hello all, i've a bunch of those chinese voltmeter 3.00-99.9volt (positive) they have only two wires one positive and one negative, they use it for self powering and measurement. i have a -12v line i would monitor, how can i convert it to positive? could someone help me please? thanks
Flipping the leads won't do as that would reverse the polarity of the power supply. Connecting them normally - positive wire to positive input - would probably work without displaying a negative sign. I have a similar module somewhere but I don't have a clear answer for the OP until I've had time to check it out more thoroughly.
These things are usually digital voltmeters with a built-in 5V linear regulator. Some have separate connectors for power and for signal while others have the two joined together by a jumper on the pcb and share the same external connector. In the latter case, the power and signal inputs can usually be separated by removing the jumper.
However, the OP says that his VM goes up to 99.9V whereas mine is rated for 30V, apparently because it uses a linear regulator.
*If* the OP's meters can really take 100V (I've seen the ads at AliExpress), they must use a different regulator. Dropping almost
100V with a linear regulator at enough current to power an LED display would generate a lot of heat for a small SMD.
Agreed, this is the correct answer. Think of it this way, a two wire meter measures the difference in potential between the two wires (and powers the unit) thus the more negative wire must go to the more negative source (-12) and the positive wire goes to the more positive side of the supply - in this case common.
It will work just fine.
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
Sure it does. The voltage feed into a reference reference and is compared with the input voltage. The result is a useful measure of the input voltage - providing that the input is above the minimum range the device needs and less than the voltage that produces smoke. I'm not sure I would like to use one over about 30 volts without looking closely at it first.
I look forward to the day when a chicken can cross the road without having
its motives questioned.