AMPROBE PICTURE

Hello,

This is the amprobe that I used to measure DC Amps while charging two 2200 MAH Lithium batteries in parallel. Charging at about 1 amp at 3.87 volts and climbing. The amprobe was set at the 6 amp AC scale. It can measure AC amps up to 300, AC volts up to 600. It also has a ohms scale up to 1000 ohms. Old unit but still pretty accurate.
Can someone explain to me why it is working?
My home made charger is not that sophisticated. I'm pretty sure it is more like a linear output then a pulsating current,


Guido.

"I'm pretty sure it is more like a linear output then a pulsating current"

I'm pretty sure you're wrong.

That Amprobe uses a current transformer which splits open so it can encircle the wire, and it will NOT read actual DC current. Period.

Typical modern clamp-on meters use a Hall-effect sensor to read the steady magnetic field surrounding a wire carrying DC current.
DC clamp on

Guido,
Didn't Columbus have one like that on the Mayflower?

Hello bob,

Is it possible the you are wrong? The output current controlled by a rheostat. It is wired to a power transistor which receives 8 volts of dc power from a bridge diode. The transformer output is about 8 volts AC, with 120 volts in the primary input. I don't know where the DC output is pulsating. The output is DC, the amprobe is reading the same current as my cheap in line meter?
Typical modern clamp-on meters is not what I have. I'm looking for a reason why this AC meter is reading DC current. Relocating electrons I can do. How the get there is not so clear to me,


Guido.

(quoted from post at 16:46:09 03/20/15) Hello bob,

Is it possible the you are wrong? The output current controlled by a rheostat. It is wired to a power transistor which receives 8 volts of dc power from a bridge diode. The transformer output is about 8 volts AC, with 120 volts in the primary input. I don't know where the DC output is pulsating. The output is DC, the amprobe is reading the same current as my cheap in line meter?
Typical modern clamp-on meters is not what I have. I'm looking for a reason why this AC meter is reading DC current. Relocating electrons I can do. How the get there is not so clear to me,


Guido.

Click to expand...

nless you have a massive capacitor that can sustain your 1 ampere for the 8 milliseconds between the line input pulses that come from your full wave bridge rectifier, then it is exactly as Bob says, a pulsing current at the rate of 120 pulses per second.

Pop the back off the Amprobe and connect a 'scope probe to the output leaks of the CT and I'm pretty sure you're gonna see the pulses JMOR is telling you about.

The CT passes those pulses on to the rectifier in the meter that then drives the meter.

The CT simply CANNOT "pass" a pure DC signal on to the rectifier, but it CAN pass pulses.

I was servicing a 05 Suburban today it had a gizmo that was attached around the battery cable near the battery its factory and a inductive pick up of some kind... What Have I missed... Never seen it before...

I have been wanting a inductive inductive clamp on meter like in you link (volt/amp) if you have one what do you use...

Here's the deal: Your charger probably doesn't do much to filter its output. You said it has a "rheostat through a mosfet", which I assume limits its peak voltage. Let's assume for a moment the mosfet isn't in the circuit, just to simplify things. If you ran the output of your rectifier bridge to a resistive load, you would see a full-wave recitified sine wave on your load. Let's assume it's about 12 volts peak (12 x .707 = 8 volts rms). But you're not connected to a resistive load, you're connected to a battery, and a battery is a voltage source. The diodes in your rectifier bridge only conduct when they're forward biased, which is only half the time (the arcsine of .707 is 45 degrees), when voltage on the transformer side of a diode is greater than the battery voltage. So the CURRENT is not DC, nor is it half-wave rectified AC. It's a bunch of rounded pulses with a 50 percent duty cycle. To your Amprobe, it looks like AC and that's why it registers current.

I have an Amprobe just like that and bought it about 30 years ago and it's still working.

On the pickup, the jaws are the iron core of a transformer. The primary winding is the wire you clamp onto. The secondary feeds the meter circuitry and I haven't popped the back so I don't know the circuit in detail.

However, being a transformer it operates on voltage/current variations that are required to produce coupling through the core. DC will not work because as soon as you exceed the "Volt-Time Integral" the core will saturate and the output will be zero as will be the meter reading.

Mark

Hello JMOR,

Are you saying that the rectified voltage is pulsing at 120 pulses per second?
Like I stated before, on this setup, readings are exactly the same on both meters,

Guido.

(quoted from post at 13:24:41 03/21/15) Hello JMOR,

Are you saying that the rectified voltage is pulsing at 120 pulses per second?
Like I stated before, on this setup, readings are exactly the same on both meters,

Guido.

