Re: ESR Meter - Roll your own - ESRrev0.JPG

[Mike Monett]

John Larkin wrote:

This was built along similar lines. The synchronous rectification
is not perfect due to phase shift in the forward signal path but
the arrangement is useful for reducing the annoying capacitive
quadrature component from Q's up to about 3 (at the 100kHz). It's
usable down to the milliohm area and proved the (cheap) 1000 off
10uF cap's I bought, had an ESR of between 1 and 3 ohms!.

Hi John,

Do you mind if I make some comments? The 1 ohm resistor, R5, can be
removed from the circuit. It is at a virtual ground node and has
little or no voltage across it. So removing it has little effect on
the circuit.

With R5 gone, and the ESR range switch in the 1 ohm position, the
180 ohm resistor, R4, is effectively in series with the capacitor
under test. The op amp merely changes the location of the ground
node, and inverts the output signal polarity.

Since R4 (180 ohms) is now in series with the capacitor, it
completely swamps out the internal ESR (0.05 ohms.) This means the
series combination of C, L, and R has negligible "Q", and there is
no quadrature or orthogonal component in the circuit.

However, as the ESR decreases, the corresponding voltage drop that
we are trying to measure also decreases. We no are faced with the
problem that the di/dt from the inductor is much larger than the I*R
drop from the ESR. This means the leading and trailing edge of the
square wave have large spikes.

If you used a diode peak detector to measure the amplitude, it would
respond as best it could to the leading edge spike, which would make
it impossible to measure the drop across the ESR. Your circuit and
Larkin's share this problem.

Using a synchronous rectifier helps a bit, but you are now faced
with trying to turn it on after the leading edge spike, and to turn
it off before the trailing edge spike. That could be tricky.

I spent this afternoon looking at these problems in SPICE, and have
come to an amazing observation. There is a hidden but very
significant feature in the bridge ESR circuit referred to at the
beginning of this thread. The links are:

1. Talino Tribuzio's page, showing the circuit from Nuova
Elettronica:

http://www.qsl.net/iz7ath/web/02_brew/15_lab/06_esr/index.htm

2. Gintaras' web page, who refers to Tribuzio's page in his readme

http://alytus.auksa.lt/esr/

The schematic is at

http://alytus.auksa.lt/esr/esr_meter_schematic.pdf

The valuable hidden feature is the bridge configuration completely
eliminates the leading and trailing edge spikes due to the capacitor
internal inductance. Since the spikes are in phase with the square
wave signal on the other side of the bridge, they simply disappear
at the output of the op amp!

This means the peak detector has a very clean square wave to work
with, and it can give a much more accurate measurement of the
signal. There is no tricky timing to fiddle with that can go out of
whack just when you need to use the tester.

If you like, I can post the analysis of your circuit and Larkin's
version showing the huge spikes that appear as the ESR becomes
smaller, and the triangular wave from the capacitance charging and
discharging. I don't really see a good way of overcoming these
problems.

As I mentioned in previous posts, the bridge circuit has significant
advantages, including low test voltage, in-circuit test, shorted
capacitor detect, etc. With the extremely clean output signal into
the peak detector, it becomes the obvious choice for hassle-free ESR
measurements.

I'll post the LTspice ASC file here along with the PLT file so you
can see how it works. I changed the bridge resistance to lower
values to allow measuring lower values of ESR.

