Grounding a 317 regulator circuit.

N

Nikolas Britton

Guest
Hello all, I need some advice on the correct way to ground a standard
LM317 circuit. Right now I have the base of the transformer and the
negative side of the DC output connected to earth ground. To me it
seems weird having to earth ground the DC negative side, but if I don't
do that I'll get 12.5Vac / 20mA from the DC negative post to earth
ground. I thought the DC section was suppose to be isolated because of
the transformer?

Parts list (so far):
*The transformer is 60Vac with a center tap to give me 30Vac, pulled
from a dead Sony amp.
*KBPC10 Bridge rectifier, metal casing, I think it's rated at 10A /
50V.
*Sprague powerlytic 2600uF 150V filter cap.
*230 ohm 1/4 watt resistor.
*10K potentiometer.
*LM317.
*Big heat sink.

BTW, IIRC, when I first tried grounding it I connect the earth ground
to the heat sink that had the rectifier and the LM317, plus side of the
DC output, mounted on it but then I got 12.5Vac / 7A on the negative
side to earth ground....

Thanks.
 
Ban wrote:
Niko, with 30Vac you will get more than 42Vdc across the cap, which
is above
the absolute max. rating(input/output differential) of the LM317.
This means
when a short occurs on the output the regulator might blow. There is
a
LM317HV version available, which is good for 60V.
Now to the connections:
1. connect the center tap to the neg. side of the cap and leave the -
tab of
the rectifier open, so only 2 of the diodes are used. you do not need
to
connect a earth. View the diagram with fixed font.

[snipped]

Be aware that your rectifier is not rated for 60Vac, better to take
some
discrete diodes with 3A/200V(1N5402) rating.
Click to expand...
Thanks.

I looked at the LM317HV and it's max output is 500mA, I need amps more
then I need volts. I payed zero dollars for the parts I have now
because I pulled them all from dead equipment than came my way. If I
had to buy new parts then my selection would be limited to whats
available at the local radio shack. What about a power resister is
series to drop the voltage down to 40Vdc, I have lots of 5 and 10 watt
resisters.

Here are the rectifiers and regulators what I have in my parts bin, I
found another box that has a lot of rectifiers, transistors, diodes,
and ICs that I forgot about so I still need to sort and catalog those
parts:
S30SC4M, 1, Schottky Rectifier
KA317, 1, Voltage Regulator TO-220AB varitable 1.5A Positive
LM7912CT, 1, Voltage Regulator TO-220AB 12V 1.5A Negative
LM7905CT, 1, Voltage Regulator TO-220AB 5.0V 1.5A Negative
LM7808CT, 1, Voltage Regulator TO-220AB 8.0V 1.0A Positive
LM340T15, 1, Voltage Regulator TO-220AB 15V 1.0A Positive
LM340T5, 1, Voltage Regulator TO-220AB 5.0V 1.0A Positive
RS603M, 1, Bridge Rectifier RS-6M 6A 200V
RS405L, 1, Bridge Rectifier 4A 600V
RS603M, 1, Bridge Rectifier RS-6M 6A 200V
KBPC804, 1, Bridge Rectifier BR-8 8A 200V
KBPC10, 2, Bridge Rectifier - 10A MaxVrms=35 MaxVrrm=50 Vr=50 Ifsm=300A

I decided on the 317 because I had the circuit diagram in a book and
had all the parts needed to build it on the spot. Also at my desposal
is a crap load of power transistors, big diodes, box full of
transformers of all sizes, large caps, etc if someone has a better way
to do what I want.
 
tempus fugit wrote:
What DC output are you trying to get from the regulator?

1.5V ~ 24V.... I need a general purpose benchtop power supply that can
Click to expand...
handle at least 3 amps

I just found these circuits that uses a 317 and power transistors to
get 3 ~ 5 amps out:
http://repairfaq.ece.drexel.edu/sam/samschem.htm#schslp2
http://repairfaq.ece.drexel.edu/sam/samschem.htm#schslp2a

What is the purpose of the diode (D1) in the 2nd link? and would it be
possible to put a switch, for safety, in series with the base of the
power transistor so you can "turn on" the extra amps only when I need
it?

I have two NTE378 PNP Transistors that might work for this circuit but
I'm not sure if they are overkill. The collector current (continuous)
is rated at 10A and I think the transformer is rated only to 7A , too
much?
 
Peter Bennett wrote:
On 21 May 2005 23:50:07 -0700, "Nikolas Britton"
nbritton@nbritton.org> wrote:


BTW, IIRC, when I first tried grounding it I connect the earth
ground
to the heat sink that had the rectifier and the LM317, plus side of
the
DC output, mounted on it but then I got 12.5Vac / 7A on the negative
side to earth ground....

Thanks.

If you look at the LM317 datasheet at
http://www.national.com/ds/LM/LM117.pdf, you will find that the
mounting tab is connected to the output terminal - you DO NOT want to
ground that tab, or the heat sink the part is mounted on (unless you
take measures to insulate the tab from the heatsink.
Click to expand...
Yes!, I found that out when I hooked my oscilloscope ground lead to it.
I disconnected the scope lead when it sparked, only a small spark, and
got my meter out to check the volts/amps, the meter showed 12.5Vac /
7Adc. I didn't believe the meter readings so I tested it with a 100ohm
resister, burned up in milliseconds!

I wanted to have everything mounted inside a metal chassis. The heat
sink I have was designed for this chassis and is directly mounted to it
but being that the 317 is in a TO-220 package it would be hard to
isolate it, unless you had plastic screws.
 
Radio Shack has (had?) a TO-220 mounting hardware with a mica
insulator, washers etc. I saw them as recently as last year. But only
a large, fully-stocked RS would have them.
 
Thanks everyone, today I built a wooden box for it and reassembled the
circuit inside of it.

I'm going to keep it, for the moment, a standard 317 circuit. The only
thing I modified from the original circuit I posted was to add an extra
sprague powerlytic 2700uF 150V cap after the regulator stage to help
stabilize devices such as audio amps. I tested the circuit with a 200
watt 12v car audio amp and I was impressed by the little LM317. Is it
possible for the 317 to output 2.4 amps?, this is what my amp meter
read....

At first I had a 4"x4"x1/8" heat sink on it but with the audio amp it
was getting too hot so I had to upgrade the sink to a 10"x1.5"x3/8"
with about 20 1"x1/8" fins. With this sink I was able to keep it in the
110 to 130 (F) range with the audio amp, also I put a dab of thermal
grease on the metal tab.... 2.4 amps?!?!
 
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