Click to expand...

reasonable approximation:

Hello MARKB_MI,

There is no filtering on the outputs on the box.
While charging from the 8 volts output into the batteries, both meters read the same amps. The 12v output, comes straight from a 10 amp diode bridge. It poweres a 9 cell pulse charger. NOW! the amp readings on the 4 cell pack is about 2.1 amps with the charger on. The amprobe is twice as much at 4.8 amps.? The reading is the same before and after the charger?
I do have a set of capacitors in the circuit for the 12 volt output, but the are switchable. When on, the output voltage is 2 volts higher. The switch is visible right next to the rheostat. It came in handy when AC line voltage dropped with many racers plugged in on a single 120V circuit.

Guido

Hello JNMOR,

I'm starting to see the light......Thanks


Guido.

Hello Texasmark1

I just replied to MARKB-MI. Look at my response.

Guido.

Hey Guido,

JMOR's drawing explains what I was trying to say quite well.

I'm not certain how your pulse charger works, but I have a pretty good idea why there's such a discrepancy between the DMM and Amprobe readings. As we've already discussed, the current out of the rectifier bridge is pulsed. The current only flows when a diode is forward-biased. Now most low and mid-range instruments assume DC to be a constant voltage and for AC to be a sine wave. But you're measuring a pulsed signal, and your two instruments treat it differently.

Digital multimeters filter DC, so they measure the average current of the pulsed signal. (True RMS meters actually calculate the root-mean-square value, but for pulsed DC the average and RMS are about the same.) So far so good.

Now let's go to your Amprobe. While the DMM uses a shunt to sense current, the Amprobe measures it indirectly by sensing the magnetic field around the conductor. The magnetic flux is not a function of the current, but rather the RATE OF CHANGE of the current. Now with a sinusoidal waveform (like household ac), current and rate of change of current are one and the same. That's because the rate of change of a sine wave is a cosine wave, and a cosine wave is just a sine wave shifted 90 degrees. But the rate of change of your pulsed current is quite different from a sine wave. At the start and end of the pulse, the slope of the waveform is nearly vertical, so the rate of change approaches infinity. During the rest of the pulse, you have a sinusoidal rate of change (due to the humps you see on JMOR's drawing). There is zero rate of change when the current is zero. So what is happening is the Amprobe is being fooled by the vertical portions of the waveform so it reports more current than is actually passing through the conductor.

Regarding your filter capacitors, they aren't actually increasing the voltage applied to the batteries under charge. What they do is to increase the AVERAGE voltage your meter sees. But remember, the battery is charging only when the diodes between it and the ac source are forward biased, and that's when the voltage is close to peak. (I assume the devices you're charging have series diodes which keep current from flowing out of the their batteries, otherwise you would see no change in the voltage when you connect the filter caps.)

There is a downside to adding big filter caps to a power supply: The bigger the filter capacitors, the shorter the duration of time the diodes are forward-biased. This means there's less time for current to flow into the capacitor, which means the peak current through the diode is greater. Add a big enough filter cap, and you'll blow the diodes.

Hello MARKB_MI,

I understand how the meter is being fooled by the high loop. Makes sense now. About the caps in the output of the power supply there are 3 in parallel. Don't remember the capacity nor the voltage. IT has been 20 plus years, I can find out for you by opening the box. I may have one around as well. The voltage increase of 2 volts was taken without a load. I can tell you that by switching them on, when 120V became 100V, the charger no longer tripped to trickle. It needs at least 12 Volts input or it would take twice as long, 40 instead of 20 minutes, to charge the packs, or just keep reverting to trickle. With a 16 Volt input it can charge nine NiCad cells in series. AT 1.5 peak voltage that meant 13.5 volts. Thanks for taking time to explain. The readings only tell part of the story. The little knowledge I have acquired it has been on my bench. Lots of smoke has been emitted through the years. Thanks again......

Guido.

Hi Guido,

I realize now that your pulse charger is probably intended to run off a battery. In this case, adding the filter caps is a big help, because it changes the rectified ac into something closer to the smooth DC a battery would provide.

I also made a mistake when I described what's going on with your Amprobe. I said the flux is a function of the rate of change of the current. That's not correct: the flux is a function of current, but the voltage detected by the Amprobe is proportional to the rate of change of the flux. In the end, the result is the same: the Amprobe is detecting the rate of change of current. But my explanation wasn't correct.

