Resonance or...

[Electronman]

Hi,

Can everybody direct me why I have a sine wave across my inductor when I connect it into 5V DC? The power supply is regulated so why my scope is showing more that 5 MHz across the inductor when the inductor is in parallel with the DC voltage?
I got almost 550 KHz when I joined a 483 cap across the inductor too!!

I know something about LC resonance, but not sure if it works with DC sources too!!

Resonance occurs when the Xl = Xc, so and in this cause the resonance freq of the L and C is equal

 

[dknguyen]

If the power supply is a switching power supply, then the ripple is coming from that (but you should see it whether or not the inductor is there).

 

[Electronman]

yea, the power supply is a switching power supply. but I don't have this problem when there is not inductor across the power supply. I have a pure DC voltage when the power supply is connected across the scope(i.e. without the inductor)
Is this possible the inductor starts resonating at the DC voltage? The frequency was exchanged when I put a ceramic caps across the inductor too!

 

[Hayato]

Well, how are you connecting the inductor?

Just DC supply + Inductor?

Then you are shorting your supply. Doing that, the ripple voltage raises too much and excites the natural ressonating mode of the inductor.

 

[Electronman]

Well I tried it with the secondary of a 15V transformer as an inductor. The inductor draws 500mA at 5V and I have a 454 kHz sine wave across it!
The power supply is able to give more than 6A at 25V

I am suspect to the power supply though!?

Is there anybody to direct me what I must expect of the output of an inductor when it is connected to a DC power source? I know that an ideal inductor is a short circuit at DC voltage but what about a real inductor? Can you let me know if I am seeing a fault on the scope due to the inductor across a DC power supply or I must expect so?

Thanks

 

[Roff]

Well I tried it with the secondary of a 15V transformer as an inductor. The inductor draws 500mA at 5V and I have a 454 kHz sine wave across it!
The power supply is able to give more than 6A at 25V

I am suspect to the power supply though!?

Is there anybody to direct me what I must expect of the output of an inductor when it is connected to a DC power source? I know that an ideal inductor is a short circuit at DC voltage but what about a real inductor? Can you let me know if I am seeing a fault on the scope due to the inductor across a DC power supply or I must expect so?

Thanks

How did you measure the 500mA? That implies that the resistance is 10 ohms, which is way too high for the secondary of a 15V transformer.
There is no good reason I can think of to connect an inductor directly across a DC supply. Why are you doing this?

 

[Hayato]

In DC, a inductor is a short-circuit, or a small resistance.

As I told you before, you are seeing frequency because the ripple is exciting the natural ressonance mode of the inductor.

Test your supply, add a 1k ohmic load without the inductor and check if there is any frequency noise there.

 

[Ubergeek63]

yea, the power supply is a switching power supply. but I don't have this problem when there is not inductor across the power supply. I have a pure DC voltage when the power supply is connected across the scope(i.e. without the inductor)
Is this possible the inductor starts resonating at the DC voltage? The frequency was exchanged when I put a ceramic caps across the inductor too!

what do you see with a resistive load?

Dan

 

[Electronman]

I tried the power supply using a 10 ohms resistor and yes I have that 454 kHz freq across the resistor to! So there is a problem with the switching powers supply.
I have bout it for 150$ so Why it has those ripples (the below pic) at its output?

Rolf,
My power supply showed me that current.
My ohmmeter shows 19ohms across the secondary of the 300mA transformer.
I just wanted to see the behavior of an inductor at DC and then put a large caps across the self and see the resonance freq via a scope.

Hayato
What do mean by this:
"ripple is exciting the natural resonance mode of the inductor"?

 

[Ubergeek63]

Well I tried it with the secondary of a 15V transformer as an inductor. The inductor draws 500mA at 5V and I have a 454 kHz sine wave across it!
The power supply is able to give more than 6A at 25V

Power line transformers are only good for audio frequencies. You need switching inductor at those frequencies.

Dan

 

[MrAl]

Hi,


You posted a scope picture but did not specify the scale factors.
You need to tell us the peak to peak ripple.

Some ripple occurs on most power supplies, it depends how much
it is before you can say it's too much.

 

[Ubergeek63]

I tried the power supply using a 10 ohms resistor and yes I have that 454 kHz freq across the resistor to! So there is a problem with the switching powers supply.
I have bout it for 150$ so Why it has those ripples (the below pic) at its output?

What is the amplitude of the ripple? Many switchers are allowed to have 100-200mV or so of ripple.

What do mean by this:
"ripple is exciting the natural resonance mode of the inductor"?

It is the resonance frequency that is the result of the inductance and parasytic capacitance of the inductor.

Dan

 

[Hayato]

It was 10 ohms at 25V or 5V?

10 ohms for 25V is very capable to generate much ripple as it can be considered a "heavy load" (62,5W = 42% of full load) .

 

[Electronman]

Here are the results at 5V and 400mA power supply (I use the said transformer here too).

Vpp=265 mV

RMS=73mV
Freq= 454kHz

Ubergeek63,

Are you trying to say that the inductor is at resonance at DC too? It does resonance even if there is no cap?

 

[Hayato]

The thing is that the "inductor" is not ideal. It does have a parasitic series resistance and parallel capacitance.

This 5V is SMPS as well?
And 265mV of ripple, is not that bad.

 

[Mikebits]

Much of your measured ripple could be due to equipment setup. Switchers generate a lot of EMI. What is your grounding like, how long is the ground lead on your scope probe? Keep your grounds short and solid. Use short scope ground leads.

A typical good quality switch supply usually has a 100mv of ripple. 50mv is better. You may be chasing a non problem depending on your application and setup.