Power supply for digital ammeter

[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