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
Ron
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
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.
Cheers
Phil
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.
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.
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)
EPLS
Re: Battery meters
Postby smokey125 » Sat Jan 14, 2017 11:20 pm
Phil_G wrote:EPLS
It is when you know what your doing!
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!