Understanding Bob Parker ESR meter schematic

[ifrenide]

Hello. I did a simple try to understand Bob Parker's ESR meter.but unfortunately ,there are some bocks which are not clear.
I will make a squar on it ,and label them with colors.
best regards.

[ifrenide]

the red squar block.I don't know completely what is its pupose.the blue one I know we can switch each single transistor at a time to make pulses and ,every single transistor has a different resistor attached with its collector. to limit current I guess. I don't know why they use different current levels. 
finaly the green squar its too complicated.
best regards.

[bob91343]

Rather than try to analyze this complex gadget I will say that the usual way of measuring ESR is to apply a high frequency voltage to a capacitor and measure the current.  Since the capacitor offers low impedance to the high frequency signal, the current is largely a function of the ESR.

The caveat is to make sure the capacitance is of low reactance compared to the ESR so that the current is largely controlled by the latter.  Otherwise you get false high readings.

My bridge, for instance, can measure capacitance at 1 kHz.  I switch it to R measurement and it reads ESR as long as the capacitance is high enough.  For most electrolytic capacitors, 1 kHz is high enough frequency for that.  If I want more precise values I can compute it from the dissipation factor.  That also includes effect of dielectric leakage but for most measurement that is negligible.  As a sanity check I can switch the test frequency to 100 Hz and repeat; the result should be very close.

Measuring ESR seldom calls for accuracy.  It's more often used as an indicator of capacitor quality.  There is also ESL, the equivalent inductance, which can pervert these measurements.  An electrolytic capacitor is often a rolled up affair with two electrodes and some insulation; this construction can create paracitic inductance which will pervert the component impedance as frequency rises.  That's the reason for putting a small capacitor in parallel with an electrolytic capacitor.

Nothing is ever as simple as we might want, and there is no substitute for understanding what's going on rather than relying on a fancy piece of test gear.

[cur8xgo]

the red squar block.I don't know completely what is its pupose.the blue one I know we can switch each single transistor at a time to make pulses and ,every single transistor has a different resistor attached with its collector. to limit current I guess. I don't know why they use different current levels. 
finaly the green squar its too complicated.
best regards.

General strategy for simplifying and reversing a schematic. Redraw it as follows:

try to eliminate anything on the schematic which is there for protection or not directly involved in the measurement..like clamp diodes, display circuitry, bypass caps, voltage regulator support circuitry etc..

replace any voltage source like "battery" or voltage regulators with a fixed voltage that is either what you know it is or what is typical

if there are switches pick a position and analyze the circuit in that configuration

if you know of any driving waveforms then draw them on the schematic. For instance if you know "P24" is always a 100KHz square wave then draw it.

re-layout the subcircuits in a way that is easier to read.

get rid of any refdes, part numbers, or values which are not that important and do not help you understand the circuit..for instance 1N4148, etc..

The idea here is to take as much load off your brain as possible so it can try to analyze only the important part of the circuit. Schematics aren't always drawn well. Something very simple could be totally obvious drawn one way, and totally obscured drawn another. And this can vary from person to person.

[Andy Watson]

This one ?  http://www.bobparker.net.au/esr_meter/esrmeter.htm

Towards the bottom of the page, under the heading "More Info", there is a link to an article that explains in detail how it works:
http://www.bobparker.net.au/esr_meter/k7214.pdf

The blue box determines a particular range of charging current for the capacitor under test - has one transistor to discharge the capacitor. The red box generates a voltage ramp, under software control. It is basically an ADC function.  The green box contains an amplifier and input protection - it's basically input conditioning.

[floobydust]

Yes, the assembly manual for Dick Smith Electronics K7214 gives more details.

Q3, Q4, Q5 are current sources 0.5mA 10-99R, 5mA 1-10R, 50mA 0-1R depending on the range needed. This sets the pulse constant-current for charging the capacitor on ramp up.
Q6 later discharges the capacitor for ramp down.

Q7 and Q8 are an amplifier for the AC signal measured across the capacitor. The signal is amplified 20X and goes into the MCU comparator input.

Q9 and Q10 are a current-mirror which charges C10 the reference capacitor. Q11 discharges or resets C10. The time for this to happen is compared to the cap under test.

[ifrenide]

1-how can the meter knows the range in automatic way ,and select between Q3,Q4,or Q5 ? I mean ,these charging current selection is based on what? and how can the microcontroller, decide which transistor to be switched ?

[wilfred]

It starts on the highest test current and if it is out of range it tries the next lower current. See the document K7214.pdf @AndyWatson linked on pp5 section "Range Changing".

[ifrenide]

thank you wilfred.  there are other questions please.
1- the ESR of capacitor under test how it affect the signal going into microcontroller .it's about timing ? Imean how many pulses per second?
2-the ramp voltage block why they use it? I label it with red squar on schematic attached here if you can take a look please.
best regards.