Electrical quiz for my MB friends. - PeachParts Mercedes-Benz Forum

ericnguyen · #1 07-09-2003, 07:32 AM

Dear friends:

1) If you take a look at multimeters, you will notice that most of them only support a maximum of 1mA to 10A for DC, and 1A to 1000A for AC, even though they support voltage range of 1V to 1000V for both DC and AC. The question is: If multimeters can only handle low current (A) loads for DC, why it can handle much larger loads for AC ?

Diesel afficionados have always wanted a multimeter that can handle at least 30 A for DC so that they can efficiently test their glow plugs easily (measuring resistance only is not deterministic of a actual condition of a glow plug). Unfortunately, such multimeters are very expensive (high end Flukes...)

2) What are the proper terms to describe the opposition to a DC current and an AC current? Do they use the same unit?

Good Luck :-)

Eric

ck42 · #2 07-09-2003, 11:19 AM

"If multimeters can only handle low current (A) loads for DC, why it can handle much larger loads for AC ?"

AC voltage is peak to peak or .707 * DC voltage.

"What are the proper terms to describe the opposition to a DC current and an AC current? Do they use the same unit?"

DC "opposition" unit of measurement is the ohm. AC's is frequency dependent and is also measured in ohms, but it is referred to as the circuit's impedance.

Mordecai · #3 07-09-2003, 11:42 AM

A trick used to measure large current loads is to place in series a very small resistor whose resistance is known very accurately. If we remember Ohm's law, V = I *R, that is the voltage drop across a resistance--like a glow plug--is equal to the current (in amps) times the resistance (in ohms).

They make low resistance, high precision, resistors just for this purpose. I have no idea where to find one however. You could make one out a short piece of resistence wire I suppose.

Before you run down to Radio Shack to a small resistor remember that the power dissipated in a resistance is P = I * I * R. (I squared R).

BTW--AC can be measured from a fluctuating magnetic field which is easy to do for large currents. DC can't be measured that way.

M.

#4 07-09-2003, 12:29 PM

Oh, I know what the deal is with the 1000Amp AC current scale. There is an accessory amprobe that you plug into the meter that allows this measurement. It clamps around one of the conductors of the AC power and measures current via inductance.

Precision, high power, low resistance resistors are readily available at Electronic component supply houses. Use 1 ohm so that there is no math calculation needed. If the circuit being driven is very low resistance (or impedance) use a .1 Ohm resistor.

Have a great day,

Mordecai · #5 07-09-2003, 12:47 PM

Quote:

Originally posted by LarryBible
Use 1 ohm so that there is no math calculation needed. If the circuit being driven is very low resistance (or impedance) use a .1 Ohm resistor.

Let's see here, if my series GP passes 30 amps and drops 1.5 volts that makes it a 1.5/30 ohm resistor. Or, 0.05 ohms. If you put a .1 ohm resistor in series with this you'll have a total circuit resistence of 0.15 ohms, or triple the resistence of the GP. Remember that a GP's resistence will change as it heats up so you won't get the full heat reading. That's why you need an ultra-precision low resistence resistor. But, for automotive work it might not matter.

Instrumentation is a b*cth.

M.

ericnguyen · #6 07-09-2003, 09:23 PM

Dear ck42:

Your answer "AC voltage is peak to peak or .707 * DC voltage." to the first question is not to the point. I was asking about DC and AC current loads. I expect you guys to show some formula manipulations that clearly show why such thing (high current loads) is possible with AC currents.

Eric

sublettm · #7 07-09-2003, 09:48 PM

Good luck finding any meter, even a good Fluke meter, that will directly measure 30A DC or AC. You could simply run your current through a shunt and measure the voltage drop across it and figure out the current as discribed above using Ohm's Law.

In the 10+ years I have been repairing and calibrating electronic test equipment I have never seen a voltmeter that measures Peak to Peak voltage. Every meter I have ever laid hands on has been a true RMS meter. You can purchase some of the new Graphical meters made by Fluke which measure RMS voltage and will convert it mathimatically to a Peak to Peak value...it does not however acutally measure p-p voltage.

If you do decide to use a shunt, or resistor, make sure it is big enough to handle the power running through it. Too small of a resistor and you will let the smoke out of it really quick.

