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Topic: Digital Ammeter problem - I'm sure there is an easy answer. (Read 2071 times) previous topic - next topic

pazu

Hi all,

I'm hoping some electronic guru can assist. I'm building a power control system for a 3d printer and part of the system will measure voltage/amperage of various parts of the system - hotbed/hotend/control system.

I've bought these cheap Ammeter units with shunts here: https://www.ebay.com.au/itm/252487705726

They have been wired up as per the diagram but I've run into a slight issue, the units need a seperate power supply to the one I am measuring otherwise it won't read the load. I'm assuming it's not liking the common ground (or common source voltage).

I really don't want to run a seperate power supply just for the LCD units - does anyone have any suggestions or tricks to isolate the LCD power but still use the same source power supply?

Cheers,
Matt

jackrae

Post the supplier's wiring diagram and a drawing showing how you actually connected it up.  Those units are generally OK for measuring both the voltage of the supply system and the current being drawn from it.

pazu

On mobile so a little hard to be accurate, but it is wired exactly as in this diagram: http://image.dhgate.com/0x0/f2/albu/g1/M01/D1/60/rBVaGVagV82AYU22AAH-V8g4_0E131.jpg

The load power supply and input power supply are wired together (from same source).

When wired like that, I get voltage but no amperage measurement.

When using a different power supply on the input power supply (just for the LCD), everything works fine.

Thanks for the quick reply!!

stuart0

On mobile so a little hard to be accurate, but it is wired exactly as in this diagram: http://image.dhgate.com/0x0/f2/albu/g1/M01/D1/60/rBVaGVagV82AYU22AAH-V8g4_0E131.jpg
With all the wires and shunt disconnected, test whether or not there is continuity between the terminal marked green and the one marked black.

pazu

Can confirm there is continuity between black and green. If I just use the red wire (LCD power +) only, LCD will power up, still don't get any amp reading though.

stuart0

Ok, it looks like it should work like that.

You say that you have had it working ok with a separate supply for the power input. Can you show a wiring diagram of how its configured when working.

Also, did you get any other data or information with the meter?

pazu

I had it wired exactly as per the original diagram posted above, with input power (top left) hooked up to a seperate power supply, everything else the same. As soon as I did that it started to work. The only difference I can tell between the two layouts is I'm essentially using two seperate grounds - even though they appear connected.

That doesn't make sense too me.

No other information besides what was in the eBay listing (similar diagram but in Chinese)

stuart0

That doesn't make sense too me.
Yes it seems odd to me as well.

Are you absolutely certain that there is no other connection (chassis ground for example) between the load negative (yellow wire) and the power supply negative (black). If there was somehow such a connection (external to what is shown on the diagram) then it would bypass your shunt. That's the only thing I can think of.

pazu

I'll triple check everything again first thing tomorrow and take better photos, but I'm 100% sure - I'm using an ATX PSU, bench testing it all. The load is a set of LEDs I'm touching between the shunt load point and positive on the power source.

jackrae

You could try permutations of positive to positive and negative to negative then document what actually happens.  It may be that the way the circuit is designed that it cannot tolerate the sense voltage having the same power supply as the drive voltage.  This used to be a common weak spot on many of the cheaper digital meters whereby the measured voltage could not be used as the power voltage.

If there isn't a simple wiring solution, one way around the problem would be to use an 'isolated' DC-to-DC converter module.  This will use the measured voltage as its input and output an isolated DC voltage to power the supply lines.  Something like one of these

edit : Just seen your last post.  It could be that the LED current is too low to register.  Can I suggest a 12volt/12 watt halogen lamp, or similar

pazu

so I've checked everything this morning and it is definitely isolated.

Here is a picture of the current setup - connections are labeled. I'm applying my load on the NEG end of the shunt and the output labelled +Hotend.



Jackrae - thanks for your advice, the LED unit pulls about 250ma (high wattage one), so it definitely registers when using a separate supply.

I think your suggestion of using an isolated DC-DC module might be the answer - I'll try and get one in and test.

Thanks for everyone's help so far.

stuart0

Just as a quick sanity test, can you check the voltage drop across the shunt with a handheld digital voltmeter with a DC millivolt scale.

Do it once with your panel meter disconnected and only the shunt in circuit. Then repeat the measurement with your complete circuit and meter in place.

BTW. What is the shunt's rating ( ? mV/A )

pazu

done - in all scenarios it has a drop of about 0.2mv

Shunt rating is 50A - overkill for what I need but most ammeters without a shunt only handle up to 10A, I need between 10-20A.

Cheers,
Matt

stuart0

done - in all scenarios it has a drop of about 0.2mv
Shunt rating is 50A - overkill
Ok, but that means your test load is only 0.5% of full scale. With a cheap meter, are you sure that you're not just looking at noise/offset there?

pazu

it's possible - I'll try with a bigger load, but using the LED load I have is repeatable in that when using it I get 0.7A each time - so its definitely measuring something.

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