Here's the LTspice ASC file:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Version 4
SHEET 1 880 708
WIRE -496 -112 -528 -112
WIRE -368 -112 -416 -112
WIRE -304 -112 -368 -112
WIRE -192 -112 -224 -112
WIRE -368 -96 -368 -112
WIRE -128 -80 -160 -80
WIRE -48 -64 -64 -64
WIRE -720 -48 -816 -48
WIRE -704 -48 -720 -48
WIRE -592 -48 -624 -48
WIRE -528 -48 -528 -112
WIRE -528 -48 -592 -48
WIRE -368 -48 -528 -48
WIRE -192 -48 -192 -112
WIRE -192 -48 -288 -48
WIRE -128 -48 -192 -48
WIRE -816 -32 -816 -48
WIRE -160 0 -160 -80
WIRE -160 0 -240 0
WIRE 128 16 96 16
WIRE 224 32 192 32
WIRE 240 32 224 32
WIRE 336 32 304 32
WIRE -528 48 -592 48
WIRE -240 48 -240 0
WIRE -240 48 -448 48
WIRE -224 48 -240 48
WIRE -96 48 -144 48
WIRE -48 48 -48 -64
WIRE -48 48 -96 48
WIRE 32 48 16 48
WIRE 128 48 32 48
WIRE -816 64 -816 48
WIRE 32 128 32 48
WIRE -720 144 -720 -48
WIRE -704 144 -720 144
WIRE -592 144 -592 48
WIRE -592 144 -624 144
WIRE -512 144 -592 144
WIRE -416 144 -448 144
WIRE -304 144 -336 144
WIRE -192 144 -224 144
WIRE -592 160 -592 144
WIRE -192 160 -192 144
WIRE 96 176 96 16
WIRE 208 176 96 176
WIRE 336 176 336 32
WIRE 336 176 208 176
WIRE 208 192 208 176
WIRE 336 208 336 176
WIRE 32 224 32 208
WIRE -592 256 -592 240
WIRE -464 272 -480 272
WIRE -432 272 -464 272
WIRE -272 272 -288 272
WIRE -240 272 -272 272
WIRE -480 288 -480 272
WIRE -288 288 -288 272
WIRE 208 288 208 272
WIRE 336 288 336 272
WIRE -480 384 -480 368
WIRE -288 384 -288 368
FLAG -96 48 DIFF
FLAG -592 256 0
FLAG -192 160 0
FLAG -816 64 0
FLAG -368 -96 0
FLAG -592 -48 E2P
FLAG -592 48 E2N
FLAG 32 224 0
FLAG 336 288 0
FLAG 208 288 0
FLAG 336 32 DC
FLAG -480 384 0
FLAG -464 272 VCC
FLAG -288 384 0
FLAG -272 272 VEE
FLAG 224 32 VOP
FLAG 32 48 VIN
FLAG 160 0 VCC
FLAG 160 64 VEE
FLAG -96 -96 VCC
FLAG -96 -32 VEE
SYMBOL res -608 144 R0
SYMATTR InstName R8
SYMATTR Value {Rb}
SYMBOL res -608 128 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R9
SYMATTR Value {Rt}
SYMBOL cap -448 128 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C3
SYMATTR Value {C}
SYMBOL res -320 128 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R10
SYMATTR Value {ERS}
SYMBOL ind -320 160 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L3
SYMATTR Value {L}
SYMBOL Voltage -816 -48 R0
WINDOW 0 42 44 Left 0
WINDOW 3 -22 -62 Left 0
WINDOW 123 15 130 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(0 4 0 {Tr} {Tr} 5u 10u)
SYMBOL res -608 -64 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5
SYMATTR Value {Rt}
SYMBOL res -512 -96 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName R11
SYMATTR Value {Rb}
SYMBOL res -320 -96 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName R12
SYMATTR Value 47k
SYMBOL res -384 -32 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName R13
SYMATTR Value 1k
SYMBOL res -544 64 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName R14
SYMATTR Value 1k
SYMBOL res -240 64 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName R15
SYMATTR Value 47k
SYMBOL cap 16 32 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C4
SYMATTR Value 2n
SYMBOL res 16 112 R0
SYMATTR InstName R16
SYMATTR Value 47k
SYMBOL diode 240 48 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL cap 320 208 R0
SYMATTR InstName C5
SYMATTR Value 2nf
SYMBOL res 192 176 R0
SYMATTR InstName R17
SYMATTR Value 470k
SYMBOL Opamps\\1pole 160 32 R0
SYMATTR InstName U1
SYMATTR Value2 Avol=1Meg GBW=100Meg Slew=100Meg
SYMBOL Voltage -480 272 R0
WINDOW 0 42 44 Left 0
WINDOW 3 47 72 Left 0
WINDOW 123 15 130 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 10
SYMBOL Voltage -288 384 R180
WINDOW 0 42 44 Left 0
WINDOW 3 47 72 Left 0
WINDOW 123 15 130 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 10
SYMBOL Opamps\\1pole -96 -64 R0
SYMATTR InstName U2
SYMATTR Value2 Avol=1Meg GBW=100Meg Slew=100Meg
TEXT -528 -224 Left 0 ;'Tribuzio Bridge ESR Circuit
TEXT -528 -184 Left 0 !.tran 0 2.2m 2m 100n
TEXT 32 -200 Left 0 !.param C = 100uF\n.param L = 2.533E-08\n.param ERS =
0.00005\n.param Rt = 100\n.param Rb = 1\n.param Tr = 100n

