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charging capacitor bank

R

Robert Morein

Jan 1, 1970
0
I'm constructing a power supply that will have 56,000 ufd per positive and
negative rail.
The caps are cans with screw terminals, two 28,000 ufd units per rail.

How many amps do I need to specify the bridge rectifier for to avoid blowout
on slow start?

I would prefer not to limit current with a resistor, though I guess I could
use a relay bypass.
But I've seen quite a few audio amplifiers that charge the bank without any
extra current limiting component.
 
J

James Meyer

Jan 1, 1970
0
How many amps do I need to specify the bridge rectifier for to avoid blowout
on slow start?

If you really mean "slow start", then the bridge current rating isn't a
problem.
I would prefer not to limit current with a resistor, though I guess I could
use a relay bypass.
But I've seen quite a few audio amplifiers that charge the bank without any
extra current limiting component.

Even if you don't have a slow start mechanism the transformer will limit
the surge current into the caps. I'd try a 50 amp bridge for starters.

Jim
 
R

Robert Morein

Jan 1, 1970
0
James Meyer said:
If you really mean "slow start", then the bridge current rating isn't a
problem.


Even if you don't have a slow start mechanism the transformer will limit
the surge current into the caps. I'd try a 50 amp bridge for starters.

Jim
Any problem with paralleling two 40's ?
 
F

Fritz Schlunder

Jan 1, 1970
0
Robert Morein said:
I'm constructing a power supply that will have 56,000 ufd per positive and
negative rail.
The caps are cans with screw terminals, two 28,000 ufd units per rail.

How many amps do I need to specify the bridge rectifier for to avoid blowout
on slow start?

I would prefer not to limit current with a resistor, though I guess I could
use a relay bypass.
But I've seen quite a few audio amplifiers that charge the bank without any
extra current limiting component.


Key tidbits are missing from your post. I'm not referring to pineapple
tidbits but rather informational tidbits. What are the relevant voltages
involved? Are the capacitors being charged from the output of a
transformer? If so, what is the transformer rated for. What kind of bridge
rectifier did you have in mind. What is your load current? What kind of
slow start system are you referring to?

Generally speaking however inrush limiting is usually not mandatory (though
in some cases desirable anyway) when charging even very large capacitors
from the output of a transformer. Usually the transformer's leakage
inductance and copper resistance will limit and absorb most of the transient
stress upon power up. Since the transfomer is physically very large it has
a lot of thermal inertia and can easily handle this extra stress without
problems. If the transformer is massively overrated for the application
then a larger percentage of the initial stress may be placed on the bridge
rectifier, thus perhaps making inrush limiting more beneficial.
 
W

Winfield Hill

Jan 1, 1970
0
Fritz Schlunder wrote...
...

Key tidbits are missing from your post. I'm not referring to pineapple
tidbits but rather informational tidbits. What are the relevant voltages
involved? Are the capacitors being charged from the output of a
transformer? If so, what is the transformer rated for. What kind of
bridge rectifier did you have in mind. What is your load current? What
kind of slow start system are you referring to?

Generally speaking however inrush limiting is usually not mandatory
(though in some cases desirable anyway) when charging even very large
capacitors from the output of a transformer. Usually the transformer's
leakage inductance and copper resistance will limit and absorb most of
the transient stress upon power up. Since the transfomer is physically
very large it has a lot of thermal inertia and can easily handle this
extra stress without problems. If the transformer is massively
overrated for the application then a larger percentage of the initial
stress may be placed on the bridge rectifier, thus perhaps making
inrush limiting more beneficial.

2 x 28000 uF implies moderate voltages of 170V or less. Robert, if this
is a direct offline rectifier, inrush-current-limiting protection will
be a wise addition. If you're using a power transformer, then as Fritz
said, you may be able to live with the intrinsic current limiting that it
provides. But 56mF at 170Vdc = 810J is a fairly-large apacitor-storage
energy to charge quickly, so inrush limiting may well be mandatory. I've
used a simple time-delay ac relay shunting a series input power resistor
with good success for transformer-rectifier charging of capacitor banks.

Thanks,
- Win

whill_at_picovolt-dot-com
 
R

Robert Morein

Jan 1, 1970
0
Winfield Hill said:
Fritz Schlunder wrote...

2 x 28000 uF implies moderate voltages of 170V or less. Robert, if this
is a direct offline rectifier, inrush-current-limiting protection will
be a wise addition. If you're using a power transformer, then as Fritz
said, you may be able to live with the intrinsic current limiting that it
provides. But 56mF at 170Vdc = 810J is a fairly-large apacitor-storage
energy to charge quickly, so inrush limiting may well be mandatory. I've
used a simple time-delay ac relay shunting a series input power resistor
with good success for transformer-rectifier charging of capacitor banks.

Thanks,
- Win

whill_at_picovolt-dot-com
Thanks. I think I can experiment using a Variac and a Tek P6021 current
probe to see if the peak current exceeds 40 amps.
 
J

James Meyer

Jan 1, 1970
0
Thanks. I think I can experiment using a Variac and a Tek P6021 current
probe to see if the peak current exceeds 40 amps.

Don't forget that a nominal 40 amp diode will have a surge rating for a
short time that is many times its continuous 40 amp rating.

Jim
 
R

Robert Morein

Jan 1, 1970
0
James Meyer said:
Don't forget that a nominal 40 amp diode will have a surge rating for a
short time that is many times its continuous 40 amp rating.

Jim
Yes, but what would that be?
 
