Analog ESR meter design

[shovenose]

Re: Analog ESR meter design

Simple it is... Reminds me of this particular quote:



Henry Ford said it.

What all analog ESR meter circuits have in common is that they use a constant AC voltage source passed thru a shunt resistor, then thru the cap under test in series. The idea is that depending on the ESR of the cap, this circuit produces a certain AC current which makes a certain voltage drop across the sense resistor, and that is shown on the meter.

Now, to me this approach seems ass-backwards, because the source resistance of the voltage source isn't exactly zero so errors creep in, then if the shunt resistor is too high a value it won't read low ESR caps, and if it's too low the signal voltage becomes too low to be useful.

So instead of doing the same old thing i decided for a different and (at least to me) more straightforward approach, which is feeding the cap thru a constant AC current source which depending on the cap's ESR produces an AC voltage across which can be read directly across the cap itself, no shunt resistor involved!

And the true gem - the voltage source circuit produced maximum voltage output when the ESR was zero. This circuit produces maximum voltage output when the ESR is high, therefore, if you calibrate it properly, it can also be used with a DMM and display a voltage proportionally to ESR, for example 1mOhm = 10mV ! Now how's that for coolness factor.

I didn't understand a work of it but it sounds cool

[Th3_uN1Qu3]

Re: Analog ESR meter design

Simple it is... Reminds me of this particular quote:

If you've always done it like that, then it's probably wrong.

Henry Ford said it.

What all analog ESR meter circuits have in common is that they use a constant AC voltage source passed thru a shunt resistor, then thru the cap under test in series. The idea is that depending on the ESR of the cap, this circuit produces a certain AC current which makes a certain voltage drop across the sense resistor, and that is shown on the meter.

Now, to me this approach seems ass-backwards, because the source resistance of the voltage source isn't exactly zero so errors creep in, then if the shunt resistor is too high a value it won't read low ESR caps, and if it's too low the signal voltage becomes too low to be useful.

So instead of doing the same old thing i decided for a different and (at least to me) more straightforward approach, which is feeding the cap thru a constant AC current source which depending on the cap's ESR produces an AC voltage across which can be read directly across the cap itself, no shunt resistor involved!

And the true gem - the voltage source circuit produced maximum voltage output when the ESR was zero. This circuit produces maximum voltage output when the ESR is high, therefore, if you calibrate it properly, it can also be used with a DMM and display a voltage proportionally to ESR, for example 1mOhm = 10mV ! Now how's that for coolness factor.

[shovenose]

Re: Analog ESR meter design

Make it simple and easy to use, like an Apple product
guaranteed success.
Not really kidding.
-shovenose

[Th3_uN1Qu3]

While there are a number of analog ESR meter designs on the web, none of them are sensitive enough for low ESR caps. Also they share a common fallacy which i'll point out below.

I won't go into the digital ones because from what i've seen so far most designs that are offered for free are also pretty dodgy, and if you're not into programming micros buying a programmer just to build one piece of kit is overkill, and most of you won't justify the expense.

So, how does an analog ESR meter work? Well, it's quite simple really. It's an resistance meter (ohm meter) just that it works in AC instead of DC.

• First we have an oscillator, which should be 100kHz as that's where today's SMPS caps are spec'd at, but there are designs on the net operating at various frequencies which will be more or less accurate depending on the type of cap you are measuring...
  
• Then we have a current amplifier which makes this AC signal low impedance, so there is no voltage drop due to the source. This has been implemented in various ways - opamps, transistors, transformers, and so on. Sometimes the oscillator and amplifier stage are one and the same.
  
• Then we have a sense resistor in series with one of the probes, which drops a known voltage. When you short the probes, there's only this sense resistor in the circuit, and you use this to set the maximum indication of your meter movement. The capacitor to be tested is put in series with this sense resistor.
  
• And finally we have a rectifier and filter, which converts the AC to DC so that the meter can read it. These ESR meter designs typically use a 50 or 100 uA meter movement.
  
• Sure there's other things such as input protection and such but the four building blocks i mentioned above are the foundation of any analog ESR meter.

Now, the first thing wrong with most ESR meter designs on the web is that the sense resistor is way too high in value, so low ESR caps cannot be read accurately - when the bottom of the range is 50 ohms there is no way you're gonna spot a 0.1 ohm difference let alone a 0.01. So let's drop the value of the sense resistor. If you do this without changing anything else you'll quickly notice something funny - the meter will not go to max scale anymore! That's because by lowering the value of the sense resistor, less voltage gets dropped by it, so there's less voltage available to drive the meter. Some designs try to get around that by adding a gain stage to feed the meter, but this opens a whole new can of worms as the whole thing becomes extremely sensitive to outside interference.

So, what's the quick and easy solution? Increase the current, duh! This solves all our problems - we can use a low value sense resistor AND still have enough voltage drop to show on the meter. As to why nobody thought it before, there are two issues:

1. It eats battery. Most meters on the web are designed to be powered off a 9 volt battery. That won't work if you're trying to draw a few hundred milliamps.
2. When applied to a cap with high ESR (almost open circuit) it will develop a voltage that will turn on semiconductors, causing erroneous readings.

Point 2 can be gotten around fairly easily. As for the first one, not quite doable... So, what i'm going to do is design a bench meter, that runs off 12 volts. Or if you have a NiMH pack handy...

Everyone post their wishlist here, and i'll see what i can make of it.