Calculating inductance

Other Hardware General Electronics
1

DC42 said:

I've done some sums, and from the datasheet I think the PWM frequency of the TLC5947 should be around 1kHz (based on 4MHz nominal internal clock and 4096 clocks per PWM cycle). If you use a 100uF decoupling capacitor, that will give an impedance of 1.6 ohms to ground. To get a 10-fold reduction in the amplitude of the unwanted signal, the impedance of the inductor should be 16 ohms, which implies an inductance of about 3.3mH

How did he arrive at 3.3mH? I can't figure out how you take 1000hz and 16 ohms and arrive at a value of 3.3.

Elsewhere he suggested that a 1000uf capacitor may be be better and I need to know how to adjust the mH for the larger capacitor.

(In case it matters, the maximum current that will flow through this inductor would be 480mA. 20mA max per LED for 24 leds.)

2

The impedance is:
Z=2pif*L

so if you want the inductance at a given impedance:

L = Z/(2pif)

In this case:
16/(23.141591000) = .00255

so .0033 is the closest standard value.

3

So using this calculator:

I see that I get 1.6 ohms for 100uf and 1khz input.

for 1000uf 1khz input I get 0.16 ohms.

To get a 10-fold reduction in the amplitude of the unwanted signal, the impedance of the inductor should be 16 ohms

So if 0.16ohms x 10 = 1.6 ohms then:

1.6/(23.141591000) = .000255

So 0.33mH or 330nH I guess? But it looks like there's a 2.7mH as a standard inductor value so I'm not sure why he didn't choose that when it was closer to the actual value.

I can use values that are larger right? I think I was told before that it's okay to use larger values than needed with inductors. I'm not really sure what a Henry is. But I'm gonna head out to Radio Shack shortly and see if I can find something I can use to fix this audio issue, and presumably they're not going to have exactly what I need.

4

0.33mH = 330uH, not 330nH.

5

Ah, I was reading mH as "microhenries".

6

You really need to learn the SI prefixes.

T = tera = 10^12
G = giga = 10^9
M = mega = 10^6
k = kilo = 10^3

m = milli = 10^-3
u = micro = 10^-6 (actually it should be a greek letter mu, which is like a u with a tail)
n = nano = 10^-9
p = pico = 10^-12
f = femto = 10^-15

The henry (lower case h), symbol "H" (upper case), is that inductance that
generates 1 volt when the current changes uniformly at 1 amp per second.

Thus 1uH needs the current to be changing at 1 amp per microsecond to generate
1 volt. (or equivalently generates 1uV when current changes at 1A/s)

7

Annoyingly, LTSpice doesn't recognize the difference between M and m. It sees both as milli, so to enter 1M ohm you must use 1000k.

Note that the "k" for kilo is lower case as are "da" for deka 10^1 and "h" for hecta 10^2, the three exceptions to the upper case "rule" for prefixes greater than 10^0. Or... I guess that makes the rule uppercase for prefixes greater than 10^3.

In electronics, you are pretty safe learning only pico 10^-12 to giga 10^9 and just looking the others up in the rare circumstance that it becomes necessary. Although femto and tera aren't bad to know, too.

http://physics.nist.gov/cuu/Units/prefixes.html

8

Will the inductor have any DC flowing thru it?
If yes, then you also must consider the maximum DC current that the inductor can handle before it saturates.

9

You really need to learn the SI prefixes.

I know the metric prefixes, I just got confused for a moment. :slight_smile:

Will the inductor have any DC flowing thru it?

It will have lots of DC flowing through it. Almost 250mA in this application, up to 480mA in others. It's on the +5v line going to my led modules and powering all my LEDs. It's there to prevent noise from the PWM from getting into the rest of the system.

The henry (lower case h), symbol "H" (upper case), is that inductance that
generates 1 volt when the current changes uniformly at 1 amp per second.

Thus 1uH needs the current to be changing at 1 amp per microsecond to generate
1 volt. (or equivalently generates 1uV when current changes at 1A/s)

I don't understand what I'm supposed to take away from this.

