My Homebrew ESR Meter

[martyb123]

c7 and c8 are for the voltage regulator i have the schematic with the voltage regulator drawn in

[Yorkie]

c7 and c8 are for the voltage regulator i have the schematic with the voltage regulator drawn in

Hi Marty,

Have you (or someone else) updated the circuit to include and additional cap (C8?) for the voltage regulator? I'd just added the voltage regulator and left the 10uF cap (C7) in place, but I think that often, when 100mA 78L05 is used in circuits, a cap is added before the regulator to prevent it behaving erratically. If that's the case, maybe I should update the PCB to accomodate that extra cap? In practice, I haven't found that the TL805 has misbehaved.

Any comments please?


David

[martyb123]

yes the image you left up the top of this page shows a c7 cap on the prototype board next to it is a space for the c8 cap which as you said goes with the 78l05. the original schematic only shows 1 cap c7 on the 5 volt incoming supply so thus means the original builder had run it off the batteries and not the regulator i am amazed that someone with your knowledge did not pick that up???? i am now building the new prototype with my 78l05 on the prototype board 2 by 3 inches and there was plenty of room for the regulator to go onboard i had the larger TO-220 package new in the spare parts tray rated at 1.5 amps so i used that,mind you depends on what 1 you have as to what one you use at the circuit will only draw the actual power it needs from the regulator. i also fitted a blue 5 mil led with a 68 ohm resistor for the on power as i do not like red led,s. so circuit layout is important to me i tend to be fussy with the layouts i choose. do not own an oscilloscope and if i did i would need to do another 5 year tecnical collage course to learn how to use it not to mention the 350 dollars to buy it new . i use protype boards for everthing i build as its easy to do your own component layouts on them. i normally build and sell computer systems i.ve done so many in the last ten years i became bored with them.so thus started this hobby on older electronic intergrated circuits electronics was a hobby when i was young and now have the time to get back to it. found some really good parts suppliers along the way which has kept the costs down. the council cleenup piles here were full of thrown out stereos i stripped the capacitors out of what i collected and now build this esr meter to check them all.:Dthanks for being here to talk to great to have someone to discuss these circuits!!

[Yorkie]

Thanks for the reply Marty.

The original designer of course ran his ESR meter from four recharegable cells, which made little sense to me as they self discharge over time, and will probably be dead when I want to use the meter, apart from which, as the battery voltage diminished, it was necessary to have a front panel zero pot to enable the meter to read FSD for zero. On the other hand, a 9V PP3 battery is very cheap, has a shelf life of years, rather than Nicads/NIMH which discharge in weeks. (An earlier ESR meter I made from a design in the now defunct 'Television' magazine still has the same PP3 in it from eight years ago!). ESR meters are used only intermittently, and then only for a few minutes at a time, and so it seemed to me that it made more sense to fit a 78L05 regulator to provide a constant 5V from a 9V PP3. It also meant that the front panel pot could be replaced with a pre-set pot on the PCB to simplfiy construction and use, by minimising off-board wiring. Once set for FSD, the preset shouldn't need any further adjustment. (The original circuit didn't show the wiring of the front panel pot - it simply said: 'To 50uA meter and 25k zero pot', which many less experienced constructors found confusing, and wired up incorrectly).

When you say 'someone of my knowledge' I fear that you endow me with abilities that I'm afraid I don't possess, and in so doing, might give me delusions of adequacy! I don't see any sign of a C8 on the circuit or original PCB - I just left the 10uF cap C7 in place, popped a 78L05 100mA regulator in front of it, and amended the PCB track layout to accomodate it. It worked fine, still does, and I've had no feedback from anyone to suggest otherwise. However, I believe that it is customary when using voltage regulators to put a cap at the input to the regulator as well as on the output or there is a risk of self-oscillation. If that is so, what value do you suggest - another 10uF tantalum? As can be seen from the photo of the actual built PCB in my posting, there is space to add a couple of pads on the tracks for another cap adjacent to the 78L05.

I think the designer went through several iterations and on his final PCB layout the component numberings have several gaps in the numbering sequence for components that don't exist, either on the circuit or on the PCB.

I used a 100mA regulator for two reasons - firstly, the current drawn by the circuit is just a few mA - in fact the LED consumes more than does the instrument itself, so a 1A regulator wasn't called for, and secondly, it fitted on the PCB more easily than a 1A regulator.

I should once more, say that I claim no credit for the original concept and design, any such credit righlty beloniging to VE7IT, the originator. All I've done is to modify it to my own requriements, and to share my efforts with others, in the spirit of home-brew.

Once more Marty, thanks for your input.

David

[martyb123]

c8 is required when the regulator is far from the power-supply filter. so normally you would get away with not having it there at all so thats why your circuit works regardless you have stated tha yours is a tl7805 whom which is made by texas instruments whom they sugest a 0.22 uf on the output and a 0.33 uf on the imput. but all that changes when using a diferent brand of 7805 regulator as diferent companies can specify differen values for these 2 capacitors . i have placed a ceramic capacitor of 100 nf (capacitor number is 104) for the value of c8 as the schematic i have for this circuit does have it in place which sugest to me that his c8 capacitor was positioned off board so his wire lengths were longer thus so he actually needed to have the c8 capacitor installed.personally when installing voltage regulators i always install the 2 capacitors 98 percent of all schematics use both capacitors on a regulator and as the second ceramic capacitor cost 0.1 cents you would be silly not to install it anyway all the rest of my caps were ceramic type caps for that reason which are rated at 50 volts and are of a smaller size than what others have used so fit nicely on the prototype board. thers is no advantage of shoving in a larger tantalum capacitor as its package size and shape vary from a ceramic but still achieve the same job as the ceramic cap. if the original circuit designer or anyone else that designs a circuit thinks they need a diferent type of cap it is displayed in that schematic by specifying it in the footnotes the 2 straight lines on schematic means a ceramic capacitor normally unless of course its a audio circuit and then the size and type of capacitors used changes greatly .

[martyb123]

so the original prototype circut simply ran off the 1 capacitor as he only intended to run it off 3 x 1.5 batteries to give the circuit 4.5 volts of unregulated power which was good enough to make it run however inacuracies develop due to the bateries running down on power thus giving less acurate readings on the meter as time went on using the 5 volt regulator meant the 9 volt battery would a t 7.5 voltd would still supply an acurate 5 volts to the circuit thus making the meter itself more acurate and thus get longer out of 1 battery instead of recharging the 3 1.5 batteries. so voltage regulators are used to supply an acurate voltage to your circuit and if they cannot achieve this voltage acuratly they are designed to shut down thus forcing 1 to replace the battery or replace the transformer or wall wart etc.the data sheets on all these parts must be read and understood to gain knowledge of how that regulator works!

[AlanBeckett]

Hmmmm.
An interesting discusion and comment.
I spent years designing this sort of circuit, and once the PCB was stable the BoM was fixed and there weren't any more problems. The product was churned out for years.
So, why should there be any problems with this PCB and its specified components?
Alan

[martyb123]

not a problem on the circuit a variation of the circuit

[Yorkie]

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

[martyb123]

spose ill have to work out how to take the plastic face off my meter without breaking it to get the label on it i,m thinking thats not possible?? or spose they sell meters with removable faces???? youv,e really got me curious now can i not just leave it alone???