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Old 07-21-2011, 08:37 PM
 
22,674 posts, read 24,663,689 times
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(Do not attempt if you do not know about electricity/electronics)

I put together a real simple device that lets you easily measure Voltage, Current and Resistance of the device in question. Comes in real handy for anything you want to test, up to 15 amps in my case.

Just bought a 3 prong (hot, neutral, earth ground) power strip with a 15 amp rating/circuit breaker. Then I installed switches in both the hot and neutral lines. this way you can easily test V/I/R without having to insert your meter/breaking a connection, saves a lot of fumbling around. The wall switches that I put on the hot/neutral lines have screws to attach the wires. I actually backed the screws way out then soldered them in place (then soldered the wires onto the screws) so as to have a very easy place to touch with the meter probes or connect alligator-clip type jumper wires if you wish to hold the meter probes that way. I also grounded the metal parts of the switches for safety. I left room between the hot and neutral so they can be separated, that way you can also use a clamp on meter to measure current.

Here is the functions:

1. Hot open/neutral closed: measure current by shunting open switch with ammeter.
2. Hot closed/neutral closed: measure voltage drop across load by touching voltmeter to hot and neutral screws.

3. Hot Open/neutral open: measure device resistance (won't work on certain devices with semiconductors) by touching ohmmeter to UNPOWERED hot and neutral screws.

Watts Law is very helpful in a lot of diagnostic situations once you have the above information.
P=I*V
I=P/V
V=P/I
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Old 07-21-2011, 10:46 PM
 
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I wouldn't use a setup like that. Resistance doesn't mean much, since many devices resist flow through inductance. Voltage drop is pretty simple to measure anyway either with the appliance case opened or by not inserting the plug into socket all the way and touching probes, and current is FAR better measured using a Kill-A-Watt, or splitting a zip cord and wrapping one of the wires around a non-contact ammeter clamp. Yeah, you can do it your way, I just would do it other ways.
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Old 07-21-2011, 11:14 PM
 
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Quote:
Originally Posted by harry chickpea View Post
I wouldn't use a setup like that. Resistance doesn't mean much, since many devices resist flow through inductance. Voltage drop is pretty simple to measure anyway either with the appliance case opened or by not inserting the plug into socket all the way and touching probes, and current is FAR better measured using a Kill-A-Watt, or splitting a zip cord and wrapping one of the wires around a non-contact ammeter clamp. Yeah, you can do it your way, I just would do it other ways.

The point was to NOT have to open anything. And you did not read.........I left a dual option for current measurement............clamp on and shunt..........best to have BOTH OPTIONS. Not inserting a plug all the way to measure voltage.............you are joking right..........ever hear of something called technique..........that is a bad one.

Resistance........uh, well it is important for many devices that are mostly resistive and develop very little inductance. Many formulas can be derived if you know the resistance. Power=current squared * resistance and power = voltage squared/resistance and resistance = Voltage squared/power.
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Old 07-22-2011, 08:15 AM
 
Location: Las Flores, Orange County, CA
26,329 posts, read 93,860,614 times
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You might be an engineer if you try to fix a $5 radio.
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Old 07-22-2011, 10:55 AM
 
23,622 posts, read 70,563,787 times
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Quote:
Originally Posted by tickyul View Post
The point was to NOT have to open anything. And you did not read.........I left a dual option for current measurement............clamp on and shunt..........best to have BOTH OPTIONS. Not inserting a plug all the way to measure voltage.............you are joking right..........ever hear of something called technique..........that is a bad one.

Resistance........uh, well it is important for many devices that are mostly resistive and develop very little inductance. Many formulas can be derived if you know the resistance. Power=current squared * resistance and power = voltage squared/resistance and resistance = Voltage squared/power.
Whatever. If you are going to get fussy about "technique", I can't think of many professional quality testing devices that purposely expose hot wires. Banana plugs and other ways of connection including probe points where you insert the probe, yes. Exposed screw terminals, not so much.

I'll plug in my Kill-a-watt and not have to worry about compensating for power factor, or playing around with formulas. Toasters are resistive. Devices using transformers, like a lot of electronics and ballasted lamps, tend to be inductive. I'm not understanding why measuring resistive load is important to you, given those other issues.

You made something that made it easier for you to do some tests. Fine. I'm just saying that I wouldn't bother with something like that. A Kill-a-watt is a nicely developed and packaged product at a low price. In push comes to shove situations, I cannot recall seeing any electrician or engineer using anything similar to what you built.
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Old 07-22-2011, 11:17 AM
 
1,402 posts, read 3,505,171 times
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Quote:
Originally Posted by Charles View Post
You might be an engineer if you try to fix a $5 radio.
Or build something that you can buy for $10 from radio-shack.
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Old 07-22-2011, 07:23 PM
 
22,674 posts, read 24,663,689 times
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Quote:
Originally Posted by harry chickpea View Post
Whatever. If you are going to get fussy about "technique", I can't think of many professional quality testing devices that purposely expose hot wires. Banana plugs and other ways of connection including probe points where you insert the probe, yes. Exposed screw terminals, not so much.

I'll plug in my Kill-a-watt and not have to worry about compensating for power factor, or playing around with formulas. Toasters are resistive. Devices using transformers, like a lot of electronics and ballasted lamps, tend to be inductive. I'm not understanding why measuring resistive load is important to you, given those other issues.

You made something that made it easier for you to do some tests. Fine. I'm just saying that I wouldn't bother with something like that. A Kill-a-watt is a nicely developed and packaged product at a low price. In push comes to shove situations, I cannot recall seeing any electrician or engineer using anything similar to what you built.

That is why god invented jumper wires and meter probes with alligator clips.....so they can be connected to THINGS like screws. And yeah, if you are testing anything you better be aware of the dangers, that is not specific to my little tool.

And yes, formulas are very useful, even when resistance is used in the equation. There is a lot more than just toasters that are almost purely resistive. And the device that I made for 5 bucks can help in measuring V/I/R not just resistance.
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Old 07-22-2011, 08:02 PM
 
Location: Las Flores, Orange County, CA
26,329 posts, read 93,860,614 times
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Gadgetbox - 10 completely useless homemade machines
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Old 07-22-2011, 11:10 PM
 
22,674 posts, read 24,663,689 times
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I do not get it, I did not say I was inventing a new microprocessor or similar.......just a little fricken tools to make taking readings easier.................SORRY.
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Old 07-23-2011, 12:15 AM
 
Location: Not far from Fairbanks, AK
20,303 posts, read 37,259,238 times
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Quote:
Originally Posted by tickyul View Post
I do not get it, I did not say I was inventing a new microprocessor or similar.......just a little fricken tools to make taking readings easier.................SORRY.
All you need to know is "PIE" and have a $5.00 voltmeter at hand. The placard on the device being tested also gives you an idea of how much power it uses
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