I dont know much about resistors

[eggi]

I am gonna try and build this **broken link removed**
but have a few questions about resistors:
- how important is it that the watt values match? can I use a 50w where Wallins use a 20w? or a 0.5 watt where he uses a 0.25watt?
- i was thinking buying a 1% resistor lot on ebay, can I use these where he uses 5% resistors?

"Soundwaves are the best waves"

 

[Exo]

I am gonna try and build this **broken link removed**
but have a few questions about resistors:
- how important is it that the watt values match? can I use a 50w where Wallins use a 20w? or a 0.5 watt where he uses a 0.25watt?

Yes, You may always replace a resistor with one that has a higher watt rating. It's the power the resistor can handle, putting in a higher-spec one is allright, it'll only be bigger

- i was thinking buying a 1% resistor lot on ebay, can I use these where he uses 5% resistors?

Again yes, This percentage is the precision of the resistor. Putting in a more precise resistor is obviously ok. They'll probabely cost a bit more then 5% ones

 

[evandude]

yes, on both counts... the wattage of a resistor is how much power it can dissipate - in other words how well it resists heating up. using a 50w instead of a 20w means it'll stay cooler if you were to push it to the limits.

and using a 1% accuracy resistor instead of a 5% is fine too, if anything, it's better... all that means is that the resistors are guaranteed to be a lot closer to their rated values.

 

[Nigel Goodwin]

yes, on both counts... the wattage of a resistor is how much power it can dissipate - in other words how well it resists heating up. using a 50w instead of a 20w means it'll stay cooler if you were to push it to the limits.

One point to bear in mind with really high wattage resistors, they are usually specified as mounted on heatsinks - large ones!. Without a heatsink you need to derate them considerably - so if it's got mounting holes, it may well need a heatsink.

 

[evandude]

which makes using a 50w instead of a 20w even better in this case. you can probably get away with NO heatsink on a 50W resistor if it's only dealing with 20 watts of power or less, saving you the hassle of mounting a heatsink, etc.

however, make sure you can FIT a 50w resistor into your project. they aren't small, by any stretch of the imagination. The ones I've been using at work are a couple inches long, about as big around as your finger, with big mounting lugs, and a bunch of cooling fins.

if you were to build the project in an aluminum (or other metal) enclosure you could simply drill a couple holes and bolt them to it, that should be a more than adequate heatsink since you're only running them at up to 40% of their rated capacity.

 

[eggi]

Great, thanks, this was what I thought too, but I wanted to be sure.
One more thing, whats the difference between a SPDT and a DPDT toggle switch? Wallins uses DPDT switches, but on some webshops they only have the SPDT switches, but Im not sure I can use them.

 

[andrew2022]

SPDT - single pole double throw - 1 input switched between 2 outputs
DPDT - same as above, but has 2 SPDT operated by the same switch. u can use a DPDT if u cant get a SPDT but no the other way round

 

[evandude]

exactly what he said. so no, you can't use a SPDT in place of a DPDT.

 

[stevez]

Wallins indicates that the 20 w resistor should be non-inductive. Substituting with common wirewound power resistors may add a bit of inductance. At audio frequencies the effect may be trivial though in a test fixture you may want to limit any sources of potential error.

Just to satisfy my curiosity I wanted to see what impact a small amount of inductance might have. I looked at a table and saw that a 5/8" diameter coil (air) with 10 TPI has an inductance of 1.7 microhenries. That might represent the inductance in a 50 w resistor. I also thought that if the inductive reactance were 10% of the resistor value that might be significant given that it is a test fixture - that's 0.8 ohms. At 10 kHz you'd need an inductor with about 13 microhenries to get 0.8 ohms. All of this presumes that my hurry-up math is correct. Where I come out in all this is that using a wirewound resistor may impact things so be ware. The result would be a load that changes slightly with frequency.

 

[eggi]

This is the 50w resistor I thought of using:

https://www.vishay.com/docs/30201/rhnh.pdf

Its the NH-50

 

[Exo]

That should do perfectly...

It's non-inductive (NH) and according to the graphic's it should be able to take care of 20W without heatsink...

 

[Roff]

That should do perfectly...

It's non-inductive (NH) and according to the graphic's it should be able to take care of 20W without heatsink...

Take a closer look at the power derating graph. The vertical axis is percent of rated power, not watts. The NH50 is rated at 40% (20 watts) with no heatsink to 25C, and goes downhill above that temperature. I would mount it on something that can radiate a little more heat.

 

[Exo]

The vertical axis is percent of rated power, not watts.

...Whoops :roll:

 

[Roff]

The vertical axis is percent of rated power, not watts.

...Whoops :roll:

Well, it was an honest mistake. I almost came to the same conclusion.