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here's the PLT file:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[Transient Analysis]
{
Npanes: 3
Active Pane: 1
{
traces: 1 {268959746,0,"V(dc)"}
X: ('µ',0,0,2e-005,0.0002)
Y[0]: (' ',3,0,0.003,1)
Y[1]: ('_',0,1e+308,0,-1e+308)
Volts: (' ',0,0,0,0,0.003,1)
Log: 0 0 0
GridStyle: 1
},
{
traces: 1 {268959747,0,"V(diff)"}
X: ('µ',0,0,2e-005,0.0002)
Y[0]: (' ',1,-1,0.2,1)
Y[1]: ('_',0,1e+308,0,-1e+308)
Volts: (' ',0,0,2,-1,0.2,1)
Log: 0 0 0
GridStyle: 1
},
{
traces: 1 {268959748,0,"V(e2n)"}
X: ('µ',0,0,2e-005,0.0002)
Y[0]: ('m',0,0,0.002,0.04)
Y[1]: ('_',0,1e+308,0,-1e+308)
Volts: ('m',0,0,0,0,0.002,0.04)
Log: 0 0 0
GridStyle: 1
}
}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Regards,

Mike Monett

 

[Mike Monett]

To complete the record, here is the LTspice files for John J. and
John L.'s ESR measuring circuits. The capacitor parameters are
chosen to resonate at 100KHz, which is also the test frequency.

As can be seen in the transient analysis, there is no evidence of
orthogonal or quadrature components in the output signal.

The reason is the external circuit resistance completely swamps the
internal ESR. The external circuit determines the current through
the series network, so there is no energy transferred back and forth
between the capacitive and inductive components, thus no phase shift
between them.

The output signal is simply the di/dt from the ESL, I*dt from the
capacitor, and I*ESR, which is what we are trying to measure. Both
approaches give almost identical results.

You can change the component values in the .param list. As can be
seen, the inductive spike is very sensitive to risetime. Trying to
measure ESR below about 50 milliohms becomes very problematic with
these approaches.

As shown in the parent post, the bridge approach removes the
inductive spike, but it leaves the capacitor charging ramp. So it
also begins to fail below about 50 milliohms ESR.

However, the bridge approach allows in-circuit testing,
automatically detects shorted capacitors, and is insensitive to the
polarity of the capacitor.

Here is the LTspice ASC file:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Version 4
SHEET 1 880 708
WIRE -64 -48 -592 -48
WIRE -32 -48 -64 -48
WIRE -736 16 -800 16
WIRE -592 16 -592 -48
WIRE -592 16 -656 16
WIRE -544 16 -592 16
WIRE -448 16 -480 16
WIRE -336 16 -368 16
WIRE -224 16 -256 16
WIRE -800 32 -800 16
WIRE -224 32 -224 16
WIRE -800 128 -800 112
WIRE -688 304 -800 304
WIRE -576 304 -608 304
WIRE -544 304 -576 304
WIRE -512 304 -544 304
WIRE -496 304 -512 304
WIRE -400 304 -432 304
WIRE -288 304 -320 304
WIRE -176 304 -208 304
WIRE -64 304 -176 304
WIRE -32 304 -64 304
WIRE -576 320 -576 304
WIRE -512 384 -512 304
WIRE -480 384 -512 384
WIRE -352 384 -400 384
WIRE -176 384 -176 304
WIRE -176 384 -352 384
WIRE -800 400 -800 384
WIRE -352 400 -352 384
WIRE -576 416 -576 400
WIRE -400 416 -416 416
WIRE -416 432 -416 416
WIRE -512 464 -512 384
WIRE -400 464 -512 464
WIRE -352 496 -352 480
FLAG -800 400 0
FLAG -64 304 Jardine
FLAG -576 416 0
FLAG -416 432 0
FLAG -352 496 0
FLAG -64 -48 Larkin
FLAG -224 32 0
FLAG -800 128 0
FLAG -544 304 Vin
SYMBOL res -592 304 R0
SYMATTR InstName R2
SYMATTR Value 1e6
SYMBOL res -592 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R3
SYMATTR Value 180
SYMBOL Voltage -800 288 R0
WINDOW 0 42 44 Left 0
WINDOW 3 -41 151 Left 0
WINDOW 123 15 130 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(4 -4 0 {Tr} {Tr} 5u 10u)
SYMBOL cap -432 288 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value {C}
SYMBOL res -304 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value {ESR}
SYMBOL ind -304 320 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L1
SYMATTR Value {L}
SYMBOL E -352 384 R0
WINDOW 0 38 42 Left 0
WINDOW 3 36 69 Left 0
SYMATTR InstName E1
SYMATTR Value 1e5
SYMBOL res -384 368 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 100
SYMBOL res -640 0 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R6
SYMATTR Value 180
SYMBOL cap -480 0 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value {C}
SYMBOL res -352 0 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R7
SYMATTR Value {ESR}
SYMBOL ind -352 32 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L2
SYMATTR Value {L}
SYMBOL Voltage -800 16 R0
WINDOW 0 42 44 Left 0
WINDOW 3 21 103 Left 0
WINDOW 123 15 130 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(0 4 0 {Tr} {Tr} 5u 10u)
TEXT -528 -208 Left 0 ;'ESR Measuring Circuits
TEXT -856 -152 Left 0 !.param C = 100uF\n.param L = 2.5330295910584E-
08\n.param ESR = 0.05\n.param Tr = 100n
TEXT -816 192 Left 0 ;NOTE:\nR2 was 1 ohm in the original. It is at
virtual ground and has little effect on the circuit. \nR4 was 56k in the
original. Reduced to 100 ohm to reduce settling time for transient
analysis.
TEXT -856 -184 Left 0 !.tran 0 100.1m 100m 250n

