personal bitch, voltage meter

LewTwo

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Houston, Texas
volt meter 100v dc.jpg
I keep looking for a small, cheap voltage meter to add to my battery pack(s). I hate those stupid LED gauges. The problem is all these 0-100 Volt DC meters use three wires. The third wire is to power the meter and requires somewhere between 5 and 30 volts. Now our friends in China are very clever. Why doesn't one of these geniuses add a couple of resistors to form a voltage divider so the darn things would only require a simple two wire connection :?: :?: :?: :?: :?: :?:
 
They make them 2 wires. I have bought a buch of them a few years ago and I use them on all my bikes. I can't find the link right now but I will.
 
LewTwo said:
Why doesn't one of these geniuses add a couple of resistors to form a voltage divider so the darn things would only require a simple two wire connection

If they added a divider to divide 100V -> 30V for supply, then the minimum voltage you could test would be 15V; as the divider would drop that to below the minimum required supply voltage of 4.5V.

If they set it to take 4.5V from an input of 12V; then it wouldn't handle inputs <12V, and the maximum input it would be able to deal with is 80V.

They probably reason that it is easier for you to select a couple of resistors to suit the range requirements of your application.
 
Buk___ said:
LewTwo said:
Why doesn't one of these geniuses add a couple of resistors to form a voltage divider so the darn things would only require a simple two wire connection

If they added a divider to divide 100V -> 30V for supply, then the minimum voltage you could test would be 15V; as the divider would drop that to below the minimum required supply voltage of 4.5V.

If they set it to take 4.5V from an input of 12V; then it wouldn't handle inputs <12V, and the maximum input it would be able to deal with is 80V.

They probably reason that it is easier for you to select a couple of resistors to suit the range requirements of your application.
OK ... how about a 5 volt zener diode and a resistor?
 
LewTwo said:
OK ... how about a 5 volt zener diode and a resistor?

It would still limit the test voltage to >5.6V. Perfect for your application, but they are selling a general purpose unit.

That's why the 2-wire versions are usually targeted at 12-24 volts or similar.
 
I just got one of these mini voltmeter with the yellow, red, black wire. I want to attach it to my 36v battery pack.
Can someone explain what wires i need to connect. Thanks

DIY Red Blue Digital LED Mini Display Module DC2.5V-32V DC0-100V Voltmeter Voltage Tester Panel Meter Gauge for Motorcycle Car
It is just like the posted by LEW TWO.
 
This is a little off topic, but I really like Luna Cycle's flat, LCD voltmeter.
LCD-Can leave hooked up all the time, no drain on the Batt.
2 wires
Very small and FLAT, easy mounting
Timed back light w/ push of a button.
$13
https://lunacycle.com/luna-voltage-meter/

Opps, looks like it's out of stock. Maybe something simular is aval.
 
Found this one 12 to 100v with extra switch.
12-100v-twist-throttle-Accelerator-with-Led-digital-display-indicator-lock-for-electric-bike-scooter-mtb.jpeg
 
If it were me, I'd just power it off a separate smaller battery hooked up to a momentary switch. The small battery will last a long time because it's only on for a second or two while you look at the voltage...

Way easier than messing around with trying to step down the voltage.
 
MadRhino said:
Found this one 12 to 100v with extra switch.
12-100v-twist-throttle-Accelerator-with-Led-digital-display-indicator-lock-for-electric-bike-scooter-mtb.jpeg

I want that but thumb throttle with voltage display and key lock would be great. Exactly like the Cyclone half throttle, but thumb throttle.


https://www.ebay.ca/itm/12-100v-finger-thumb-shifter-Accelerator-with-digital-display-lock-key-scooter/272972386138?hash=item3f8e6c0f5a:g:UOUAAOSwZuRZxcpV
^Good, but red voltage numbers would be better. Easier to see in sun.

https://www.ebay.ca/itm/scooter-thumb-finger-shifter-throttle-with-digital-display-accelerator-12-100v/272946172864?hash=item3f8cdc13c0:g:rfQAAOSw6YtZUJmM
^Kinda Perfect! No key though
 
LewTwo said:
Why doesn't one of these geniuses add a couple of resistors to form a voltage divider so the darn things would only require a simple two wire connection?
Because that would take a lot of power and would always be a poor compromise. Better (IMO) to use that power to move the bike, rather than using it to heat up resistors.
 
billvon said:
LewTwo said:
Why doesn't one of these geniuses add a couple of resistors to form a voltage divider so the darn things would only require a simple two wire connection?
Because that would take a lot of power and would always be a poor compromise. Better (IMO) to use that power to move the bike, rather than using it to heat up resistors.
A lot of power?? Two1/4 watt resistors would not be much of a power draw.
 
LewTwo said:
A lot of power?? Two1/4 watt resistors would not be much of a power draw.
A 1/4 watt voltage divider won't work. Do the math. Assume a current draw between 80ma (displaying 18.8 volts) and 30ma (displaying 11.1 volts.) Even assuming a perfectly stable voltage you're not going to be able to find a combination that will give you decent regulation without drawing a lot of power (watts.) The problem you will run into is that your divider has to draw enough current so that the changing current of the display is negligible compared to the current drawn by the divider. So that means ~500ma divider current draw (that would give you roughly 10% regulation.) And 500ma at, say, 36 volts is 18 watts in the divider resistors. That's a lot of power to throw away.

You can do better with a zener plus a resistor, but you still have to plan for the lowest expected voltage. And when you hit the highest expected voltage you are going to see a lot of dissipation in the zener as it regulates. Example - a 30 to 42 volt system with a 5.1 volt zener. To work at 80 ma / 30V you need 312 ohms at 2 watts and no power in the zener - not too bad. But at 30ma / 42V you are at 4.4 watts on the top resistor and .45 watts in the zener; total of 5 watts wasted.

A simple resistor + zener + PNP regulator will get you much lower power overall, since you will only dissipate the "wasted" power (worst case of 2.9 watts at highest voltage/current; goes down from there.) A small switcher will give you the lowest possible power use - at a cost.
 
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