Power supply for digital ammeter

[DocHeb]

I am trying to use a Digital AC Ammeter to measure the amount of current running through my Automatic Standby Generator Panel. It was sold on ebay and is a AC 200mA Ammeter with a 1000:1 coil.

This Ammeter has the following specifications:

"Required Supply Voltage AC/DC 8-15V. Supply current can not exceed 150mA or this meter might be damaged."

I am having problems finding a transformer that has <150 mA. I don't understand the concept of excess supply current. If I were to plug a light into a 20A houshold circuit it doen't mean that the light will pull all 20 amps. Why would this digital unit pull too much current? Where can I buy an appropriate power supply?

 

[SnowRidge]

The ad says that the meter power requirements are 8-15 vdc or 8-15 vac with a maximum draw of 150 ma. I would be suspicious of the 8-15 vac part, unless there is a data sheet accompanying the device that confirms this. That may be a typo in the ad.

Just power it is with a wall wart that supplies 9-12 vdc at 150 ma minimum, and you should be good to go.

 

[quicksandfarmer]

That's a very unusual spec for a power supply, unusual to the point where I think it is a mis-print. In any case, using the rating of the power supply is the wrong way to go -- the current rating is the minimum that the supply is guaranteed to deliver, not the maximum. The usual way to limit current is to use a fuse. Fuses that small are somewhat rare but they do exist, and you can find them with Google.

 

[RonMar]

I read those specs the same way SnowRidge does. It needs 8-15 VDC and up to 150MA. Your light bulb analogy is correct, you could plug a 12 VDC 500MA supply into this thing, but it is only going to draw up to 150MA...

 

[DocHeb]

Here is the info from the data sheet:

AC 200mA Ammeter

This Digital AC Ammeter Panel Meter was fabricated with LSI IC with highly
stable components, and adopted advanced measurement theory:

Specifications:
Required Supply Voltage: AC/DC 8-15V. Supply current can not exceed 150mA or
this meter might be damaged.
Dimension (LxHxD): 3 1/8 x 1 5/8 x 1 inch.
Installation opening (LxH): 3 x 1 1/2 inch.
LED tube size (HxW): 9/16 x 3/8 inch.
Accuracy: +/- 0.3% + 1 digit, at 23 degree Celsius +/1 3 degree, RH 45%~75%. 10
degree temperature change won't cause reading over this spec.
Brightness: High Intensity, 15mm Red LED.
Digits: 3 1/2 digits display max.: 1999.
Auto-zeroing: Yes
Auto-polarity: Yes
Sampling rate: 400 millisecond
Over range indication: Either 1 or -1 at the left most digit.
Ambient temperature requirement: 0 ~ 50 degree Celsius.
Decimal point position: Can be altered by shorting any of following points:
(However, max. measurement limit keeps the same)
G for 1 decimal point
S for 2 decimal points
B for 3 decimal points.

 

[RonMar]

That would be a weird spec for the voltage input that powers the meter.

It however would not be an unusual requirement for the sense input. That 150 MA could be for the sensed line that is being monitored to generate the display and is most likley the maximum range of the meter, just like mechanical meters have a maximum input limit beyond which damage to the meter movement will occur. This input sense current will be provided by the 1000-1 current tap transformer. If you exceed 150A on your monitored circuit, the Current Tap will exceed 150MA output to the meter.

 

[jinman]

Chris, I think they are using this caution to tell you to not use a transformer like many of those used to charge emergency battery packs. They will have an open circuit output of over 20VDC and can supply 500 mA. You need one that puts out less than 15VDC and will drop off if you pull over 150 mA. The internal power supply of your meter cannot regulate the voltage down to it's working level without dropping excess voltage by pulling current from the unregulated supply transformer. If your transformer can supply greater than 150 mA while supplying its maximum voltage, the internal power supply will overheat. I'd look for a low power 12VDC supply and try that.

 

[DocHeb]

Jim,
So a plug-in cellphone transformer, 12VDC - 300mA should be able to power two of these ammeters? I've got one that size in a drawer.

 

[jinman]

Jim,
So a plug-in cellphone transformer, 12VDC - 300mA should be able to power two of these ammeters? I've got one that size in a drawer.

Well....
What bothers me is that the meter might really like 9VDC to operate. That means it has to drop 3 volts on the 12 volt supply. 3 volts at 300 mA is the same power as 6 volts at 150 mA. So, technically, it should work really well on two meters. It should also be okay for one meter since the power drop should not be that excessive. If it were mine, I'd sure try it.

 

[SnowRidge]

That would be a weird spec for the voltage input that powers the meter.

It however would not be an unusual requirement for the sense input. That 150 MA could be for the sensed line that is being monitored to generate the display and is most likley the maximum range of the meter, just like mechanical meters have a maximum input limit beyond which damage to the meter movement will occur. This input sense current will be provided by the 1000-1 current tap transformer. If you exceed 150A on your monitored circuit, the Current Tap will exceed 150MA output to the meter.

I think the EBay listing said it was a 0-200 milliameter, so the sense input should be able to handle more than 150 ma. Unfortunately, the link seems to be gone, so I can't go back and take another look. 0-200 is a common range for digital meters, so I think those numbers are likely correct.

I have to agree that the spec is "weird." It does not appear to have been written by an electronics professional with a complete command of the English language. I suspect whoever wrote it (or the original data sheet that it was copied from) was not a native English speaker.

The statement that supply current in excess of 150 ma may damage the meter may have been intended to reinforce the power supply voltage limitations. In other words, saying something like, "Don't go over 15 volts or it will draw too much current."

To be safe, just use a regulated 9 or 12 volt DC supply. Here are links to two of them at Jameco Electronics:

Jameco Electronics Power Supplies & Wall Adapters: VARIOUS: DCU090025F8451

Jameco Electronics Power Supplies & Wall Adapters: JAMECO RELIAPRO: DCU120020G2420