Mike

ck42 · #8 07-09-2003, 10:07 PM

Actually, I don't recall there being a difference in AC current capabilities for digital meters.

blah...throw me a bone.

Chris
'85 300sD

JimSmith · #9 07-09-2003, 10:26 PM

Eric,

You apparently missed the post by Larry Bible. Larry noted AC current is not necessarily measured directly, it can be calculated from the voltage it induces in a coil aound the conductor carrying the AC current in question, with known frequency and impedance characteristics. Therefore the instrument never has to operate with 30 Amperes (or even more than milliamperes) on the circuit boards to accurately measure Ampere values of this magnitude.

Larry also noted DC current cannot be measured that way as the current flow, being constant, will not induce a voltage in a coil around the conductor. Other means that bring the measured Amperes being measured into the instrument body are often used. The capacity of the innards of the instrument are based on cost, which is typically tied closely to production volume.

The ranges available in the common Multi-Meter work fine for most users. Household AC circuits are much more likely to exceed 10 Amperes than the DC circuits most users of Multi-Meters will encounter. To hold down cost, because none of us is likely to buy a $75 Multi-Meter for casual use when a $35 unit is offered and will suffice, the manufacturers do not make the instruments more capable than the customer will buy in a competitive environment. For those needing additonal features, there are more capable Multi-Meters. At added cost of course. Hope this helps. Jim

ericnguyen · #10 07-20-2003, 04:57 PM

Dear Mr. JimSmith and everyone:

For the first question:
Thanks a lot for reminding me of Mr. LarryBible's reply to my post in the tech help forum. Yes, measuring AC current amperage is best done via the indirect measurement of the voltage induced in a coil around the conductor carrying the AC current, and that's what my original question was trying to address.

However, Mordecai's trick of using a low resistance and high precision (but must be NOT variable due to temperature increase)resistor to trigger a voltage drop can only be best applied to DC because that trick will become more complicated and involved for AC (formula manipulation and equation solving). Why?, because an AC current I is equal to the ratio of the RMS voltage to the impedance (not the simple resistance). An AC circuit impedance is dependent upon [the AC frequency, the inductance, the capacitance, the resistance and the circuit schematic]

For the other question:

Yes, the resistance concept for DC is mostly replaced the impedance concept for AC, because the inductance (an inductor such as a coil), or the capacitance (a condenser/capacitor) imposes a resistance-like and frequency-dependent effect on AC. DC treats an inductor as a regular resistor and a capacitor as an insulator (infinite resistance), i.e. a DC circuit with a resistor, an inductor (a coil) and a capacitor in series will have a DC current of zero ampere because of the capacitor's infinite resistance.

I will post a more detailed explanation later this weekend (using some formula manipulations). I hope someone will try to refresh his/her memory about AC behavior and formulaes and post a clearer and more informative reply before I do. It has been more than 10 years since I learnt about AC, so I think it's good for me and everyone to refresh our memory (closed book of course :-)

Best regards,

Eric

dabenz · #11 07-21-2003, 07:54 PM

1) Dollars to donuts you're looking at two different circuits under the hood of your multimeter. Mine are. Check the circuit diagram for your meter if you have it. Most current measuring circuits are actually voltage measuring circuits with a little behind-the-scenes math. Five to ten seconds is the most time most multi-meters can handle higher currents, regardless of the cost.

If you have $180 then you can buy a clamp meter from the big boys (like Newark) that can do both AC and DC current. A clamp meter uses a Hall-Effect sensor, which means it "measures" the strength of the magnetic field generated by the current. No big deal for DC. Remember the right hand rule: wrap your right hand around the wire with your thumb going in the direction of the current. The magnetic field goes in the direction of your fingers. Now change the direction of your hand 60 times a second for household AC, or 10,000 times a second for the fancy stuff. That's why that fellow Hall gets the credit for his sensor that he developed in the 1980s. That's Nobel prize level of work, in my opinion.

2) Opposition to current is resistance. Resistance is resistance in DC, and is generally known as a passive resistance in ohms. For AC, the resistance has an active and passive component. How they interact depends on the type of load and type of power source. Remember phasors, real and imaginary numbers, and terms like inductance, reluctance, and phase angles?

Good luck in your hunt.