7915 doesn't give me -15v...what's wrong?
Richard Hult
I have a question regarding the regulator 7915:
I have built a +/-15v power supply using a 7915 and 7815.
The problem is that I get +15v and -21.9v. I have checked
the pin layout many many times, and I have tried to replace
the 7915 with a new one; I still get -21.9v.
I read the spec. and it said that maximum neg input voltage is
-35vdc. I feed it with -30vdc, and I use a 33uF electrolytic
cap at the input and a 10uF eletrolytic cap in parallel with
a small ceramic cap at the output.
Does anyone have a clue of what I am doing wrong??
/Richard
--
Winfield Hill
Richard Hult, <d4h...@dtek.chalmers.se> said...
>
> I have a question regarding the regulator 7915:
>
> I have built a +/-15v power supply using a 7915 and 7815.
> The problem is that I get +15v and -21.9v. I have checked
I assume you noticed that the 7915 pinout is totally different from the
7815? Incidentally, the IC designers did this because the metal tab and
center pin must be bonded to the IC substrate for best thermal conduction.
Generally the substrate must be at the most negative circuit voltage.
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Lance Corey
d4h...@dtek.chalmers.se (Richard Hult) wrote:
>
>I have a question regarding the regulator 7915:
>
>The problem is that I get +15v and -21.9v. I have checked
>the pin layout many many times, and I have tried to replace
>the 7915 with a new one; I still get -21.9v.
>
>Does anyone have a clue of what I am doing wrong??
>
>/Richard
>
Aside from the volt meter, do you have a load on the output ? Although most
voltage regulators will produce some output with a load, many cannot
regulate the voltage without a load being present.
Lance Corey
Dept. Pharmacology & Therapeutics,
University of British Columbia,
2176 Health Sciences Mall,
Vancouver, B.C. V6T 1Z3 Canada
la...@neuro.pharmacology.ubc.ca
tadedek
the 78 and 79 series regulators are load independant regulators.
They don't need a minimum load
Sheldon
Roy McCammon
Both the LM7815 and LM7915 are specified down to load
current of only 5ma. Its considered conservative design
practice to provide at least a 5 ma load.
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Opinions expressed herein are my own and may not represent those of my employer.
Ted Davis
In article <5csnie$1...@nyheter.chalmers.se>, d4h...@dtek.chalmers.se says...
>
>
>I have a question regarding the regulator 7915:
>
>the pin layout many many times, and I have tried to replace
>the 7915 with a new one; I still get -21.9v.
>
>-35vdc. I feed it with -30vdc, and I use a 33uF electrolytic
>cap at the input and a 10uF eletrolytic cap in parallel with
>a small ceramic cap at the output.
>
>
>/Richard
>
It sounds to me as if you have a pair of leads reversed - you are aware that
the two don't have the same pinout, aren't you?
--
T.E.D. (tda...@umr.edu)
Dave Baldwin
tadedek (TAD...@flash.net) wrote:
: the 78 and 79 series regulators are load independant regulators.
: They don't need a minimum load
79xx regulators are notorious for Not regulating without a load.
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Jens Brungot
All unloaded 78xx reg's I've used have given the supposed voltage +
some, e.g. 5,1V for a 7805, no load.
JB, JB Electrics, Norway
Andre' Mielke
Jens Brungot wrote:
> > 79xx regulators are notorious for Not regulating without a load.
> Never been a problem for me...
> All unloaded 78xx reg's I've used have given the supposed voltage +
> some, e.g. 5,1V for a 7805, no load.
>
> JB, JB Electrics, Norway
Notice he said 79xx.. why don't we just ask the original poster to put
1.5k across the output and then measure the volts? Does this fix the
problem?
Nick
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Uday Kamath
Richard Hult (d4h...@dtek.chalmers.se) wrote:
: I have a question regarding the regulator 7915:
: I have built a +/-15v power supply using a 7915 and 7815.
: The problem is that I get +15v and -21.9v. I have checked
: the pin layout many many times, and I have tried to replace
: the 7915 with a new one; I still get -21.9v.
OK, the input power is ok, if your using 30volts,
it should give you -15 volts.
Just one note, the 7815 pinout is in ground out
the 7915 is ground in out, where in is -30 input and out is
-15volts. Another note, you have to have atleast 100ma to
get the regulator regulate!!, so use a resistor and check out
the voltage. This is unlike the 7815 which usually gives
15volts without a load.
-Uday<kam...@expert.cc.purdue.edu>
.
: --
Richard Hult
>Just one note, the 7815 pinout is in ground out
>the 7915 is ground in out, where in is -30 input and out is
>-15volts. Another note, you have to have atleast 100ma to
>get the regulator regulate!!, so use a resistor and check out
>the voltage. This is unlike the 7815 which usually gives
>15volts without a load.
I did not have a load, and that was the problem. I read the
data sheet once more and it said that I need a 5mA load. I
heard that it was load independant, so I didn't think of
that at first.
Thanks to all of you who tried to help me!!
Richard
>-Uday<kam...@expert.cc.purdue.edu>
--
Nick Ciarallo
Uday Kamath wrote:
>
> Richard Hult (d4h...@dtek.chalmers.se) wrote:
>
> : I have a question regarding the regulator 7915:
>
> : I have built a +/-15v power supply using a 7915 and 7815.
> : The problem is that I get +15v and -21.9v. I have checked
> : the pin layout many many times, and I have tried to replace
> : the 7915 with a new one; I still get -21.9v.
>
> OK, the input power is ok, if your using 30volts,
> it should give you -15 volts.
>
> the 7915 is ground in out, where in is -30 input and out is
> -15volts. Another note, you have to have atleast 100ma to
> get the regulator regulate!!, so use a resistor and check out
> the voltage. This is unlike the 7815 which usually gives
> 15volts without a load.
>
> .
>
> : --
Also, don't forget the tab is connected to the middle pin
on both the 7815 and 7915. Most people when using these
regulators screw the tabs to a heat sink. This is normally
OK except for thr 7915. Notice that the tab (and middle
pin) are at the - supply input. Just make sure that the
tab on the 7915 is isolated from the heat sink...
Nick
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Terry King
Uday Kamath wrote:
>
> Richard Hult (d4h...@dtek.chalmers.se) wrote:
>
> : I have a question regarding the regulator 7915:
>
> : I have built a +/-15v power supply using a 7915 and 7815.
> : The problem is that I get +15v and -21.9v. I have checked
> : the pin layout many many times, and I have tried to replace
....
Make SURE you have bypass capacitors (say .1 uF) on the input and output
of these 3-terminal regulators to ground. MOST of the time I've seen
one giving the 'wrong' DC voltage it's actually oscillating. If you have
a scope, look at the 'DC' output to be sure it's all DC!
--
Regards, Terry King ...In The Woods In Vermont
Equipment Engineer
Little Castle Studio
Box 633 2173 Shelburne Road
Shelburne, Vermont 05482
tk...@together.net
Ray Gannon
I haven't got the data sheets to hand, but I think the 7815 and 7915 have
different pinouts - it's worth a double check.
sha...@ratchet.demon.co.uk
>I haven't got the data sheets to hand, but I think the 7815 and 7915 have
>different pinouts - it's worth a double check.
looking from the front...
7815 in - com - out
7915 com - in - out
Shakey
Richard Steven Walz
In article <5dsk5u$n...@gcsin3.geccs.gecm.com>,
Ray Gannon <ray.g...@gecm.com> wrote:
>I haven't got the data sheets to hand, but I think the 7815 and 7915 have
>different pinouts - it's worth a double check.
Das right! Da man's nailed it for ya!! Everybody trips on THAT one!!
-Steve
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Terry King
...
Richard Steven Walz wrote:
> >I haven't got the data sheets to hand, but I think the 7815 and 7915 have different pinouts - it's worth a double check.
> -------------------------------
> Das right! Da man's nailed it for ya!! Everybody trips on THAT one!!
> -Steve
A little more about more subtle problems with negative regulators
(From Bob Peases's excellent book TROUBLESHOOTING ANALOG CIRCUITS):
These negative regulators have more critical needs for the OUTPUT
capacitor: They need "tens of Uf (say 47uF) of TANTALUM capacitor, or
a few hundred uF of aluminum electrolytic" and "Avoid ceramic or
film capacitors (here)".
There can actually be a problem with TOO MANY ceramic discs on the
output
bus (typically on a PC Board) that need to be 'swamped with large value
capacitors'.
Pease designed a lot of these IC regulators for National and has seen
it ALL (He'd say 'well, almost')...
Lots to be learned about all of this stuff!