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Building a Voltmeter/Ammeter for an Electric Bicycle

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AusEbiker

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Hi all,

I would like some advice please.

I am building an electric bicycle and I want to monitor the current draw and the battery voltage.

My intention is to use a Digital Panel Meter which can be switched from voltmeter to ammeter mode.

The bike will have two 12 Volt (sealed lead acid) batteries in series to give 24 volts (nominal).

I have purchased a 3.5 digit Digital Panel Meter (200 mV full scale deflection - uses a separate 9v battery to power the meter).

I will therefore need to use scaling resistors to give a 0-200 Volt range on the meter.

From my research I believe that this means that I need a voltage ratio of 1,000 (200 Volts divided by 200 mV).

I have drawn the attached circuit diagram based on an application example given in the instructions for the panel meter.

The values of the resistors are;
R1 – 9.99M ohm
R2 – 100K ohm

From what I can find available locally it would be impractical to achieve the 9.99M ohm value (I can only find 1M ohm and 10 M ohm resistors – which means I would need to have a clumsy series of 1M ohm and smaller value resistors).

My question therefore is;
Can I substitute a 10M ohm resistor for R1 and a 100K ohm plus a 10 ohm resistor (in series) for R2?

Using the formula;
Voltage ratio = (R1+R2)/R2, I calculate that this gives

10,000,000 + 10,000 + 10)/(10,000 + 10) = 10,010,000/10,010 = 1,000.001

Maybe this is a silly question – but if it is this close to desired ratio (and considering the error factor of 1% for the metal film resistors that are recommended) why wouldn’t the manufacturer suggest this as an alternative?

The other matter that confuses me is the fact that resistors have a watt rating as part of their specification. What is the significance of this?

Thanks in advance for any help you can give me – once I have this part sorted out I will add to this post and include a circuit diagram of how I intend to switch this set up into ammeter mode.
 

Attachments

  • Voltmeter for Electric Bicycle.jpg
    Voltmeter for Electric Bicycle.jpg
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In your circuit diagram (Voltmeter for Electric Bicycle.jpg): R1=9.99M, R2=10K.
In your text: R1=9.99M, R2=100K.

Yes, you can use R1=10M and increase R2 to obtain desired voltage ratio.
Your method is OK, but you have to be more careful in calculation.
 
why don't you just use a trimmer pot to get the precise resistance? probably a good quality multiturn unit.
 
10Megs and 10k, 1% each, will give you a maximum error of 2%, if each resistor is off by the maximum tolerance and in opposite directions. Do you really need any better than that? If so, you will need to use either tighter tolerance resistors, or a pot and a reference for setting it.
How precise is the reference in the voltmeter? Keep in mind that, even with a perfect divider, you are still limited by the internal reference of the meter.
 
Ron H said:
10Megs and 10k, 1% each, will give you a maximum error of 2%, if each resistor is off by the maximum tolerance and in opposite directions. Do you really need any better than that? If so, you will need to use either tighter tolerance resistors, or a pot and a reference for setting it.
How precise is the reference in the voltmeter? Keep in mind that, even with a perfect divider, you are still limited by the internal reference of the meter.

Thanks to all of you for the help so far.

Firstly, the value of R2 should be 10k ohm (pardon my typo).

Secondly, accuracy of the set up is not very important, a 2% error would be quite acceptable. The documentation for the panel meter claims an accuracy of + or - 0.5%.

I have attached to this posting two more circuit diagrams - one showing the set up in Ammeter mode, and the other in Voltmeter mode. The active circuit in each is shown by thick lines and the inactive wires shown thin.

Please tell me if anything is wrong - I am quite good at drawing diagrams, but my electronic knowledge is pretty basic.

I am still hoping that someone can explain why resistors have a wattage.

I would also like to know if this setup will consume a significant amount of power.

Thanks again in advance.
 

Attachments

  • Ammeter Mode.jpg
    Ammeter Mode.jpg
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  • Voltmeter Mode.jpg
    Voltmeter Mode.jpg
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It's always cool to build it yourself but there is at least one meter on the market that will do what you want and more at a pretty reasonable price. I've been working to become a dealer for this product but haven't heard back from them on my status. It's worth a look. It will read 0-60 volts, 0-100 amps, and more.

**broken link removed**
 
Hi Ron,

Thanks for the link regarding wattage rating - that has explained it very well.

Now can anybody out there tell me if my circuit design is valid, please?
 
You seem to be over complicating the switching.
See my (crudely) edited version below.

JimB
 

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  • Ammeter Mode Edited.JPG
    Ammeter Mode Edited.JPG
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