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Digital Multimeter : General / Basic questions (DC amps/volts)

In my electronics enlightenment journey, I find myself indulging into the fundamentals of understanding the basics of circuits---starting with understanding DMM readings.

I want to make sure I have the fundamentals first before I blow any fuses or circuits from making simple measurements.

Surfing the various threads regarding the Digital Multimeter, I understand that Volts must be measured in paralell with the black lead (com) on the negative and the read lead (Volt 200mA max)

And amps must be measured in series with the actual current running through using blac (com) and red (either 10 ADC 10 amp max unfused or the Volt-ohm 200 mA max fused)

As far as measuring VDC. With the DMM set to 20 DCV, I measured some fairly new batteries (one 9v and 6 1.5volt AA size in series = 9volt)but for both of these the readings showed less than 9v...aprox 8.3 - 8.7 volts. Then I tested an old

9volt battery that couldn't even power the DMM. This gave a reading of a little more than 4 volts. So why do voltages vary. I understand that batteries are depleted of amp/hours, etc., but wasn't aware that it applied to voltage. Can someone put light to these observations...

Now, the tricky part... Measuring amps. I haven't yet gotten to the level of measuring individual components, resistors, transistors, diodes, etc. so no need for the related ambiguous jargon just yet . Anyhow, The scales confuse me. On my particular model (YUGO DT-830B)the DCA selection is 200 microamps, 2000 micro amps, 20 miliamps, 200 miliamps, and 10 amps.

Does, 10 amps mean that it's the maximum amps it can read, 200 miliamps is the maximum amperage this selection will take...or??? What if we don't have a clue how much amps a certain load draws, let alone how to read the DMM scales.

For example, I hooked up a 3 volt motor with 2 AA batteries. As I don't have a clue whether a 3 volt motor will draw more than the maximum 200 mA of the one red terminal, I assume it was better to use the 10 amp max unfused red terminal. So the positive terminal from the battery with the red lead, and the black lead in series on to the load (motor).

All kinds of varying readings jumping around. And I'm not sure how to interpret them. With the DMM set to 10A, I get average readings between .09 all the way up to .25 Then with it set to 200 m, the readings are between 1.1 to 2.8.

So how do I interpret these into amps/miliamps? The assumption is that such a small motor can't possibly be drawing 1 or 2 amps, well, I don't really know. That's why I'm posting.

So on top of interpreting the confusing scales of amps, also I would like to know why I can't get a steady still reading of volts or amps with the running motor.

Also, is it possible to measure amp hours of battery cells with the DMM? If so, how must one proceed?

Thanks in advance.

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Reply to
greenwanderer108
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The batteries increase their internal resistance as they are depleting. Think of them as a voltage source with a resister in series. The ranges on your DMM show maximum for that setting. If you are set to the 200mA range, the meter will measure 200mA maximum on that range. A reading of 1.1 on that scale means 1.1 mA. If you load that little motor even slightly the current will go up. You can not measure AH with that meter. The best that you can do is load the battery with its rated load and see if the voltage drops below the manufacturers spec. Good Luck, Tom

Reply to
Tom Biasi

--- No. There is no such thing as "Volt 200mA max".

Here are the meter's spec's:

DCV: 0-200m ±0.25% -2-20-200-1kV ±0.5% ACV: 0-200V-750V ±1.2% DCA: 0-200u-2m-20m ±1.0% -200mA ±1.2% -10A ±2.0% Fused R: 0-200-2k-20k-200k ±0.8% -2Mohm ±1.0% hFE measurement Transistor test Diode test Battery: 9V

Looking at DCV (DC Voltage) we have 5 ranges:

0-200mV ±0.25% 0-2V ±0.5% 0-20V ±0.5% 0-200V ±0.5% 0-1000V ±0.5%

When you measure voltage you place the leads in parallel with whatever you want to measure, but the placement of the leads isn't critical since all that will happen if you get them backwards is that the meter will indicate a negative voltage. What you _should_ do is place the black lead (after making sure that it's plugged into the black jack on the meter) on what you want your reference to be.

---

--- Or any of the other current ranges.

---

--- As someone else posted, when batteries become depleted their internal resistance rises, so the voltage they can deliver to a load decreases because of the votage drop across the internal resistance.

---

--- The different current ranges indicate what the maximum current is for that range before the meter will indicate an overload. The 10 amp range is unfused, so you can put a great deal of current through that range. Be especially careful on that range, since it's totally unprotected and you could damage the meter, or worse, if you let a lot of current through there.

---

--- That's exactly right. If you're unsure of the voltage or current, always start on the highest range and work your way down.

---

--- With the meter set on the 10A scale, the readings are displayed in amperes, so .09 would be 0.09 amps (90 milliamps) and .25 would be

0.25 amps (250mA, or one quarter of an amp).

On the milliamp scales, the readings are displayed in milliamperes, so 1.1 would be 1.1 milliamperes, and 2.8 would be 2.8 milliamperes.

---

--- The motor is making a huge amount of electrical noise when it's running, disrupting the meter readings. Try putting a large-valued electrolytic capacitor (10,000µF or more, and watch the polarity) across the motor and see what happens.

---

--- It's not possible.

-- John Fields Professional Circuit Designer

Reply to
John Fields

On a chart with amp-hours across the horizontal axis, and with voltage on the vertical axis, then a battery curve tends to hold voltage constant until battery approaches end of life. Then battery voltage starts dropping quickly. What voltage is too low? For a nine volt battery, some appliances declare a battery at 8.7 volts or lower to be dead.

Voltage to amp-hour relationship requires that chart. Also that relationship changes when a rechargeable battery wears out. This curve between voltage and amp-hours is unique for different battery technologies.

For meter readings - always go to the maximum setting for first measurement. With experience, then one has a feel for what need be set. Voltage half of meter is very forgiving. Current side of meter is not. Always go to maximum setting -

10 amps - for your first current measurement.

You will learn that some batteries of same voltage but different technologies put out large currents whereas others output low currents. First type are designed for high power applications - ie electric drill. Second type for long term, low power applications - ie smoke detector. There are even high current and low current versions of NiCd batteries. So which current setting do you use first? Highest because you don't know and have respect for your meter.

BTW, d> In my electronics enlightenment journey, I find myself indulging

Reply to
w_tom

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