capacitor

"ivan" wrote in news:eol84r$slt$ snipped-for-privacy@news.flashnet.it:

Why do you keep ignoring the ESR? With only 22 ohms from other resistance, the ESR of the cap (which doesn't have to be in any way related to the voltage it can stand) will have a significant effect on the Q of a tuned circuit. I've said it at least twoce, and I think others have mentioned it, so I agree that you're barking up the wrong tree if you continue to ignore ESR.

Whoa, do you have that RLC triad hanging off in a vacuum?

Most circuits are hooked up to other things which will affect the Q.

And are you tuning the frequency for a peak?

If you give us the whole circuit diagram we'll be able to make more informed judgments.

"Lostgallifreyan" ha scritto nel messaggio news:Xns98BBA17DC9A34zoodlewurdle@140.99.99.130...

it,

I have short circuited R and saw an increase of 15Vpp. By changing the cap type I see from worst (50Vpp) to best (120Vpp). Would ESR variance be high enough? What are the typical values of ESR? Unfortunately I do not have the means to do a proper study on this here...

You mean C=2.20 nF ?

What happens when C = 2.22 nF or 2.18 nF ? The resonant frequency will shift and the current and voltage will no longer 'peak' in the same way as with 2.20 nF ( assuming it's set up for that exactly ).

Bear in mind that the average L can't be trusted to be 737.00 uH either btw !

With a Q of ~ 26 you most certainly aren't looking at capacitor dielectric loss issues.

You should do as another poster suggested and add a small trimmer cap in parallel with your C and tune for resonance. That may open your eyes as to what's going on.

Graham

Ah! Finally, some numbers! The 2n2 is very likely 2.2 nF, or .0022 uF, or 2200 pF (which are just different ways of writing the same thing). And

100V sounds about right - is it about the size of your thumbnail?

Cheers! Rich

and

But remember to use a non-magnetic screwdriver ;)

Cheers

PeteS

Values for capacitor ESR vary wildly depending on the device material; it also varies with applied voltage (more for some than others), applied signal frequency and rms current.

ESR values in excess of 10 ohms are not unusual for certain materials, but at a mere 125kHz I wouldn't expect it to be quite that high - however, it could easily be a couple of ohms - a matter of 10% in your case.

Indeed, the ESR of a cap can vary wildly for one cap with different stimuli.

Cheers

PeteS

John Fields wrote in news: snipped-for-privacy@4ax.com:

That is so cool, I've never used anything like that before. I like it.

Re linkage, putting LTSPICE into Google gives the right link at top of results.

--- Earlier you stated that your circuit looked something like this: (View in Courier)

Er / +----[7.37E-4H]--+--[2.2E-9F]--+--[22R]----+ | | [GEN1] [GEN2] | | GND GND

Where GEN1 and GEN2 are square-wave sources with their outputs 180 degrees out of phase and Er is the voltage measured, to ground, from the junction of the capacitor and the resistor.

From the values you've given, the circuit is resonant at:

1 1 f = -------------- = ------------------------------------ 2pi sqrt(LC) 6.28 * sqrt (7.37E-4H) * (2.2E-9F)

~ 125kHz.

and the reactances of the capacitor and inductor will be:

Xc = Xl = 2pi fL = 6.28 * 1.25E5Hz * 7.37E-4H ~ 578 ohms

Now, since the signs of the reactances are opposite to each other, at resonance they'll cancel and you'll be left with only the resistance of the 22 ohm resistor, the resistance of the wire in the inductor and the equivalent series resistance (ESR) of the capacitor to limit the current through the circuit. Just for grins, if we assume an ohm for each, the total series resistance in the circuit will be 24 ohms.

Now, in order to determine the current in the circuit we'll need to determine its impedance, thus:

Z = sqrt (R² + (Xl - Xc)²)

= sqrt (24² + (578 - 578)²)

= 24 ohms

Assuming you're driving your circuit with 5V sources, the maximum current you'll be able to pump through the 24 ohms will be:

E 5V I = --- = ----- = 0.208 ampere R 24R

but because the reactance of the capacitor is 578 ohms, the voltage dropped across it will be a startling:

E = IR = 0.208A * 578R ~ 120V

Now, assuming that your generators can deliver the current and you short out the 22 ohm resistor, the reactances will still cancel and we'll be left with 2 ohms of resistance to oppose the current. That means the current in the circuit will be:

E 5V I = --- = ---- = 2.5 ampere R 2R

and since the reactances haven't changed, the voltage dropped across the capacitor will be:

E = IR = 2.5A * 578R = 1445V.

