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.
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.