Resistor voltage after switch-on

[potcore]

Dear Engineers,

I have recently been testing some inductor values at work by switching 5V across the inductor in series with a known resistor and noting the rate of voltage rise across the resistor with a scope on "trigger and hold" mode.

-Anyway, while doing this i noticed an interesting effect and wondered if readers could verify my explanation.

First of all, please think of the following simple test circuit......
A 20 ohm resistor and a switch are placed across 5V DC...
The switch is then closed.
The voltage on the resistor "should" fairly immediately go to 5V and stay there.
-However, it does not.
-When the switch is closed, the resistor voltage goes to 5V and stays there for about 500uS. The resistor voltage then goes down to zero volts for about 2 to 4 ms. Then the resistor voltage goes up to 5V and stays there.

Here is the complete circuit...

Here is the Resistor voltage waveform immediately after switching S1 ON....

I would be very grateful if readers could verify my following explanation about why the resistor voltage goes back down to zero, and then back up again...

I believe that the first rise to 5V is caused by current discharging out of the Electrolytic smoothing capacitor. However, when this is discharged, the current has to come from the mains transformer. However, at first, the mains transformer does not supply current to the circuit, since it is simply "magnetizing" itself (ie, taking in current, and converting it into its magnetic field)...-During this "magnetizing phase", there is no current flowing through the 20 ohm resistor even though switch S1 is closed, therefore, the voltage across this 20R has gone back down to zero. Next, when the transformer has "magnetized" itself, it then supplies current to the circuit, and the resistor voltage goes up to 5V as would be expected.

I would be very grateful indeed if this explanation could be verified.
(Unfortunately i don't know the valus of the electrolytic smoothing capacitor as its inside the "wall-wart" adaptor plug.)

 

[itsmoked]

The timings you are showing are very typical of switch bounce. The switch closes, rebounds briefly ,and closes again. The varying time frame also fits with applied finger effort.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DanDel]

If your transformer is connected to 240VAC, then it has already become 'magnetized', and will already have its secondary voltage applied to the rest of the circuit.

I would be curious to see what happens to the voltage comming out of this unit when the switch is closed.
Or, try the circuit using 5V of batteries.

 

[itsmoked]

Change switch brands or style and see how things change. You could even try a copper strap that you just mash down on another strip.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[potcore]

Thankyou for your replies.
Thinking about the "wall wart" adapter plug in this circuit. I just took it alone, placed 20 ohms across its 12V output, and the waveform was obviously DC 12V, but with 1.6V peak to peak ripple which was sawtooth shaped. The sawtooth shape suggests some kind of SMPS rather than my simple diode bridge representation of it, -apologies for this.
This adapter is called "Universal Unregulated AC/DC adapter, 1000mA 23W 230V-50Hz, 12VA Max, L82BF "

Thanks very much for the recommendations, i will look into these.

 

[itsmoked]

If it is a switcher your it could just have horrendous output response but my money is still on switch bounce.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[potcore]

Thanks very much for your replies...it was indeed switch bounce.....i now use a pic to software de-bounce and have a mosfet in series with the RL circuit.

Incidentally though, the "wall wart" adaptor plug's sawtooth ripple (5th thread) has a period of 10ms. This is extremely low for a SMPS. I am wondering if any Engineers have any idea about what type of SMPS it may be? I would only have expected a ripple period of 10ms from a full wave diode bridge output -not a SMPS.

 

[ScottyUK]

It's using a plain diode rectifier. 10ms equates to 100Hz: what you are seeing is just ripple from the mains, which in your location is 50Hz. The rectifier produces ripple at double the line frequency unless it is a half-wave rectifier using a single diode. Yours is a full wave rectifier which probably uses a bridge rectifier but just might use a centre-tapped push-pull configuration. No SMPS in there!

The waveform is sawtoothing because the capacitor charges rapidly near the voltage crest but discharges more slowly giving a steep rise and a slower decay. If you vary the load you will probably see the waveform change shape: as load increases the decay will be steeper with a deeper trough; the peak-peak ripple will be greater.


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