Why the small resistor in series with power?

[loki]

I've seen in several schematics the habit of having a small resistor (usually 47R or 100R) from the +9V connection to the circuit components that require power. Why is it used?
I'm no electronics expert but I know that a resistor reduces power by some amount, depending on the resistor value. A 100R resistor what difference can it make?

[anotherjim]

It does 2 things.

Primary - Forms a low pass filter with the power supply capacitors - helps reduce audio and radio frequency noise on the power supply.

Secondary - Protection - It will limit supply current in a short circuit fault. It may well go up in smoke, but it might have saved everything else. If it's only momentary fault, the protection may work long enough to stop any damage ever happening. A fuse on the other hand will blow - and that's the end of that until you change the fuse.

[davent]

Should also be an electrolytic cap, 47u, 100uF....  to ground after that resistor which together will filter out noise from a power supply. Small value resistor so voltage drop across it is minimized. I power the LED from before filter as the LED has the potential to be the most power hungry part in the pedal.

Circuits dead quite powered by a battery can get noisy even from a great power supply, this little filter can kill the demons.

SLow typist here.

[Seljer]

And a typical effects pedal circuit hardly ever consumes more than a couple of milliamperes (unless it's something fancy with clocks and LFOs like a chorus or a delay).

Let's say theres a 100ohm resistor installed in series there and the you've got a fuzz pedal that draws 3mA.

as per Ohm's law: 100ohms * 3mA = 0.3V

Which is how much supply voltage you "lose" over that resistor in exchange for better filtering of noise from the power supply.

[loki]

Thanks a lot for the replies!  

[Haze13]

1/2*Pi*R*C = cut off frequency. Larger cap, larger resistance - lower cut off. Larger resistance = bigger voltage drop across it, so if your effects consumes more current than smaller the resistance should be (don't forget about his power dissipation) and larger the value of the cap.

[darron]

In some instances I've learnt it won't practically matter how large of a power filter cap you use until you start using the series resistor before it to complete the low-pass filter. Exactly like how it would be silly to add a smal value RF filter cap at the beginning of a circuit without a series resistor prior.

One particular circuit I was building (pnp with negative ground) would always be noiser on power supplies than an INTETNAL (shielded) battery. Even with a 470uf filter cap. Add a 47 ohm resistor and presto! Perfect.

[chi_boy]

So I have a follow-up question on this topic.

Some power supplies will use a diode is series with the supply as a reverse polarity protection.  Depending on the diode, there is always a small voltage drop across the diode. 

In that scenario, is the diode functioning like a resistor, and if so, will the diode followed by a large cap to ground form a low pass filter?

[darron]

The diode won't really work as a resistor. There's a minimal amount of resistance (like any conductor) though. But I treat them as none for pedals.

The voltage lost across a diode is due to the forward voltage drop. That is, the voltage needs to be beyond a certain figure before it can start to pass through the diode. So any voltage lower than that, say 0.6/0.7V didn't get to pass and is subtracted from the output. It's still sitting there though and not burnt up like a resistor would.

A lot of circuits just leave it out anyway though

[PRR]

>M is the diode functioning like a resistor

In a nearby thread, someone discovered that his pedals average 32mA.

At 32mA the *dynamic* resistance of a solid-state diode is just under 1 Ohm.

If you are filtering hum/buzz you need your R-C product to be far-far below 100/120Hz (or 50/60Hz if your wart is real cheap). Say 10Hz. 10Hz at 1 Ohm is 16,000uFd. Rather large and costly.

If you can spare just 0.3 Volts you can use a 10 Ohm resistor and now you only need 1,600uFd. Maybe large for pedal but very common in simple hi-fi plans.

If your pedal sucks 32mA, then a 47 Ohm resistor drops 1.5V, which is a lot in a 9V world. However many-many pedals are much less than 32mA. At 10mA a 47 Ohm is only a half-volt, and you get great filtering with just 340uFd.

[bluelagoon]

Isn't it more beneficial to reduce power drop from the resistor to instead use aa inductor in series at the power input to achieve same as the resistor only without the unwanted power drop?

[Steben]

Isn't it more beneficial to reduce power drop from the resistor to instead use aa inductor in series at the power input to achieve same as the resistor only without the unwanted power drop?

Resistor is usually the common and easy part in any value.
The voltage drop is only important in very fragile circuits.