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here is the PLT file:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[Transient Analysis]
{
Npanes: 2
{
traces: 1 {524292,0,"V(jardine)"}
X: ('µ',0,0,1e-005,9.99998048950707e-005)
Y[0]: ('m',0,0.03,0.002,0.058)
Y[1]: ('_',0,1e+308,0,-1e+308)
Volts: ('m',0,0,0,0.03,0.002,0.058)
Log: 0 0 0
GridStyle: 1
},
{
traces: 1 {524290,0,"V(larkin)"}
X: ('µ',0,0,1e-005,9.99998048950707e-005)
Y[0]: (' ',3,2.025,0.001,2.039)
Y[1]: ('_',0,1e+308,0,-1e+308)
Volts: (' ',0,0,3,2.025,0.001,2.039)
Log: 0 0 0
GridStyle: 1
}
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Regards,

Mike Monett

 

[john jardine]

Hi John,

Do you mind if I make some comments?

[my pleasure Mike :] The 1 ohm resistor, R5, can be

removed from the circuit. It is at a virtual ground node and has
little or no voltage across it. So removing it has little effect on
the circuit.

Can't lose the 1 ohm as the other ranges need it. If you look at the feed
current you'll notice with the 1 ohm either in or out, the current to the
virtual earth is the same at 20ma. So the 4V signal, 180 ohms has the
same drive effect as 20mV, 1 ohm. Switch and wiring hence kept simple.

With R5 gone, and the ESR range switch in the 1 ohm position, the
180 ohm resistor, R4, is effectively in series with the capacitor
under test. The op amp merely changes the location of the ground
node, and inverts the output signal polarity.

I'm puzzled. The 180 ohm sees only 0V (the V.E.). The capacitor sees
similarly. Currents essentially isolated. Phase changes notified by opamp
output voltage.

Since R4 (180 ohms) is now in series with the capacitor, it
completely swamps out the internal ESR (0.05 ohms.) This means the
series combination of C, L, and R has negligible "Q", and there is
no quadrature or orthogonal component in the circuit.
However, as the ESR decreases, the corresponding voltage drop that
we are trying to measure also decreases. We no are faced with the
problem that the di/dt from the inductor is much larger than the I*R
drop from the ESR. This means the leading and trailing edge of the
square wave have large spikes.

For some reason LTspice is refusing your .asc text list, so I can't picture
where that 'L' and 180 ohms are. Can you repost it as a pic' somehow?.
I had a play with my unit and it is sensitive to (a lot) of added series
inductance. There's a 5% reading change with 2 foot of coiled test leads
(about 200nH). This is basically a 'ring down' (70MHz) at the opamp output
and seems related to the opamp stability (the THS is something like 150MHz
GB).
On the 1 ohm range it is surprisingly difficult to add noticable test
inductance without adding serious amounts of lead/wire resistance.
Essentially the meter is reading lead resistance with a bit of
capacitor ESR thrown in.
[Poly cap and s/c link. actual Vout -.063V. Link replaced with 1206 0.01ohms
and Vout=0.052V].
As test I've looked for inductive 'spikes' using the 'scope and about 20
different test capacitors (good through to rubbish). I saw nothing
untowards.
Presumably confirming that capacitors are not inductive (other than the
trivial effect of their leads and the test socket wiring).

