Re: Where are all the ESR meters?

On Jul 24, 4:05 am, Winfield <winfieldh...@xxxxxxxxx> wrote:
On Jul 23, 9:31 am, MooseFET <kensm...@xxxxxxxxx> wrote:



On Jul 22, 8:23 pm, ehsjr <eh...@xxxxxxxxxxxxxxxx> wrote:

Jeff L wrote:
"Winfield" <winfieldh...@xxxxxxxxx> wrote in message
news:1185103461.604452.53880@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Jim Thompson wrote:

Winfield wrote:

I'll post mine, when I get enough energy to transcribe
it from my paper scratchings, calculations and notes.
Remember, it must be four terminal, and handle high DC
voltages when probing in-circuit storage capacitors.

How high is "high"?

Perhaps a better question is, how big is big?

Several designs we've been considering have a
pair of diodes to discharge the test capacitor
and limit the circuit voltages, but I've heard
these can fail with large, charged capacitors.

I think the issue isn't necessarily how high
the voltage (tube amplifiers get to hundreds
of volts), or how high the current delivered,
but how much energy is going to be dissipated
in the protection components that discharge
the guilty capacitor.

I'd say the answer is, the size of two fists.
I think we're talking about ~ 100J of energy.
Isn't that more than enough to blow out a
common glass diode and/or a 1/4-watt resistor?

Protection is easy - read the voltage across the cap in question with a
comparator, if it is more then a few volts, have a indicator displayed to
discharge the cap first. If it is below a few volts, connect the cap to the
metering circuitry with a couple of low RDS on FET's.

Just use a spring return spdt toggle.
Toggle to test.

Ckt ---o
o<--o---- to DUT
|
Gnd ---+-------- to DUT

If the DUT is a large capacitor charged to several hundred volts, that
would have to be one heck of a switch.

I think it would be far better to make a current source and voltage
preamp that can both withstand several hundred volts.

Neither the current source nor the voltage preamp needs to be super
good they just have to not crosstalk. Assuming we have well regulated
+/- 12V supplies, the current source can be just a rail to rail output
driving a resistor. For a measurement current of 1mA, a 12K resistor
would be required. Assuming we have to withstand 500V, we need a
resistor that can take a 21W pulse.

Operating at 1mA results in about a 1uV measurement requirement. This
would let us use a resistor on the order of 100K on the input of the
preamp without getting swamped with noise.

Your numbers don't compute for me. I'd like a 100mA
signal if it worked out, but have accepted 10mA as a
compromise. 1mA seems very wimpy,

I accepted a 1mA to keep the power in the resistor within reason. If
I would like a bigger signal and to be rich and good looking. I don't
think I will get wither wish.

but assuming that,
your 12V suppiy (while a pain for batteries) doesn't
allow proper operation if there's very more than 12V
sitting on the cap.

"proper operation" in this case I defined as not going BANG not making
a measurement was not a problem to me in this situation.

And your 12k resistor would mean
a 1-min discharge time constant with say 5000uF, yawn,
waiting to get within the operating-voltage range.

So long as the meter tells you what is wrong, I will accept the job of
discharging the capacitor. It makes the meter design a lot simpler
not to have to deal with that issue.


I'm going for 25 ohms of discharge resistance, using
another set of diodes to +/-3V rails. Of course, the
rails must have a way to get rid of any excess energy.

I wouldn't put the energy onto the power rail and then try to figure
out how to get rid of it. It would be better to discharge to the
ground more or less directly


One side could look like this:


Vcc----/\/\----
! e PNP
Probe--\/\/--->!-------+--- ------GND
\ /
--- Perhaps TIP-36
!
Vcc

.