Not Understanding AC Circuits

oldrayj
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Not Understanding AC Circuits

Post by oldrayj » Mon Jun 17, 2013 8:27 pm

In building my enclosure I want to include controls for all powered equipment. Specifically I want to control the spindle with a toggle switch instead of the tool-mounted switch on my Dewalt 660. I have, on occasion, nudged the tool off center when turning it on. I also plan use and auxiliary plug for a vacuum.

Since I don’t understand all I know about circuits, I will appreciate someone checking the diagram below.
EnclosureWiring.png
EnclosureWiring.png (130.93 KiB) Viewed 2560 times
P1 is an IEC 320 inlet plug with a fused switch (S1). All the rest of the plugs and switches are standard. I have designed the circuit so that S1 provides power to the bus and turns on the AC fan (Fan1) and the power supply before anything else is activated. The DC fan (Fan2 is also activated) and power will be available for the Wall Wart (to be used to power the Uno when I build my optocoupler). S2 provides DC power to the shield via the Panic button. I can then use S3 and S4 to control the spindle and vacuum.

I used the Ohm function on my millimeter to do a continuity test. I got all ones and zeros in the right places and no zeros in the wrong places.

Next comes my major point of confusion. To check the polarity I used a 9-volt battery connected to P1. The polarity checks were OK, but when S3 is off and S4 is on, I read about 2 volts at P2. The converse is true with S4 off and S3 is on. I don’t see where the voltage comes from. I have not been able to find any shorts. Puzzled.

Will appreciate guidance on where I’m going astray.

cvoinescu
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Re: Not Understanding AC Circuits

Post by cvoinescu » Mon Jun 17, 2013 8:41 pm

Your diagram looks good to me, although the fact that you used the same color wires for low-voltage DC and for mains is a bit confusing. If you can, have S1 interrupt both line and neutral. It's slightly safer that way. Never interrupt the ground!

By polarity, I assume you're checking that what should be line is line, what should be neutral is neutral, and what should be ground is ground. You've checked this already during your continuity test, haven't you? To answer your question, you probably have an electronic voltmeter. Some have insanely high input impedance, and even a slight leakage or static charge measures well into volts. You can test this, while still using the 9 V battery, by using your hand to touch both probes at the same time. The stray voltage should reduce to practically zero. Or, if your meter has a battery test range, use that instead.

To make sure it's not an actual problem, when you check continuity you should use the lowest ohm range, but when you check insulation (things that should not be connected), use the highest range. You should measure open circuit even on a 20 megohm scale. If not, something is fishy.
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no_hazmats
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Re: Not Understanding AC Circuits

Post by no_hazmats » Mon Jun 17, 2013 10:05 pm

So, if I understand correctly, you are attaching a 9V battery to P1 and checking the voltages throughout the circuit to ensure the correct polarity. Is the DC Power Supply connected while you are doing that? Although it is an AC input on the line IN terminals, it will still have some significant impedance with a 9VDC battery connected to the input - which would look like a big load or a short circuit I'm guessing.

Try removing one of the leads that goes from your AC terminal block to the Power Supply and see of that makes a difference.

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oldrayj
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Re: Not Understanding AC Circuits

Post by oldrayj » Tue Jun 18, 2013 6:25 pm

Thank you both for your inputs.

Sorry for the poor color-coding. I also failed to mention that no equipment is hooked up yet (power supply, fans, etc.). With respect to having S1 interrupt both line and neutral, I realized that my diagram is also not entirely correct. S1 is an integral fused switch and plug as pictured below.
InletPlug.png
InletPlug.png (178.85 KiB) Viewed 2511 times
The way the switch is wired, I believe it controls both line and neutral.

I did the leakage voltage test, and the reading for the closed switches was about 3 volts. When I touched both probes, the voltage jumped around a lot and went as low as .8 Volts but seemed to average out around 1.5 volts.

I did the continuity test with my multimeter set as suggested. With the setting at 2000K ohms the readings across the closed switches are OK. However, on the 20 megaohm scale, I get readings in the order of 2.95 across the closed switch . I don’t know if that’s indicative a problem with an AC circuit?

I also tried another meter with similar results (both meters are of the el cheapo variety).

