On Thu, 11 May 2006 01:04:23 GMT,
wrote:

Limiting inrush current is mostly to protect the diodes as the first
charge the caps. For the first few ac cycles those caps look like a
dead short or mightly close. Under those conditions the diodes have
to stand the short circuit current the transformer can provide which
can easily exceed diode rating if care is not taken.

Also if a toroid mains transformer is used, the transformer start-up
current can be huge for a large (say 1 kVA) transformer, which could
harm the house wiring or burn the fuses in the main distribution
panel.

Depending at what mains cycle phase angle the transformer was
previously disconnected and on what phase angle it is reconnected to
the mains, the start-up peak can be insignificant or the current can
be huge, which is limited just by the copper resistance of the
primary.

Paul OH3LWR

Limiting inrush current is mostly to protect the diodes as the first
charge the caps. For the first few ac cycles those caps look like a
dead short or mightly close. Under those conditions the diodes have
to stand the short circuit current the transformer can provide which
can easily exceed diode rating if care is not taken.
Also if a toroid mains transformer is used, the transformer start-up
current can be huge for a large (say 1 kVA) transformer, which could
harm the house wiring or burn the fuses in the main distribution
panel.


As far as harming house fuses or wiring, here in the USA and I'd bet
most elsewhere the electrical codes provide for time delay fuses
for surge loads and house wiring that is sufficently robust to
withstand the rated current of the distribution fuses. That should
not be an issue and the power supply should have it's own fusing.
It could be a nusance if the power supply has a massive enough
surge to open the mains fuse but there is not safety issue unless
the mains fuse or wiring is improper.
================================
When switching on an Astron 13.8 V - 50 Amperes PSU ,the surge current
is that high that it (usually)trips the mains 'over-current breaker ' in
the switchboard. I shall now include a soft start facility ( resistor in
230V supply) which is by-passed as soon as DC output is established.

Talking about 'mains fuses' : In Britain and many other European
countries most (modern) domestic supply boards no longer have
traditional (melting) wire fuses but (resettable) fast acting 'over
current' cut-out devices .
There are 100 Amperes wire fuses in each phase of the incoming supply
cable ,but these can only be replaced by the electricity supply company
,since these fuses are each fitted inside a separate 'wired-lead-sealed'
box.

BTW the over-current breakers are separate from earth leak detection
cut-out devices ,officially called residual current detectors (RCDs).

Frank GM0CSZ / KN6WH

Paul Keinanen wrote:

Also if a toroid mains transformer is used, the transformer start-up
current can be huge for a large (say 1 kVA) transformer, which could
harm the house wiring or burn the fuses in the main distribution
panel.

Depending at what mains cycle phase angle the transformer was
previously disconnected and on what phase angle it is reconnected to
the mains, the start-up peak can be insignificant or the current can
be huge, which is limited just by the copper resistance of the
primary.

This might be a good time to mention that light bulbs, which I've seen
recommended here as a test load, have a resistance when cold that's only
about a tenth their resistance when hot. So you'll get some pretty
awesome current when you first connect one. A supply with foldback
current limiting probably won't light one at all unless the voltage is
raised slowly.

Roy Lewallen, W7EL

Roy Lewallen wrote:

This might be a good time to mention that light bulbs, which I've seen
recommended here as a test load, have a resistance when cold that's only
about a tenth their resistance when hot. So you'll get some pretty
awesome current when you first connect one. A supply with foldback
current limiting probably won't light one at all unless the voltage is
raised slowly.

That's a cool thing to know.

Hmmmm ...

Bobbie Barmore was discussing IOT power supplies and their crowbars on
the GowBugs list a few months back. Seems that the test to see if the
crowbar's good is pretty simple:

1) Turn everything off
2) Safety tag and padlock all the breakers and the START switch
3) Ground everything that even might think about biting
4) Put a piece of (?) 32-gauge wire between HV and ground.
5) Pull the shorting sticks, button everything up, unlock and
untag the breakers, untag the START switch
6) Make sure everyone's clear and safe
7) Push the START switch

The power supply's good for something like 700 mA at 35 KVDC, and it is
a _really_ bad idea to get across it if you might pass less than abount
700 mA. But the wire's good for more than 700 mA, and so will look like a
short to the PS.

If the wire melts, vaporizes, or explodes, then the crowbar circuit is bad.
It should be unaffected by the test. Put 35 KV across a piece of 32ga wire
without blowing it up.

Hydrogen thyratrons to dump the power, and fast-acting circuit breakers to
Shut Things Down NOW, appear to be the way to protect the wire from the PS.
Fascinating technology.

Homebrew _that_.

--
Mike Andrews, W5EGO

Tired old sysadmin

On Thu, 11 May 2006, Mike Andrews wrote:

Roy Lewallen wrote:

This might be a good time to mention that light bulbs, which I've seen
recommended here as a test load, have a resistance when cold that's only
about a tenth their resistance when hot. So you'll get some pretty
awesome current when you first connect one. A supply with foldback
current limiting probably won't light one at all unless the voltage is
raised slowly.

That's a cool thing to know.

FYI, vacuum tube filaments show the same thing. Lower ohms when cold.

More below...

Hmmmm ...

