19-03-2012, 12:48 PM
I've just completed my meter kit and boxed it up, and I must say it's an excellent little kit at an unbeatable price. At first, the readings were a bit erratic, then I recalled Alan's comments that the instructions recommend that particularly for low value capacitances, the meter should be screened in a metal box and the box connected to the meter ground. As is my habit when making these little boxes from scrap oak offcuts using double sided scrap PCB sheet, it dawned on me that the PCB front panel itself forms a capacitor. Out of curiousity I measured the capacitance of the 4" x 5" front panel and it was 480pF! I connected the underside to the ground terminal and that cured the erratic readings.
In use, the 'zero' button is pressed and 'Co' appears on the display for a few seconds, then the display reads zero till a cap is connected. The term 'Co' means 'compensation' which compensates for the capacitance of the test leads. This reading is then stored in memory until such times as the 'zero' button is pressed again. The display shows the value of the cap under test annotated with 'p', 'n' or 'u' according to the capacity range.
Alan referred to one of the caps - C5, 10uF 16V on the output of the voltage regulator, being too tall to enable the meter to be housed behind a panel with the display through a window in the panel. The kit is designed to be used unboxed as well as boxed up. I didn't appreciate the cap height until I'd soldered it in, but was able to gently unsolder the leads and fit a new cap laying down on the PCB, where there's plenty of space for it. This then enable the display module to fit into a window cut-out in the front panel flush with the front of the panel, with the PCB mouted on spacers.
It's a high quality plated-through-hole PCB, but once soldered in, it's very difficult to de-solder leads and pins without damaging the PCB, so if anyone contemplates buuidling this kit, do make sure that you get components in the right place with the right orientation, testing the resistors to make sure that you've selected the correct value. (There's only one diode to fit, and the crystal fits either way round). Alan mentioned using an IC socket for the 20-pin IC, which is a very wise move, but my kit included a socket anyway.
As it comes, the kit included an on-off switch and an on-board 2.1mm power input socket to be powered from a 9V DC 'wall wart' but like Alan, I left these components off the PCB and just connected front panel switches and powered the meter from six 1.5V cells as the consumption is no more than 30mA so the 5V regulator should be happy with that arrangement for many hours of use, given that it's only going to be used intermittently for a few moments at a time.
There no much more that I can add to what Alan said at the start of this thread, other than to say that anyone wanting a cheap but useful cap meter that auto-ranges from 1pF to 500uF will find this a nice little kit to build and use. I've attached a couple of pics, which will look very similar to Alan's. I've shown a 1,000pF (1 nF) 20% tol cap on test, which would be within tolerance between 800pF - 1,200pF. As can be seen, it reads 1,030 pF (displayed as 1.03nF) so is within 3% of its stated value. As a rule, I've always found polystyrene and silver mica caps to be close to their stated value and to remain stable for decades. Which si good news, given that they're often in innacessible places such as front end switchery or inside IFT cans!
Hope that's of interest.
In use, the 'zero' button is pressed and 'Co' appears on the display for a few seconds, then the display reads zero till a cap is connected. The term 'Co' means 'compensation' which compensates for the capacitance of the test leads. This reading is then stored in memory until such times as the 'zero' button is pressed again. The display shows the value of the cap under test annotated with 'p', 'n' or 'u' according to the capacity range.
Alan referred to one of the caps - C5, 10uF 16V on the output of the voltage regulator, being too tall to enable the meter to be housed behind a panel with the display through a window in the panel. The kit is designed to be used unboxed as well as boxed up. I didn't appreciate the cap height until I'd soldered it in, but was able to gently unsolder the leads and fit a new cap laying down on the PCB, where there's plenty of space for it. This then enable the display module to fit into a window cut-out in the front panel flush with the front of the panel, with the PCB mouted on spacers.
It's a high quality plated-through-hole PCB, but once soldered in, it's very difficult to de-solder leads and pins without damaging the PCB, so if anyone contemplates buuidling this kit, do make sure that you get components in the right place with the right orientation, testing the resistors to make sure that you've selected the correct value. (There's only one diode to fit, and the crystal fits either way round). Alan mentioned using an IC socket for the 20-pin IC, which is a very wise move, but my kit included a socket anyway.
As it comes, the kit included an on-off switch and an on-board 2.1mm power input socket to be powered from a 9V DC 'wall wart' but like Alan, I left these components off the PCB and just connected front panel switches and powered the meter from six 1.5V cells as the consumption is no more than 30mA so the 5V regulator should be happy with that arrangement for many hours of use, given that it's only going to be used intermittently for a few moments at a time.
There no much more that I can add to what Alan said at the start of this thread, other than to say that anyone wanting a cheap but useful cap meter that auto-ranges from 1pF to 500uF will find this a nice little kit to build and use. I've attached a couple of pics, which will look very similar to Alan's. I've shown a 1,000pF (1 nF) 20% tol cap on test, which would be within tolerance between 800pF - 1,200pF. As can be seen, it reads 1,030 pF (displayed as 1.03nF) so is within 3% of its stated value. As a rule, I've always found polystyrene and silver mica caps to be close to their stated value and to remain stable for decades. Which si good news, given that they're often in innacessible places such as front end switchery or inside IFT cans!
Hope that's of interest.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
