I used the wrong designator for the 100mA 5V regulator in my earlier post. I should of course have correctly referred to it as '78L05' so have now edited that. You're right Marty about 78L05 regulators and the choice of caps either side of them in almost all designs. In poking around for application sheets on this simple basic regulator in the 78xxx/79xxx series, I found this info, which is interesting and relevant:
http://stuff.nekhbet.ro/2006/06/18/how-t...-7805.html
When I get time, even though many have been built, and I've had no reports of problems experienced, I'll amend the PCB layout around the regulator to include two caps. That they were omitted has nothing to do with keeping costs down, when little caps are literally 'ten a penny'. Put it down not to penny pinching, but to technical ineptitude on my part. As the bard said a long time ago: 'In the kingdom of the blind, the one-eyed man is king':D
As to the choice of 10uF tatalum by the original designer, I haven't a clue why he chose it or what its role is - I just left it in place. BTW, he used four - not three nicads, and never used a 5V voltage regulator at any time - in his footnote he rightly says:
Quote:
'Several buildrers have e-mailed me with modifications [I wasn't one of them] to use a low current regulator to keep the voltage more constant. Since my unit runs on four nicad cells there is not enough headroom for a 5V dropout regulator. The charge voltage of my nicads is 5.6V but most packs are usuable down to 4.7V. I added a 15 Ohm 0.5 Watt resistor and 4.7V Zener to ground. This really helps to keep the readings more stable as the nicads discharge. I had an isse driving the meter to full scale so ended up reducing the resistor R17 down to 4K7.
End quote.
I remain puzzled as to why he chose to used four nicads in a unit that draws so little current, in which a cheap PP3 battery, even if the unit is used daily, will last for ages, whereas within a month or so, the nicads will be flat - probably at a most inconvenient time, but that's a matter for the designer. The discharge and self-discharge characteristics of NIMH cells are better than Nicads, so would be a batter choice, but not so much better.
As to getting the meter to read FSD, I found that I needed to reduce R17 down from 10K to 3K3.
'Colin' (Colin Armstrong), who may be known to other forum members, as he is to me, who is registered on this forum, but who due to heavy work commitments gets no time to participate, designed an excellent neat dial for the 50uA meter movement which he published elsewhere. I've been in touch with Colin to request his permission to re-publish it on here. I'm pleased to say that he's very happy for me to do that, so I've attached it, and hope that others will find it useful.
Neat eh? I wish I could do stuff like that!!
I'm pleased that more than a year after I first posted details of this handy little project, which pays for itself in no time at all, that it's still arousing interest.
Once more, thanks for your input Marty, and to Alan.
David
http://stuff.nekhbet.ro/2006/06/18/how-t...-7805.html
When I get time, even though many have been built, and I've had no reports of problems experienced, I'll amend the PCB layout around the regulator to include two caps. That they were omitted has nothing to do with keeping costs down, when little caps are literally 'ten a penny'. Put it down not to penny pinching, but to technical ineptitude on my part. As the bard said a long time ago: 'In the kingdom of the blind, the one-eyed man is king':D
As to the choice of 10uF tatalum by the original designer, I haven't a clue why he chose it or what its role is - I just left it in place. BTW, he used four - not three nicads, and never used a 5V voltage regulator at any time - in his footnote he rightly says:
Quote:
'Several buildrers have e-mailed me with modifications [I wasn't one of them] to use a low current regulator to keep the voltage more constant. Since my unit runs on four nicad cells there is not enough headroom for a 5V dropout regulator. The charge voltage of my nicads is 5.6V but most packs are usuable down to 4.7V. I added a 15 Ohm 0.5 Watt resistor and 4.7V Zener to ground. This really helps to keep the readings more stable as the nicads discharge. I had an isse driving the meter to full scale so ended up reducing the resistor R17 down to 4K7.
End quote.
I remain puzzled as to why he chose to used four nicads in a unit that draws so little current, in which a cheap PP3 battery, even if the unit is used daily, will last for ages, whereas within a month or so, the nicads will be flat - probably at a most inconvenient time, but that's a matter for the designer. The discharge and self-discharge characteristics of NIMH cells are better than Nicads, so would be a batter choice, but not so much better.
As to getting the meter to read FSD, I found that I needed to reduce R17 down from 10K to 3K3.
'Colin' (Colin Armstrong), who may be known to other forum members, as he is to me, who is registered on this forum, but who due to heavy work commitments gets no time to participate, designed an excellent neat dial for the 50uA meter movement which he published elsewhere. I've been in touch with Colin to request his permission to re-publish it on here. I'm pleased to say that he's very happy for me to do that, so I've attached it, and hope that others will find it useful.
Neat eh? I wish I could do stuff like that!!
I'm pleased that more than a year after I first posted details of this handy little project, which pays for itself in no time at all, that it's still arousing interest.
Once more, thanks for your input Marty, and to Alan.
David