Bobbie Barmore was discussing IOT power supplies and their crowbars on
the GowBugs list a few months back. Seems that the test to see if the
crowbar's good is pretty simple:

1) Turn everything off
2) Safety tag and padlock all the breakers and the START switch
3) Ground everything that even might think about biting
4) Put a piece of (?) 32-gauge wire between HV and ground.
5) Pull the shorting sticks, button everything up, unlock and
untag the breakers, untag the START switch
6) Make sure everyone's clear and safe
7) Push the START switch

The power supply's good for something like 700 mA at 35 KVDC, and it is
a _really_ bad idea to get across it if you might pass less than abount
700 mA. But the wire's good for more than 700 mA, and so will look like a
short to the PS.

If the wire melts, vaporizes, or explodes, then the crowbar circuit is bad.
It should be unaffected by the test. Put 35 KV across a piece of 32ga wire
without blowing it up.

Hydrogen thyratrons to dump the power, and fast-acting circuit breakers to
Shut Things Down NOW, appear to be the way to protect the wire from the PS.
Fascinating technology.

Homebrew _that_.

35 KV at 700 ma? Exploding wires might not dump all the charge (what size
capacitor?). Maybe need 2-3 dozen carbon resistors in series with a
milliamp meter to monitor remaining voltage?

--
Mike Andrews, W5EGO

Tired old sysadmin

wrote:
Limiting inrush current is mostly to protect the diodes as the first
charge the caps. For the first few ac cycles those caps look like a
dead short or mightly close. Under those conditions the diodes have
to stand the short circuit current the transformer can provide which
can easily exceed diode rating if care is not taken.

That's what was cool about choke-input filters. Hmmm, wonder if I have
a 100 mH, 100A choke in the junkbox...

--
All relevant people are pertinent.
All rude people are impertinent.
Therefore, no rude people are relevant.
-- Solomon W. Golomb

Mike,
I'm confused. The "crowbar" I am familiar with is an over voltage
protection device. If the supply output voltage goes up too high, it
triggers, turns on providing a very lo resistance load and effectively
shorting the output. Thus it initiates the over current protection device,
whatever that is.

If I read this correctly, it appears you are PLACING a short on the supply
and testing the over current protector...

It sounds like "Crowbar" is also used for an over- current protector...

I always thought the term came from using a real 1 inch diameter steel
crowbar (the tipe used in Christmas Story to open the crate containing the
leg lamp) to short the output of a power supply as a really good way to get
to zero volts in a hurry - but only in principle, of course.

73, Steve, K9DCI



"Mike Andrews" wrote in message
...
Hmmmm ...

Bobbie Barmore was discussing IOT power supplies and their crowbars on
the GowBugs list a few months back. Seems that the test to see if the
crowbar's good is pretty simple:

1) Turn everything off
2) Safety tag and padlock all the breakers and the START switch
3) Ground everything that even might think about biting
4) Put a piece of (?) 32-gauge wire between HV and ground.
5) Pull the shorting sticks, button everything up, unlock and
untag the breakers, untag the START switch
6) Make sure everyone's clear and safe
7) Push the START switch

The power supply's good for something like 700 mA at 35 KVDC, and it is
a _really_ bad idea to get across it if you might pass less than abount
700 mA. But the wire's good for more than 700 mA, and so will look like a
short to the PS.

If the wire melts, vaporizes, or explodes, then the crowbar circuit is
bad.
It should be unaffected by the test. Put 35 KV across a piece of 32ga wire
without blowing it up.

Hydrogen thyratrons to dump the power, and fast-acting circuit breakers to
Shut Things Down NOW, appear to be the way to protect the wire from the
PS.
Fascinating technology.

Homebrew _that_.

--
Mike Andrews, W5EGO

Tired old sysadmin

Steve N. wrote:
Mike,
I'm confused. The "crowbar" I am familiar with is an over voltage
protection device. If the supply output voltage goes up too high, it
triggers, turns on providing a very lo resistance load and effectively
shorting the output. Thus it initiates the over current protection device,
whatever that is.

If I read this correctly, it appears you are PLACING a short on the supply
and testing the over current protector...

It sounds like "Crowbar" is also used for an over- current protector...

I always thought the term came from using a real 1 inch diameter steel
crowbar (the tipe used in Christmas Story to open the crate containing the
leg lamp) to short the output of a power supply as a really good way to get
to zero volts in a hurry - but only in principle, of course.

73, Steve, K9DCI

You're on the money as to what gets done to save the IOT. It's a lot
more expensive than the entire power supply, and so the PS gets to
suck it up between the time the hydrogen thyratron (or ignitron or
whatever) turns on and the time the overcurrent limiter or magnetic-
controlled breaker on the PS input decides to Turn Things Off Right
Damnit _Now_.

You're also on the money about crowbars being used as overcurrent
protection as well as overvoltage protection.

My experience with crowbars involves both the voltage-foldback flavor
and the current-foldback flavor. Sometimes you'll be running something
at constant current and want to limit the voltage across it; for that,
you fold the curent back to keep from exceeding the max voltage.
Sometimes you'll be running something at constant voltage and want
to limit the current through it; for that, you fold the voltage back
to keep from exceeding the max current. In the extreme case, you cut
the voltage across, or current through, a device off by putting a
short across the PS output. We had some lab PSes at the NASA Manned
Spacecraft Center with adjustable voltage _and_ current limits,
though none with high-capacity crowbars involving ignitrons, hydrogen
thyratrons, and such (to me) exotic devices. But both voltage and
current limits came in very handy.

--
Mike Andrews, W5EGO

Tired old sysadmin