# Why does the DMM make this circuit "work" ? (Power issues : servo & bluetooth)

wvmarle:
Size is relative. The smallest inductor is huge compared to a 0805 resistor;

No it is not.
Surface mount inductor on the left 0805 resistor on the right.

Chetan_:
I just want to know which 3.3V regulator you used I am also with the same project.

i’m still prototyping and haven’t even settled on how to supply the power properly - at the moment i’m using the LD1117A(3v3) - it’s a LDO type. (although also using a DC-DC step-down converter - based on LM2596 chip)

Grumpy_Mike:
No.
You put the filter between the break out board and the servo.

so you mean like this ?

meanwhile, on your webpage that you linked, it says using inductance values of tens of milli-henries.

i’ve been calculating what length and radius of coil that would involve and it’s huge !

is my current circuit really needing that high a value - i can’t imagine that’s what my DMM was “contributing” ?

what sort of inductance/choke levels would be reasonable for 6V spikes of 100mA ?
(is there a formula for this and what would the ‘phenomena’ be called ?)

so you mean like this ?

No.

Like this:-

The square boxes are the servos.

i can’t imagine that’s what my DMM was “contributing” ?

Your right it wasn’t so that suggests you don’t need that much inductance so you only need a small value. That was a general purpose comment for heavy motors.

what sort of inductance/choke levels would be reasonable for 6V spikes of 100mA ?

You can’t say without first defining the period of the pulse and you don’t know that.

is there a formula for this and what would the ‘phenomena’ be called

This is called “inductive reactance” https://www.electronics-tutorials.ws/inductor/ac-inductors.html and is the effective resistance an inductor presents when fed with an AC signal. This is a pure inductance where the actual resistance is considered zero.

The formula is inductive reactance in ohms = 2 * Pi * f * L
where f is the frequency in Hz and L is the inductance in Henries

BabyGeezer: meanwhile, on your webpage that you linked, it says using inductance values of tens of milli-henries.

i've been calculating what length and radius of coil that would involve and it's huge !

My 10 mH inductors are still the standard 10mm radius, 12mm tall. Not too bad. Ohmic resistance is increasing of course (I'm sure the larger the induction the thinner the wire used to be able to have more coils, so more resistance per length and more overall length) so I'm normally using 330µH for my filters. Just 2-3Ω so not too much losses.

Grumpy_Mike:
No.

Like this:-
The square boxes are the servos.

yes, that was kind of my original point about having to “insert” the Pi circuit and not being able to (be lazy and) “just plug in the servo connection”

Grumpy_Mike:
Your right it wasn’t so that suggests you don’t need that much inductance so you only need a small value. That was a general purpose comment for heavy motors.

i see - okay then, i’ll try and hand make a coil just to see if it holds back the jitter.
(more to do with “see what happens” than designing the circuit ideally.)

by my calculation, a simple straight (air core) coil of 20mm radius, 10mm long with 20 turns should give about 63 micro-H. (that’s already 2.5 meters of the enamel wire needed(!), i don’t have any ferrite (nor iron) core - now i get the “don’t bother making your own” comment !)

Grumpy_Mike:
You can’t say without first defining the period of the pulse and you don’t know that.

the effective resistance an inductor presents when fed with an AC signal. This is a pure inductance where the actual resistance is considered zero.

so, good’ol V=IR applies here where the (supressing) “resistance” required would be an equivalent of (6V / 100mA) = 60 Ohm ?
and if i can get the frequency; the (minimum) inductance would be L = 60 / (2*Pi * f)

would the inverse of the pulse(time) be considered the ‘frequency’ ?
(am just realizing that AC doesn’t have to be a constant period, and that a “DC spike” is also AC !)

An inductor basically tries to keep the current constant - it uses energy stored in its magnetic field for that. That's also why you need that flyback diode on inductive loads: so the current can continue to flow.

When you have a pure DC current flowing, all you see is the ohmic resistance, as in the normal resistance of the wire used. Note that when the power is switched on the inductor resists, so current starts up a bit slower as the magnetic field builds up.

So it rejects all changes in current, that includes spikes. For the same reason an inductor has a much larger resistance for AC current - it changes all the time. This is normally called "impedance" which includes many more effects than just ohmic resistance.

would the inverse of the pulse(time) be considered the 'frequency'

Yes.

Reactance is just one aspect of impedance but for this application you don’t care about impedance.

Without a ferrite core an air core inductor is too big and does not work as well as the ferrite is lossy and absorbs some of the energy in the spikes.

ok then - thanks a lot, Mike and wvmarle !

new adventures into the world of coils will have to be planned soon.