quick circuit bending question



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quick circuit bending question

Postby Confuzzled » Tue Oct 10, 2017 1:13 pm

Years ago when I first started dabbling with circuit bending a friend of mine at work (who happened to be an engineer in electronics) wired up a 1/4" output to a toy he helped me bend and put a resistor attached to the plug. I'll take pics and add it to the post, but now that I'm getting back into it I have to ask why?

Is it always needed? How do you judge when it's needed?
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Re: quick circuit bending question

Postby oscillateur » Tue Oct 10, 2017 9:41 pm

Typically toys like that have a speaker, which has a resistance (usually 8-16 ohms).
That resistor basically simulates that (and makes the signal a bit less hot I think).

It's almost always a good idea to do that.
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Re: quick circuit bending question

Postby crochambeau » Thu Oct 12, 2017 9:14 pm

Series resistances can limit the amount of damage a semi conductor will risk if everything goes "wrong". I know lots of people who just make direct connections with wires and have a blast. I've also noticed sometimes stuff gets broken in the process and never recovers.

There might be a correlation there.

(hint hint)

Resistance produces different results though, so, it's up to you. I agree it's good engineering practice to have limiting resistances in place.
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Re: quick circuit bending question

Postby BetterOffShred » Fri Oct 13, 2017 11:10 am

I'm no Scientist of Electrical Elegance or anything, but a fine example for me is the LED... if you don't run a current limiting resistor in front (or behind) an LED, they will pull "infinite current" and pop in a matter of seconds or less. Any device that "pulls current" need some sort of resistive load to limit what happens with the current. E=I*R So if you give it 9 Volts (E) and 1000 Ohms of resistance.. I = E/R.. or .009 amps. 9 miliamps. boom. Conversely, if your R is approximately 0.. any number divided by approximately zero is infinity. Obviously we can't divide by zero, but even a piece of wire has a _little_ resistance, the point being your current will go as high as the power producing device will allow, and things break as crochambeau says :)

It's the same thing in some power networks on pedals, the 9V in runs right through a 100R resistor to "stabilize" the power source. I had a couple pedals that were noisy as balls, threw a 100R in series with the + wire, way quieter. Theres science and big words, but I don't know them. I tinker
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Re: quick circuit bending question

Postby eatyourguitar » Mon Nov 13, 2017 2:39 pm

BetterOffShred wrote:I'm no Scientist of Electrical Elegance or anything, but a fine example for me is the LED... if you don't run a current limiting resistor in front (or behind) an LED, they will pull "infinite current" and pop in a matter of seconds or less. Any device that "pulls current" need some sort of resistive load to limit what happens with the current. E=I*R So if you give it 9 Volts (E) and 1000 Ohms of resistance.. I = E/R.. or .009 amps. 9 miliamps. boom. Conversely, if your R is approximately 0.. any number divided by approximately zero is infinity. Obviously we can't divide by zero, but even a piece of wire has a _little_ resistance, the point being your current will go as high as the power producing device will allow, and things break as crochambeau says :)

It's the same thing in some power networks on pedals, the 9V in runs right through a 100R resistor to "stabilize" the power source. I had a couple pedals that were noisy as balls, threw a 100R in series with the + wire, way quieter. Theres science and big words, but I don't know them. I tinker


I can agree with you that current limiting resistors are generally a good idea to prevent damage. I don't want anyone to be misinformed about why a 100R resistor makes a 9v guitar pedal less noisy. it is not because of a current limiting resistor, it is because the resistor is in series to the power supply feeding a load that has some capacitance. this creates an RC filter. these are two unrelated topics. the 100R used in the RC filter will blow if there is a short in the guitar pedal connecting the power rails. the only thing these two concepts have in common is that they share parts while doing two jobs.

you might have an idea that limiting the current is also limiting the noise. this is actually a false preconceived notion. the resistance of the load will determine the load. it is as simple as that. even simpler. the load will determine the load. the current consumption will never change unless the load is changed or the power supply voltage is changed. this is because I = V/R (yes this is the same as I = E/R). so while the voltage drop on a 100R resistor on a 9.5v power supply is typically 0.05v we can also say that the power supply has not changed a significant amount from 9.5v to 9.45v. therefor the attenuation of noise beyond the corner frequency of the power supply RC filter is a much more significant explanation for the reduction in noise floor and power supply ripple. you can do the math for an RC filter of 100R + 100uf. you will see that it attenuates quite a lot of AC all the way down into the audio range.
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