Resistor Wattage

[DW1961]

Can you use 1W resistors for any application that requires less than a 1W specification, or is there something else a at play when choosing a resistor for wattage? I figire instead of buying 1/4 watt, 1/2 watt, and so on, I could just buy 1W and be done with it?

[rsjsouza]

If the resistor fits and you are operating in low frequency, the higher power will only hurt your pocket.

[BeBuLamar]

Unless you use a lot of resistors the pockets don't hurt much but you must have space for the larger resistors.

[bob91343]

Larger parts have greater parasitics and so may not be suitable for higher frequency work.  Different types of resistors have different temperature characteristics.

The best procedure is to understand the parts and the application so as best to find the correct use.

[TimFox]

Subject to the universal law of "all other things being equal", there should be no problem substituting a part with a higher power rating, so long as you have space for it.  The high-power resistor should run cooler for a given current than the low-power unit.
If parasitic reactances are important to the application (e.g., RF), then you must be careful about the details of the resistor.
For example, some people worry about the inductance of wire-wound resistors in the audio range, but my measurements often find that at 10 to 100 kHz the net reactance is capacitive, not inductive.
Another example:  old-style Allen-Bradley carbon composition resistors.  The conductor is a mixture of carbon and talc, with more talc at higher resistance values.  At high frequency (certainly above 5 MHz for a 1 megohm 1/2 W unit), the talc-powder capacitance tends to short out the local carbon granules, leaving a component with net capacitive reactance and a reduced parallel resistance compared with the DC value.
A rare example of an axial leaded resistor with data-sheet discussion of impedance at high frequency is  https://www.vishay.com/docs/28769/mbasmahf.pdf  for a series of non-helixed thin-film resistors between 1.5 and 470 ohms (see last graph).
The larger cases of higher-power resistors can also have a higher surface breakdown voltage, which is useful in vacuum-tube circuits.

[DW1961]

Subject to the universal law of "all other things being equal", there should be no problem substituting a part with a higher power rating, so long as you have space for it.  The high-power resistor should run cooler for a given current than the low-power unit.

Hi Tim,

Good to hear from you here. If that's the case, then I'll look at 1 watt resistors. I have a box of assorted 1/4 watt already.

[TimFox]

What type (metal film, carbon film, cermet, etc.) are you buying?  For general purpose, I prefer metal film, but the others have their uses.

[bdunham7]

The high-power resistor should run cooler for a given current than the low-power unit.

With the seemingly obvious but occasionally overlooked caveat that it will produce exactly the same amount of heat....

[DW1961]

What type (metal film, carbon film, cermet, etc.) are you buying?  For general purpose, I prefer metal film, but the others have their uses.

Hey Tim,

They are metal film, and I get them free from a review program I am in. I got 1/4 watt, but only because they didn't have 1/2 watt. They don't have any 1 watt offers currently, but they now do have 1/2 watt. If it weren't for having to pay taxes on the 'free' reviewed items, I'd just get the 1/2 watt also and wait for a 1 watt offering, if they ever have it. It seems like mostly it's 1/4 watt.

This is the assortment: https://www.amazon.com/gp/product/B08NY3XR96

[DW1961]

The high-power resistor should run cooler for a given current than the low-power unit.

With the seemingly obvious but occasionally overlooked caveat that it will produce exactly the same amount of heat....

A very important observation, too.

[TimFox]

Death, taxes, and heat...

[wizard69]

I would tend to suggest going with the 1/2 watt set anyways.    The reality is that the 1 watt resistors are big enough to get in the way if you don't need them.

As for buying assortments like this, I tend to tell people not buy if the actual quantities per value are not itemized.    Some vendors will ship such assortments with far to many high value resistors that you MIGHT not need.   It appears that one of the commenters have said that they bias toward the lower ohmic values, but that is not the same thing as the vendor stating exactly what you will get.   I highlighted "might" above because you haven't really said what you are interested in electronics wise.

Now considering the "might" qualifier do you really need another resistor assortment?   If you have a 1/4 watt collection why not just order replacements in packages of 50, 100 or a thousand, when you need them?   who knows you next project might call for a 10 watt 8 ohm resistor or 20, 10K ohm, 1 watt resistors of a certain construction.   Past usage and future projects are the best predictors of what makes sense when spending money on "stock".

As for the larger resistors as many have already pointed out it does matter what the construction of the resistor is.   Wire wound resistors can be very inductive for example.   The important thing here to realize is that no electronic component is perfect.   Capacitors for example has series resistance.   Carbon composition resistors can be noisy.   It is better to understand how a component works and impacts the over all circuit design.   For some projects you may need to consider the voltage rating of a resistor.   The point is each device is an island unto itself, even 1 watt metal film resistors will have different characteristics from manufacture to manufacture and from series to series.   For many home projects it doesn't matter but you still need to be aware.

