15-12-2011, 10:19 AM
I‘m afraid that the amount of information imparted in previous posts may have made this useful little project seem rather more complicated than it is. In the transition of the forum from ‘GF1’ to ‘GF2’ the pictures and PCB artwork have been lost from the thread, without which there is insufficient info for anyone who might be contemplating building the project. Furthermore, based on feedback that I’ve received, I felt that two slight amendments to the design were called for:
Firstly, the addition of a 0.1uF capacitor on the input to the 78L05 voltage regulator, which is considered good practice, though I’ve had no indication from anyone that the lack of this capacitor has caused any problems.
Secondly, the addition of what is often (perhaps unkindly!) referred to as an ‘idiot diode’ to protect the meter from accidental polarity reversal should a constructor decide to use a ‘wall wart’ to power the unit rather than a PP3 9V battery. I don’t personally see the need to use a wall wart as the meter is only used occasionally and for a few minutes, and draws only a few mA is use. It only requires a 5V supply, via the TL805, so a PP3 can drop by a few Volts before it ceases to power the unit correctly. That said, it is a fact that some constructors prefer to use a wall wart, and have inadvertently reversed the polarity, which will destroy the TL805, the 74HC14N IC and 2N2222A transistor. Not the end of the world – all three will cost no more than a pound in total, but the addition of a protection diode – a 1N4007 say, costing just a few pence, will obviate the risk. (Such diodes are now routinely incuded in all designs in Everyday Practical Electronics).
I’ve therefore modified the PCB artwork to incorporate these two additions, and also have highlighted the differences of the component layout as compared to the original. In other respects, the component layout is as per the original project.
I also found a source of a larger 50uA meter – the original 50uA meter from ESR Electronics Ltd was 60mm x 45mm, but I’ve found that Rapid Electronics stock one at a similar price which is larger (70mm x 60m) and hence, the dial is less cramped and clearer to read. With my limited artwork skills, I’ve managed to enlarge and re-calibrate the dial to suit this meter. The dial can be printed onto photo paper, and if the scale-plate of the meter is carefully removed, can be stuck on the rear of the plate and the plate fitted back again. Removal of the dial only involves two crews, and the scale-plate, another two crews.
Some constructors have found difficulty in obtaining the 0.47uF 400V capacitor ‘C5’ which is the ‘DC isolation capacitor’ that protects the meter should it be inadvertently applied to a charged capacitor. Such capacitors come in various case styles with both axial and radial leads. I’ve therefore amended the layout of the connectors points at the RH end of this cap on the PCB, with three holes as options for different sizes. (The overlay mentions just two, but the PCB has three options). The polyester capacitor that I use is available from Rapid at 38p.
To ensure that the new layout is satisfactory, I’ve built a meter and have attached a few pictures which show the meter and a close-up of the PCB. I’ve also attached a pic of the PCB artwork, the component overlay highlighting where it differs from the original designer’s layout, and a copy of the updated dial artwork.
The link to the Rapid 50uA 70mm x 60mm meter is here:
http://www.rapidonline.com/SearchResults...kw=48-0300
The link to the 0.47uF 400V capacitor is here:
http://www.rapidonline.com/SearchResults...kw=48-0300
I should add that for those who aren’t equipped to make PCBs, the layout isn’t in any way critical and could be built on ‘perf-board’ using a similar layout, connecting component wires beneath the board. Not perhaps as neat as a PCB, but nevertheless, functional. In earlier posts I’ve covered in depth the construction, testing, troubleshooting and use of the meter, but if anyone does build it and have trouble getting it to work, I’ll be happy to help.
All of the components apart from the optional LED, the switch, battery and meter, are now mounted on the PCB, with four pairs of pins for connections – two for the battery, two for the meter, two for the cap test leads, and two for the LED. When built, the only adjustment needed to set up the meter is to temporarily short out the cap test leads and adjust the pre-set pot on the PCB to get full scale deflection. (Meter needle to the far right of the dial). As with an analogue Ohm-meter, the dial reads from right to left – lowest readings to the right – higher readings to the left.
