REALLY basic question...

[Dave]

Trying to hook up a 7805 +5V regulator as well as a 7905 -5V regulator to my
circuit, thus providing the +/- voltages for the 74HC4053 triple 1 of 2
multiplexor. Problem is, I'm not sure how to do this. The 7805 has its
three pins labeled in the datasheet (from left to right)
input/ground/output, and I'm assuming that the input is +voltage, ground is
negative, and the output is the advertised +5V. This at least makes sense
to me.

The 7905 is another matter. From left to right its pins are labeled (in the
datasheet) GND/input/output. Assuming (hate that word) that it's input is a
negative voltage (because it's output is negative) that makes the GND a
positive voltage. Can that be right? Or do I make GND negative, and the
input positive? Somebody please help with this. One person told me that
the 7905 can have an input of either positive or negative, which would lead
to the ground being interchanbably positive or negative. That doesn't sound
right, but I've been wrong before. One thing seems obvious- I cannot have a
negative ground for my 7805 and a positive ground for my 7905. This, it
seems, would fry my 74HC4053. (Back to the negative ground and positive
input for a -5V output...) What to do?

Ignorantly yours,

Dave

 

[Andrew Holme]

"Dave" <db5151@hotmail.com> wrote in message
news:13oq79vr2e7r133@corp.supernews.com...

Trying to hook up a 7805 +5V regulator as well as a 7905 -5V regulator to
my circuit, thus providing the +/- voltages for the 74HC4053 triple 1 of 2
multiplexor. Problem is, I'm not sure how to do this. The 7805 has its
three pins labeled in the datasheet (from left to right)
input/ground/output, and I'm assuming that the input is +voltage, ground
is negative, and the output is the advertised +5V. This at least makes
sense to me.

The 7905 is another matter. From left to right its pins are labeled (in
the datasheet) GND/input/output. Assuming (hate that word) that it's
input is a negative voltage (because it's output is negative) that makes
the GND a positive voltage. Can that be right?

All voltages are relative. Yes, GND on a 7905 is positive with respect to
its other terminals; but there is only one GND in your circuit. Connect the
7905 GND and the 7805 GND to the one and only GND. GND or 0V is a reference
point. 7805 inputs and outputs are positive w.r.t. GND whilst 7905 inputs
and outputs are negative w.r.t. GND.

 

[Dave]

"Andrew Holme" <andrew@nospam.com> wrote in message
news:fmj88n$d4f$1$8300dec7@news.demon.co.uk...

"Dave" <db5151@hotmail.com> wrote in message
news:13oq79vr2e7r133@corp.supernews.com...
Trying to hook up a 7805 +5V regulator as well as a 7905 -5V regulator to
my circuit, thus providing the +/- voltages for the 74HC4053 triple 1 of
2 multiplexor. Problem is, I'm not sure how to do this. The 7805 has
its three pins labeled in the datasheet (from left to right)
input/ground/output, and I'm assuming that the input is +voltage, ground
is negative, and the output is the advertised +5V. This at least makes
sense to me.

The 7905 is another matter. From left to right its pins are labeled (in
the datasheet) GND/input/output. Assuming (hate that word) that it's
input is a negative voltage (because it's output is negative) that makes
the GND a positive voltage. Can that be right?

All voltages are relative. Yes, GND on a 7905 is positive with respect to
its other terminals; but there is only one GND in your circuit. Connect
the 7905 GND and the 7805 GND to the one and only GND. GND or 0V is a
reference point. 7805 inputs and outputs are positive w.r.t. GND whilst
7905 inputs and outputs are negative w.r.t. GND.

Hmmm. Well, that at least makes sense. THANK YOU Andrew. For giving my
searching mind a little peace. It is greatly appreciated. And this fits
with something someone else told me, that the 7805 and 7905 can share a
common ground. Wow. Stretches my mind, but I guess that's a necessary
step...

Best regards,

Dave