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inrush current

W

Wiebe Cazemier

Jan 1, 1970
0
The NTC thermistors I've salvaged from a whole range of scrap PC monitors
have ranged from a few hundred Ohms to as high as 12k at room temperature,
the running resistance is usually 5 Ohms or less.

These were also inrush limiters? Monitors also use these things in their
degausing system, if I'm not mistaken. I can imagine their values needing to
be a lot higher, to allow the alternating current to diminish to zero. Yet,
you'd think they'd use PTC resitors for that (to let the current start high,
end low when the resistor is hot), so perhaps I'm wrong here :)

But that is quite a difference. Mine don't differ at all when in use or not.

The 9-10 Ohms explains why there is "5.0" written on both of the NTC's BTW :).
I wonder if they mean that's their room temperature resistance, or running-hot
resistance.
 
W

Wiebe Cazemier

Jan 1, 1970
0
Its among 20 or so Gb of data sheets on my other PC, but next time I'm there
I'll see if I can remember which folder to look in!

Where did you get that? That's quite a useful archive. Mine is only a few MB
big. I download them one by one when I need them...
 
R

Ross Herbert

Jan 1, 1970
0
These were also inrush limiters? Monitors also use these things in their
degausing system, if I'm not mistaken. I can imagine their values needing to
be a lot higher, to allow the alternating current to diminish to zero. Yet,
you'd think they'd use PTC resitors for that (to let the current start high,
end low when the resistor is hot), so perhaps I'm wrong here :)

But that is quite a difference. Mine don't differ at all when in use or not.

The 9-10 Ohms explains why there is "5.0" written on both of the NTC's BTW :).
I wonder if they mean that's their room temperature resistance, or running-hot
resistance.


The component you are referring to is sometimes called an Inrush
Current Suppressor. It is actually a low-ohmic NTC resistor,

RS Components in Australia have them as shown here
http://www.rs-components.com.au/electronic-components-au/index621.html

It is highly likely that RS in UK/Europe will also have them.
 
I

I.F.

Jan 1, 1970
0
Wiebe Cazemier said:
Where did you get that? That's quite a useful archive. Mine is only a few
MB
big. I download them one by one when I need them...

First the good news - after hours of searching I found the document, its a
Shindengen MJ2400. Now for the bad news! - The chip is based on a modified
bridge rectifier with integral thyristor to shunt the inrush limit resistor
after start up. Sorry I made a mistake, this chip is only suitable for SMPSU
circuits that have a mains bridge rectifier and smoothing cap input - it
won't work with a transformer.

My hoard of data sheets has likewise been downloaded on an as needed basis,
I usually get any other sheets on the page while I'm at it and often phone
chip manufacturers and scrounge their current data book CD.
 
I

I.F.

Jan 1, 1970
0
Wiebe Cazemier said:
These were also inrush limiters? Monitors also use these things in their
degausing system, if I'm not mistaken. I can imagine their values needing
to
be a lot higher, to allow the alternating current to diminish to zero.
Yet,
you'd think they'd use PTC resitors for that (to let the current start
high,
end low when the resistor is hot), so perhaps I'm wrong here :)

But that is quite a difference. Mine don't differ at all when in use or
not.

The 9-10 Ohms explains why there is "5.0" written on both of the NTC's BTW
:).
I wonder if they mean that's their room temperature resistance, or
running-hot
resistance.

There are 2 common types of degauss thermistor, the most common of these has
2 PTC elements one of which is in series with the degauss coil the other is
connected across the mains input - this is constantly heated to ensure that
the degauss PTC proper is heated above its cut off temperature. This ensures
there is no magnetic ripple on the picture in normal operation.

The less common type contains both NTC & PTC elements, the current drawn by
the equipment flows through the NTC to keep the PTC element warm during
normal operation.
 
W

Wiebe Cazemier

Jan 1, 1970
0
First the good news - after hours of searching I found the document, its a
Shindengen MJ2400. Now for the bad news! - The chip is based on a modified
bridge rectifier with integral thyristor to shunt the inrush limit resistor
after start up. Sorry I made a mistake, this chip is only suitable for SMPSU
circuits that have a mains bridge rectifier and smoothing cap input - it
won't work with a transformer.

Hours of searching? You sure are willing to invest time in helping people. You
have my thanks, even though I can't use the chip.

Anyway, Instead, I'm gonna make a time delayed relay driver, which bypasses a
block of resistors (which are in series with the primary coil) a short time
after power up. I'll put a thermal fuse on the resistors to make sure it
disconnects should the transformer develop a short. A normal fuse, slow blow,
in series with the resistors, rated at lower value than what the resistors
would use without the transformer in series with them might also be a good
idea. If I time it right, I should be able to use a fast blow fuse as main
fuse then (as noted by the PDF Ross Herbert pointed to). Experimention is in
order.
 
I

I.F.

Jan 1, 1970
0
Wiebe Cazemier said:
Hours of searching? You sure are willing to invest time in helping people.
You
have my thanks, even though I can't use the chip.

Anyway, Instead, I'm gonna make a time delayed relay driver, which
bypasses a
block of resistors (which are in series with the primary coil) a short
time
after power up. I'll put a thermal fuse on the resistors to make sure it
disconnects should the transformer develop a short. A normal fuse, slow
blow,
in series with the resistors, rated at lower value than what the resistors
would use without the transformer in series with them might also be a good
idea. If I time it right, I should be able to use a fast blow fuse as main
fuse then (as noted by the PDF Ross Herbert pointed to). Experimention is
in
order.

