I Keep on Blowing Up Panel Meters! - Page 1
Forum section: electronics / projects

author: German_EE
author_rank: Super Contributor
timestamp: October 04, 2016, 06:34:27 pm
content: OK, crazy problem time.Take one dual channel LM317 based power supply, the regulators share a bridge rectifier and common ground but their outputs can be individually set between 1.5V and 25V. The output of this supply is clean, noise free and doesn't oscillate on either channel.Add two panel meters. These are the cheap 200mV variety where an isolated supply is required so I feed them from a separate transformer and rectifier. There is no connection between this supply and either regulator. To give 200V full scale the inputs have a 1M resistor in series and a 1K ohm across the input, not exact but close enough.When wired to the variable supply one meter lasted about five minutes before it died showing 'overange' on the display even when the supply output was at 5V. This was replaced then an hour later the other meter died showing a blank display, I gave up and went home. This morning I replaced the second meter and now another one has failed with a blank display.The meter specification demands an isolated supply and I've arranged this. Are these meters being killed by being fed through a common ground or is there something else? Remember that they are being fed via a 1M ohm resistor so any currents are in the microamp range.

author: madires
author_rank: Super Contributor
timestamp: October 04, 2016, 06:55:54 pm
content: AFAIK, each panel meter needs its own isolated power supply, not a shared one.

author: StillTrying
author_rank: Super Contributor
timestamp: October 04, 2016, 08:23:26 pm
content: To give 200V full scale the inputs have a 1M resistor in series and a 1K ohm across the input, not exact but close enough.Until you figure what's going wrong, it might be safer to put a 2k2 across the meter's -/+ Vin, and a 1M1on eachof the inputs, that should give the common mode Vin more chance to float to where it wants to.Link to the exact meter ?

author: MickM
author_rank: Regular Contributor
timestamp: October 04, 2016, 10:22:23 pm
content: "Are these meters being killed by being fed through a common ground ?"I would have to say yes they are.Isolated includes the ground.Try a 9v battery and 7805.Mick M.

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2016, 10:26:24 pm
content: Dumb question, but are the 1 M resistors able to drop 200 V without breakdown?(Edit: OK, you do say you are not actually feeding 200 V to the meters. So my question may not be relevant.)

author: German_EE
author_rank: Super Contributor
timestamp: October 08, 2016, 08:30:14 am
content: An update. I changed around the internal power supply arrangements so that each meter was fed from an independent supply but the common ground on the measurement input remained. The panel meters no longer die on me. During my research I found a data sheet from Murata where they showed the internals of their meter and, assuming that the design has common elements, I see the problem, something to do with the input ADC. Note that mine are NOT Murata devices but no name meters from Conrad.For future reference. If you purchase a 5V supply panel meter you should be OK no matter what you do, but a meter with a 9V supply needs to be totally independent from everything else including other panel meters.

author: electr_peter
author_rank: Unknown
timestamp: October 08, 2016, 10:07:16 am
content: Many panel meters require isolated power supply to work correctly. If two meters of such kind are used in, for example, PSU to measure voltage and current, 3 completely independent power sources are required - 1 for PSU, 1 for voltage meter, 1 for current meter. Voltage and current panel meters must have isolated power sources.It is a common situation. In practice such problem is solved by having extra transformer windings or several isolated DC/DC converters.OP problem seems to be that circuit provided only one isolated power source for two panel meters while two isolated power sources were required.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 08, 2016, 04:47:59 pm
content: Quote from: German_EE on October 08, 2016, 08:30:14 amBeware of ones that are multi-power too! Blew up all of mine with 160V because the isolation failed!

author: Behemot
author_rank: Contributor
timestamp: October 14, 2016, 11:57:55 am
content: So if I understand it correctly, if I wanted to use 10 panel meters (models with independent PS+/PS- and VIN+/VIN- inputs) I can not use comon PS insulated from the measured circuits, but I still need 10 separate PSUs separated also from each other? Like rly?

author: Andy Watson
author_rank: Super Contributor
timestamp: October 14, 2016, 12:12:31 pm
content: Quote from: Behemot on October 14, 2016, 11:57:55 amWith respect to the meter, are Vin+Vin- inputs independent of PS+PS- ? Unless the datasheet says that Vin is floating I would assume that it is NOT, - i.e. VIN+VIN- should lie within the range defined by PS+PS-.

author: Behemot
author_rank: Contributor
timestamp: October 14, 2016, 01:09:18 pm
content: I have no idea if it has any datasheet at all, but it has these two inputs. The fleebay description statesQuoteSo if I select the power supply voltage to be slightly higher than then measured voltage, say 14 V for PS and maximum about 12 V for the measured circuit, I should be on the safe side? And power all the meters from single PSU shared by all of them?Also for negative voltage I assume I'd plug GND to VIN+ and the negative voltage to VIN- and the meter will than show the absolute value.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 14, 2016, 02:24:57 pm
content: Quote from: Behemot on October 14, 2016, 01:09:18 pmInteresting! But, I would still make the assumption that Vin needs to be within the range Ps- to Ps+.QuoteSame as above, I would ensure both VIN- and VIN+ are within the range defined by PS. If you use a common PS power supply for all the meters then voltages that you are measuring must also share a common reference and be within PS- to PS+.Could you reverse-engineer the meter circuit?

author: Behemot
author_rank: Contributor
timestamp: October 14, 2016, 02:34:32 pm
content: Don't have them yet so that's why I am asking, to not waste the money while I may end with useless meters.However according to the photos they seem to be simple, I can see two ICs and couple SMD components.To be exact, I'd like to measure voltage of 5 rails from a power supply, those have common ground so that's OK. Than I also want to measure current using precision current shunts so there I will only measure voltage drop over the shunts (with 5 more panel meters) with no ground. All 10 of them will be powered by some other external PSU. I thought that would be just OK but after reading this thread it makes me wonderNext thing which comes to play is that while the two PSUs may be actually galvanically separated from mains and each other, most of them are ground-referenced. So should I keep ground reference, or use un-grounded (class II) PSU for the meters? Questions, only questions

author: Behemot
author_rank: Contributor
timestamp: October 17, 2016, 10:50:45 am
content: As could be expected, no reasonable answer from the seller…not even the manufacturer name

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 23, 2016, 11:16:35 pm
content: Use other panel meters !From Ebay/Ali you can get 5 digit panel meters for < USD5. These have a STM8, microchip adc & led displays.No Isolated Vcc troubles there.