[ElectricDruid]

Isn't it more beneficial to reduce power drop from the resistor to instead use aa inductor in series at the power input to achieve same as the resistor only without the unwanted power drop?

I did actually see an example of that just recently, on the Madbean/VFE relay bypass board:



It's not that common though, for the reasons Steben gave: Everyone has resistors, but not many of us have inductors around, and resistors are cheaper anyway.

[Rob Strand]

Also for 50Hz you need very high inductances to match the performance of a resistor.    Low current inductors with large inductances can end up with high DC resistances.  If the current is significant then such an inductor will be large and much more expensive.  For high frequencies the inductors can help but they still aren't that small in value.

It's not clear what the inductor on the madbeans circuit is trying to achieve because it filters the incoming supply but not the local supplies.  IMHO the layout of the tracks and caps around IC2 would be important and if done correctly you might not need the inductor.

Watch out for stuff like this in small inductors,

https://www.inductorchina.com/inductor-types-fixed-al0307-getwell.html

1mH 26 ohms

Whereas

https://www.farnell.com/datasheets/2156400.pdf

1mH 2.5 ohm

[johngreene]

I've seen in several schematics the habit of having a small resistor (usually 47R or 100R) from the +9V connection to the circuit components that require power. Why is it used?
I'm no electronics expert but I know that a resistor reduces power by some amount, depending on the resistor value. A 100R resistor what difference can it make?

I had to add a small resistor to designs to protect against high frequency noise generated by many switch mode power supplies. Some of these cheap ones are really bad. You just have to be careful to power any components that change their current consumption due to switching (status LED) or modulation (tremolo, chorus, etc.) that use LEDs to either modulate a LDR or show status or both, as this will modulate the power supply if powered from the output side of this series resistor! Sometimes a series protection diode has enough variation in series resistance to cause the supply to modulate.

[bluelagoon]

What about this one at around $0.40 AUD, its cheap enough, the inductance is 470uH, and its resistance is only 3.4 ohm, a much less power drop than any of the series resistors offer.

https://www.digikey.com/en/products/detail/abracon-llc/AIAP-01-471K-T/3060653

Also alluding to PRR's earlier post where the smaller the resistor the more huge the capacitor value needs be, but not with a series inductor, you get by with just a 100uF in the power supply with a 3.4 ohm inductor being used. isn't this a better deal?. since the voltage  drop with the series resistor does add up, especially when you start using voltage doubling converters, you end up with twice the voltage drop. probably never too significant by any means in an effect pedal, but maybe if using a battery to power. but even for the practicality of a need for a much smaller Filter cap to achieve same end is an advantage.

[PRR]

> inductance is 470uH

What is the inductance of 470uH in the audio band? Anybody know math(s)?

I get 0.15 Ohms at 50Hz, 15 Ohms at 5kHz. Well, 3.5r and 18r counting resistance.

There are situations where a choke makes sense. Low-current low-production guitar pedals, it is hard to make a case.

[Rob Strand]

> inductance is 470uH

What is the inductance of 470uH in the audio band? Anybody know math(s)?

I get 0.15 Ohms at 50Hz, 15 Ohms at 5kHz. Well, 3.5r and 18r counting resistance.

There are situations where a choke makes sense. Low-current low-production guitar pedals, it is hard to make a case.

That was my earlier point you need enormous inductors and small inductors with even mild inductance will often end up with high resistance.  The DC resistance will do more than the inductance!

[bluelagoon]

Yes all good and well the mathematical equations being considered, but in all practical intent, wouldn't that 470uH inductor do a satisfactory job at filtering as much as a series resistor would do, and wouldn't it do the job without taking as much a voltage drop as the resistor would do, and isn't it also true that an inductor does not need the same exceedingly large sized filter capacitor to achieve the same result as the resistor and capacitor choice. Even though it may be overkill, if it works and has some advantage as not requiring as large a filter cap, and if it saves some extent of power loss over the resistor choice, then isn't it a viable alternative at maybe a few extra cents over the series resistor.?

[FSFX]

The significance of the filtering provided by having a series protection diode should not be ignored or underestimated. Its effective forward resistance in combination with decoupling capacitors can provide a considerable amount of filtering of noise.


The filtering effect of series diodes can be seen here compared to that of a 5 ohm and 50 ohm series resistor. In this case, there is a 9mA load and Nishicon 100uF capacitor and a Worth 1nF NPO ceramic capacitor for decoupling.