AVR-Chat

Thanks Graham - that was some excellent information. I was a bit of a
chicken and finally decided to open the screen to check for voltage
regulators - seems ok with the 15V so far.

Many thanks again though for your feedback on this - you may have even
saved my life.

Regards
Dean Claxton

-----Original Message-----
From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On
Behalf Of Graham Davies
Sent: Saturday, 21 May 2005 3:37 AM
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] Re: A little off topic, but PSU question....

--- In AVR-Chat@yahoogroups.com, "Claxton, Dean J" 
<dean.j.claxton@t...> wrote:

> ... There is a current mode PWM
> chip on ... the board - M62281FP
> ... adjust the output voltage 
> ... by playing ...

What you've probably got here is something like the application circuit 
at the end of your data sheet.

You need to know first of all that there are voltages in here that are 
way more hazardous than what comes out of your power outlet. Even if 
you've had a jolt from 110 volts AC, do not assume you will survive 
electrocution by the higher DC voltages inside this power supply. Don't 
touch anything while it's on. Plug it into its load and wait for it to 
discharge even after you've turned it off and disconnected it from the 
outlet before you touch it. Wear safety glasses. I'm serious.

What you need to find is the circuit that senses the output voltage and 
feeds it back to the chip. Look for the optoisolator and the voltage 
reference shown at the bottom right of the application circuit. If 
there are two optoisolators, the one connected to CT(OVP) is over-
voltage protection. It's the one connected to EA OUT that you want. 
Trace this part of the circuit and draw out the schematic. Assuming it 
looks like the application circuit, what you want to do is to make 
small adjustments to the ratio of the two resistors at the right that 
form a voltage divider across the power supply output. To reduce the 
output by 10%, try putting a resistor in parallel with the upper of 
these two that is ten times the value of the one that's already there. 
Hands off, stand back, safety glasses on and see what you get. Power 
off, disconnect, discharge and adjust accordingly. Don't go far from 
the original design voltage. You're playing with the voltage at the VCC 
pin of the chip as well.

Graham (no liability accepted).





 
Yahoo! Groups Links

--- In AVR-Chat@yahoogroups.com, "Claxton, Dean J" 
<dean.j.claxton@t...> wrote:

> ... that was some excellent information ...

It just happens that at work we have a chance to sell a product in a 
country that uses 240 volt power so the workings of off-line flyback 
switching power supplies is very much on my mind.

> ... I was a bit of a chicken ...

You were smart. You did the right thing. I've taken 240 volts AC from 
one hand to the other (live to neutral) but I'm not up for messing with 
similar DC voltages. I hear your muscles clamp up and you can't get 
loose. Maybe I should check that, but I'm still not up for the 
experiment.

Graham.

On Wed, May 25, 2005 at 01:54:49AM -0000, Graham Davies wrote:

> similar DC voltages. I hear your muscles clamp up and you can't get
> loose. Maybe I should check that, but I'm still not up for the
> experiment.

Geez!  Don't even think about messing with that, Graham.  At least not
on purpose.  It only takes a few milliamps to stop your heart,
a.k.a. electrocution.

-Brian
-- 
Brian Dean
BDMICRO - ATmega128 Based MAVRIC Controllers
http://www.bdmicro.com/

On Wed, May 25, 2005 at 01:54:49AM -0000, Graham Davies wrote:
> similar DC voltages. I hear your muscles clamp up and you can't get
> loose. Maybe I should check that, but I'm still not up for the
> experiment.

Geez!  Don't even think about messing with that, Graham.  At least not
on purpose.  It only takes a few milliamps to stop your heart,
a.k.a. electrocution.
-Brian

Once bitten *forever* shy. Many years ago whilst a techie at an elevator
company, I had the misadventure to be bitten by 130Vdc. Apart from the
initial shock, one thing I do remember about it, is that the muscles in my
arms and chest ached for days afterwoods. I consider myself extremely lucky.
Zapped with 240Vac is one thing, but high voltage dc is a whole different
ballgame... RUN A MILE!  

If you have to work on live equip to fault find, do yourself a favour and go
get some good quality latex gloves, snug fitting ones, they can litterally
save your life!

/~Darren.

Darren Worley said:
> On Wed, May 25, 2005 at 01:54:49AM -0000, Graham Davies wrote:
>> similar DC voltages. I hear your muscles clamp up and you can't get
>> loose. Maybe I should check that, but I'm still not up for the
>> experiment.
>
> Geez!  Don't even think about messing with that, Graham.  At least not
> on purpose.  It only takes a few milliamps to stop your heart,
> a.k.a. electrocution.
> -Brian
>
> Once bitten *forever* shy. Many years ago whilst a techie at an elevator
> company, I had the misadventure to be bitten by 130Vdc. Apart from the
> initial shock, one thing I do remember about it, is that the muscles in
> my arms and chest ached for days afterwoods. I consider myself extremely
> lucky. Zapped with 240Vac is one thing, but high voltage dc is a whole
> different ballgame... RUN A MILE!
>
> If you have to work on live equip to fault find, do yourself a favour
> and go get some good quality latex gloves, snug fitting ones, they can
> litterally save your life!
>

We were told early in our electrical engineering career by a professor
that there is a certain current range that is lethal.  Too little current
results in no harm, and too much locks the muscles in your chest and
protects your heart somehow.  I believe the dangerous value was around 15
mA. Everyone has different skin resistance, so it's hard to determine how
it will affect each individual.  I know one person that can hold two wires
from a 120 VAC cord, one in each hand, and while he feels it, it does not
cause him any harm nor visible reaction.  Another college friend has
related a story about how they would hook each wire from a 120 VAC cord to
two beer cans, then a whole group of people would hold hands and the
people at the two ends would each hold a beer can.  Obviously they had
drank the beer first!

The most painful shocks that I have received either involved horizontal
flyback amplifier tubes (remember when TVs used tubes?) or transmitting
tubes...

Mike