summary: power supply and chassis

[Martin Czech]

Thank you for your input! One step after another, we get it together.

There are a couple of motivations to write a summary:

-I think the power supply item will frequently come up on this list.
-For the not so experienced builders there may be also a health or life
 risc when toying with mains power.  
-There are things that I also was not so sure about.

Please comment any wrong statement with explanation why it is wrong
and sucessive better formulation!
Did I forget anything?

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What power supply and chassis requirements do we have
for a typical supply?

PE: protective earth
PSUGND: power supply ground, usually middle tap of the +- xV transformer


0. High voltage secondary
Note: In case of high voltage secondary (>40V), eg. for tube equipment,
PSUGND and signal ground have to be hardwired to PE. Everything else is
forbidden and a risk to your life! Option 3b must not be applied!
Use audio transformers to avoid groundloops in such a case.


1. Transformer
Use a "safe isolated" power transformer, these have very high insulation
breakdown voltage between primary and secondary, thus no hazardous
voltages may ever appear at the secondary. The transformer should also
have a shield between secondary and primary, this is to be connected to PE
(at least same gauge as power line, yellow/green colour).
Never go the cheap way to take directly a "down divided" mains voltage,
like in cheap old radios and tvs. One day it will kill you.


2. Enclosure
Use a metal enclosure for your device. This will damp rf interference.
Try to have as few holes in the enclosure as possible. Otherwise
rf signals may sneak in. 


3a. Protective earth
This is recommended:
Use a 3 prong IEC male connector/rf filter/switch/fuse device as power
input. This way you have maximum balanced and unbalanced line noise
rejection and mechanical strain safety. You can plug the power cord off
(transportation). Hardwire your chassis to the PE outlet (at least same
gauge as power line, yellow/green colour). If the chassis has several
parts/metal sheets and a good electrical connection is not enshured
(washers and screws), give them seperate lines.  

Always connect PSUGND to the chassis PE via a cap, say 10nF, 400V rating,
special mains voltage self healing type.  This will short out any rf dirt.
The PSUGND is now dc wise floating. This could pose some problems.
There are some options:

-You may add a resistor about 100Ohm from PSUGND to PE in order to avoid
large potential differences.  Try different values.
-if you don't like such a low resistance, you can also apply antiparallel
diodes to PE (conduction only for large voltages, high current rectifier
types, like 1N4001, just for safety), together with a 10kOhm parallel
resistor. You can try different values.
-you may also hardwire PSUGND to PE if you use transformer isolated
balanced I/Os.


3b. Protective earth, second choice
This is only for people who know what they do, but however, it is
not recommended for hobbyist usage.  If you go this way, you should be
quite experienced. Most industrial 19" bricks work the way. Ground loops
are avoided to a great deal by this scheme.  I would not recommend it,
however.

If you can make sure that no primary leads can ever touch your metal
enclosure it is allowed not to apply PE to the metal enclosure.
This means you have to provide protection against mechanical stress
for the power chord as well as epoxyd (or other methods) insulated bare
primary wires, plastic insulator sheets below or above the pcb where we
have primary voltage etc.

Always connect PSUGND to the chassis PE via a cap, say 10nF, 400V rating,
special mains voltage self healing type.  This will short out any rf dirt.
The PSUGND is now dc wise floating. This could pose some problems.
There are some options:

-You may add a resistor about 100Ohm from PSUGND to PE in order to avoid
large potential differences.  Try different values.
-if you don't like such a low resistance, you can also apply antiparallel
diodes to PE (conduction only for large voltages, high current rectifier
types, like 1N4001, just for safety), together with a 10kOhm parallel
resistor. You can try different values.


4. Switching loads
Switching transformers on/off may cause exessive arcing. Therefore add a
snubber network at the transformer primary clamps.  A series connection
of a xxxnF 400V rating, special mains voltage self healing type cap
and a 100Ohm resistor will damp away any switching transients. 

Large supplys look like a short to mains if switched on. This may lead
to a system crash in a nearby computer, or to house circuit breaker
trip. Excessive currents may also damage transformers, diodes and
caps. Therefore a "safe turn on" relay circuit should be provided.
The transformer starts with a series resistor, limiting the current to
1A or so. After several mains cycles the relay shorts this resistor.
The relay may be driven by the unregulated PSU voltage.  The resistor
does not need to have the full power rating, since it is only on during
a short amount of time. I use 4W wire types. It has to be ensured that
this assumption is always valid (thermistor fuse), otherwise fire hazard
is will result! I believe that the overstressed resistor will finally
open, but who knows. There are resistors with build in solder junction,
this will melt open if the resistor gets to hot. Such a simple scheme
will not protect against fast on/off/on/off sequences, a better circuit
will be needed for such fools.


5. Transients and module protection.
Power lines have nasty transients up to severall kV by lightning
and switching of large loads. Therfore use a suitable varistor after
the rf mains filter.  This will eat up most of the transient energy.

It is a good idea to go even further. Your modules represent such
a time/money value that you really should protect them.  It is a
good practise to use separate regulators for each module.  This gives
maximum dc PSU decoupling, and keeps the regulators pretty cool. 317/337
regulators are still recommended high drop types. Susceptible, high
power or noisy modules may also need extra supply RC or even RLC filters
(eg. smd ferrite coils) (logic) for ac decoupling. Transzorb diodes
protect your precious circuits from remaining transients, this is also
recommended for all I/Os. This should also lead to a short when a voltage
regulator goes berserk, so a fuse for each regulator is also usefull. Even
a totally failing module will not stop the synth from working.  A good,
low inductance local ground connection is required to make all measures
really work.  It is reasonable to provide several types of local
ground, eg. logic ground, analog ground, relay ground, led ground,
PSU filter ground und the module pcb, with optional mutual connections
and connections to the ground mekka. This way you have a chance to cure
problems without changing the pcb.


6. I/Os
Unbalanced connections: Always use isolated jacks. This means: the signal
and ground pins should be isolated from chassis. The jack ground should
connect to your signal ground ("ground mekka").

Balanced connectors do not use the shielding as signal path. If both,
signal source and drain are have decoupled PE and signal ground the
shield pins should be tied to chassis. Of course not the two balanced
signal pins. This is the right way to avoid rf interference. Doing
this the signal source and drain enclosure and cable shield together
look like a single "bone shaped" enclosure. In hostile environments
(large distances, separate mains power (i.e. motor driven generators,
faulty PE sheme), large relaxation currents may flow through the cable
shield.  Only in such an extreme case a ground lift option can make sense.

Unfortunately even today you can buy wrong constructed balanced gear that
will force you to ground lift. Such hostile enviroments may force the
use of transformer balanced connections,because electronically balanced
senders and recievers can't cope with high common mode voltages.


m.c.

----------------------------------------------------------------------
m.c. (alias mad scientist): 3 CDs à 72 min., Elektronic Music info, 
synth do it yourself info, Synth-Diy Mailing List Archive (Grey Pages) 
all @ http://www.geocities.com/SoHo/Museum/4459/index.html, enjoy!