If you used a diode peak detector to measure the amplitude, it would
respond as best it could to the leading edge spike, which would make
it impossible to measure the drop across the ESR. Your circuit and
Larkin's share this problem.

Using a synchronous rectifier helps a bit, but you are now faced
with trying to turn it on after the leading edge spike, and to turn
it off before the trailing edge spike. That could be tricky.

The test spikes I forced were a transient effect and only occupied a few %
of each complete cycle. Yes, a peak detector (without windowing) would have
severe problems on such a waveform. The niceness of the PSR is that it
can take on allcomers and average the spike energy out over each full
cycle.
It's not exact, as there's a 15% reading error with Qs up at 2.5. The opamp
clips at Q=3 but these are good quality poly' capacitors.

I spent this afternoon looking at these problems in SPICE,

Luxury!. I had a bad cap' problem. Result was a ESR meter. Cost me a day to
design and build. I've only used the damned thing once in the past year

and have

come to an amazing observation. There is a hidden but very
significant feature in the bridge ESR circuit referred to at the
beginning of this thread. The links are:

1. Talino Tribuzio's page, showing the circuit from Nuova
Elettronica:

http://www.qsl.net/iz7ath/web/02_brew/15_lab/06_esr/index.htm

2. Gintaras' web page, who refers to Tribuzio's page in his readme

http://alytus.auksa.lt/esr/

The schematic is at

http://alytus.auksa.lt/esr/esr_meter_schematic.pdf

The valuable hidden feature is the bridge configuration completely
eliminates the leading and trailing edge spikes due to the capacitor
internal inductance. Since the spikes are in phase with the square
wave signal on the other side of the bridge, they simply disappear
at the output of the op amp!

This means the peak detector has a very clean square wave to work
with, and it can give a much more accurate measurement of the
signal. There is no tricky timing to fiddle with that can go out of
whack just when you need to use the tester.

If you like, I can post the analysis of your circuit and Larkin's
version showing the huge spikes that appear as the ESR becomes
smaller, and the triangular wave from the capacitance charging and
discharging. I don't really see a good way of overcoming these
problems.

As I mentioned in previous posts, the bridge circuit has significant
advantages, including low test voltage, in-circuit test, shorted
capacitor detect, etc. With the extremely clean output signal into
the peak detector, it becomes the obvious choice for hassle-free ESR
measurements.

I'll post the LTspice ASC file here along with the PLT file so you
can see how it works. I changed the bridge resistance to lower
values to allow measuring lower values of ESR.
[...]

Regards,

Mike Monett

 

[Winfield Hill]

Hi John,

Do you mind if I make some comments? The 1 ohm resistor, R5, can be
removed from the circuit. It is at a virtual ground node and has
little or no voltage across it. So removing it has little effect on
the circuit.

With R5 gone, and the ESR range switch in the 1 ohm position, the
180 ohm resistor, R4, is effectively in series with the capacitor
under test. The op amp merely changes the location of the ground
node, and inverts the output signal polarity.

Since R4 (180 ohms) is now in series with the capacitor, it
completely swamps out the internal ESR (0.05 ohms.) This means the
series combination of C, L, and R has negligible "Q", and there is
no quadrature or orthogonal component in the circuit.

However, as the ESR decreases, the corresponding voltage drop that
we are trying to measure also decreases. We no are faced with the
problem that the di/dt from the inductor is much larger than the I*R
drop from the ESR. This means the leading and trailing edge of the
square wave have large spikes.

If you used a diode peak detector to measure the amplitude, it would
respond as best it could to the leading edge spike, which would make
it impossible to measure the drop across the ESR. Your circuit and
Larkin's share this problem.

Using a synchronous rectifier helps a bit, but you are now faced
with trying to turn it on after the leading edge spike, and to turn
it off before the trailing edge spike. That could be tricky.

I spent this afternoon looking at these problems in SPICE, and have
come to an amazing observation. There is a hidden but very
significant feature in the bridge ESR circuit referred to at the
beginning of this thread. The links are:

1. Talino Tribuzio's page, showing the circuit from
Nuova Elettronica:

http://www.qsl.net/iz7ath/web/02_brew/15_lab/06_esr/index.htm

2. Gintaras' web page, who refers to Tribuzio's page in his readme

http://alytus.auksa.lt/esr/

The schematic is at

http://alytus.auksa.lt/esr/esr_meter_schematic.pdf

The valuable hidden feature is the bridge configuration completely
eliminates the leading and trailing edge spikes due to the capacitor
internal inductance. Since the spikes are in phase with the square
wave signal on the other side of the bridge, they simply disappear
at the output of the op amp!

