Atten TPR3005T-3C lab power supply GND problems?

[Aido4ever]

Hi everyone.

I bought this power supply from aliexpress.

It's a 30v 5A 3 channel power supply with channel 1 and 2 at those specs and a third one is a select-able 2.5v/3.3V/5V channel!

The power supply output exactly what i want when i check between the - and the + terminal of each channel (in channel 1 and 2). But when i check between Terminal + and GND i get only AC voltage at fixed Amplitude which can not be changed!

Also, i found out the GND in the two channels, which suppose to be independent, are connected!

here is a link to the product in Aliexpress:

I can not find any use for the GND Termilas. since they aoutput nothing on DC voltage and A fixed  amplitude AC Voltage of 10v pp (3.3v rms)

http://www.aliexpress.com/item/Atten-TPR3005T-3C-Adjustable-voltage-constant-current-DC-power-supply/607481340.html

[electr_peter]

Hi, Aido4ever,

From your description of the issue few things are apparent:

• you have insufficient knowledge about power supplies and mains earth issues - this is very important SAFETY ISSUE and practical consideration
• you bought a power supply with isolated outputs and it performs as it should.

As for 1), please check this forum topics, Dave's videos about power supplies and mains earth issues with power supplies and oscilloscopes, isolated(floating) vs earthed power supplies, etc.
Again, this is very important issue and your questions shows that you do not understand it. You have to know your equipment so that you would be certain about both your safety and potential damage to other equipment by power supply.

How are you measuring AC voltage between GND (green terminal) on PS and +/-? Typical DMM has ~10MOhm input impedance and it shows ghost voltage - it is normal behaviour with mains powered systems.
If you load this ghost voltage (some DMMs has low input impedance mode, like Keysight/Agilent U1272, some Fluke DMMs), it will go away. If ghost voltage turns out to be low impedance AC source (which is unlikely), you have a problem.

GND (green) is connected(should be connected) to instument chassis, which is connected (must be connected) to mains earth. Both green terminal are shorted together. It is normal and it should be so. Your PS still has independant floating channels - these can be made dependant by connecting GND (green) to either + or - terminals.

But before your do anything with PS, please educate yourself about these topics

[Aido4ever]

Thank you for your reply.

I thought GND was suppose to be the mid point between + and - terminals in cirquits whcih need need negative and positive voltages. Not to be EARTH GROUND.. . (as in cirquits the tern GND is the -).

thank you for clerefying that.

I've tested the AC voltage with fluke 289 and with my rigol ds2202 oscilloscope. so it's not a ghost voltages is a bit distorted sinusoidal wave.

[electr_peter]

I thought GND was suppose to be the mid point between + and - terminals in cirquits whcih need need negative and positive voltages. Not to be EARTH GROUND.. . (as in cirquits the tern GND is the -).

+/- on your supply are supposed not to be connected to GND (green terminal = chassis ground = mains earth). Supply output is isolated by default (via transformer) until you remove this isolation by connecting "+" or "-" terminal to GND(green).
Different supplies use different markings. To be 100% sure, one should read manual and check with DMM for continuity.

I've tested the AC voltage with fluke 289 and with my rigol ds2202 oscilloscope. so it's not a ghost voltages is a bit distorted sinusoidal wave.

Did you use LoZ function on Fluke 289? LoZ mode is supposed to load circuit by ~1-3 KOhm load and remove ghost voltages. However, this LoZ mode is supposed to be used on mains voltages (50-300 V AC), and thus is inaccurate on low AC levels.

Your scope also should be grounded to mains earth, so it may be the best tool to debug this problem. Scope also has ~1-10 MOhm impedance (very low loading on ghost voltage).

If you are sure about your measurents (I still would do more measurements just to be sure), mentioned symptoms suggests that the is something wrong with mains/earth wiring and instument connections to mains:

• one possibility is that your instruments for some strange reason use only 2 pronged plugs (should be 3 pronged plugs for power supply and oscilloscope) AND/OR mains sockets have only two connections (no earth connection).
• other possibility is that your mains wiring do not have real earth at all (not connected/broken for some reason). If there is no earth in the room/house with mains wiring, all kinds of strange ghost voltages appear where there should be no AC voltages.
• some other issue with mains/earth wiring (maybe some AC motor close by is miss wired, etc.).

I would still say that the most likely cause is ghost voltage from mains to floating PS output. Check your setup carefully.

[Aido4ever]

Hi. thank you for your kind help!

While testing it again with the fluke 289 using loZ it showed 0 voltage....

Do i need to connect between the scope earth to the power supply earth and test again using the scope? or the testing i did with the scope is enough to know that there is something wrong with the power supply grounding?

Thank you!

[electr_peter]

Hi. thank you for your kind help!

While testing it again with the fluke 289 using loZ it showed 0 voltage....

That's great. If it shows ~0 volts on LoZ mode AC and DC between +/- terminals and GND, then your power supply is isolated correctly and there is nothing to worry about.

Do i need to connect between the scope earth to the power supply earth and test again using the scope? or the testing i did with the scope is enough to know that there is something wrong with the power supply grounding?

Thank you!

I think testing with scope is not needed. Scope has too high impedance to eliminate ghost voltages, it picks up all kind of noise and it can be dangerous (because of mains voltages).
Just to be sure, make these measurements with your multimeter:

• disconnect your power supply from mains. Test resistance between earth terminal of mains plug and +/i/gnd connections on power supply.
  
  • resistance between "+"(red) and mains earth on the plug - should be high, >10MOhm
  • resistance between "-"(black) and mains earth on the plug - should be high, >10MOhm
  • resistance between "gnd"(green) and "mains earth on the plug - should be low, ~0-1Ohm
• disconnect your scope from mains. Test resistance between earth terminal of mains plug and gnd connections on scope BNC connector (outer shell). Resistance should be low, ~0-1Ohm
• test voltages on LoZ mode on your electric socket
  
  • voltage between phase and mains earth - should be high, 120 VAC-240AC (depending on local mains voltage)
  • voltage between neutral and mains earth  - should be low, 0-5 VAC
  If you see voltages close to half of mains voltage (~60 VAC if mains is 120 VAC, ~120 AC if mains is 240 VAC) with above measurements, something is wrong with mains wiring.
• connect scope and power supply to mains. Check resistance between scope BNC outer terminal and power supply "gnd"(green) - should be low, ~0-2Ohm

By the way, you power supply has floating outputs. You can think of these outputs as a separate batteries with "+" and "-" terminals exposed. Now it is up to you to connect those "batteries" in a manner that is most useful for your.
If your need single power supply, any single power supply output would do.
If your need dual power supply with 0 reference (say +10 V / 0 V / - 10V), set voltages to 10 V on both outputs and connect one "+" to other "-". This connection can be said to be reference of 0V. Unconnected "+" would be +10V, "-" would be -10V.

Series/parallel configurations of power supply outputs can be accessed via buttons on some supplies -  check manual of your power supply.

GND terminal (green) is used when your need to reference your power supply rails to mains earth. If you do not need reference to mains earth, do not connect this terminal to anything.

Hope this helps.[/list]