Electrolytic capacitor ESR at Zero Hertz.....i.e. DC?

[eem2am]

Hello , this capacitor's ESR is 0.141R at 120Hz.
(Rubycon TLV series , 1.5mF, 16V)

-but do you know what is its ESR at DC (Zero Hz) ?

Datasheet on web:-
https://www.rubycon.co.jp/cn/catalog/c_pdfs/aluminum/c_TLV.pdf

 

[keith1200rs]

A capacitor cannot have an ESR at DC. If the voltage isn't changing there is no current - that capacitor isn't doing anything.

Keith.

 

[eem2am]

the reason for asking is that this capacitor will be charged up and then used to actuate a latching solenoid valve by discharging it through the coil.

...so its essentially DC when its discharging.

But i fear that the Zero Hz ESR may be well high and poorly toleranced from part to part (or batch to batch?)

 

[keith1200rs]

The ESR will be very small, but the instantaneous current rise will probably be limited by the coil inductance. When you connect the capacitor to the coil I am sure ESR will not be an issue. It is not DC any more at that time.

Keith.

 

[eem2am1]

Sorry Keith but i must disagree.

An Electrolytic cap only has low esr when it has high frequency ripple...............if it's just discharging , (current in one direction only) then this is going to mean the Electrolytic capacitor has very high ESR.

-you will need to check the esr is not so great that it reduces the current required to latch the solenoid.

-i do not know how you will find out the esr value you require because datasheets dont quote the zero hertz esr.....they only quote it at 120hz typically.

 

[alexan_e]

Sorry Keith but i must disagree.

An Electrolytic cap only has low esr when it has high frequency ripple...............if it's just discharging , (current in one direction only) then this is going to mean the Electrolytic capacitor has very high ESR.

-you will need to check the esr is not so great that it reduces the current required to latch the solenoid.

-i do not know how you will find out the esr value you require because datasheets dont quote the zero hertz esr.....they only quote it at 120hz typically.

It is kind of funny that you answer to your own thread (opened with your old identity) and you say that you disagree with the person that answered your thread,
I have no idea which answer is correct but I assume you have solved your circuit problem .

Alex

 

[chuckey]

I have done this sort of thing in the past and the limiting value is the current X time to operate the solenoid versus the voltage V time of the capacitor. I used a 50mF to operate a rotary wave guide switch (28V rating @4A) from a 24V PSU, 100% reliable for at least 5 years! I would guess that the ESR must be > 10% of the resistance of the solenoid before you get worried.
Frank

 

[FvM]

You may want to tell about the time constant of the said capacitor-coil (LRC) circuit, but according to the switching speed of most coils, I won't expect more than a few ten or maximum one hundred ms. So obviously, the involved frequency range is much above "zero" hertz. Actually, it isn't too far apart from usually specified ESR frequency range.

Of course, it's possible to determine a capacitors complex impedance (including ESR) over a much wider frequency range, from µHz or mHz up to MHz. Based on these measurement, you can design equivalent circuits and predict their behaviour very exactly. But I doubt, that you need it for your application.

 

[Orson Cart]

The ESR of a standard or good quality aluminium electrolytic cap is quite constant at low frequencies, the 120Hz ESR value will essentially be the same for 120Hz to 120uHz, it will change more due to temp than freq at these frequencies. Thus if you wish to compute IR volt drop when discharging (or charging) the 120Hz value will be fine, Regards, Orson Cart.

 

[FvM]

The ESR of a standard or good quality aluminium electrolytic cap is quite constant at low frequencies.

Did you measure it? The usual manufacturer specification doesn't allow to determine the behaviour. Constant ESR would refer to a simple LRC equivalent circuit of the capacitor. The loss factor graphs, as far as shown by manufacturers suggest, that the loss factor ~f slope is at least approaching a constant value a low frequencies, which would in fact involve an increasing ESR, somewhere between constant and ~ 1/f, and a RC ladder in the equivalent circuit.

 

[Orson Cart]

Yes we measure the intrinsic resistance of the capacitors, and find that the temp makes far more difference than the frequency - below 100Hz, Regards, Orson Cart.

 

[eem2am]

Thankyou for all these replys, and of course, any more greatly appreciated.

On another point, chuckey, you kindly described that you used a 50mF capacitor at 24V.

I have also emailed yourself about this, as i would be most grateful to know how you charged up the 50mF?, especially if it was charged up by an isolated offline PSU....because just about every pesky offline PWM controller these days comes with built in "auto-restart", which shuts down the chip before such big secondary caps get charged up.
(basically , the modern PWM chips "think" that the big secondary cap is an overload on the secondary, and so shut themselves down)

 

[FvM]

every pesky offline PWM controller these days comes with built in "auto-restart", which shuts down the chip before such big secondary caps get charged up.

It's up to you either to implement true current limit operation on the secondary, or setup the primary peak current to a reasonable level without triggering a shut-down.

 

[chuckey]

eem2am, The problem was that our telemetry was controlling a satellite uplink, the unit looked at all the alarms and sent them back to the control centre on a RS232 link on which we received controls to force the link to change over to reserve etc. The supplied changeover unit had this big rotary wave guide switch in it that was rated at 28V 4A, and our power supply was rated at 24V 1 A. We could have bought and fitted another higher power power supply, but that might have led to more problems. So I simply put the 50mF across the switch with a set of relay contacts in circuit. To charge the cap I used a 20 ohm resistor from the 1 A supply. So the switch worked great but there was 3 second delay (while the cap was charging) before it would (could?) operate again. As once the "main" amp had failed and we switched to the "reserve", there would be little point in being able to immediately switch back the the broken "main" amp. However that option was there after the three seconds.
In testing there was a confusing side effect, after we switched the telemetry unit off, it actually continued to operate for 30 seconds or so due to the big cap back feeding its supply line via the 20 ohm resistor.
Frank

 

[eem2am]

Thanks chuckey,

I think that is maybe how the shower company i worked for should have done it.

They have 30mF on the secondary rail, for closing the water valves in the case of mains failure.