I know that several forum members have these meters, and I hope these notes might encourage a few others who like home-brewing to have a go at building this cheap but useful little item of test gear.
Firstly, the addition of a 0.1uF capacitor on the input to the 78L05 voltage regulator, which is considered good practice, though I’ve had no indication from anyone that the lack of this capacitor has caused any problems.
Secondly, the addition of what is often (perhaps unkindly!) referred to as an ‘idiot diode’ to protect the meter from accidental polarity reversal should a constructor decide to use a ‘wall wart’ to power the unit rather than a PP3 9V battery. I don’t personally see the need to use a wall wart as the meter is only used occasionally and for a few minutes, and draws only a few mA is use. It only requires a 5V supply, via the TL805, so a PP3 can drop by a few Volts before it ceases to power the unit correctly. That said, it is a fact that some constructors prefer to use a wall wart, and have inadvertently reversed the polarity, which will destroy the TL805, the 74HC14N IC and 2N2222A transistor. Not the end of the world – all three will cost no more than a pound in total, but the addition of a protection diode – a 1N4007 say, costing just a few pence, will obviate the risk. (Such diodes are now routinely incuded in all designs in Everyday Practical Electronics).
I’ve therefore modified the PCB artwork to incorporate these two additions, and also have highlighted the differences of the component layout as compared to the original. In other respects, the component layout is as per the original project.
I also found a source of a larger 50uA meter – the original 50uA meter from ESR Electronics Ltd was 60mm x 45mm, but I’ve found that Rapid Electronics stock one at a similar price which is larger (70mm x 60m) and hence, the dial is less cramped and clearer to read. With my limited artwork skills, I’ve managed to enlarge and re-calibrate the dial to suit this meter. The dial can be printed onto photo paper, and if the scale-plate of the meter is carefully removed, can be stuck on the rear of the plate and the plate fitted back again. Removal of the dial only involves two crews, and the scale-plate, another two crews.
Some constructors have found difficulty in obtaining the 0.47uF 400V capacitor ‘C5’ which is the ‘DC isolation capacitor’ that protects the meter should it be inadvertently applied to a charged capacitor. Such capacitors come in various case styles with both axial and radial leads. I’ve therefore amended the layout of the connectors points at the RH end of this cap on the PCB, with three holes as options for different sizes. (The overlay mentions just two, but the PCB has three options). The polyester capacitor that I use is available from Rapid at 38p.
To ensure that the new layout is satisfactory, I’ve built a meter and have attached a few pictures which show the meter and a close-up of the PCB. I’ve also attached a pic of the PCB artwork, the component overlay highlighting where it differs from the original designer’s layout, and a copy of the updated dial artwork.
The link to the Rapid 50uA 70mm x 60mm meter is here:
http://www.rapidonline.com/SearchResults...kw=48-0300
The link to the 0.47uF 400V capacitor is here:
http://www.rapidonline.com/SearchResults...kw=48-0300
I should add that for those who aren’t equipped to make PCBs, the layout isn’t in any way critical and could be built on ‘perf-board’ using a similar layout, connecting component wires beneath the board. Not perhaps as neat as a PCB, but nevertheless, functional. In earlier posts I’ve covered in depth the construction, testing, troubleshooting and use of the meter, but if anyone does build it and have trouble getting it to work, I’ll be happy to help.
All of the components apart from the optional LED, the switch, battery and meter, are now mounted on the PCB, with four pairs of pins for connections – two for the battery, two for the meter, two for the cap test leads, and two for the LED. When built, the only adjustment needed to set up the meter is to temporarily short out the cap test leads and adjust the pre-set pot on the PCB to get full scale deflection. (Meter needle to the far right of the dial). As with an analogue Ohm-meter, the dial reads from right to left – lowest readings to the right – higher readings to the left.
I know that several forum members have these meters, and I hope these notes might encourage a few others who like home-brewing to have a go at building this cheap but useful little item of test gear.
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'
