Repost: Battery meters

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Repost: Battery meters

Post by smokey125 » 24 Feb 2018, 23:31

Battery meters
Postby smokey125 » Thu Jan 12, 2017 4:40 pm

Having read through Flynn Futaba conversion and the various comments about the original battery meter and making sure you don’t put too much current through them.
I want to try and avoid burning out the meter in either if my transmitters.
I have found on the NORCIM website a circuit schematic for one of the Fleet systems. That shows a 39K resistor between +ve and the meter. Based on my rudimentary electronics that seems to give the right sort of current. The schematic shows the –ve side of the meter going onto various other components which are part of the transmitter circuit. Do I need these or can I go straight to –ve from the meter without risking damaging it?
Re: Battery meters
Postby ceptimus » Thu Jan 12, 2017 5:07 pm

Many transmitters used the meter to display the RF output of the transmitter rather than the battery state. I think that Fleet circuit is doing that.

If you're using the same meter to monitor your battery then I think a 39K won't be enough.

You also need a means of setting the meter so that it 'goes into the red' at whatever battery voltage is appropriate for your chosen battery - with a 2-cell Lithium battery that might be about 6.6 V.

I would use a 100K potential divider between the power rails to scale the battery voltage down - and then connect the pot wiper via another resistor to the meter + terminal with the - terminal of the meter to ground. That second fixed resistor should be at least 39K, but I'd fit a 100K to start with and only reduce it if the pot doesn't allow you to set the meter reading high enough.

Re: Battery meters
Postby smokey125 » Thu Jan 12, 2017 5:27 pm

:) Thanks for another really helpful reply. Even better I actually followed what you your saying! Looks like I'm going to have to get busy with the breadboard to try and figure this out!
Every time I think I'm making progress I find another challenge! I'll get there in the end!

Re: Battery meters
Postby ronstv » Thu Jan 12, 2017 7:53 pm

you might be better off using a 6v2 zener so you get an expanded scale reading.
I use this circuit regularly and its perfect for a 2s

Circuit courtesy of Phil


Re: Battery meters
Postby Phil_G » Thu Jan 12, 2017 8:11 pm

In the Fleet diagram the 100 ohm resistor and the 6.2v zener do just that Ron, so 39k resistor current is:
(battery - 6.2) / 39000 so if your battery is by some lucky chance 10.1v :D
its 10.1 - 6.2 which is 3.9v divided by 39000 ohms is 100uA, and its a 100uA FSD meter.
The meter full scale deflection (FSD) is therefore 10.1 volts and zero deflection 6.2 volts.

39k is good for that particular 100uA meter in the diagram Doug, there was no 'standard' meter so yours might be different.
For trials, too big a resistor wont hurt anything. Too small might! Its not the coil, its a 100uA meter but 300, 500 uA wont burn it out, its not that that does the damage, its the physical impact of the needle hitting the end stop very hard when too much current passes that usually breaks them.

Re: Battery meters
Postby ceptimus » Fri Jan 13, 2017 12:01 am

Ah, so I was wrong about it being an RF meter in the Fleet circuit diagram. I do wonder why they drew the meter and its associated zener/resistor there in the circuit diagram all mixed up with the RF circuitry rather than putting it off to one side to make it easier to understand. I suppose it was because the 6.2V rail was used for other stuff besides being the lower voltage side of the meter connection, but it could still have been laid out more clearly.

Re: Battery meters
Postby smokey125 » Fri Jan 13, 2017 2:53 pm

Ah so it’s the age old thing, the push doesn’t hurt just the sudden stopping. :lol:

Ok I think I’m following this! The 6.2 zener creates the 6.2v rail Martin mentioned. The 100Ω resistor provides the correct current for the zener and the 39KΩ provides the right current for the meter based on the voltage drop created by the zener?
The bits to either side of the zener are parts of the RF circuit?
The diagram was drawn by someone who essentially reverse engineered it from a complete board. I’ve found it very helpful in some ways but not in others. In amongst the meter and LED shown top left there is what I believe to be a DPDT switch although all the pair of terminals are joined at the back so it is effectively a SPDT switch. There are multiple wires connected to this for various different purposes. I’ve been trying to trace them to help me figure out what was happening with the meter. With the answers on this thread I’ve realised I’ve missed something and I think I know what now. :idea:
So if build it up exactly as is but using a 2s lipo the meter will just show partially used when fully charged but take a long time to go down? Alternatively if I went for a 3s LiFe it would be much as it was originally.

Interestingly despite the age difference between the two Fleet transmitters the RF board is very similar in both of them with a lot of the components and layout common to both. The big difference is the modulator boards!

Re: Battery meters
Postby Phil_G » Fri Jan 13, 2017 4:43 pm

Its easier than that Doug.
Choose a zener of the same voltage as your required 'minimum' on the meter.
Choose a series resistor worked out using:

(battery max voltage) minus (zener voltage)
Meter FSD current

So say you want to indicate the state of a 2S lipo.
Your minimum will be say 6.8v and so you use a 6.8v zener. The maximum is 8.4v, which is a difference of 1.6v

Assuming a 100uA meter, 1.6v/.0001A is 16k ohms.

