Question on cheap lcd panel meter. - Page 1
Forum section: electronics / beginners

author: sureshot
author_rank: Regular Contributor
timestamp: August 04, 2018, 11:52:40 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: August 04, 2018, 01:42:29 pm
content: 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.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 04, 2018, 02:33:48 pm
content: 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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 04, 2018, 07:08:41 pm
content: 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.

author: mariush
author_rank: Super Contributor
timestamp: August 04, 2018, 07:54:20 pm
content: 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 boardYou 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?

author: Ian.M
author_rank: Super Contributor
timestamp: August 04, 2018, 08:02:41 pm
content: As I said, it *may* be difficult to, (at the same time), power it from a non-isolated rail in the PSU, get positive current readout for current flow from the PSU to the load and avoid its own current consumption being displayed if no load is connected.What's the configuration of your PSU's secondaries, and rectifiers, with RMS voltages? Its almost certainly possible (and cheap) to add a capacitively coupled rectifier to provide it with a supply that has no DC connection to the main unregulated and 0V rails.

author: rstofer
author_rank: Super Contributor
timestamp: August 04, 2018, 09:52:14 pm
content: The difference is that in one situation, Diagram 2, the MINIMUM load voltage is 4V. So, you can't turn the load off and have the meter display. In the other situation, Diagram 1, the minimum voltage is 0V - you are using a separate supply. The meter will stay on if the load voltage drops to 0V.The separate supply will always be preferable.

author: sureshot
author_rank: Regular Contributor
timestamp: August 04, 2018, 11:02:23 pm
content: I know there cheap, but on a radio forum I'm on the guys convert server power supply to run high current amplifiers. I know they hold up reasonably well, both up to 10 Amps with no external shunt, and up to 100 Amps with external shunt. I see what Ian is saying about not being viable to run off the supply its measuring. I probably will opt for a dedicated power supply, I should just be able to squeeze it in. I'm just curious if it would run off a separate regulator from the rectified and filtered input voltage. Its 15 Volts AC so around 21 Volts after rectification and filtering. I was going to try a 7812 off the filtered input. Diagram 2 does work, I wired it up to an atx power supply with a 50 watt halogen lamp. I'm thinking meter power wires to added 7812, yellow sense wire to main output. And current wires red black thick wires on negative side of the output. I've no idea what a capacitive rectifier is. Couple of what I'm messing about with. 5 Amp and 10 Amp units, in various stages of completion.

author: Jwillis
author_rank: Super Contributor
timestamp: August 04, 2018, 11:50:41 pm
content: In the bottom left corner of the PCB of the panel meter is a SMD/SMT SOT-223-4 regulator for the LED display and the controller. These are rated just as any regulator with a maximum input around 35 volts. So with if your voltage is below 35 volts to the load you can use the supply voltage for the panel meter from the load supply .If the load has higher voltage ,the panel meter needs a separate supply that doesn't exceed 35volts but typically 30V. Just a guess but could be a LM340MP-5.0

author: Ian.M
author_rank: Super Contributor
timestamp: August 05, 2018, 12:20:50 am
content: I assume you've got a 12V regulator in there to keep the fan happy. Test diagram 2 with the red wire fed from your +12V regulated supply. If it reads other than 0A with no load on the main output, or reads one count high for very small loads, or jitters by one count you need an isolated supply for the meter.In that case, add an isolated DC-DC converter, fed from the 12V rail, with its output wired to the meter as per diagram 1. A cheap 1W 12V to 5V one will do the job if the max meter supply current is under about 100mA, otherwise get a beefier one. As they are typically about the size of a cafe sugar cube you should have no problems fitting it in.

author: StillTrying
author_rank: Super Contributor
timestamp: August 05, 2018, 12:22:51 am
content: From the 2 images in the 1st post it must be like this. ( but don't blame me if it blows up! )The current measuring shunt being in the ground line can be a bigger problem than supplying 4-30V to the thin red wire.

author: sureshot
author_rank: Regular Contributor
timestamp: August 05, 2018, 12:58:28 am
content: Yes I was thinking of a dropping resistor for the fan from the rectified filtered input, but a 7812 is a better option. I'm going to try that with the meter added sourcing its power. Then wire up the rest of the meter for sense and current measuring. If it goes pop, or starts acting up I will go with a buck converter from the input, or maybe even a separate small chassis mount transformer. The meter is said to use less than 20mA for its current. It will come together somehow. Thanks for the help from everyone, appreciated.