How to find ESR of cap?

[rainman1]

I'm not sure how to conclude the ESR of the capacitor from its spesifications.
what i did is:
R = 1 / [tan(delta) * 2PI * 120(Hz) * Cap(F)]
For a 350V rated, 330uF capacitor, it turned out that R = 16ohm, which is not reasonable.

By the way, voltage rating's units arent Vrms? why do they call it WVDC?

Thanks.

**broken link removed**

 

[Nigel Goodwin]

A capacitors specifications will list the ESR - you can't calculate it.

BTW - higher voltage capacitors often have pretty abysmal ESR.

 

[duffy]

Tsk, tsk, Nigel, such a smart guy and you don't know tangent of delta?

Spec sheets don't always give you an ESR because it is dependant on frequency. See that tangent of delta term up there? That's the ratio of ESR to Xc.

tan δ = ESR/Xc

ESR = Xc tan δ

 

[rainman1]

i had a mistake in calculation apparently.
that is exactly what i did.

 

[duffy]

Yeah, that's was the wrong formula, use mine. Should get 1 ohm... ugh, is this a homework problem? I'm always suspicious of magic numbers like that.

They call it WVDC because the capacitance changes (slightly) when it gets charged, so that's Working Voltage DC. It changes because the charge on the plates exert a compressive force on the dielectric.

 

[crutschow]

By the way, voltage rating's units aren't Vrms? why do they call it WVDC?

It's an electrolytic capacitor which is polarized. You can not apply AC to it (unless it's riding on a DC bias). Thus the voltage rating is given in working volts DC.

 

[rainman1]

Thanks guys.
I was looking also for inductors of above 100mohm, and that can induct 2Arms.
In each inductor's datasheet, i see only DC current limit, how come?

 

[rainman1]

What i meant is that i'd like to purchase an inductor that can induct 1.8Arms AC current, but i cant find in datasheet its AC current limit.

 

[crutschow]

What i meant is that i'd like to purchase an inductor that can induct 1.8Arms AC current, but i cant find in datasheet its AC current limit.

Normally inductors are used to filter DC current so they are rated for that. To prevent the inductor from saturating with the AC current peak, use the dc value as the peak allowed AC current value, i.e. maximum DC current rating equals 1.4 times the desired AC rms current, or 2.5Adc for your desired 1.8Arms.

 

[rainman1]

Thanks.
Does the saturaion of the inductor have any effect on it contibuting a -90C degrees to the total phase of the circuit?

 

[duffy]

Thanks.
Does the saturaion of the inductor have any effect on it contibuting a -90C degrees to the total phase of the circuit?

Don't use Celsius degrees for phase angle, use Fahrenheit.

 

[rainman1]

You are right
An inductors contributes Pi/2 radians to the total phase of the circuit.
An inductor reaching or not reaching to saturation has any influence of the fact that it contributes Pi/2 radians to circuit's phase?

 

[Nigel Goodwin]

Spec sheets don't always give you an ESR because it is dependant on frequency. See that tangent of delta term up there? That's the ratio of ESR to Xc.

While ESR as such is dependent on frequency, the specifcation isn't, because it's specified at a common frequency - 100KHz.

 

[rainman1]

I just cant design that circuit well
I'm trying to design a serial RLC circuit with a 235Vrms voltage source.
The circuit current should be 1.8Arms, and the resistor shouldn't be larger than 5-6 ohms becaues than its power consumption would exceed 20W and i wouldnt be able to find such a small tolerance resistor (or it would cost too much).
The circuit phase should be Pi/3 radians.
Is it possible?

 

[duffy]

While ESR as such is dependent on frequency, the specifcation isn't, because it's specified at a common frequency - 100KHz.

Which makes it useless unless you happen to be operating the cap at 100khz.

 

[crutschow]

You are right
An inductors contributes Pi/2 radians to the total phase of the circuit.
An inductor reaching or not reaching to saturation has any influence of the fact that it contributes Pi/2 radians to circuit's phase?

When the inductor starts to saturate its inductance is reduced. Thus the phase angle will change if you are depending upon a particular value of inductance and capacitance to determine the phase angle of the circuit.

 

[btcg]

Which makes it useless unless you happen to be operating the cap at 100khz.

I think Nigel's right.

Also: according to Mr Vette (who made the original ESR meter) readings at 100k and 50k are just as accurate.

 

[duffy]

I don't think you understand the problem. At OP's freq, 120Hz, the ESR is an ohm. At 100kHz, it's .0012 ohms. Big difference.

 

[crutschow]

The ESR (Equivalent Series Resistance) should be a minimum at low frequencies (as opposed to the capacitor reactance). It's caused by the intrinsic internal resistance in the cap.

 

[Nigel Goodwin]

I don't think you understand the problem. At OP's freq, 120Hz, the ESR is an ohm. At 100kHz, it's .0012 ohms. Big difference.

Would be if it was true - but it's not - ESR will be nowhere near as low as that.

As far as I know ESR at low frequencies would be lower than at 100KHz, certainly it's never mentioned or considered a problem at low frequencies.