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

 

[DocHeb]

Thanks to everyone for your advice. You confirmed what I suspected, but sometimes those know-it-all doctors need to step back and admit that they might not know everything.

Although the meter spec page lists it as a 200mA ammeter, the eBay page was selling it as a 150mA ammeter, so I suspect that the sense input current should be limited to 150 mA, rather than the power supply input. I ordered two of the 9V units SnowRidge suggested.

I don't know why the eBay page I linked to was removed by the moderators - I wasn't trying to sell something here, but - oh well!

 

[Soundguy]

I agree with jinman and snowridge.

Use a walwart that will throw up to 15vdc.. but is regulated.. IE.. open circuit unloaded voltage is still max of 15vdc. As jinman mentions.. some power packs have low or unloaded v-output much higher than their rated output. And that digital meter is a -very- small draw.. and perhaps not enough to load an unregulated wallwart down to a safe voltage level.

For safety.. get a 9-12v adapter... if it's output unloaded is higher than 15v, then buy a 2$ TIP cased 7812 regulator IC from radio shack plus a 2 pack of capacitors.. anything from 50uf to a few hundred uf is enough.. make sure dc volt range is 15 or better. With the tip packaged regulator on your desk, face up, it will have a mount tab on top, and 3 legs pointing down. The middle leg is common with the mount tab and is ground (- polarity). The tab on the left is V-in the tab on the right is V-out.

From your walwart, clip the wire. run negative to the ammeter negative power supply lug, also run a wire to the middle (-) pin on the vreg .. run positive from the walwart to the V-In pin, and from the V-out pin, run that to the ammeter positive power supply lug... voltage there should stay VERY close to 12v, as long as the wall wart can produce 12.6v+. Take your first capacitor and wire it's + side to the V-in pin, and it's (-) side to the middle (-) pin.. Take the other cap and wire it's (-) pin to the middle (-) pin, and it's + pin to the V-out pin. This provides some waveform filtering in case the walwart is 'cheap, and is half or fullwave with no capacitor inside to 'smooth' the bumps. The vreg chip will run much better with a filtered v-in pin.

Soundguy

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?

 

[Soundguy]

It's been posted here many times that links to EBAY auctions are expressly not allowed.

soundguy

I don't know why the eBay page I linked to was removed by the moderators - I wasn't trying to sell something here, but - oh well!

 

[DocHeb]

I guess I don't get to enough forums, I don't recall seeing that. I searched FAQ for "ebay" and "link" and got no hits describing this, and it isn't in the general forum usage section - oh well!

 

[DocHeb]

Use a walwart that will throw up to 15vdc.. but is regulated.. IE.. open circuit unloaded voltage is still max of 15vdc. As jinman mentions.. some power packs have low or unloaded v-output much higher than their rated output. And that digital meter is a -very- small draw.. and perhaps not enough to load an unregulated wallwart down to a safe voltage level.

For safety.. get a 9-12v adapter... if it's output unloaded is higher than 15v, then buy a 2$ TIP cased 7812 regulator IC from radio shack plus a 2 pack of capacitors.. anything from 50uf to a few hundred uf is enough.. make sure dc volt range is 15 or better. With the tip packaged regulator on your desk, face up, it will have a mount tab on top, and 3 legs pointing down. The middle leg is common with the mount tab and is ground (- polarity). The tab on the left is V-in the tab on the right is V-out.

From your walwart, clip the wire. run negative to the ammeter negative power supply lug, also run a wire to the middle (-) pin on the vreg .. run positive from the walwart to the V-In pin, and from the V-out pin, run that to the ammeter positive power supply lug... voltage there should stay VERY close to 12v, as long as the wall wart can produce 12.6v+. Take your first capacitor and wire it's + side to the V-in pin, and it's (-) side to the middle (-) pin.. Take the other cap and wire it's (-) pin to the middle (-) pin, and it's + pin to the V-out pin. This provides some waveform filtering in case the walwart is 'cheap, and is half or fullwave with no capacitor inside to 'smooth' the bumps. The vreg chip will run much better with a filtered v-in pin.

Soundguy

And they say that doctors don't speak English!

Is the black three-legged thing in the picture a "TIP 7812"?

 

[Soundguy]

My wife told me I should go back to school and get another 4ys college and go from 'engineer' to 'doctor' based on my handwriting talents ( or lacktherof )!!

While I can't read the numbering on the package.. it is deffinately a TIP package. and is 'probably' a VREG chip. The round black part above it is a full wave bridge as you can see it has 2 ac lines denoted by ~~ and then a +/- .. so the 3 legged item is probably a tip VREG.. what model it is is anybodies guess.

78XX vregs are positive voltage vreg.. IE.. ground polarity is negative. ( voltage 'above' ground potential.

79XX vregs are negative voltage vreg.. IE.. ground polarity is positive. ( voltage 'below' ground potential.

The XX## designation is the vreg regulaton output level. IE.. a 7805 is a +5v reg.. a 7912 is a -12v reg.

Most of the 78/79 series vregs will tolerate into the mid 20's voltage range on the input, reguardless of the output... check your individual package to make sure of the specs before you use it..e tc.

Soundguy

 

[Soundguy]

I guess I don't get to enough forums, I don't recall seeing that. I searched FAQ for "ebay" and "link" and got no hits describing this, and it isn't in the general forum usage section - oh well!

I'm not sure if it is in a faq or 'sticky' message somewhere.. but I did find at least one reference to it from Hakim in post #10 in the following link.

soundguy

http://www.tractorbynet.com/forums/...biggest-used-4wd-tractor-12k.html#post1236206

 

[SnowRidge]

Doc shouldn't need any of that stuff with the regulated 9 vdc supplies he ordered.

 

[Soundguy]

I missed the link for the PS he ordered.. i was just making a generalized comment.. I usually just garb an xformer 'close' to the V-out range i need, and FWB of fthe shelf and drop in the filter caps and a vreg chip.. perhaps an inductor ( for PI or L filter setup ), and then go. Depending ont he supply voltage i need, or the MA output desired, occasionally building is cheaper than buying.. Of course.. if I'm working on something with a big enclosure with internal room for the power supply section, that's fine.. if not, IE for a small project.. then the wallwart is better.. ( though small switching PS with some triacs can get pretty small and light/cheap by getting rid of the copper ball in there..)

Soundguy