Not a 'repair' question as such but I was thinking of using an old skool 250uA FSD moving coil panel meter to read the 10 to 15V DC range (a lead acid battery charge / discharge indicator), next to a 0-30A Ammeter (inc shunt etc).
I was thinking of using either a voltage reference such as LM4040DIZ-10.0 or could I get away with a simple zener (BZX55C?) with associated resistor(s) please?
Once I have the 10V (that might need it's own drive resistor?) for the
-ve side of the meter I think I'd need a 20k resistor in series with the meter to my 10-15V supply (a lead acid battery)?
R = V/I = 5 / 250 x 10^-6 = 20k ohms.
Or ... I could use a straight potential divider on the grounds that a moving coil meter isn't going to be that accurate in the first place?
Maybe: -Ve, 10k, 5k, +Ve with 20k in series with the meter across the
There should be a way to rig a simple resistor to do what you need. You may have to re-scale the meter, but that should do it. Perhaps an adjustable w ire-wound resistor that you could tweak until *your* voltage range matched the meter range?
formatting link
As a on e-off, the cost would not be too excessive. And the meter would be just as 'accurate' as before. But there should be no reason to get fancy. It would all depend on how much current is going through the resistor(s).
Your total error is all the errors added together, so you'd need some excep tional circuitry to not add anything significant to the meter's error, whic h might be 1% or so. More realistic would be to keep the electronic errors within 1%.
To get that you'd be using an opamp to knock 10v off, with 0.1% resistors e verywhere it matters. Avoiding anything getting hot always helps keep the e rrors down, so anything that gets warm heatsink it, and operate parts way u nder their limits. Not that that will be a problem with this meter.
Several others from similar sources with a table of resistance values to make the desired range (in volts) and either blank faces or sequential numeric faces. Sorry for pointing to an Euro-meter, but just that they are out there.
Had I known, the "meter man" at Kutztown would have had exactly what you want in the size you want it, new or used. He has hundreds. Now you have to wait until spring.
Most people just use a Zener and maybe a few normal (or SB) forward volt drops to trim the offset.
I was thinking the TL431 programmable Zener could be calibrated to exactly what you need - but it needs a minimum current through it (possibly 1mA or so).
You can probably incorporate a TL431 into some kind of bridge circuit with the movement spanning the 2 arms, but it will almost certainly need some kind of power supply.
As long as the final result calibrates correctly, does it matter what the tolerances are / were?
With 250uA FSD I can't anything much getting warm?
Quite.
So, precision reference, zener, resistor bridge?
With a precision reference and zener my meter wouldn't start doing anything till the measured voltage started to exceed 10V, even if I was ramping from 0 to 15V (and that's fine for a 12V lead acid).
With a resistor bridge with a 1/3 split I should be able to get FSD at my 15V but anything less wouldn't work (would it?) ... so it's going to have to be either of the first two ... or something else?
Basically, as long as I hold the negative of the meter at the same voltage as the positive at anything 10V and below, the meter won't do anything. Then as the +ve of the meter exceeds 10V and goes to 15V, a meter set to read 0-5 volts should follow ok, giving me reasonable / best possible accuracy across the working range of the battery.
The only prerequisites are to ensure the -ve of the meter is held exactly at +10V (from a supply ranging from 10.5 to 14.4V) and that the meter current limiting resistor is correctly matched (to give the
250uA required for FSD at 5V PD).
As long as any reference would be suitably driving at anything above
10V and not overdriven at 15V then we should be good to go?
Because the reference voltage will be too close to the minimum voltage expected Ian?
If that is the case I could use a very light bang converter to bring the supply up to say 15 V and power it from the source that is being monitored in use (running an electric outboard from a 93Ah battery).
build it accurately I don't know which suits you better. Long ago I used to use selected 5% resistors where 1% were wanted, but they don't stay as accurate as 1%ers.
no :)
If you had a perfect-knee perfectly accurate voltage zener then sure, but where will you find one of those?
or resistors on the input of an opamp so it only turns on once 10v is reached
Ok, so with our source at 15V we get 240 uA and ~FSD on our meter. That's 14.88V across the resistor and 0.12V across the meter, therefore the meter is ~500 ohms?
At 10V we get 2/3 of 240 uA (160uA) and so our voltage across the meter is now 0.0256 (~1/5th of FSD) and so the meter will read 1 volt and not 0?
The problem is that the voltage across the meter will fall in proportion to the voltage across the entire network, not just across the meter component (which is why I ruled it out in my second post). ;-)
Maybe not but can appear as such to some (I think). ;-)
No that's 15 volt across the resistor only so it's ~240 uA. With the internal meter resistance it would be slightly less current.
No, it's still assuming the meter resistance is close to zero. at 160 uA, it will read 2/3 scale at 10 volts.
No, you're NOT measuring voltage across the meter. You're measuring current through the meter.
