AC / DC Amp Meter Question

[ndemge]

Riddle me this....

I have a string of 130 VDC batteries. (10 12v batteries at full 13.0 charge)

I hook up a Standard 120vac Lightbulb. It burns just fine.
Clamp Amp meter reads nothing.

-this same meter, when clamped around the wire and hooked to AC, shows the .83 amps that it is pulling.

Why does it not read? Is there a different between an incondecent light bulb made for AC and for DC that causes this?

I use this meter for measuring AC and DC all the time, works great.

 

[Coors]

Re: AC / DC Amp Meter Question

Does the clamp read dc?

 

[ndemge]

Re: AC / DC Amp Meter Question

Yup.

If I'm charging the batteries, I can clamp it on, reads just fine.

 

[i386]

Re: AC / DC Amp Meter Question

I'm stumped. This is interesting.

 

[ndemge]

Re: AC / DC Amp Meter Question

Was just messaging with an EE friend of mine,,, he asked me all the obvious questions, led to the same . hmmmmmm

I can take the load, plug into AC and it reads fine
I can take the load, plug into DC and it does not read.

amp meter reads DC just fine when under a normal DC load.

...off to find a new resistive load.

 

[ndemge]

Re: AC / DC Amp Meter Question

... Just took a small fan forced space heater.
The heating coils heated up, still no load showing on the meter.

(was 7.4 amp running on AC)

Fan didn't turn.

...And then the coil over heated and let out the magic smoke. -so my last fan driven space heater is now out of the house. I feel safer already



Tinker for the sake of tinkering.
If it ain't broke- Take it apart to find out why.

 

[JustJason]

Re: AC / DC Amp Meter Question

what is the make and model of both your DMM and amp clamp?

 

[Coors]

Re: AC / DC Amp Meter Question

Me thinks it is broke.

 

[dolluper]

Re: AC / DC Amp Meter Question

Always U get get these scre-wy electrical problems why you??

 

[JCF350]

Re: AC / DC Amp Meter Question

... Just took a small fan forced space heater.
The heating coils heated up, still no load showing on the meter.

(was 7.4 amp running on AC)

Fan didn't turn.

...And then the coil over heated and let out the magic smoke. -so my last fan driven space heater is now out of the house. I feel safer already



Tinker for the sake of tinkering.
If it ain't broke- Take it apart to find out why.

You trying to measure DC amps on one or both of the conductors (wires).
A/C fan won't turn because there is no change in polarity.

 

[JB]

Re: AC / DC Amp Meter Question

Not broke.

The clamp senses current by electromagnetic induction by measuring the change in the magnetic field around the wire. With pure DC current the field does not change. With pulsing DC (the battery charger output) the field changes just as in AC and the induction takes place.

To measure pure DC current you have to break the circuit and place an ammeter in series with the load.

 

[ndemge]

Re: AC / DC Amp Meter Question

JB,

So when I use this meter to measure the DC output of bateries going to my inverter, does it read because the current is being used in pulses?

 

[JCF350]

Re: AC / DC Amp Meter Question

Not broke.
To measure pure DC current you have to break the circuit and place an ammeter in series with the load.

Not so with a DC amp clamp, they measure the strength of the magnetic field around the conductor. Use them all the time, much faster than installing a shunt/or a meter in series with the circuit.
http://www.myflukestore.com/p3721/fluke_88-v.php?p_tab=accessories#products_categories_16


But this may his problem as he might not be using the right kind of amp clamp.
BTW DC amp clamps are directional.

 

[SS MAYFLOAT]

Re: AC / DC Amp Meter Question

Those pulses would be 30 times per second on way, then 30 pulses a second the other way, thus 60 hz. DC motors need brushes to run. Brushless motors or normally AC only. Thats why the motor didn't run on your heater.

Noel, look a a schematic for the amp meter wiring for a boat. You will need to use your leads vs the amploc. I've never had any luck getting a ture reading with a clamp style meter for reading DC.

 

[whywhyzed]

Re: AC / DC Amp Meter Question

[....

 

[ndemge]

Re: AC / DC Amp Meter Question

Found some specs on the meter. Turns out, it's not meant to measure DC current. So aparently I can only use it to measure dc current, IF it's to power an inverter, or being charged by AC power. -And it does that rather acuratlly I might ad.

Interesting.



Meter is a Sperry DSA-500


Spec Summary:

Withstand Voltage: 3,700VAC (RMS, 50/60Hz) for 1 minute between electrical circuit and housing case.
Insulation Resistance: 10M Ohm or greater at 1,000V between electrical circuit and housing case.
Dimensions:7"L x 1.75"W x 1.1"D (184 x 44 x 27mm)
Weight: Approx. 7 oz (190g) including batteries

AC Current
Ranges: 40A (0-39.99A)
Ranges: 400A (0-399.9A)
Basic Accuracy: ?2.0%rdg ?6dgt (50/60Hz)

AC Voltage
Ranges: 400V (0-399.9V)
Ranges: 600V (150-599V)
Basic Accuracy: ?2.0%rdg ?5dgt (50/60Hz)

DC Voltage
Ranges: 400V (0-399.9V)
Ranges: 600V (150-599V)
Basic Accuracy: ?1.5%rdg ?5dgt

Resistance
Ranges: 400 Ohm (0-399.9 Ohm)
Ranges: 4,000 Ohm (150-3999 Ohm)
Basic Accuracy: ?2.0%rdg ?5dgt

 

[JustJason]

Re: AC / DC Amp Meter Question

put your meter on DC milivolts
put 1 end on the negative battery cable

here's where it gets screwy....

pierce your negative battery cable exactly just shy of 19 1/8 inches with the other test lead.

What the meter reads in milivolts is your amp draw.

or.... what's the wattage of the bulb?

divide the wattage by the voltage and thats your amp draw.

 

[QuadManiac]

Re: AC / DC Amp Meter Question

put your meter on DC milivolts
put 1 end on the negative battery cable

here's where it gets screwy....

pierce your negative battery cable exactly just shy of 19 1/8 inches with the other test lead.

What the meter reads in milivolts is your amp draw.

or.... what's the wattage of the bulb?

divide the wattage by the voltage and thats your amp draw.

This dimension (19 1/8") will only work with one guage of wire and one brand/type of meter. It's just using the resistance of the wire in parallel with the shunt resistance of a particualar meter as a current divider. Trying this with another meter or different guage wire may well blow the meter's fuse or give a completely false reading.

 

[JustJason]

Re: AC / DC Amp Meter Question

This dimension (19 1/8") will only work with one guage of wire and one brand/type of meter. It's just using the resistance of the wire in parallel with the shunt resistance of a particualar meter as a current divider. Trying this with another meter or different guage wire may well blow the meter's fuse or give a completely false reading.[/qoute]

hmm... should work with all meters...
I know it works with my Fluke 8, my fluke 88V, my es530.... i've even done it with borrowed crappy blue point meters.
Internally i don't believe the voltage input jacks on the meters are fused... just the current inputs.

 

[i386]

Re: AC / DC Amp Meter Question

I'm glad y'all figured this out.

Like JB said, I thought clamp meters only measured AC so I did a little reading. According to this article at Wikipedia...

"Newer units often read both AC and DC current. A Hall effect device is used to detect the DC magnetic field induced into the clamp. The signal from the Hall effect device is amplified and displayed using a built-in DVM. Typical handheld units can read currents as low as 200mA, and units which can read down to 1mA are available."