Hello MARKB_MI,

The power box has a 15V 3.5 amp charger, which I used to charge a battery to power the charger. Trying to avoid low input to the charger.
Having an automotive background, I always thought that a dc current would have zero frequency. That olds true if it is coming from a battery and to fixed load. When used for intermitted control, like injection, then it would be pulsed. Look at the picture. With the charger feed from the battery, charging 6 cells about 1.75 amps output, amp probe ZERO. With the charged on trickle/pulse the amprobe reads .6 amps of course on and off with the pulses of the charger. Now I have a clearer understanding of these invisible gremlins thanks to you,

Guido.
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V as a function of the rate of change of current:

V sensed = L (inductance of instrument or what have you) di (differential of current aka change) /dt differential of time the current changed the amount it did.

I guarantee you the transformer will follow an input signal to some Voltage across the windings, occurring in some time until that function drives the core flux up or down the BH curve (curve showing the core magnetization properties) until it saturates. When it does, the inductive reactance (XL) of the transformer goes to zero and as does the coupling, as does the output voltage and the primary current is only limited by the dc resistance of the primary winding. In the case of the amprobe that is nothing.

For a transformer to function properly the applied voltage that produces a corresponding current has to move the core flux up and down the BH curve to keep the transformer a transformer, within the limits mentioned above.

On pulsed vs rms as I said earler and as Mark B is stating, a short pulse with a fast rise time effects the Root-Means-Square voltage or current depending on what you are sampling differently.

The equation is: Under the root radical you have voltage 1 squared + voltage 2 squared + etc till you get them all counted. Add up the squared values for a sum total and take the square root of that.

A short pulse of high amplitude may contain the same area as a sine wave, but since the amplitudes are squared, a second order equation, they will register higher as you are noticing.

Like 2+2 =4 and 3+3 = 6, but if you square 2 it's 4 and if you square 3 it jumps up to 9. So obviously your meter will show you 2 different values.

Hello,

I learned quite a bit with this post. Thanks to the inputs of the more learned members and their patience. When you asked for an amprobe years back, that is what you got! The two black leads are for voltage measurements. Does any one know what the yellow leads are used for?

Guido.

Look on the meter scales and see if there is any numbering that is in degrees F or C. I think that's a thermocouple and self registering like the voltage scale just like the amperage scale, so you still don't need a battery and get all these functions. I really liked that with an instrument that I have had for years and use seldom. Nothing like a seldom used battery powered instrument being destroyed by a leaking battery.

I enjoyed the exchange. I've been retired 10 years as of Jan 1 this year. You forget a lot. This helps to keep things going.

Mark

Hello Texasmark1,

It does take a battery to use that function of the meter. I have a battery in the case though. I also know what that function is. It is like a hidden tool challenge. I have also enjoyed the sparky conversation and appreciate the education.

Guido.

(quoted from post at 14:04:18 03/20/15) Hello,

This is the amprobe that I used to measure DC Amps while charging two 2200 MAH Lithium batteries in parallel. Charging at about 1 amp at 3.87 volts and climbing. The amprobe was set at the 6 amp AC scale. It can measure AC amps up to 300, AC volts up to 600. It also has a ohms scale up to 1000 ohms. Old unit but still pretty accurate.
Can someone explain to me why it is working?
My home made charger is not that sophisticated. I'm pretty sure it is more like a linear output then a pulsating current,


Guido.

Click to expand...

The clip on AC ammeter must be picking up some ripple on the DC in order to read current. Smooth DC won't read anything on those.

Now isn't that what we just got through saying???????

Looks like he's a few days late & dollar short?

Hello Texasmark1,

I posted for some answers for two reasons. Not only was the probe giving me a reading but it was correct!. Well, it matched the in line digital meter anyhow! Changing current flow to a higher output, both meters read the same. I may have used a multiple tap transformer. I would have to open the box to verify it though. Would that make a difference? The other output, from the same transformer, is 12 volt but not adjustable?.

Guido.

Opinion: You'd have to get an oscilloscope or one of the Fluke brand (to name a name) that has a LCD of the volt-time picture of the voltage that it is measuring; in essence a built in scope. Then you would have some insight as to why the readings changed.

RMS is used normally because it's the same heating effect as DC. Makes it easy to compare apples to apples. Again, spikes get hotter for the same "area under the curve" for the reasons I said previously about how RMS is calculated. The closer the wave forms are to one another, especially in amplitude, the closer they will be.

Mark

Hello Texasmark1

I can see why RMS is used. I do not have a scope. Would be nice to have one though.
I'm now working on a capacitor spot welder. I'll post separately for that,

Guido.

Good luck. Doubt I can help you with that. But you never know. Ask the questions and maybe you can get an answer from someone.

Mark