This means the peak detector has a very clean square wave to work
with, and it can give a much more accurate measurement of the
signal. There is no tricky timing to fiddle with that can go out of
whack just when you need to use the tester.

[ snip ]

John, wrt the alytus.auksa.lt schematic -- I don't see exactly
how it can work. Considering its complete symmetry: two 22-ohm
resistors to ground and 4.5mA square-wave drive, identically on
each D.U.T. pin, there can be no current through the capacitor.
Are we looking at the same drawing?

 

[Mike Monett]

John, wrt the alytus.auksa.lt schematic -- I don't see exactly
how it can work. Considering its complete symmetry: two 22-ohm
resistors to ground and 4.5mA square-wave drive, identically on
each D.U.T. pin, there can be no current through the capacitor.
Are we looking at the same drawing?

Win,

If you are referring to http://alytus.auksa.lt/esr/esr_meter_schematic.pdf,
the right side of the capacitor, the CAP2 pin on J6, goes to ground, and
not to the junction of R12 (1K) and R17 (22).

The left side of the cap is connected to the left side of the bridge, at
the junction of R11 (1K) and R16 (22).

There seems to be an optical illusion that makes you believe the cap is
connected across the bridge, and I made the same mistake the first time I
looked at it. But of course, it wouldn't work if it was connected that way.

The cap really is connected from the left side of the bridge to ground.

Regards,

Mike Monett

 

[Winfield Hill]

Win,

If you are referring tohttp://alytus.auksa.lt/esr/esr_meter_schematic.pdf,
the right side of the capacitor, the CAP2 pin on J6, goes to ground, and
not to the junction of R12 (1K) and R17 (22).

The left side of the cap is connected to the left side of the bridge, at
the junction of R11 (1K) and R16 (22).

There seems to be an optical illusion that makes you believe the cap is
connected across the bridge, and I made the same mistake the first time I
looked at it. But of course, it wouldn't work if it was connected that way.

The cap really is connected from the left side of the bridge to ground.

Regards,

Mike Monett

Thanks, now I can see again! Praise the Lord!

 

[Mike Monett]

Thanks, now I can see again! Praise the Lord!

He is busy on other things, but asked me to thank you for the kind
thought

Regards,

Mike Monett

 

[Winfield]

Win,

If you are referring tohttp://alytus.auksa.lt/esr/esr_meter_schematic.pdf,
the right side of the capacitor, the CAP2 pin on J6, goes to ground, and
not to the junction of R12 (1K) and R17 (22).

The left side of the cap is connected to the left side of the bridge, at
the junction of R11 (1K) and R16 (22).

There seems to be an optical illusion that makes you believe the cap is
connected across the bridge, and I made the same mistake the first time I
looked at it. But of course, it wouldn't work if it was connected that way.

The cap really is connected from the left side of the bridge to ground.

iz7ath says the esr tester circuit came from an Italian magazine,
Nuova Elettronica N212. It would be interesting to see their
writeup, because its operation seems backwards to me. If no
capacitor is connected, the bridge is balanced, and there's no
signal to the meter, which reads 0. With a capacitor connected
the bridge becomes unbalanced. A perfect capacitor, esr = 0 ohms,
maximally unbalances the bridge, and presumably pushes the meter
to full scale. A poor capacitor reads slightly less than full
scale. For example, with esr = 1 ohm (which is not a very good
capacitor by today's standards) we still get a nearly full-scale
meter reading, because 1 ohm is so much less than 22 ohms, and
is still pretty close to zero ohms by comparison. So the meter
is hard to read, showing little change for various capacitor esr
values in the sub-1-ohm region of interest. In fact, 1-ohm and
0.1 ohm capacitors would have nearly the same reading. Not good!

 

[Fred Bloggs]

iz7ath says the esr tester circuit came from an Italian magazine,
Nuova Elettronica N212. It would be interesting to see their
writeup, because its operation seems backwards to me. If no
capacitor is connected, the bridge is balanced, and there's no
signal to the meter, which reads 0. With a capacitor connected
the bridge becomes unbalanced. A perfect capacitor, esr = 0 ohms,
maximally unbalances the bridge, and presumably pushes the meter
to full scale. A poor capacitor reads slightly less than full
scale. For example, with esr = 1 ohm (which is not a very good
capacitor by today's standards) we still get a nearly full-scale
meter reading, because 1 ohm is so much less than 22 ohms, and
is still pretty close to zero ohms by comparison. So the meter
is hard to read, showing little change for various capacitor esr
values in the sub-1-ohm region of interest. In fact, 1-ohm and
0.1 ohm capacitors would have nearly the same reading. Not good!