What type (metal film, carbon film, cermet, etc.) are you buying?  For general purpose, I prefer metal film, but the others have their uses.

Hey Tim,

They are metal film, and I get them free from a review program I am in. I got 1/4 watt, but only because they didn't have 1/2 watt. They don't have any 1 watt offers currently, but they now do have 1/2 watt. If it weren't for having to pay taxes on the 'free' reviewed items, I'd just get the 1/2 watt also and wait for a 1 watt offering, if they ever have it. It seems like mostly it's 1/4 watt.

This is the assortment: https://www.amazon.com/gp/product/B08NY3XR96

[DW1961]

I would tend to suggest going with the 1/2 watt set anyways.    The reality is that the 1 watt resistors are big enough to get in the way if you don't need them.

As for buying assortments like this, I tend to tell people not buy if the actual quantities per value are not itemized.    Some vendors will ship such assortments with far to many high value resistors that you MIGHT not need.   It appears that one of the commenters have said that they bias toward the lower ohmic values, but that is not the same thing as the vendor stating exactly what you will get.   I highlighted "might" above because you haven't really said what you are interested in electronics wise.

Now considering the "might" qualifier do you really need another resistor assortment?   If you have a 1/4 watt collection why not just order replacements in packages of 50, 100 or a thousand, when you need them?   who knows you next project might call for a 10 watt 8 ohm resistor or 20, 10K ohm, 1 watt resistors of a certain construction.   Past usage and future projects are the best predictors of what makes sense when spending money on "stock".

As for the larger resistors as many have already pointed out it does matter what the construction of the resistor is.   Wire wound resistors can be very inductive for example.   The important thing here to realize is that no electronic component is perfect.   Capacitors for example has series resistance.   Carbon composition resistors can be noisy.   It is better to understand how a component works and impacts the over all circuit design.   For some projects you may need to consider the voltage rating of a resistor.   The point is each device is an island unto itself, even 1 watt metal film resistors will have different characteristics from manufacture to manufacture and from series to series.   For many home projects it doesn't matter but you still need to be aware.

What type (metal film, carbon film, cermet, etc.) are you buying?  For general purpose, I prefer metal film, but the others have their uses.

Hey Tim,

They are metal film, and I get them free from a review program I am in. I got 1/4 watt, but only because they didn't have 1/2 watt. They don't have any 1 watt offers currently, but they now do have 1/2 watt. If it weren't for having to pay taxes on the 'free' reviewed items, I'd just get the 1/2 watt also and wait for a 1 watt offering, if they ever have it. It seems like mostly it's 1/4 watt.

This is the assortment: https://www.amazon.com/gp/product/B08NY3XR96

The reason for not buying them is because I'm getting them for free (I have to pay income tax on them), but you can't sit around too long or they leave the queue. The ones I got are: 0, 10, 15, 22, 30, 39, 47, 68, 75, 100, 150, 220, 270, 330, 360, 470, 510, 680, Ohm - 1k, 2k, 202k, 303k, 4.7k, 5.1k, 6.8k, 10k, 15k, 22k, 30k, 47k, 51k, 68k, 100k, 220k, 300k, 470k, 680k & 1M Ohm and vary from 25 each to several most popular at 50 each. Most of them are 25 each.

The 1/2 watt offer is about the same.

They both come in individual packages with specifications on each bag.

[JustMeHere]

What voltage are you working with?  A 5k resistor is still under 250mA at 30V.  Do the Ohms law math.

[DW1961]

What voltage are you working with?  A 5k resistor is still under 250mA at 30V.  Do the Ohms law math.

Well, that was my next investigation, but to answer your question, DC and not more than 30V.

[Zero999]

Resistors also have a voltage rating, which trumps the power rating. A 1M ¹⁄₂W resistor rated to 500V, will never be able to dissipate the full power rating, because it will exceed the voltage rating.

Resistors can also normally withstand dissipating more power, than the rating, for a short length of time. A 1W resistor can be overloaded by several watts for a second, or so, without being damaged. The reason for this is there is a temperature, at which the resistor will change value, smoke and catch fire and power is converted to heat in the resistor. It also means that at higher ambient temperatures, the resistor will need to be de-rated.

[TimFox]

For a given series of resistors, there is a “critical resistance value”, where the voltage and power limits are equal for that resistance.  Above that value, the part is limited by its maximum voltage rating;  below, the power limit rules.  A good data sheet often quotes this parameter.
It gets more complicated, however, since it is generally a good idea to derate the power in operation, and the voltage rating behaves differently.

[wizard69]

maybe I'm not following you, but if you are paying price per resistor (as tax) then these kits might be a rip off.   You can buy resistors in bulk from the big USA suppliers for 10 cents a piece in order quantities of 1.   Buy a whole reel and you will end up paying a fraction of a cent.   Maybe I misunderstood your cost numbers but if the "tax" comes out to 25 cents a piece you are basically paying retail.  The whole bit about tax for an item form China is a bit perplexing as I'm not sure what is going on there.

