RE: Bleeder (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 22 Oct 2007 13:31:01 +0000
From: "Lau, Gary" <Gary.Lau@xxxxxx>
To: 'Tesla list' <tesla@xxxxxxxxxx>
Subject: RE: Bleeder (fwd)

Resending, not sure it made to the List the first time.

-----Original Message-----
From: Lau, Gary
Sent: Saturday, October 20, 2007 8:56 PM
To: 'Tesla list'
Subject: RE: Bleeder (fwd)

Hi Bart:

Sorry, but I must disagree.  Bleeder is the function these resistors serve.  They do absolutely nothing towards equalizing the voltage between capacitors in the string in the application that we use them for.

To sanity check this, I dusted off Microsim and simulated a circuit of ten series caps, each in parallel with a resistor, and powered the string with a 20kV 60 Hz sine wave.

In the first case, all resistors were 10Meg, 9 caps were 0.1uF, and one was 0.2uF.  The voltage across the 0.1uF cap was 2.1kV, the voltage across the 0.2uF cap was 1.05kV.  The voltage was not equalized despite identical resistors.

In the second case, all 10 caps were 0.1 uF.  9 of the resistors were 10Meg, the 10th was 20Meg.  The voltage was a 2.0kV sine wave across each of the caps, regardless of the resistor value.  Having dissimilar but high-value "equalizing" resistors did not affect the division.

The AC impedance of the caps at 60Hz is far lower than the resistor impedance, so the resistors do nothing as far as establishing the voltage division ratio.  The voltage division is established solely by the ratios of capacitance values.  If DC was used, that would be a different story.

I didn't follow your discussion of how a cap's ESR value relates to the proper bleeder resistor value.  The ESR (Equivalent Series Resistance) value of a cap is the value of resistance that if put in Series (not parallel) with a "perfect" capacitor, would behave the same as the actual physical cap.  The actual ESR value of our caps is probably in the milliohm range.  I'm not sure how a series resistance variation of a fraction of an Ohm would affect voltage division in the string, or bleeder-discharge rate.  The value for the equivalent PARALLEL resistance of these caps is so high that it really doesn't matter, many times higher than the bleeders we would use.  The spec for MMKP376 caps (I didn't have the CD cap spec handy, but this is a similar PP pulse cap) just states >100,000Meg.

I advocate the use of bleeder resistors for 2 reasons.  "Some" capacitor types will develop an asymmetrical, residual charge on the caps in a series string, despite being matched in value.  The cause of this is unknown but is suspected to be related to coronal rectification.  CD caps did not exhibit this behavior when I tested them, but as the underlying mechanism is not understood, it's best to try to dissipate this charge with a simple bleeder rather than hope it doesn't happen to your caps.

But mostly I use bleeders because I'm human and I will eventually fail to short my cap's terminals before touching it.  The likelihood of my bleeder failing open is much, much lower than the likelihood that I'll not bother to short the cap before touching it.  I'd be interested to learn the particulars of how you got zapped due to a failed bleeder resistor.

Regards, Gary Lau
MA, USA


> From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: Bleeder (fwd)
>
> MMC, 1/2W, 10M ohms (if ESR is similar to common CD capacitors). Mount
> directly across each cap if you have enough caps in series to minimize
> the voltage across each cap. If only a few MMC's in series (less than
> 10), it may prudent to use a couple 5M resistors in series (to increase
> arc voltage across the resistors).
>
> As far as "bleeder", maybe not the correct term in this case. Equalizing
> resistors would be a better term although bleeding also occurs (but the
> time duration is long, so don't think of it in terms of a "bleeder").
> The transformer does the real bleeding. Disconnect the transformer from
> the charged cap at the right time and the bleed will be very slow. You
> can get zapped in that situation. A bleeder is designed to bleed down a
> charge in some amount of time, but bleeders can fail open. NEVER rely on
> bleeding as a safety device. Been there, done that, got ZAPPED!
>
> Why 10M ohms? Due to common CD caps ESR. This value provides a low loss
> resistive approach to equalizing the voltage across the string without
> waisting a lot of power across the resistor. But, if you use a cap with
> a vastly different ESR value, you will need to adjust the equalizing
> resistor for the same efficiency.
>
> BTW, the cap will work without the resistors. Are you more likely to
> kill a cap? Of course you are. Will it actually kill the cap? Depends on
> the caps ESR and the number of caps in series. The equalizing resistors
> are simply a low cost safety device for the caps themselves. Their not a
> human safety device.
>
> Imagine 10 resistors in series and 10,000V across the series string.
> Obviously your going measure 1000V or very near across each resistor.
> That is the basic idea of equalizing the voltage across each cap. To
> keep the voltage drop across each one as "similar" to the next as
> possible. Although any value resistor will do this, it is important to
> get the value high enough so that power is not waisted in the resistor
> itself. The ESR of each cap plays a role in that voltage drop because it
> is in parallel with each resistor. We arrive at 10 Meg resistors to
> achieve this situation with Cornells common MMC capacitors. If a
> different capacitor is used, there might be a better resistor value to
> optimize the string.
>
> Take care,
> Bart
>
>
>
> > From: Anthony R. Mollner <penny831@xxxxxxxxxxxxx>
> > To: Tesla list <tesla@xxxxxxxxxx>
> > Subject: Bleeder
> >
> > I'd like to ask a question and find out what the common consensus is
> > regarding bleeder resistors. What does the group recommend for bleeder
> > resistors on an MMC bank? 1/2 watt, 5 watt, 1 meg, 10 meg etc.??
> > Tony