With this setup the meter will display from 6.8v (pointer at zero) to 8.4v (pointer at 10)


Re: Battery meters
Postby smokey125 » Sat Jan 14, 2017 11:20 pm

Phil_G wrote:EPLS :D
It is when you know what your doing! :lol:
Does Rons picture show the right layout. From what I've read you would normally have 8.4v, the resistor then the zener with the meter wired + to 8.4v and - to 6.8v. Hope that makes sense, might be easier for me to draw what I'm thinking!
Ignore the above I've worked out that it's me being thick again! :oops:

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Re: Repost: Battery meters

Post by iflylilplanes » 14 Feb 2020, 01:29

Phil, are most meters from the early 1970s have the same values?, I have a couple of Futaba's a 4ch and a 5ch similar to your Sprengbrook with the internal mains charger, original power was 8 cell NiCad pack 9.6v, would like to have the meters working as well, I'm going to power the transmitters with 2 18650 Li-Ion cells.




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Re: Repost: Battery meters

Post by WernerL » 14 Feb 2020, 06:37

Regarding: do all meters have the same value?
Just a data point, but for car radios:

Airtronics Championship Series:
R = 670 Ohm
Full scale: 400mV / 600uA
Orange to full: 200mV / 300uA
Red to orange: 140mV / 200uA
The needle moves until 500mV (needle at far right)

Futaba FP-2F:
R = 400 Ohm
Full scale: 450 mV
Red to orange: 225 mV
The needle moves until 480mV (needle at far right)

For my "headless transmitter" ( I am using the microcontroller to drive the meter. A PWM output controls the voltage, and I have some "calibration" in the software that allows me to set maximum on the meter when the battery is fully charged. The meter is connected to the PWM pin with an appropriate series resistor, and a capacitor as filter.

cheers, Werner

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Re: Repost: Battery meters

Post by Phil_G » 14 Feb 2020, 11:15

I was converting my Sprengbrook tonight and when I got to doing the meter, I thought maybe a video...

This isnt limited to retro-transmitters of course but its a question that is frequently asked...
When we run a battery 'flat' its not completely exhausted, but it is down to the minimum that our equipment is happy with to run properly & predictably.
For the common 2S lipo or LiFe that we use a lot in transmitters, the useful voltage range is 8.4v when full, to 6.2v when flat. You could run lithium cells down below 3.1v per cell but utimately this would damage the cells.
So ideally, a battery meter would read from 6.2v to 8.4v across the full travel of its indicator needle.
To do this we use a zener diode and a 'multiplier' resistor in series with the meter. The zener directly determines the low reading, ie 6.2v with a 6.2v zener. For the upper end of the scale, subtract the zener voltage from the desired full scale voltage, say 8.4v - 6.2v = 2.2v

The meters FSD current is usually printed on the rear. In my case its a 100uA movement so using ohms law we can find the value of the multiplier resistor:
R=2.2v/.00001A = 22k ohms.

For the Sprengbrook I decided on a full scale of 10 volts as I've been testing using a PP3 Duracell:
For the upper end of the scale, subtract the zener voltage from the desired full scale voltage, 10v - 6.2v = 3.8v
The meter is (say) a 100uA movement so using ohms law we can find the value of the multiplier resistor:
R=3.8v/.00001A = 38k ohms (nearest 'preferred' value is 39k)

One more example for say a 9.6v NiMh. Fresh off charge these can reach over11 volts, and you wouldnt want to go below 1.1v per cell, ie 8.8 volts. So a scale of 8.8 to 12 would make good use of the meter range. For the low end we use an 8.8v zener (or the nearest standard value). For the upper end of the scale, subtract the zener voltage from the desired full scale voltage, 12v - 8.8v = 3.2v

The meter is (say) a 100uA movement so using ohms law we can find the value of the multiplier resistor:
R=3.2v/.00001A = 32k ohms. Actually 33k is the nearest preferred value.

Just substitute your own preferred upper & lower voltages, and choose a zener voltage and resistor value to suit. Or choose the 'low end' zener and use either a preset pot or a selection of 'test' resistors to set the upper range if you're not sure of the meter FSD current. Start high and reduce the resistor to give FSD at the desired voltage.

Note that the banded end of the zener goes towards the battery positive.

iflylilplanes wrote:
14 Feb 2020, 01:29
Phil, are most meters from the early 1970s have the same values?
Hi Dave, not necessarily but 100uA is a common value, just substitute whatever FSD value you have into the R=V/I equation. Or cheat and use a preset pot to set FSD! ;)
WernerL wrote:
14 Feb 2020, 06:37
...For my "headless transmitter" I am using the microcontroller to drive the meter.
Hi Werner, welcome to the forum!
Thats a very nice project, I can see a lot of work has gone into it, very impressive ;)

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