Grounding a 317 regulator circuit.

Discussion in 'Electronic Basics' started by Nikolas Britton, May 22, 2005.

1. Nikolas BrittonGuest

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
*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.

2. BanGuest

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.

30V+30Vac
-. ,--------.
)|( | ____
)|( | | | +1.25...37V
120Vac )|(---. | +-+-------o------|317 |-------o--o
)|( | | A A | |____| |
)|( | '-+ | |+ | ___ |
-' '---)------(-+ === .----o----|___|-o
| A A /-\ | | |
| +-+-- | |+ .-. ---
| - open | === | |<-. ---
| | /-\ | | | |0.1u
'----------------o | '-' | |
| | | | |
'---o----o---o------o--o
10u/50V
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
Be aware that your rectifier is not rated for 60Vac, better to take some
discrete diodes with 3A/200V(1N5402) rating.

3. Nikolas BrittonGuest

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.

4. Peter BennettGuest

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.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter

5. tempus fugitGuest

What DC output are you trying to get from the regulator?

6. Nikolas BrittonGuest

1.5V ~ 24V.... I need a general purpose benchtop power supply that can
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?

7. Nikolas BrittonGuest

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.

8. kellGuest

insulator, washers etc. I saw them as recently as last year. But only
a large, fully-stocked RS would have them.

9. Roger JohanssonGuest

The alternatives:

Isolate the heat sink from the chassis, and leave a warning label
inside saying the heat sing is not isolated from the circuit.

Isolate the chassis against outer touch, without reducing its heat
emissing properties too much. A thin layer of plastic, maybe?

Choose another regulator circuit, where the collector of the power
transistor remains at ground potential.

I have designed several such power supplies, The main part is a npn
power transistor sitting between ground (output ground) and the
negative end of the rectifier bridge, which is NOT grounded anywhere.

The positive end of the rectifier becomes the positive output from the
regulator.

Now we only need to control the base of the power transistor so the
voltage over the transistor is what needs to be subtracted to get the
output voltage we want between the ground (the collector of the
transistor) and the positive supply.

First I add a series resistor in the emitter of the power transistor,
and a smaller transistor which controls the current by pulling down the
base of the power transistor. This sets the maximum current this
circuit will allow through the power transistor.
The small transistor, e to minus, c to base of power transistor, b to
the top of the current sensing resistor, the emitter resistor of the
power transistor. 0.6V/1Amp=0.6Ohm, for example.

rectifier plus Output plus

Output ground
rectifier minus

Then I need to compare the output voltage with the reference voltage.

The reference I use is often a zener, or led's used as zeners.

The output voltage is floating compared to the regulator circuit and
that poses a little problem.
The sensor or output voltage needs to float with the output, but send
control signals to the regulator part, which is completely under
ground, so to speak because it is below the zero voltage of the
output.

This is not so difficult, we can use current or even light to control
it.

The "sensor" must take a sample of the output voltage difference, and
use it to control a current in a wire to the regulator unit.

There are many fine engineers here who can help you with the rest if
you want to continue this lesson in power supply design.

Well, even if we complete this design and build it the chassis will be
grounded, and some care is needed for that chassis not to get in touch
with anything which it could short. We prefer boxes which are not
electrically active in any way.

For example a toaster where the chassis is grounded, and you poke some
bread crumbs out with metal wire. If you hold the other hand around the
toaster's chassis you can die if the chassis is grounded metal. If it
is isolated, and even better, made out of plastic, you have a better
chance of survival.

Sometimes we need direct contact between the metal chassis and the air
so much that we can sacrifice the rule that a chassis should be
isolated.

I built a computer monitor once, designed a 95 Volt 1A power supply as
described above, and bolted the output transistor directly to the sheet
metal outer chassis I made for the tube.
Its collector was the ground point in the regulator I built so there
was no need for isolated mounting, maximizing the possible power and
reducing the working temperature.

10. Nikolas BrittonGuest

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