Protection Circuit &
Probes Connected & for
(disconnect, discharge cap, reconnect)
No Ohms beep.
|4.0SI Front Panel
The "R" pot is for setting the zero of the resistance range.
The "C" pot is for setting the zero of the capacitance range.
1 beep per Ohm.
Dot Matrix LCD
|Version 4.0 Dot Matrix LCD no longer
||Inside Version 4.0
The newest model 4.0SI has a protection circuit (I) that's very comprehensive. You can see in the photo 4.0SI Inside (above) that there is a new PCB in the lower left. This circuit detects that the capacitor has some voltage and disconnects the ESR meter and at the same time lights a red LED telling you to remove the probes as soon as possible. In addition there's a green LED that tells you that the probes are in contact with something. This is part of the "I" upgrade.
With a 220 uF cap and 400 VDC the protection circuit survives, but at 2200 uF at 1500 VDC it fails. One way to characterize this is to look at the energy stored in the capacitor.
Joules = 1/2 * C * V2 So for the above two cases:
Joules = 0.5 * 220E-6 * 400 * 400 = 17.6 Joules
Joules = 0.5 * 2200E-6 * 1500 * 1500 = 2,475 Joules
So 10 Joules should be safe and maybe 1,000 Joules lets the smoke out. Don't know about 100 Joules.
Since this meter is capable of measuring very small resistance values it's important that the zero be set to correspond to how the measurement is to be made.
With the meter off place the test probes on the center left DIP socket terminals (a white link is shown between them) and while holding them (like chip sticks) power on the meter and wait a few seconds until the zero calibration is finished.
If you measure in the socket using a calibration done as described above for test probes the resistance value will be low by a fraction of an Ohm because of the test lead resistance.
So insert a short "U" shaped piece of test lead wire on the right side of the DIP socket so that it forms a short. Power up the meter as before and wait for it to finish the zero calibration.
If a capacitor is in the DIP socket when the meter is powered up it will do a calibration using the ESR value of that capacitor as the reference resulting in an inaccurate reading afterword.
ESR = 0.06 Ohms
Cap = 1237 uF
When Version 4.0SI is connected to a battery the protection circuit senses the voltage and disconnects the meter so it can not be used on batteries.
Warning: Reverse Polarity will blow the protection diode and maybe some traces, so this is not recommended for the faint of heart.
# of LED
This came DOA with a DC short at the 24 Volt Input.
After removing the rear finned high power assembly that included the J3 input connector it was clear using a Fluke 87V DMM that the problem was not in the finned high power assembly but in the main chassis. Two large blue caps were tested with the ESR-Cap meter with these results:
C1= 0.24 Ohms - 1530 uF [s.b. 01132U050AK - 1300uF 50V] so looks OK
C2= 0.00 Ohms - 1720 uF (Fluke 87V DMM shows falling resistance not short) ESR meter now reads 0.40 Ohms - 1720 uF [s.b. 101172U100AJ2A - 1720 uF 100V] so OK
After taking the chassis apart to gain access to the Input DC Power Supply Filter assembly is was looking like there may be something wrong with the SCR, but none of the DMM measurements made sense, so the ESR meter was used. The following circuit diagram was drawn:
|At first I thought that the SCR was the
problem (and was thinking that the L1 choke terminal
"1" was the input and "2" was the output, but when I
came back to the drawing noticed the ESR readings that
I've highlighted in red. The L1 coke measures
0.43 Ohms and it's input terminal "2" shows 1710 uF
& 0.48 Ohms to ground. The L1 choke output
terminal "1" shows 0.00 Ohms to ground and 1720
uF. That means the problem is either with the A1
9V battery charging circuit or with C2. But C2
is showing 1720 uF which is exactly the label
value. What's going on????
After removing the bracket and unscrewing the negative terminal on C2 (see photo on AM-7176A web page Fig 15) both C2 and the rest of the circuit tested OK. What's going on??
When reassembling C2 the answer came. When the screw holding the terminal lug was tightened the terminal rotated to the right causing the many black wires attached to push into the other terminal. While there was not a metal to metal short the insulation on maybe a couple of wires was heavily deformed, not enough to make a hard short, but enough to cause a short when a voltage over maybe 10 Volts was applied. By using the shank of a pocket Phillips screwdriver to reform the black wires they were moved away from the positive terminal and the problem is solved.
A minor concern is the 1k resistor that is between the 2N685 SCR gate and cathode (ground) which measures 37 Ohms in either polarity using the DMM and ESR-Cap meter. It looks like the only way the SCR can turn on is if the input voltage goes over 200 so this may be part of the EMP (wiki) protection scheme. This is similar to the crowbar protection circuit used to short the output of a power supply when it's voltage exceeds some threshold, but in this case the gate voltage will never be as high (or higher) than the anode and so will never turn on by the gate. That only leaves the breakdown voltage as a turn on mechanism.
There has been some discussion of where to place surge protectors (Wiki) and the answer seems to be as close to the input as possible. In this case it's just after the circuit breaker.
|on the back side of
the PCB is the CR2032 coin cell and on the front
side (shown at left) is the AT Tiny microcontroller.
|Showing good (right
hand LED is green)
when connected to Loop antenna.
(high Q at 60 kHz)
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Page created 2 Feb 2010.