Resistors in a circuit--does it matter much?

January 29, 2012 1:55 PM

Noob electronics question: Can I replace a 3Mohm 1/2 W resistor on a guitar pedal circuit with a 3.3Mohm resistor?

Or even a 3Mohm 1/4W resistor? Will this cause the circuit to not work? Does it matter that much? I'm new to DIY electronics and can't find the answer with a google search.

posted by sleepy pete to technology (18 answers total)

Yes. Resistor values are generally very loose; it's common for carbon resistors to only be held to 20% of rated value, and in a circuit like your guitar pedal the exact value really isn't critical. You technically shouldn't downgrade the power rating, because they probably used a 1/2 watt resistor for a reason, but a good way around that is to use two 1.5m resistors in series or 6m in parallel to get the power rating. Or just put in the 1/4 watt resistor and cross your fingers. Power ratings are typically very conservative too, and the worst that's likely to happen is that the resistor might fail and you'll have to replace it again.
posted by localroger at 2:24 PM on January 29, 2012

The first part of your question isn't really answerable, unfortunately. It all depends on the rest of the circuit, and how tolerant it will be to having that resistance be less than it was designed. It might work, but it also might not - no way to tell without looking at the schematic or just trying it - with the risk of course being that with a lower resistance you might fry something else in the circuit.

The second part is also unanswerable, other than to say that in general, doing that is a bad idea. If the circuit was designed for a half-watt resistor, that means that more than likely, the designer was expecting as much as a half-watt there (in other words, volts* amps > .5), and you should replace it with one that size or bigger to prevent frying this resistor and having to start all over again.

TL;DR: Don't. It's a bad idea for a number of reasons.
posted by deadmessenger at 2:25 PM on January 29, 2012

Resistors are typically spec'd at 5% or 10% (or even 20%) accuracy, so a 3.3M for 3.0M (or vice versa) is probably OK. You shouldn't switch in an underpowered resistor unless you understand the circuit, though. However, it seems unlikely that a 3.3M resistor would be running anywhere near its power limit in a circuit that you're going to connect a guitar to. If I'm calculating right, you'd need to be working with voltages around 288V before you have to worry about overheating a 3M resistor.
posted by spacewrench at 2:26 PM on January 29, 2012

He's considering a higher, not lower value resistor so the chances of frying anything are pretty close to zero. And as spacewrench thoughtfully calculated it's unlikely that there are any voltages in there capable of frying the resistor; they may have just used 1/2w because it was available. In an analog circuit it's extremely unlikely that the difference in resistance between 3m and 3.3m will cause a problem.
posted by localroger at 2:30 PM on January 29, 2012

it's common for carbon resistors to only be held to 20% of rated value

Are you thinking of potentiometers? Cheap resistors have 5% tolerance. Most audio projects specify 1% or better.

The correct replacement resistor costs $0.15. Expensive for a resistor, but cheap for a pedal fix.
posted by b1tr0t at 2:30 PM on January 29, 2012 [1 favorite]

ha ha, thanks b1tr0t--I'd forgotten to check Mouser. And thanks to the rest of you as well.
posted by sleepy pete at 2:33 PM on January 29, 2012

The resistor I previously linked is backordered, but you can get a 3M Ohm at 5% right now for $0.15.
posted by b1tr0t at 2:34 PM on January 29, 2012 [1 favorite]

You may also want to check out the forums at Freestompboxes, DIY Stompboxes and ElectroMusic. You may be able to find a thread about your specific issue. If not, you can probably find people who are interested in discussing your specific pedal and resistor.
posted by b1tr0t at 2:38 PM on January 29, 2012

Thanks again. I came to askme because I'm used to this place, but I'll check those boards out (I've used them for looking up information, but rarely for asking questions).
posted by sleepy pete at 2:41 PM on January 29, 2012

The power rating of the resistor in this application is irrelevant. To push a 1/2 watt through a 3-meg ohm resistor would require a voltage of over 1200 volts. I can guaranteed you don't have 1200 volts in your guitar pedal. Most likely it is only 12 volts or possibly a maximum of 40 volts. Your resistor will need to dissipate less than a milliwatt.
posted by JackFlash at 2:46 PM on January 29, 2012

Oh, and I should say, if you're wondering, I'm going to try the 3.3M first. Even though I searched for the 3M and didn't find one (thanks again, b1tr0t), the shipping is ridiculous for a $.35 component. I'll let you know how it works out.
posted by sleepy pete at 2:50 PM on January 29, 2012

Which pedal are you fixing?

I've been holding off on a Mouser order myself. I've got most of the BOM for Handmade Electronic Music picked out, and am working on finishing up the BOMs for some Eurorack modules as well as a random AVR project or two.
posted by b1tr0t at 2:56 PM on January 29, 2012

I'm building the Trill Tremelo. Next, I'm going for the "psychtar" pedal.

I just got a copy of the book you mention. Haven't made it through there yet, but will soon.

If you wouldn't mind throwing on a 3M resistor when you order and mailing it, I'll definitely get ya back (or pre-back... I can send you a few bucks). You can hit me up at the gmail addy in my profile.
posted by sleepy pete at 3:19 PM on January 29, 2012

Yeah, that 3M Ohm doesn't even need to be 1/4 W. Switching it to 3.3M Ohm may impact the speed range of your tremolo in some way, but it should get you up and running.
posted by b1tr0t at 3:28 PM on January 29, 2012

Are you thinking of potentiometers? Cheap resistors have 5% tolerance.

I remember when 5% resistors cost 10 times as much as 20% resistors, and 1% resistors were the stuff of mil spec and space exploration. With rare exceptions (think multi-pole filters) analog circuits do find with 20% resistors.
posted by localroger at 3:39 PM on January 29, 2012

You do realize you can just solder three 1 M ohm resistors together (in series, not parallel) if you can't find the 3 M ohm. Not tidy and will add a few cents to the cost (but shipping is always the thing anyway).
posted by Kid Charlemagne at 9:37 PM on January 29, 2012

If the 3.3MOhm doesn't work, memail me. I should have a 3MOhm resistor that I can quite easily put in an envelope for you. :)
posted by luckynerd at 9:36 AM on January 30, 2012

what's the tolerance of the resistor you're replacing? the last colour band will tell you. Also, If it's super important there might be a potentiometer elsewhere you can adjust to compensate.
posted by kg at 2:56 PM on February 6, 2012