I watched a few tutorials on inductors and I think I get what they're doing, but even knowing that H = (V * s) / A, I don't know how to apply that to this circuit.

This was th only inductor I was able to get at Radio Shack:
http://www.radioshack.com/product/index.jsp?productId=2103978

100uH, 2A max. It says "rated at 1khz" as well but I don't know what that rating is for.

Anyway, I placed that in my circuit, in series with the 5V to the LED module, and with a 270uf capacitor I had already put from 5V to ground on the module side and it helped a little, but the noise is still clearly there.

Do I need to use a smaller inductor here? Or is it just that my capacitor is too small? I bought a 470uf and 1000uf capacitor to test with. I guess I'll try those next.

10

Well, the 470uf helped a bit. The 1000uf helped more, and a 4700uf helped even more. The combination of the inductor and capacitor are clearly better, but the noise is still audible even with the 4700uf cap, and I simply cannot stick such large a large capacitors on my led module, nor would I expect to have to.

When I used that ground loop isolator in my other circuit the problem disappeared entirely. Since I clearly can't get rid of all the noise form the led modules (unless my problem is not using the right inductor for the job) perhaps the solution is to do something different on the audio side of the circuit.

11

I wish I could see your circuit in front of me. I feel certain I (and many others here) could help. OK, first you will need to build a telepresence robot, and we'll build VR goggles and waldo arms....

I'm reluctant to use inductors to reduce noise, as you can end up with a resonance between the inductor and circuit capacitance that can actually make things worse.

12

Well that's just grand. I just fried the MP3 module. And I don't even know why.

I noticed the little 10W amp I have has a 470uf cap on the power input but no inductor. So I decided to take that Radio Shack inductor, and stick it on the +12v input to that. It didn't seem to have an effect at first but I didn't think I'd made a secure connection with the new ground line I'd run to the amp so I plugged that in more securely, but perhaps I broke the connection instead. Either way, the MP3 module went up in smoke.

13

Now this is odd. I wanted to make sure that I didn't have the polarity reversed anywhere, so I unplugged my micro, and led moudle from the circuit and motor driver, leaving just the amp, the audio connector plugged into it, and the power connector. Then I disconnected the power source too.

Then I checked for continuity from the ground line to both wires on the audio line. Both beep indicating continuity. (Neither beep when touching the positive lead as expected.)

Why would both leads connect to ground? Both being wired in series with capacitors I'd get. In that case I should get no continuity. Or maybe continuity for a moment but not constant. Or, one being wired to ground would also make sense. But both having continuity to ground? What's going on there? Maybe that is why my mp3 player got smoked and why I'm having issues with noise. I'm kinda afraid to try connecting anything else to the circuit to test this though, but I guess I'll have to sacrifice another microcontroller to see if the amp is functioning properly with the new power connection with the inductor in place.

Oh, and when I read the power connection I do get +12v when I have the positive probe on the inductor, so that's in the right place.

14

Hm, well with power on, I get 12v with the red probe on the inductor and the black probe on what I appear to have correctly assumed was the audio line connected to ground on the amp.

And when I touch ground and the other audio line, I get a small voltage which drops over time. Presumably a capacitor discharging.

15

Oh great I fried my Arduino Micro too at the same time as I fried the MP3 module.

This doesn't even make sense.!

Let's say I had accidentally disconnected the ground to the amp. I don't think I did, at least not at the time I fried it. But let's say I had.

In that case the path to ground would have been through the audio line through the mp3 player ground that is attached to the Arduino ground pin. But those are just traces. No current should have flowed through either chip as a result of that.

At the same time, I have a 0.1uf capacitor in series on the + audio line. So if I managed to put 12v DC there, it should have been stopped by the cap.

I wish I knew what the hell I did to cause this. But I guess the only thing I can do right now is grab one of my other microcontrollers with built in sound and test that with the amp and see if the same thing happens. I hate to kill a $50 board though. :confused: