lektric.dan@gmail.com
Guest
Mon Aug 30, 2010 11:21 pm
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
Tim Williams
Guest
Mon Aug 30, 2010 11:21 pm
You need a negative supply to operate a 7805 at less than 5V, or LM317 at
less than 1.25, etc. I don't recall if there are any three-terminal
regulators that go to zero on a single supply.
You may be better off using a TL431, op-amp and power transistor.
Tim
--
Deep Friar: a very philosophical monk.
Website:
http://webpages.charter.net/dawill/tmoranwms
<lektric.dan_at_gmail.com> wrote in message
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Quote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
Nik
Guest
Mon Aug 30, 2010 11:23 pm
Hi ! According to...
http://www.national.com/ds/LM/LM340.pdf
... you can take the control input to a voltage divider between the outpu
and ground so making it adjustable above the nominal 5 V setting.
---------------------------------------
Posted through
http://www.Electronics-Related.com
Jamie
Guest
Tue Aug 31, 2010 12:28 am
lektric.dan_at_gmail.com wrote:
Quote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
you need to generate a -5 Volt signal to connect to terminal 2.. This
signal of course can be scaled via a pot. So, you would place the wiper
on terminal 2 of the 7805 and low side to common, high side to the
output of your -5 supply..
If you don't have a -5 supply available and, are using a
single/bipolar supply? you can then employ a very basic circuit
via a 555 timer to generate a -5 volts into a 5.1 V zener shunted
reg source to drive the top side of the pot...
Working with 555's are great because they're cheap, plentiful and
quick to get you out of trouble fast..
Just the other day I had a fixed (-) supply via a 555 to bias a pot
for +/- control to drive a small low end AC drive that didn't have a
(-) reference, nor could it offset it in software..
Have a good day.
langwadt@fonz.dk
Guest
Tue Aug 31, 2010 12:54 am
On 30 Aug., 22:21, "lektric....@gmail.com" <lektric....@gmail.com>
wrote:
Quote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
if you look at the datasheet you'll see that the output voltage isn't
very
accurate, 4%/2% so your 4.9 looks to be within with in spec.
you could tweeak the output voltage to be higher with a resistor in
the ground pin,
if you keep the input voltage and th output current resonably
constant, because the
current in the ground pin changes with output current and input
voltage.
first datasheet I found says 5mA so try 20R
and all the specs are for >5mA Io so I think you'd want to load it at
least that
-Lasse
John Larkin
Guest
Tue Aug 31, 2010 1:25 am
On Mon, 30 Aug 2010 13:21:03 -0700 (PDT), "lektric.dan_at_gmail.com"
<lektric.dan_at_gmail.com> wrote:
Quote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
Sure. Just lift the "ground" pin up above ground, and the output will
track that. A series resistor, or better yet a voltage divider from
its own output, will work. To go lower, you'll have to pull the
regulator gnd pin negative.
I like to stack regulator gnd or adj pins on top of the outputs of
other regs. You can make 1.25 and 2.5, for an FPGA, by stacking two
LM1117s, no resistors required.
John
langwadt@fonz.dk
Guest
Tue Aug 31, 2010 2:32 am
On 31 Aug., 02:25, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
On Mon, 30 Aug 2010 13:21:03 -0700 (PDT), "lektric....@gmail.com"
lektric....@gmail.com> wrote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
Sure. Just lift the "ground" pin up above ground, and the output will
track that. A series resistor, or better yet a voltage divider from
its own output, will work. To go lower, you'll have to pull the
regulator gnd pin negative.
I like to stack regulator gnd or adj pins on top of the outputs of
other regs. You can make 1.25 and 2.5, for an FPGA, by stacking two
LM1117s, no resistors required.
John
when xilinx first came out with 1.2V fpgas, AFAIR the 'normalt' 1.25V
ref
regulator with tolerances was above max spec voltage.
-Lasse
John Larkin
Guest
Tue Aug 31, 2010 2:52 am
On Mon, 30 Aug 2010 17:42:31 -0700 (PDT), "langwadt_at_fonz.dk"
<langwadt_at_fonz.dk> wrote:
Quote:
On 31 Aug., 02:25, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 30 Aug 2010 13:21:03 -0700 (PDT), "lektric....@gmail.com"
lektric....@gmail.com> wrote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
Sure. Just lift the "ground" pin up above ground, and the output will
track that. A series resistor, or better yet a voltage divider from
its own output, will work. To go lower, you'll have to pull the
regulator gnd pin negative.
I like to stack regulator gnd or adj pins on top of the outputs of
other regs. You can make 1.25 and 2.5, for an FPGA, by stacking two
LM1117s, no resistors required.
John
when xilinx first came out with 1.2V fpgas, AFAIR the 'normalt' 1.25V
ref
regulator with tolerances was above max spec voltage.
-Lasse
Move voltage just makes them faster!
John
John Ferrell
Guest
Tue Aug 31, 2010 3:18 pm
My experiments in using a 7805 regulator at higher voltages by
shifting the ground pin up with resistors, diodes and zeners all
resulted in very rubbery regulation. best use a 317 or similar for the
task...
On Mon, 30 Aug 2010 13:21:03 -0700 (PDT), "lektric.dan_at_gmail.com"
<lektric.dan_at_gmail.com> wrote:
Quote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
John Ferrell W8CCW
Michael A. Terrell
Guest
Tue Aug 31, 2010 4:46 pm
John Ferrell wrote:
Quote:
My experiments in using a 7805 regulator at higher voltages by
shifting the ground pin up with resistors, diodes and zeners all
resulted in very rubbery regulation. best use a 317 or similar for the
task...
Did you have a resistor across the output and common terminals?
--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.
George Herold
Guest
Tue Aug 31, 2010 6:00 pm
On Aug 30, 4:21 pm, "lektric....@gmail.com" <lektric....@gmail.com>
wrote:
Quote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
Yeah as others have said you can tweak it up in voltage, down is a
bit more difficult. If you are just using this for a reference (very
little current) then a real 5 volt reference might be better. REF02,
or even one of the zener references LM4040. You then don't get the
voltage variation that a voltage regulator gives you.
George H.
whit3rd
Guest
Tue Aug 31, 2010 9:23 pm
On Aug 30, 1:21 pm, "lektric....@gmail.com" <lektric....@gmail.com>
wrote:
Quote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so.
If you need more accuracy (the uA7805 only promises 4.75 to 5.25V)
there are tighter-specified regulators. Consider LT1083, for instance
(it's
an adjustable 3-terminal regulator with about +/- 1% accuracy).
If you need REAL accuracy, three-terminal regulators are not
ideal (four-terminal ones are better, they allow point-of-load
sensing).
If you just need to cover the 5.00 volt setting on your divider,
a 7806 regulator would guarantee that...
Grant
Guest
Wed Sep 01, 2010 6:49 am
On Tue, 31 Aug 2010 10:18:30 -0400, John Ferrell <jferrell13_at_triad.rr.com> wrote:
Quote:
My experiments in using a 7805 regulator at higher voltages by
shifting the ground pin up with resistors, diodes and zeners all
resulted in very rubbery regulation. best use a 317 or similar for the
task...
Did you pour extra current down the reference lead to swamp the regulator's
quiescent current? Like running a LM317 with 5V ref. instead of 1.25V?
Opamps would do even better.
Grant.
Quote:
On Mon, 30 Aug 2010 13:21:03 -0700 (PDT), "lektric.dan_at_gmail.com"
lektric.dan_at_gmail.com> wrote:
How does one slightly adjust a fixed-voltage 3-pin regulator? I've
got a 5 volt (7805) that I'm using to drive a voltage divider (no
current, just a control voltage). The circuit should go from 0-5 but
the regulator only puts out about 4.9 or so. Will a resistor between
the ground pin and actual ground allow tweaking of max. voltage out?
I should just try it, but I can't get to the circuit right now, and
don't have the parts on hand to breadboard another. Thanks.
John Ferrell W8CCW
