Archive of the former Yahoo!Groups mailing list: MOTM
Subject: Mysteries of The Universe: Ground
From: mbedtom@...
Date: 2002-04-01
I'll take a stab at explaining "ground". But this IS April 1st, so if I royally screw this up I'm gonna cop-out and say it was a "joke", OK?
"Ground" is very confusing because everyone has heard the term in differing contexts. In each context it means something different. Here are some examples:
1. (From an electrician) "Ground" is that wire attached to your cold-water pipe as it enters your house. (If a Chicago area electrician he would say "yaw, dat ting over dare".) That wire connecting to the cold-water pipe goes to your breaker panel and then connects all of the 3rd lugs on your AC outlets. If you don't have one of them on your pipe, you'll have a six or eight foot ground rod pounded into the dirt just outside your foundation. That wire connects to that ground rod. It is a safety thing to help you from getting electrocuted when a toaster or a drill shorts out.
2. (From your car mechanic) "Ground" is the negative terminal of your car's battery. It connects through a real heavy cable to the chassis of the car. Usually to somewhere on your engine, too.
3. (From a father) "Grounded" is what his 15-year old daughter is when he finds out she's been dating a 27-year old guy she met at the rock concert last month.
4. (Context of analog synthesizers) "Ground" is nothing more than a common reference point. All like "ground" symbols on a schematic are simply connections tied together on the physical circuit. It is sort of a shorthand used in drawing schematics. Usually the +15V and -15V connections are also shown in a shorthand format. They rarely show a connection to the power supply itself. Anyway, "ground" is the "common" connection to a power supply. Since it is THE reference point for everything else, it will always have a voltage of zero... it is THE reference. So when someone says to measure the voltage at the end of R57, they mean that the black wire of your voltmeter should connect to "ground" and the red wire of the voltmeter should be probing R57. The middle two pins of the MTA-156 connector on all MOTM modules is "ground". So is the shield portion of the coaxial cables that connects the inputs/outputs on the PC board to the 1/4" Switchcraft jacks affixed to the front panels.
Think of a MOTM 900 power supply as if it were two batteries connected in series. The positive terminal of battery A is the +15V volt terminal of the power supply. The negative terminal of battery B is the -15V terminal of the power supply. The negative terminal of battery A -and- the positive terminal of battery B are connected together. This junction of the negative terminal of battery A -and- the positive terminal of battery B shall be known as "Ground" (signal ground, actually). When you touch the common terminal of your voltmeter (usually with a black wire) to "ground", you are arbitrarily establishing a reference point. If the black wire on your voltmeter connects to the junction of the negative terminal of battery A -and- the positive terminal of battery B (which we are now calling "signal ground") you will measure +15V on the positive terminal of battery A and -15V on the negative terminal of battery B. That is the ordinary convention for most circuits. BUT, you could connect you voltmeter's black wire to the negative terminal of battery B. When you then touch the meter's positive lead (usually, red) to the junction we labeled "ground", you will read +15V. Leaving the black lead of the voltmeter on the negative terminal of battery B, touch the positive terminal of battery A: you will read +30V. Did that surprise you?
Attached is a .PDF document "ground.pdf". It is a little helper to go along with the explanation I just attempted. Perhaps one picture is worth a thousand words. Or if you write software, one picture is worth 1024 words :o)
Anyway, there are two identical schematics drawn. One uses the conventional "shorthand" using ground and voltage supply terminals. The other schematic connects everything with wires (no "shorthand"). Hope this at least starts in the right direction. (Yes, I know I misspelled "connection" on the PDF. Too late.) Be brave and prototype those circuits! If you want, you could use two 9V batteries wired as shown, in place of the MOTM 900 power supply to actually make the amplifier shown. Yes, it will work. And if you use batteries, you can rest easy knowing that playing around with circuits won't kill off your synthesizer if something goes wrong. After a couple of circuits you'll buy a pair of 9V wall-warts instead of batteries to save money. Then you'll wonder why a so-called 9V wall-wart puts out 14 volts! But that is another story.
Cheers!
Tom Farrand