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Basic Electronics » What is -VS?

March 14, 2010
by Ralphxyz
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I really know so little about electronics but surely I know this but am confused

How would I get a value from -VS?

I am thinking about having a stable signal from a LM34 and the spec sheet examples reference -VS/50 to get a resistor value. LM34 typical

How would I know what the temperature value would be?

Thanks for the help, Ralph

March 14, 2010
by Shanytc
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+Vs == Voltage Source

when you are building your circuit, you usually know what Vs you are using. However, when you are supplying Voltage to chips, it is better to read the chip's datasheet as to what are the range of Voltages that it can accept before overheating/melting/exploding/not working.

March 14, 2010
by Ralphxyz
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Thanks for the reply, but I did not ask what +VS was!

What is -VS? How would I use it to get the value of the resistor R1?


March 14, 2010
by mongo
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the -VS refers to the negative side of the power supply. Typically where used in the linear circuitry like amps and such. There is a positive supply with a negative 'ground' and a negative supply with the positive ground. This would make it three leads, +, -, and ground.

Look up application notes for the old LM747 op amps and you may find some classic examples.

It's also known as a double-ended power supply, as opposed to a single-ended supply.

Even your computer has a version, with +12V, -12V, +5V and maybe even a -5V.

Think of it as a couple of batteries where you would also draw power from between them.

The value of the resistor is spelled out in the formula. It just depends on the voltage level of the negative power supply. This gives you the opportunity to measure temperatures below zero. If -VS is the same as ground, as with a single-ended power supply, then the readings will be for above zero only.

March 15, 2010
by Ralphxyz
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Thanks that is what I thought -VS was but wasn't sure.

Probable this is not the method I want/need to use.

I tried using CAT5 wiring as demonstrated by hijohng The Long Arm of the Nerdkit but instead of using two RJ45 couplings/connectors I tried a RJ45 Jack at the breadboard instead of one of the couplings. This worked (for a first try) but the LCD readout was not stable, it was boucing between 130 - 500 on the ADC readout. I thought the Jack might be inducing a capacitance imbalance so I was looking at the LM34 spec sheet for filter ideas.

What is the best way of calming jitters?


March 17, 2010
by Ralphxyz
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duh, turns out one method to calm the jitters is to read the instructions!

I had "assumed" the Radio Shack RJ45 Jack would work like a tele punchdown block it turns out the wires have to be stripped to work properly, as the instructions clearly state.

I stripped the wires and now, ta dah, the RJ45 Jack works as expected/planned.

Thanks everyone for the feedback it really helps me to learn.


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