[sdiy] Equation question

[Scott Gravenhorst]

Justin Owen <juzowen at googlemail.com> wrote:
>Hello,
>
>When voltage - or a percentage of voltage (i.e VCC2) - is used as 
>part of an equation in a single supply circuit, then Voltage 
>(obviously) = Voltage. 
>
>When using a bipolar supply (i.e. +/- 12 Volt) is total voltage 
>(or say, a third of the supply voltage) calculated using 12V or 
>24V? 
>
>Hope that makes sense - thanks.
>
>Justin

If I understand the question, it depends on the reference.  The voltage
is always measured from a reference point to the source point.  If the
reference is the ground point between the supplies, then the voltage is
the difference between ground and the supply in question (because the
supplies do not necessarily have to be equal in absolute value.  In
some circuits, e.g. the Iabc path for an OTA like LM13700, then the
reference point might not be ground - in this case it isn't, it's the
negative rail.

Here's another example:  Let's say you have a dual supply of +/- 15
volts.  A resistor from +rail to ground will have it's current
calculated at a voltage of 15 volts.  Note the reference, ground. 
However, if the resistor is connected from +15 to -15, then the voltage
across the resistor is 30volts.  Again, the reference is important. 
This simple case is found often in circuits that use a pot to generate
a voltage.  You want to choose the pot's value to conform to it's power
rating so you don't smoke it.

-- ScottG
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-- Scott Gravenhorst
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