Looks like there will be lots of SR error too. It appears that the ideal
transfer function at the output will be a peak of ~Vcc*(1-ESR/22) which
is not too bad, it can be worked but he needs to change some things
around...such as subtracting the voltage at the junction of R11/12.

 

[Mike]

iz7ath says the esr tester circuit came from an Italian magazine,
Nuova Elettronica N212. It would be interesting to see their
writeup, because its operation seems backwards to me. If no
capacitor is connected, the bridge is balanced, and there's no
signal to the meter, which reads 0. With a capacitor connected
the bridge becomes unbalanced. A perfect capacitor, esr = 0 ohms,
maximally unbalances the bridge, and presumably pushes the meter
to full scale. A poor capacitor reads slightly less than full
scale. For example, with esr = 1 ohm (which is not a very good
capacitor by today's standards) we still get a nearly full-scale
meter reading, because 1 ohm is so much less than 22 ohms, and
is still pretty close to zero ohms by comparison. So the meter
is hard to read, showing little change for various capacitor esr
values in the sub-1-ohm region of interest. In fact, 1-ohm and
0.1 ohm capacitors would have nearly the same reading. Not good!

The same meter was published by Marvin Smith in the July 2001 edition of Poptronics.
A quick google search found many references to the Poptronics article, but no copy of it.
I know it's the same meter since I have a PDF copy of the Poptronics article.

Mike



When truth is absent politics will fill the gap.

 

[Winfield]

The same meter was published by Marvin Smith in the July 2001 edition
of Poptronics. A quick google search found many references to the
Poptronics article, but no copy of it. I know it's the same meter
since I have a PDF copy of the Poptronics article.

We'd love to see a copy of the article. Failing that, a posting
of the relevant portion of the meter-reading interpretation and
of the circuit-operation explanation would be helpful. Be sure
to include the author's full names for credit.

 

[The Phantom]

SNIP

We'd love to see a copy of the article. Failing that, a posting
of the relevant portion of the meter-reading interpretation and
of the circuit-operation explanation would be helpful. Be sure
to include the author's full names for credit.

Over on ABSE, Mike posted what appears to be a bad English translation of
some of the article:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The project came from an italian magazine (Nuova Elettronica N212);

It's very simple but interesting; I've built it and tested some
capacitors, so I think it's very useful: build it; It measures the
ESR (Equivalent Serie Resistance) of capacitor (electrolytic and
not); pratically you can see if a capacitor is good or not.

It's a bridge circuit that work at 100Khz; there're the following
possibilities:

1) The electrolytic capacitor is good: (low ESR) the bridge will
stay balanced and the meter will indicate the maximun current.

2) The electrolytic capacitor is not good: (high ESR) the bridge
will be unbalanced and that will cause the meter to indicate less
current; as less the meter will indicate as higher will be the ESR;
After few measure you'll be able to decide if a capacitor is good or
not.

3) There is a short circuit in the electrolytic capacitor: the meter
will indicate the maximum current and the red LED will lamp;
capacitor is not good.

4) The electrolytic capacitor is broken: the meter will not move.
Capacitor is not good.

http://www.qsl.net/iz7ath/web/02_brew/15_lab/06_esr/index.htm

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's interesting how the author calls the case where a perfect capacitor
is connected, "balanced".

 

[Mike]

We'd love to see a copy of the article. Failing that, a posting
of the relevant portion of the meter-reading interpretation and
of the circuit-operation explanation would be helpful. Be sure
to include the author's full names for credit.

Ok, I posted the article on a.b.s.e along with a link to a totally different meter that was sold by Dick Smith
electronics. I'd be interested to hear any comments on the Dick Smith version.

Mike



If there is no absolute truth then nothing can be known.

 

[DaveM]

Ok, I posted the article on a.b.s.e along with a link to a totally different
meter that was sold by Dick Smith
electronics. I'd be interested to hear any comments on the Dick Smith version.

Mike



If there is no absolute truth then nothing can be known.

The DSE meter was designed by Bob Parker, an aussie with a good head on his
shoulders. The meter has been marketed as both a kit (best bang for the buck)
and fully assembled and tested models. Most service techs that I know and those
that have posted on the sci.electronics.repair NG swear by this meter.
I bought and built a kit a number of years ago, and still use it. It's paid for
itself many times over in the time that I've owned it.

