: http://www.muyiovatki.dk *
******************************************* |
|
| Back to top |
|
|
James Lehman
Guest
|
| Posted:
Sat Oct 22, 2005 9:58 pm Post subject:
Re: Cheap Dummy Loads |
|
|
Ha! that's funny.
Years ago... like about 25... I made a dummy load for my Fender all tube
guitar amp out of an old 2" voice coil in a sealed jar of distilled water.
It works great!... Not quite 8 ohms, but still reactive in the high end,
like a real speaker.
James. :o)
"Peter Larsen" <SPAMSHIELD_plarsen@mail.tele.dk> wrote in message
news:435A307D.A3DE0DE3@mail.tele.dk...
| Quote: | Arny Krueger wrote:
Forced-air cooling is insufficient to reduce the temperature
rise to acceptable levels with so-called cheap resistors. An
oil bath is better, but still insufficient,
Why not a water bath? - in a glass jar if electrical insulation is an
issue.
Kind regards
Peter Larsen
--
*******************************************
* My site is at: http://www.muyiovatki.dk *
******************************************* |
|
|
| Back to top |
|
|
|
|
|
|
Profile
Log in to check your private messages
Log in