QQ: 1/8th watt resistors

[karbomusic]

I'm building an Echobase and noticed the 1.5M resistors I ordered are 1/8th watt. That's no big deal for this build since I had enough 1/4th watt versions already. But now since I have 30 of the 1/8ths I thought I might ask if these would typically be a problem in pedal circuits where they are not involved in obvious areas such as voltage regulators etc. and closer to the outputs where an opamp might be chugging away. I'm assuming basic signal stuff before  amplification etc occurs would be generally OK? I know one could do the math on the circuit in question on a case by case basis but thought there might be some general "stay within this fence and you are probably OK" advice.

[ashcat_lt]

Ohm's law, dude.  Even the full 9V across a 1M resistor is only .009A, right?  Times 9V is nowhere near 1/8W.

[armdnrdy]

Yeah dude! 

Just for the record...the Ibanez AD-80 is built with all 1/8th watt resistors and runs at 12 volts.

I believe the only reason we use 1/4 watt is availability. 1/4 watt are much more common in all sizes.

[karbomusic]

Ohm's law, dude.  Even the full 9V across a 1M resistor is only .009A, right?  Times 9V is nowhere near 1/8W.

I assumed but figured but also need to increase my post count. Thanks for the assurance, it's been long enough that I've been out of this that I thought I'd confirm my suspicions. you look familiar btw.

[karbomusic]

Yeah dude!  

Just for the record...the Ibanez AD-80 is built with all 1/8th watt resistors and runs at 12 volts.

I believe the only reason we use 1/4 watt is availability. 1/4 watt are much more common in all sizes.

Thanks! So even the LM386-4 "1/4 watt" ruby type amp I built? That's what made me stop and check for exceptions because other than the power, it's pretty much a simple opamp pedal running off of 9/12v. I'll check the math later, just chiming it as to what made me stop and consider the possibility.

[GibsonGM]

It was a VERY good question, and for lower values, one can easily run into trouble.

But virtually everywhere you have a 1.5M pulldown resistor, you'll be safe to use those babies.  Normally I'd suggest that someone get a full set of 1/4W resistors, AND 1/2 watt, and never bother with 1/8, but in these locations there is NO current flowing to speak of.  And anyway, the 1/8 are smaller, so you gain some space in your builds...seeing as you only have that one value (right?), don't worry about it.

Power = V*I.....the current flowing into a base, gate, cathode or opamp input is miniscule, and therefore will not amount to anything significant.  Usually we start to 'think' more when we see a plate, drain or collector resistor, which is handling the power for the device...

We can even look at something like this:  how about a voltage divider using 2 of those resistors, and a source of 24 volts.  What current will flow thru this circuit made up of a total of 3M of resistance?   24v/3 million = .000008 amps!   Times 24 = .000192 watts!  Almost 1,000 times less than what the little guys can handle. 

Same 24V and two 1K resistors in the divider instead:  24v/2000 = .012A, 12mA.   *24v = .288W, in excess of even a quarter watt unit!   

Just work 'em thru, and it'll come back, karbo!

[ashcat_lt]

you look familiar btw.

From the Reaper forum, no?  I use the same screen name all over, sometimes without the underscore.

[R.G.]

P = V²/R

P = I²R

P= I * V

[armdnrdy]

I always cheat. 

http://www.electronics2000.co.uk/calc/power-calculator.php

[GibsonGM]

P = V²/R

P = I²R

P= I * V

Yes, it certainly does!   For some reason I always end up with voltage & current, never use the others much, but they are there for the using...

[Ice-9]

I wish I could find somewhere I could buy 1/8th watt resistors, they seem impossible to find in the UK or even Europe

[GibsonGM]

Really??   Can you order online from Ebay??  There are kits of all the common values, like this:  http://www.ebay.com/itm/50value-500pcs-1-4W-Carbon-Film-Resistor-5-Assortment-Kit-/251270449946?pt=LH_DefaultDomain_0&hash=item3a80e2cb1a

Reasonably priced, too.

[Ice-9]

Really??   Can you order online from Ebay??  There are kits of all the common values, like this:  http://www.ebay.com/itm/50value-500pcs-1-4W-Carbon-Film-Resistor-5-Assortment-Kit-/251270449946?pt=LH_DefaultDomain_0&hash=item3a80e2cb1a

Reasonably priced, too.

Those are 1/4 watt and are available in huge numbers in the Uk without problem, it was the 1/8 watt ones i was talking about, now if you have a good link for those. 

Sorry if this going a bit off topic.

[GibsonGM]

For some things, Ebay is great, lol.  For others, nah....but here is something:  http://www.ebay.com/itm/Lot-taped-precision-long-leaded-resistors-U-Pick-1-4-watt-or-1-8-watt-/230672295081?pt=LH_DefaultDomain_0&hash=item35b523e4a9

Lurking this evening, it's cold, dreary and rainy out...

[Ice-9]

For some things, Ebay is great, lol.  For others, nah....but here is something:  http://www.ebay.com/itm/Lot-taped-precision-long-leaded-resistors-U-Pick-1-4-watt-or-1-8-watt-/230672295081?pt=LH_DefaultDomain_0&hash=item35b523e4a9

Lurking this evening, it's cold, dreary and rainy out...

I've been lurking tonight as well, cheers for the link Mike, but again on looking at it they don't ship to UK lol. There is lots of stuff i can't get here in the Uk (i tend to buy 1000 of 1 value at a time with resistors).
I don't want to hijack this thread so I might start a new thread on sourcing different parts from different countries soon.

[GibsonGM]

That would be a GREAT idea, Mick...lots of ppl in your neck of the woods could use something like that.  Good luck!

[PRR]

> Ohm's law, dude.  Even the full 9V across a 1M resistor is only .009A, right?

Um, off by a Thousand.

9V across 1,000,000 Ohms is 9/1,000,000 or 0.000,009 Amps.

9V squared divided by a million is 81 micro-Watts.

2/3 of that for karbomusic's 1.5meg.

The maximum voltage across a 1meg before burn-up is 353 Volts. (But there is probably a 250V limit because the thing is so tiny.)

I can not think of any "sane" application where a 1Meg or 1.5meg can't be 1/8W.

[ashcat_lt]

> Ohm's law, dude.  Even the full 9V across a 1M resistor is only .009A, right?

Um, off by a Thousand.

9V across 1,000,000 Ohms is 9/1,000,000 or 0.000,009 Amps.

I'd like to play my sleep deprivation card now, please.  At least I was wrong in the right direction...

[IvIark]

I wish I could find somewhere I could buy 1/8th watt resistors, they seem impossible to find in the UK or even Europe

Try these: http://www.ebay.co.uk/itm/320412554577
I got them just to use for 3 row spans on vero so I wasn't being limited with the layouts to the 4 rows you have to span with 1/4W to get them to lie flat.

[armdnrdy]

I was checking out the assortment and noticed that the values jump from 330K to 1M.

Here's another assortment by a different seller that includes those missing values.
http://www.ebay.com/itm/221249349392?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Nothing is perfect as you would have to fill in common values with larger amounts, (1K, 10K, 100K, etc.) and there are some other useful values that are missing.....but.....it's still a great start and a good deal!