An easy way to make a time delay is with a long tailed pair of transistors
like a differential amplifier stage and add a high value cross coupling
resistor so it acts as a Schmidt trigger and avoids relay chatter. Connect
one base to a zener about half the voltage of the secondary rail used and
the other base to a potential divider shunted by a time delay capacitor -
don't forget a discharge diode from the capacitor to Vcc, this ensures that
as Vcc falls below 1/2 value at switch off it discharges the cap. A relay
driver transistor tapped off the collector resistor of the Schmidt output
transistor completes the line up of only 3 transistors, be sure to remember
also the back emf diode on the relay coil, the relay of course must provide
adequate isolation between primary & secondary supplies!
 
R

Ross Herbert

Jan 1, 1970
0
The component you are referring to is sometimes called an Inrush
Current Suppressor. It is actually a low-ohmic NTC resistor,

RS Components in Australia have them as shown here
http://www.rs-components.com.au/electronic-components-au/index621.html

It is highly likely that RS in UK/Europe will also have them.


Sorry, that url was not correct. This one appears to work (very long
url watch wrap)
http://www.rsaustralia.com/cgi-bin/...tscape&3322216306=3322216306&catoid=-82218076
 
W

Wiebe Cazemier

Jan 1, 1970
0
An easy way to make a time delay is with a long tailed pair of transistors
like a differential amplifier stage and add a high value cross coupling
resistor so it acts as a Schmidt trigger and avoids relay chatter. Connect
one base to a zener about half the voltage of the secondary rail used and
the other base to a potential divider shunted by a time delay capacitor -
don't forget a discharge diode from the capacitor to Vcc, this ensures that
as Vcc falls below 1/2 value at switch off it discharges the cap. A relay
driver transistor tapped off the collector resistor of the Schmidt output
transistor completes the line up of only 3 transistors, be sure to remember
also the back emf diode on the relay coil, the relay of course must provide
adequate isolation between primary & secondary supplies!

I have a circuit in use which is sensitive to relay chatter, so anything better
is welcome.

My circuit design skills are somewhat limited (I really need to work on it), so
I can't completely follow what you mean. Especially the LTP, I never really
understood how they work. Could you draw up a schematic? You can post images
to www.imageshack.us, should you be unable to send attachments. Thanks in
advance.
 
W

Wiebe Cazemier

Jan 1, 1970
0
Sorry, that url was not correct. This one appears to work (very long
url watch wrap)
[long url]

This URL contains a session which now longer exists. But, I found it by
clicking through from the first link you gave me.

Anyway, I have two of them in use now, but I'm gonna use a softstarter circuit,
because the NTC's don't reduce resistance... I guess the idle current is too
low.
 
R

Ross Herbert

Jan 1, 1970
0
Sorry, that url was not correct. This one appears to work (very long
url watch wrap)
[long url]

This URL contains a session which now longer exists. But, I found it by
clicking through from the first link you gave me.

Anyway, I have two of them in use now, but I'm gonna use a softstarter circuit,
because the NTC's don't reduce resistance... I guess the idle current is too
low.


The NTC will remain high resistance (if you call 5 or 10 ohms max
high) during the first few cycles of the inrush current period at
switch-on no matter what the load is. It's temperature will increase
and the resistance will fall due to the high surge current but if the
load on the DC supply is negligible or non existent then the NTC
resistance will rise again to almost its nominal cold value. However,
one would imagine that a 600VA transformer would normally have an idle
load current of several hundred milliamps (depends on the application
though) which would be sufficient to keep the NTC in its low
resistance range providing you choose the right NTC. The fact that the
idle current is low and the NTC resistance not at its minimum during
low idle current is not really a problem since it has already done its
job in protecting the fuse. When the normal load is subsequently
applied the current drawn will definitely cause the NTC to go to its
minimum value and that is what you want to happen.

If the NTC method isn't to your liking then you will have to use a
delayed switch-on circuit which uses a relay contact to short out a
resistor in the primary winding a couple of seconds after switch-on.

A typical delay circuit such as shown here
http://www.jcscript.de/projects/eaton.html could be used where the
relay contact will short circuit say a 10 ohm 10W resistor in the
primary circuit of the transformer shortly after switch-on thus
preventing the surge current from damaging anything.
 
I

I.F.

Jan 1, 1970
0
Wiebe Cazemier said:
I have a circuit in use which is sensitive to relay chatter, so anything
better
is welcome.

My circuit design skills are somewhat limited (I really need to work on
it), so
I can't completely follow what you mean. Especially the LTP, I never
really
understood how they work. Could you draw up a schematic? You can post
images
to www.imageshack.us, should you be unable to send attachments. Thanks in
advance.

A better alternative is to re-post on alt.binaries.schematics.electronic as
I have sketched a basic circuit, but as I find math highly tedious I usually
do just enough to get by and determine most of the values by experiment.
There are some great engineers on a.b.s.e so you might get some even better
circuits to try.
 
W

Wiebe Cazemier

Jan 1, 1970
0
A better alternative is to re-post on alt.binaries.schematics.electronic as
I have sketched a basic circuit, but as I find math highly tedious I usually
do just enough to get by and determine most of the values by experiment.
There are some great engineers on a.b.s.e so you might get some even better
circuits to try.

I'm going with this one: http://sound.westhost.com/project39.htm

If all goes right, the MOSFET switch should prevent chatter, because of it's
fixed turn on voltage.
 
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