Now don't get me wrong here there is value in having a set of resistors nicely packaged up like is shown.    I just don't get the song and dance about tax.     

I would tend to suggest going with the 1/2 watt set anyways.    The reality is that the 1 watt resistors are big enough to get in the way if you don't need them.

As for buying assortments like this, I tend to tell people not buy if the actual quantities per value are not itemized.    Some vendors will ship such assortments with far to many high value resistors that you MIGHT not need.   It appears that one of the commenters have said that they bias toward the lower ohmic values, but that is not the same thing as the vendor stating exactly what you will get.   I highlighted "might" above because you haven't really said what you are interested in electronics wise.

Now considering the "might" qualifier do you really need another resistor assortment?   If you have a 1/4 watt collection why not just order replacements in packages of 50, 100 or a thousand, when you need them?   who knows you next project might call for a 10 watt 8 ohm resistor or 20, 10K ohm, 1 watt resistors of a certain construction.   Past usage and future projects are the best predictors of what makes sense when spending money on "stock".

As for the larger resistors as many have already pointed out it does matter what the construction of the resistor is.   Wire wound resistors can be very inductive for example.   The important thing here to realize is that no electronic component is perfect.   Capacitors for example has series resistance.   Carbon composition resistors can be noisy.   It is better to understand how a component works and impacts the over all circuit design.   For some projects you may need to consider the voltage rating of a resistor.   The point is each device is an island unto itself, even 1 watt metal film resistors will have different characteristics from manufacture to manufacture and from series to series.   For many home projects it doesn't matter but you still need to be aware.

What type (metal film, carbon film, cermet, etc.) are you buying?  For general purpose, I prefer metal film, but the others have their uses.

Hey Tim,

They are metal film, and I get them free from a review program I am in. I got 1/4 watt, but only because they didn't have 1/2 watt. They don't have any 1 watt offers currently, but they now do have 1/2 watt. If it weren't for having to pay taxes on the 'free' reviewed items, I'd just get the 1/2 watt also and wait for a 1 watt offering, if they ever have it. It seems like mostly it's 1/4 watt.

This is the assortment: https://www.amazon.com/gp/product/B08NY3XR96

The reason for not buying them is because I'm getting them for free (I have to pay income tax on them), but you can't sit around too long or they leave the queue. The ones I got are: 0, 10, 15, 22, 30, 39, 47, 68, 75, 100, 150, 220, 270, 330, 360, 470, 510, 680, Ohm - 1k, 2k, 202k, 303k, 4.7k, 5.1k, 6.8k, 10k, 15k, 22k, 30k, 47k, 51k, 68k, 100k, 220k, 300k, 470k, 680k & 1M Ohm and vary from 25 each to several most popular at 50 each. Most of them are 25 each.

While it is nice to have a good assortment of resistors it is better to have what you are likely to use on hand.    It does look like the creators of this assortment went to great length to make sure you got padded at the lower value range.    In my mind that can be a good thing for somebody doing beginner level electronics.   I'm just not seeing a good reason to over stock ones self with every power rating in existence.

The 1/2 watt offer is about the same.

They both come in individual packages with specifications on each bag.
(Attachment Link)

The packaging does seem to be nice and frankly it cost money to pack kits like that, so we can't dismiss the extra value there.   Still the nonsense about tax bothers me.   I mean how much is that tax and how much can you get for that price buying stuff retail.  Lets take an example:   Say you have a resistor that can be had for $0.005 each in bulk (probably a thousand), 1000 would cost you $5.    Mind you these are retail prices in the USA.  So how much is the "tax".   

By the way the listing apparently does not specify the manufacture nor the series of resistor.   While maybe not a big deal as a hobby level purchase it might make a difference for some projects.   If you look up 100 ohm resistors on digikey you will find that they have like 29 pages listed in their search system (just for axial lead metal film).   Those resistors can range in price from a fraction of 1 cent each to several dollars a piece.   You literally have hundreds of 100 ohm fractional watt, metal film resistors to choose from that meet various specs.   Add in the other compositions and you end up with thousands of choices.    So you see selecting the right resistor can be very involved if you need a good fit for an application.   This is probably not important to you as you are just looking for cheap hobby components.

Part of being cheap hobby wise is making sure you don't piss away good money on stuff you don't need.   This is where kits become a problem in my mind.   This vendor looks like they where thoughtful enough not to load the kit with crap you don't need.   Still form what you have indicated you already have one assortment of resistors so I don't see a need to buy another even if you are just paying "tax".   

