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[SOLVED] (Back to basic) Initial condition of R-L circuit

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kudjung

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Hi Everyone,

This might be just a simple question.
When measuring voltage of a simple resistance serie with inductance(R--L) circuit. The scope was probed at the inductance.
When powering this circuit with DC power supply, voltage will rise rapidly almost to power supply voltage. With the setup I have, maximum voltage is about 80% of the power supply. And the risetime is about 100 nsec.

I wonder what is the parameter that control this rising time and maximum votage at initial startup. Is it scope probe or parasistic capacitance/resistance of the inductor?

Thanks,
KJ
 

FvM

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I guess, it's mainly the risetime of the power supply voltage.
 

kudjung

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Hi FvM,

I'm using a DC power supply and turing on/off by using a toggle switch in front of the circuit. Power supply has never turned off. So, it shouldn't be from the rise time of the power supply.

Thanks,
KJ
 

KerimF

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You mean a mechanical switch but with no glitches (after all, 100 nsec is too short even for any possible glitches).
But are you connecting a rather (ideal) inductor to a DC voltage source (supply)?! As you know, the steady DC current will be Vcc/R_series.
For instance, do you have an idea on the inductor value in Henry?
 

FvM

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You'll face serious difficulties to generate a clean edge with a mechanical switch, unless it's a mercury wetted contact. I would prefer to see the real switch output waveform before I assume, that it has "zero" risetime. For an exact analysis, all circuit parameters should be known, however.
 

kudjung

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Thanks all.
-I tried both mechanical switch and using a power supply lead to quickly touch the positive supply lead of the circuit. Using a power supply wire to touch the lead of the circuit seem to giving me more cleaner edge. Toggle switch give a lot of bouncing waveform.

My circuit under test is 4.7 Kohm serie with 4.7 mH inductor. Scope probe is placing across the inductor. Circuit is wired on a protoboard.

I think I've confirmed that this rising time is caused by a scope probe capacitance and contribute a little bit by capacitance of the protoboard. Scope probe I'm using has a label of 8-15 pf. Breadboard, especially the supply and ground connection seem to have a significant capacitance(comparing to the resistor I've used).


I've confirmed this by following method.

-Wire everything using soldering not in the protoboard.
-Remove inductor and connect the other end of 4.7 Kohm to ground to mesaure the power supply risetime(connect oscilloscope probe across 4.7Kohm resistor). This give risetime on the scope under 10 nsec with some ringing.
-I then just serie the 4.7 Kohm with the oscilloscope probe, this give rise time on the oscilloscope about 130 nsec.
-I then wire this back on a proto board(4.7Kohm serie with oscilloscope probe), this give rise time on the oscilloscope about 140 nsec.

The scope probe state to has capacitance about 15 pf(max), calculating time constant of this capacitor serie with 15 pf of the oscilloscope porbe give about 70 nsec.
Still a little bit off from the the measured 130 nsec measured above but this is probably the only reason that I can think of right now.

I didn't has lot of selection of probe but changing to different probe indeed give me different rise time.

Does this sound make sense?

I can't imagine such a simple thing can giving me trouble.

Thankyou all for your input.

KJ.
 

BradtheRad

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The voltage across an inductor is supposed to drop after power is connected. Don't know why it's rising in your case.

The induced EMF is first at maximum. Then it declines as the inductor gradually lets current increase.

The rate is in accord with the L/R time constant.

Can you apply the L/R time constant anywhere in your observations?

Does the rise time change when you use less series resistance?
 
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kudjung

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Hi BradtheRad,

Votage at the inductor is gradaully decay as it suppose to be. Before that, there is a short interval of rise time of the signal before it start to decay. This rising time was puzzled me. As mentioned, My theroy right now is the rise time of the signal seem to be from the capcitance on the scope probe.

Lower resistance also provide lesser rise time.

---------- Post added at 09:47 ---------- Previous post was at 09:27 ----------

Forgot to mentioned that I've also measured the time constant with different resistor instead of using oscilloscope to calculate the rise time and it seem to prove the theory.

With 1 Kohm resistor, I measured about 15.4 ns, this give about 15.4 pf cap.
With 4.7Kohm resistor, I measured about 69 ns, this give about 14.7pf cap.

Reading the probe label, it said 15 pf max which look about right.
 

ninju

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What oscilloscope did you use? Tell the R and C that are specified in the front of it, also, tell the kind of probe (10:1 , 1:1 , like that) used. Sometimes, due to high impedance, the voltage on the oscilloscope can be more than twice the actual voltage.
 

FvM

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The scope probe state to has capacitance about 15 pf(max), calculating time constant of this capacitor serie with 15 pf of the oscilloscope porbe give about 70 nsec.
Still a little bit off from the the measured 130 nsec measured above but this is probably the only reason that I can think of right now.

The numbers sound more reasonable than I expected from your initial post. Simply assume some parallel capacitance of the inductor, and you are done. You would need to consult the inductor data sheet and calculate the capacitance from a resonance frequency specification, if given at all.
 

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