It's a series circuit. (The meter and the series resistor.) As such in this case, the series resistance is much higher than the internal resistance of the meter, and that defines what the current will be for a given voltage.
How things work: Amp meters, by definition are suppose to be low resistance items. When you're measuring current through a circuit, you don't want to waste any of the available voltage across the meter.
As an example of this, current shunts for meters are usually rated at X-amps for 50-100 mV across the shunt.
On the other hand, volt meters are put in parallel with a load, and to be accurate, they should be many time higher than the load resistance so they don't pull the supply voltage down that you're trying to measure.
Back in the "good old days" for example, they would specify taking a voltage reading with a "minimum 20K ohms/volt" meter. If your trying to measure 100 volts, if you have a 150 volt range on the meter, this would be a load resistance of 3 megohms across the circuit you're measuring.
In the example given for a 250 uA meter, that's only 4K/volt. But for measuring 10-15 volts, most supplies are good for anywhere from 1-25 amps, so the extra 250 uA is hardly noticeable.
But adjustability sometimes brings with it lower precision (noise).
Ok.
Whatever works best on the day. ;-)
Ok.
;-)
Pass, hence the questions.
Ok, but you still need to hold the -Ve end of the meter *at* 10V as the source goes between 10 and 15V? A LM4040DIZ-10.0 works like a zener so that would probably do.
If I've read the datasheet properly the (10V) device requires a minimum of 100uA and a maximum of 15mA.
formatting link
So, if we ignore the meter and assume a worst case battery voltage of
10.5V then we have .5V to get 100uA so that's 4k7 and giving us 1mA at
15V. We would also have 20500 ohms (20k series resistor and 500 ohms of the meter) in parallel with the chip feed to help ensure it stays over 100uA.
I think about all that would work (without affecting the accuracy / functionality) is a very small series fuse and reverse biased diode across the input to the meter circuit ?
But still, ohms law applies and with just a series resistor to limit the current for FSD, zero on the meter (re-marked to display 10V) won't reflect the required 10V.
For the meter to read zero there would have to be zero volts across it and with just your series resistor that would only happen when the supply voltage = zero.
So, a simple series resistor won't work for a meter reading from 10 (not 0) to 15V (but I'll be happy for you to explain it otherwise). ;-)
So, you need something to hold the -ve of the meter at 10V (and the
+ve of the meter to supply via a ~20k resistor) to allow the meter to work in the range of 0 to 5V, marked to show 10 - 15V.
OK, Let's go back to engineering 101. What's the VERY MOST important part of a project? It's the SPECIFICATION.
Exactly, precisely, concisely, unambiguously, completely stated requirements. This does NOT change. If you change your mind and change the spec, you start a new project.
How do you know when you're done? You write acceptance criteria. Exactly how the user, in this case you, is going to test the unit to determine if you met the spec? If it passes the written acceptance criteria, you succeeded. If it passes the test criteria, but doesn't work, the specification was the problem. You get paid for this project and start a new one with new specifications.
I can't emphasize how important the specification is. It's the number one cause of project failure. You can't design for criteria that were not specified. It's easy to say, "I know what I'm designing for me...I don't need no stinkin' specification." You'd be wrong. This thread is what you get. Lots of rocket scientists when you really need a boat. The very last place you want to discover that your forgot some important requirement is when the completed device is deployed.
Words like exactly, "as good as possible" have no place in a specification.
Specs are numbers and test methods. Specs include allowable variations due to initial component tolerances, component age, temperature, vibration, misuse. An example of misuse is, "what happens when the user hooks it up backwards." I learned about vibration when I carried a voltmeter on my motorcycle. When I needed it, it failed. I took it apart and it poured out like sand.
The engineer doesn't need to know WHY you want what you want. That's not his job. HOWEVER...if you state your objectives, why you want to do this, you might find the bevy of rocket scientists have a much better way to accomplish your objectives. I can't count the number of times a user wanted an complicated gizmo, but his objective could be achieved with a much different and much simpler approach by repartitioning the system.
End rant.
First question I'd ask is, "are you gonna' stare at the meter 24/7?" If not, get a cheap digital panel meter off EBAY, put a push button in series and be done with it. If you're not battery powered, you can probably tolerate the load and don't need the push button.
But, back to your original request. Take your current meter. Put a series resistor to set the max voltage to 5V. Put a 10V "battery" in series. The remaining question is, "how do you implement the 10V battery?" Can your measured voltage ever get below 10V? If so, your problem just got a LOT more complicated. See "misuse" above.
If it were me, I'd use a 10V zener diode. If you can find an affordable temperature compensated one that is accurate over the range of current from zero to the max indication on your meter and the ambient temperature range, do that. Characterize the calibration. Fire up your CAD program and make a new meter face. Since you're making a new face, any stable zener voltage works. Your meter is as accurate as your care in making the new face.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.