So, it looks like just shorting out the resistor will get you a voltage increase of:

1445 ------ = 12 = 1200% 120

There are also some second order effects like the inductance of the inductor changing as the current (and frequency) through it changes, and the parametric effects of dielectrics causing capacitance change with voltage which will spoil resonance.

Of course there are always the primary effects, which in your case involve the capacitance tolerances of the capacitors you're using and their ESR's (losses) which could easily explain the results you've been getting.

Here's an LTSPICE circuit list of your circuit which you may find interesting:

Version 4 SHEET 1 880 680 WIRE -288 144 -352 144 WIRE -160 144 -208 144 WIRE -48 144 -96 144 WIRE 80 144 32 144 WIRE -352 240 -352 144 WIRE 80 240 80 144 WIRE -352 384 -352 320 WIRE 80 384 80 320 WIRE 80 384 -352 384 WIRE -352 416 -352 384 FLAG -352 416 0 SYMBOL res 48 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 22 SYMBOL cap -96 128 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 2.2e-9 SYMATTR SpiceLine Rser=1 SYMBOL ind -304 160 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L1 SYMATTR Value 737e-6 SYMATTR SpiceLine Rser=1 SYMBOL voltage -352 224 R0 WINDOW 3 24 44 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value PULSE(0 5 0 0 0 3.998e-6 7.996e-6) SYMBOL voltage 80 224 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(5 0 0 0 0 3.998e-6 7.996e-6) TEXT -386 506 Left 0 !.tran 1e-3 uic

If you don't have LTSPICE you can download it, free, from Linear Technology's web site. Sorry, I can't get the link right now.

-- JF

Firstly you need to match the caps to better than +/- 1% to avoid capacitance value being a contributing factor.

Graham

that should be from EPCOS capcaitors,

check the following link

stalin

ivan wrote:

AND he better tell us what kind of cap meter he's using. Many $4.99 DVM's from the corner gas station have a cap measuring feature, but they are actually measuring how quickly it oscillates in a RC oscillator. Said frequency is only vaguely related to the actual capacitance, especially for lossy or leaky capacitors.

"John Fields" ha scritto nel messaggio news: snipped-for-privacy@4ax.com...

cap

high

the

ok I understand, It looks like this capacitor has a 2R series resistance then. and the other which swings at 80V has a series resistance of 14.125R as:

80V / 578R ~ 0.138A

5V / 0.138A = 36.125R

36.125R - 22R = 14.125R

yeah I see what you are saying now...

I get about 160Vpp here actually...

for those who asked, the complete circuit is roughly as shown at

in Figure 4-1. The signal and the resonant branch I refer to are COIL1 and COIL2...

ok I'll try this...thanks!

"Ancient_Hacker" ha scritto nel messaggio news: snipped-for-privacy@q2g2000cwa.googlegroups.com...

capacitance

it is a fluke 185, it has three digits after comma when measuring nF...

I changed the resistor with a 18R so I would expect 144.5V swing, but I get only a few volts increase... why?

ivan

--- Dunno...

You might be off resonance, your cap may have a higher ESR than you think it does, the coil's resistance might be higher than you think it is, and/or the impedance of your scope probe might be loading down the circuit.

Also, take a look at the spec's for the coil drivers in the U2270.

They're rated for an absolute maximum of 200mA out, but rated for a typical output voltage of 4V with a 100 mA load, so you'll be running them pretty much balls-to-the wall with that 22 ohm resistor in there, (neglecting the real part of the LC impedance) and certainly you'll be bottoming them out when you short out the resistor!

-- JF

"John Fields" ha scritto nel messaggio news: snipped-for-privacy@4ax.com...

get

60-70Vpp is safe enough then...thanks!

Nice meter, BUT:

Most meters have better accuracy on voltage ranges than on cap ranges. So all those digits on the display don't mean the cap readings are that accurate.

The manual doesnt seem to mention exactly HOW it measures capacitance. If it uses a RC oscillator, as most of them do, rather than an actual bridge, all bets are off. It's not measuring capacitance so much as charge transfer rates. If you put a battery or resistor or inductor across the terminals, it will *still* read out a capacitance value. That should be a clue it's not exactly the world's best cap meter.

The manual states the accuracy isnt so good on the cap ranges, like plus or minus one percent plus or minus 5 counts.

If you really want to investigate the properties of capacitors, get yourself a real cap meter, one that can show actual capacitance and dissipation factor. The better ones can show the characteristics to like four decimal places. A good old HP 4xxx goes for not very many bucks, has neat nixie-tube readouts, and won't fool you.

Bye. Jasen