Thanks again.

cvoinescu
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Re: Not Understanding AC Circuits

Post by cvoinescu » Tue Jun 18, 2013 8:00 pm

There's nothing wrong with cheap meters, in general. They all use the same IC (the "7107"), a very good design, still in production after 20+ years, available from a large number of manufacturers. With just a few capacitors, you can make a 2 V voltmeter; add a rotary switch and a dozen precision resistors, and you have a complete multimeter with volts, ohms and amps DC; with a couple more resistors, you can measure diode voltage drop and transistor hFE, and, if you don't care about precision, just add a few diodes and you get AC volts and amps ranges too. Pretty nifty for one chip and 10 cents* worth of passive components. :)

* $20 if you buy them singly from Radio Shack.

It's not normal to measure that low a resistance in open circuit. There shouldn't be that much leakage through the switches or between wires. If you haven't soldered the switches, can you take one of them completely out of the circuit and measure it, separate from the rest of the stuff, when open? If it doesn't measure open circuit, there's something fishy about the switch. Maybe it has a built-in indicator lamp or suppression circuit that throws the meter off, or maybe the insulation just isn't good.
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bjbsquared
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Re: Not Understanding AC Circuits

Post by bjbsquared » Tue Jun 18, 2013 8:46 pm

Does that switch light up? Perhaps when it is on?

If so, it could be the source of leakage.
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Tom Smith
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Re: Not Understanding AC Circuits

Post by Tom Smith » Tue Jun 18, 2013 10:36 pm

bjbsquared wrote:Does that switch light up? Perhaps when it is on?

If so, it could be the source of leakage.
That is definitely an illuminated switch - two of the poles are power, and the other two are for the bulb. I wired up a couple of these for my control box and it took a lot of tense testing, seasoned with some cursing, and one blown breaker switch. However, once I managed to figure it out, the switches control power, and light up when on, which is nifty :)

I don't recall the exact connections required to get the illumination, but will check if I remember tonight when I get home.

Tom

PS You don't have to wire up the bulb, it will work as a regular switch if you choose to use it that way. Just ignore the second set of poles.
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Tom Smith
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Re: Not Understanding AC Circuits

Post by Tom Smith » Wed Jun 19, 2013 5:15 am

Tom Smith wrote:but will check if I remember tonight when I get home.
OK, this picture might not actually clarify anything, but I wired up my main power switch as shown, and it lights when on, and is dark when off. Handy to know if the system is armed or not.

The layout is a little different than yours (the switch and plug receptacle are separate) but the principle should be the same. Don't worry about the capacitors or the green bead, they are not necessary for this to work, but came with the salvaged parts, so I left them in place.

The switch is to the left, and the power input is to the right, in case it isn't obvious :)
illuminated_switch.jpg
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oldrayj
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Re: Not Understanding AC Circuits

Post by oldrayj » Wed Jun 19, 2013 3:28 pm

Tom thanks for the inputs on the switch wiring. I will compare my wiring to yours.

I’m just using a 9-volt battery to check the circuit, so, I don’t expect the light to illuminate.

My main problem is that when S3 is off and S4 is on, I read 2 to 3 volts across the pins of plug P2. The converse is true with S4 off and S3 is on (voltage across the pins of plug P3).

In the continuity test noted above (S3 off and S4 on) I reported a reading of 2.95 (across the contacts of switch S3 when off) on the 20 megohm scale. Doesn’t that equate to 2,950,000 ohms? While infinite resistance would be ideal, is there a minimum threshold of resistance for a 110-volt circuit that that would constitute an open switch?

Thanks again for your patience in dealing with my lack of knowledge.

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Re: aRe: Not Understanding AC Circuits

Post by no_hazmats » Wed Jun 19, 2013 6:05 pm

There's really no need to measure in MegaOhms for this. Inexpensive multimeters with a Mohm range might not be precise at those levels to be of much use. You will be picking up minor surface conductance across the insulating materials. Using Ohm's law, if the resistance were 9Mohms, then the leakage current at 110VAC would be 0.0000012 amperes. This will not effect any operation and for your purposes, you may approximate this as zero. In the real world, if there were some speck of dust conducting enough to give this reading, it would be annihilated as soon as power is applied without you ever noticing.

If you use a 1,000 to 10,000 ohm scale and it reads as approx zero, that is good enough. In AC power wiring, you should be worried about dead shorts or open circuits.

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