W

Winfield Hill

Jan 1, 1970
0
Winfield Hill wrote...
James Meyer wrote...

That's for only one half of a single 50 or 60Hz cycle.
It may take much longer to charge a large capacitor bank.
Some detailed calculations are in order.

For example, a half-cycle of 400A peak half-sine charging
will get Robert's capacitor bank voltage up to only about
0.64 400A 8ms / 0.056F = 36V. This means many ac cycles
would be required to complete the task, and an AC off-line
directly-connected rectifier would be over stressed. If
we assume 200W constant-power charging, it would take four
seconds or 240 ac cycles (at 60Hz) to get the required 810J
of energy into the caps.

Thanks,
- Win

whill_at_picovolt-dot-com
 
R

Reg Edwards

Jan 1, 1970
0
The switch-on surge current in a diode + big capacitor circut is limited by
the internal impedance of the AC source, usually R+jwL. If a transformer is
the source then leakage inductance and primary and secondary winding
resistance must be taken into account.
 
W

Winfield Hill

Jan 1, 1970
0
Reg Edwards wrote...
The switch-on surge current in a diode + big capacitor circut
is limited by the internal impedance of the AC source, usually
R+jwL.

In the case of the AC line, typially it can easily deliver 400A
and do so cycle after cycle, creating a race between the circuit-
breaker opening or the recifier dying. The capacitor's esr helps
a bit, but not much for a 56mF bank of large electrolytics.
If a transformer is the source then leakage inductance and primary
and secondary winding resistance must be taken into account.

Must? Haha, in Robert's scene, he should be very glad indeed to
_get_ to take it into account. The transformer's high copper-wire
heat capacity means it can be massively over-stressed for a few
seconds without damage. But in a typical configuration it'll
likely not lower charging currents enough to protect the rectifier.

In my 250kW pulse generator I designed a series 200V 2A ac current
limiter in parallel with a 25-ohm 50W resistor to solve the problem,
but a timer to close a relay across the resistor could also work.

Another approach is to manually slowly turn up an input variac,
but one risks blowing stuff out if he ever forgets.

Thanks,
- Win

whill_at_picovolt-dot-com
 
J

John Crighton

Jan 1, 1970
0
I'm constructing a power supply that will have 56,000 ufd per positive and
negative rail.
The caps are cans with screw terminals, two 28,000 ufd units per rail.

How many amps do I need to specify the bridge rectifier for to avoid blowout
on slow start?

I would prefer not to limit current with a resistor, though I guess I could
use a relay bypass.
But I've seen quite a few audio amplifiers that charge the bank without any
extra current limiting component.
Hello Robert,
here is something that may be of interest to you.
There is the welding transformer, diodes and capacitors.
A limiting inductor is used in series with the capacitor bank,
four by 10,000 MFD capacitors but not in series with the
output.

http://www.welding.com.au/PDFE/Cp105 0 and 1.pdf
(the circuit diagram of this mig welder is on page 15 so you
don't have to download the whole booklet)

I couldn't get any information out of the WIA company as to
the value, and make up and exact purpose of the inductor.
Very secretive they were. The use of a "limiting inductor"
in your project, may reduce the initial surge current as the
capacitors charge up when your power supply is first
switched on. Just a thought.

Regards,
John Crighton
Sydney
 
R

Robert Morein

Jan 1, 1970
0
[snip]
Very secretive they were. The use of a "limiting inductor"
in your project, may reduce the initial surge current as the
capacitors charge up when your power supply is first
switched on. Just a thought.

Regards,
John Crighton
Sydney

Thanks John, and to everyone else.

Does anyone have a time delay relay circuit handy?
 
R

Robert C Monsen

Jan 1, 1970
0
Robert Morein said:
[snip]
Very secretive they were. The use of a "limiting inductor"
in your project, may reduce the initial surge current as the
capacitors charge up when your power supply is first
switched on. Just a thought.

Regards,
John Crighton
Sydney

Thanks John, and to everyone else.

Does anyone have a time delay relay circuit handy?

Here is a simple way to do it:

http://home.comcast.net/~rcmonsen/misc/slowstart.GIF

I've never built it, or tried it out, so be careful. The current limit
resistor for the charging phase must have a huge power rating, and the relay
must be capable of handling the large charging currents when finally
switched in without melting.

Regards,
Bob Monsen
 
J

James Meyer

Jan 1, 1970
0
I can. They won't current-share.

What about an NTC thermistor?

Good Luck!
Rich

Can you give me a reason why they won't current share?

Are you aware that a diode's forward voltage drop is proportional to the
current it's passing? It's not a fixed 0.7 volts regardless of its current.

Jim
 
W

Winfield Hill

Jan 1, 1970
0
James Meyer wrote...
Rich Grise posted this:


Can you give me a reason why they won't current share?

Are you aware that a diode's forward voltage drop is
proportional to the current it's passing? It's not a
fixed 0.7 volts regardless of its current.

The diode has a traditional logarithmic component, which
is both current and temperature sensitive, and a series
resistive component, which follows ohms law. At high
currents the resistive component by far dominates. If
two high-current power diodes are paralleled, the current
sharing will be largely determined by the relative values
of their intrinsic series resistance. While for a given
type of diode these resistance values will likely be the
same, they are not guaranteed to be the same, and may in
fact not be, especially if they have different date codes.
The series-resistance component of diodes I have measured
have varied by up to 50%.

Thanks,
- Win

whill_at_picovolt-dot-com
 
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