Electronics > Beginners
Question on cheap lcd panel meter.
sureshot:
I'm wanting to use one of those cheap Chinese lcd panel meters that measure up to 100 volts at 10 Amps. The application is for a variable power supply, i think its diagram 1 of the two schematics I've added below. Although not 100% sure, as my power supply will go below the operating voltage of the meter. Am i right in thinking it needs its own DC power supply for that operating configuration ? And if it was a fixed power supply above 4.5 Volts then it would be diagram 2 I'd have to buy a small transformer to power the meter if its diagram 1 but just wanted to check before doing that.
Thanks for reading, a pointer in the right direction appreciated.
Ian.M:
I think you'll find the thin black wire is internally connected to the thick black wire.
Diagram 1 is definitely preferable, as if you use diagram 2, you'll have a lot of trouble getting the meter to read positive voltage and current without showing its own power consumption on its current display when no load is connected.
For diagram 1, it shows the meter powered by a battery. It could also be powered by an isolated DC-DC converter or by an auxiliary capacitively coupled bridge rectifier if the main supply is powered by a line frequency mains transformer.
Doctorandus_P:
First:
These meters almost always have LED displays, not LCD's.
The thick and thin black wires are almost certainly connected internally, but there is a good chance that the shunt resistor is in between them.
The red wire is positive power supply for the module.
If you have a voltage between 4V and 30V available in your power supply then you could use that.
Those modules are not all the same, even they lok the same. There will be performance & resolution differences between differnt "brands" of modules.
sureshot:
Thank you for your replys. Yes I think there are different variants of them. I've powered it up as per diagram 2 and that was just to test it functioned. I hooked it up to the 12 Volts of a converted atx power supply. I was under the impression that it could not be powered and read from the same source, but figure diagram 2 seems to contradict that idea. What I was going to do is power it from a very small chassis transformer that's rectified and filtered, and maybe use a 7806 for it. But I was wandering if it could be powered from the unregulated filtered output of the variable power supply, via a 7812 regulator. As at the filtered stage its constant and not variable. So dropping the 21 Volts filtered voltage with an additional 7812 which stays constant. But that might come under not wanting to read and be powered from its own source.
Thanks again for the help.
mariush:
Those cheapo lcd panel meter things typically have a very cheap current shunt, so if you have currents like let's say over 4-5A for long periods of time, you may cook that current shunt and the circuit board
You should either get one of those hall effect sensor chips or current transformers and measure the voltage reported by these.
As for up to 100v range ... I'd say you'd have to use a voltage divider or something to bring that down to a lower voltage range and then you could use an ADC or a multimeter IC to measure that voltage and display it... for example you could get a PIC and set the voltage reference to 4.096v and use the 10bit ADC to measure it and multiply that by 25 (4.096v x 25 = 102.4v) ... downside is fairly low precision... it's what... 100mV steps?
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