I understand the Dick Smith has stopped selling the meter, but Bob has found
other outlets for it. John's Jukes (http://www.flippers.com/) in Canada sells
them (that's where I bought my copy).

I don't think you'll find a bad note from anyone about this meter.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra

 

[Winfield]

The DSE meter was designed by Bob Parker, an aussie with a good head on his
shoulders. The meter has been marketed as both a kit (best bang for the buck)
and fully assembled and tested models. Most service techs that I know and
those that have posted on the sci.electronics.repair NG swear by this meter.
I bought and built a kit a number of years ago, and still use it. It's paid
for itself many times over in the time that I've owned it.

I understand the Dick Smith has stopped selling the meter, but Bob has found
other outlets for it. John's Jukes (http://www.flippers.com/) in Canada
sells them (that's where I bought my copy).

I don't think you'll find a bad note from anyone about this meter.

I'd love to have one of those meters. The design seems respectable,
and the most sensitive range, 0.00 to 0.99 ohms, looks ideal for
working with serious switching-supply capacitors. I see it uses a
50mA test current and amplifies the resulting esr signal by about
25x before presenting it to a comparator, the other side of which
gets a slow 20V/ms ramp (9.5uA and 470nF), to measure the signal.

The difference between Bob Parker's design for Dick Smith Elec.,
and the Marvin Smith esr meter we were discussing is dramatic.

 

[Winfield]

Ok, I posted the article on a.b.s.e along with a link to a totally
different meter that was sold by Dick Smith electronics. I'd be
interested to hear any comments on the Dick Smith version.

From Mike's abse post, with the DSE / Bob Parker link:

There is another meter kit that was available from
Dick Smith Electronics. The manual can be found at
http://mainelectronics.com/pdf/k7204inst.pdf

 

[The Phantom]

I'd love to have one of those meters. The design seems respectable,
and the most sensitive range, 0.00 to 0.99 ohms, looks ideal for
working with serious switching-supply capacitors. I see it uses a
50mA test current and amplifies the resulting esr signal by about
25x before presenting it to a comparator, the other side of which
gets a slow 20V/ms ramp (9.5uA and 470nF), to measure the signal.

The difference between Bob Parker's design for Dick Smith Elec.,
and the Marvin Smith esr meter we were discussing is dramatic.

See more about the Bob Parker meter here:
http://members.ozemail.com.au/~bobpar/esrmeter.htm

Get the .pdf for the latest model here:
http://members.ozemail.com.au/~bobpar/k7214.pdf

Go have a look at a forum all about bad caps here:
http://www.badcaps.net/

I've only just started looking around there, and there's a lot about ESR.
For example:
http://www.badcaps.net/forum/showthread.php?s=01febe89f8a9b210661a6ca2498e21d8&p=32706#post32706

 

[Fred Bloggs]

I'd love to have one of those meters. The design seems respectable,
and the most sensitive range, 0.00 to 0.99 ohms, looks ideal for
working with serious switching-supply capacitors. I see it uses a
50mA test current and amplifies the resulting esr signal by about
25x before presenting it to a comparator, the other side of which
gets a slow 20V/ms ramp (9.5uA and 470nF), to measure the signal.

9.5u/470n=20V/ms??? Something's not right there. And the ESR pulse
amplifier DC bias schematic seems screwed with about 0.6V output offset.

 

[Winfield]

9.5u/470n=20V/ms??? Something's not right there. And the ESR pulse
amplifier DC bias schematic seems screwed with about 0.6V output offset.

Excuse me, I meant to write 20mV/ms. Yes I saw the schematic said
0.6 volts quiescent at the amplifier output, but I calculated about
2.8 volts (5*220/320 - 0.65), so that's a puzzle. Moreover, it's a
non-inverting amplifier, and with low duty-cycle 50mA pulses (for
low battery consumption), we'd expect positive-going output signal
pulses from the amplifier. Something indeed looks wrong there.

 

[The Phantom]

Winfield,

You still have an HP4194, don't you?

Try doing a sweep of capacitance and ESR from 60 Hz to 1 MHz of some caps
from a defective mother board. I visited my local computer repair guy and
he gave me a couple of defective boards destined for recycling.

Especially sweep some of the really small, high capacity, low voltage
caps. I saw some unusual characteristics with the physically small caps.

Some of the ones I tried had self-resonance at 100 kHz.