To look at it this way there is not a resistor shortage and frankly the technology has not been eclipsed by other tech.    Compare this with tubes which are often in short supply and with few remaining manufactures, if you come across a stash of tubes for sale and you have a use for them, it might be a good idea to buy the lot and keep them in storage.   In the case of tubes you can sell them at a profit if you are careful about purchase price or keep them to use as the need comes up.   Tubes can be hard to get in some designs so their is a justification to hording if you will.   Resistors are dime a dozen and nearly commodity in manufacture and sale, so there is no reason to have a massive stock on hand.    You will want some on hand of course but you will also want to order as needed.

[TimFox]

Reading carefully, he is getting some sort of freebie, but has to pay tax on it as income.  The “25” is the quantity per package (imprecisely worded).  It’s controversial if this type of kit is a worthwhile purchase, and I am leery of components without data sheet, but they may be worth a very low cost.

[JustMeHere]

What voltage are you working with?  A 5k resistor is still under 250mA at 30V.  Do the Ohms law math.

Well, that was my next investigation, but to answer your question, DC and not more than 30V.

And I should have said 250 mW.  Not mA.

[DW1961]

maybe I'm not following you, but if you are paying price per resistor (as tax) then these kits might be a rip off.   You can buy resistors in bulk from the big USA suppliers for 10 cents a piece in order quantities of 1.   Buy a whole reel and you will end up paying a fraction of a cent.   Maybe I misunderstood your cost numbers but if the "tax" comes out to 25 cents a piece you are basically paying retail.  The whole bit about tax for an item form China is a bit perplexing as I'm not sure what is going on there.

Good points. I think I' just stay on the 1/4 resistors and if the need arises, then buy what I need.

So let me explain the 'tax' song and dance.

The review program I am in is Amazon Vine. It used to be, before 2015, that you received items you could request, did the review, and then after (officially) 6 months, you could do whatever you wanted with them (most people sold the stuff immediately). What happened was that professional journalist were being indoctrinated into the Vine program and peeling off hundred of reviews each month, and actually making a living selling the stuff (I just saw a $2, 000 business table as an item, for instance). It was more lucrative than their shitty 'journalist' jobs.

Anyway, the IRS got wind of it and Amazon and the US IRS went at it for several years until Amazon submitted.

Now, each item you get is a 1099 item sent the IRS. So if you have $1, 000 in "free" items, the minimum tax on "your business," because that's what the IRS claims you now have, is 12%. So, that scales up fast. Let's say you are in a 25% tax income bracket. Well, that 1099 stuff, what the IRS says is your business, gets added to that income. So, assuming you have no write offs and you need to pay taxes on that additional $1, 000.00 of Vine products, you will pay 25% on it, for $250.00. Needless to say, it has basically destroyed the Vine program except for people now in it, if they really do need something, then they will review it and get it for a discount (pay the tax on it). You can write off anything related to your new 'business,' such as internet and phone expenses, since that's how the 'business' game works in the USA.Most people don't even know that, but I have had my own businesses and I still retain my old CPA.  For instance, if you call your friend to ask him a question about a product you are reviewing, then you can write that yearly phone expense off 100%. If you are out for dinner and discuss a Vine review with someone, you can write the dinner off, gas mileage to get there, etc. Most people have internet, which you can write off, a phone, and maybe some products they bought and used to compare the Vine review product. You can write those off too.

Back to the resistors, they were 20 bucks for 1200 resistors. If I pay 20% for them (tax bracket) they will cost me 5 bucks. If I can show enough write off to offset the product tax, then I pay nothing for them. I have a $40.00 a year phone expense and a $600.00 internet expense, so immediately I can write off 900.00 of 'free' stuff, without even looking for other things. Last year I built my own computer and wrote that off too. Basically, the more you can write off, the more 'free' the stuff gets.

The point is you have to be really careful because even those 'free' $20.00 items add up fast. I had 4k of items last year in 6 months. This year I'm being a LOT more picky, and for reasons other than taxes (i.e., my house if filing up with table, lamps, soldering irons,  stereo speakers, amps, laptops, PC cases, monitors, etc.) Some of it I can sell on CLs pretty easily, but other stuff doesn't sell well at all, such as tables (or very rarely). I'm trying to just request stuff I really need, like a circular saw and scroll saw.  I currently have no less than 10 DMM, from the CEM Amazon brand (2) to Unity, and all points inbetween. I don't need all of those. It's just clutter. I did learn a lot by tearing them down here on EEV and asking questions, but no more. I'm going to try and sell them all but two, but even if I can get 50% for them, I think the most expensive one was something like 60 bucks. So it's almost not worth messing with the Craig's List bullshit (and there is lots of it to weed through).

Anyway, that's the tax dance.

[DW1961]

Tim, spot on. So far, I haven't requested enough stuff to pay taxes on it, since my write offs will exceed the additional item cost. So, the resistors were indeed free. But each time I add another item. . .you see the point