whiz115 said:
Hi
i can't understand what's going on, i think that if i disconnect C3 then i shouldn't be able to see any output! if this is true then it's ridiculous what is happening here
i'm checking all the time the PCB for bridges but i don't see any!
what should i do?
As I said before, don't despair, we will find the problem.
Ok, so if when you completely disconnect C3 but you still see a significant voltage on your ESR meter, this could be because:
- the voltmeter is faulty or poorly calibrated
- the wiring to your board is acting as an antenna for local AM broadcast stations and putting significant AC voltage into your bridge rectifier
- your voltmeter or ESR board is picking up 60 Hz magnetic field from nearby mains wiring and converting that into a DC voltage
The first thing to do is to simply disconnect the meter from the ESR circuit and observe what voltage it shows with its input wires unconnected to anything. Is it still 220mV? What is the value shown?
When you left R17 out of the circuit, you have invited problems. By using a digital or analog multimeter instead of the specified 50uA panel meter, you may also be inviting problems. But we can cope with this as long as we understand what is going on. The original circuit was designed using a 50 uA meter along with a 25Kohm zero pot built into the meter (as far as I can tell). Such analog meters will have an internal resistance of about 1600 ohms (typical). Because it is an analog panel meter and with such low resistance, the meter itself is not sensitive to nearby electric or magnetic fields and so won't pick anything up by itself. Digital multimeters are more complex with internal circuits plus they are much higher input resistance, so it is relatively easy to pickup a voltage due to stray fields. Analog multimeters are simpler and so less prone to problems, but they still have very high input resistance when measuring volts and so still are not ideal.
Note that the original design uses internal batteries. This is an advantage compared to your design because it keeps the wiring short and isolates the circuit from the mains supply, which means there is much lower chances of picking up outside noise or signals.
In order to use a current meter, the current must be limited by an external resistance and that is why the schematic uses R17. R17 is critical to setting the total load resistance placed on the bridge rectifier. We can estimate that the load across C6 due to R17 plus the meter is about 11.6K ohms. If you are using a digital multimeter in place of the panel meter and measuring volts instead of uA, then you should have a resistor across C6 to take the place of the missing 11.6 K ohms load. Your digital multimeter does not do this because it's input resistance is probably more like 1 M ohm or higher (when measuring volts. If you use the multimeter to measure microamps, then you would need R17 only).
So, if you will be using a volt meter for this ESR device, then add a 10 K ohm resistor in parallel with C6. You mentioned that when you added a 100K ohm resistor at that point, the reading on the voltmeter went down to 18.5 mV. This is good. With no connection of a capacitor or short circuit across TP1 to TP2, we expect to see zero or nearly zero on the meter. 18.5 mV is nearly zero.
Then, when you short TP1 to TP2 with a capacitor or simply with wire, the reading on your meter should go up to some higher voltage. Please let us know what that voltage is. I expect it to be 0.7 volts.
