What's this surge protection component?

[El Meda]

DaveC <me@privacy.net> wrote:

Is this component really appropriate in this application? With the inrush
current this amp has, I just have a hard time seeing any of these components
handling the current.

Is there a better component? A simple resistor, maybe?

Thanks,

You can use a high wattage resitor -a 100W lamp, by example- and a
relay that short circuits the resistor when the power supply
capacitors are charged at o near its working voltage.
---
El Meda.

http://ingemeda.tripod.com/

 

DaveC wrote:

On Fri, 14 Nov 2003 13:03:15 -0800, ehsjr@bellatlantic.net wrote
(in message <3FB542F8.D10AEAD0@bellatlantic.net&gt:

A resistor is not a suitable replacement for one of these
limiters. Also, while using a jumper, as was mentioned in
another reply, can be a good diagnostic effort, I would
recommend against leaving it in as a permanent solution.

Why is it not a good idea to leave it jumpered rather than replacing it with
another NTC varistor? I know the inrush current will be high, and possibly
blow the slow-blow fuse, but if I notch that up another amp or so, what's the
down side? Is this too much current for the rectifier, transformer windings,
etc., in the PS circuit?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

Dave,

The downside is unnecessary stress on the components -
diodes, xformer, caps. You could jumper it and leave
it that way and run it for the next 50 years with no
problems - or you could have premature component failure
and be back inside the amp in the near future replacing
something else. A second reason: generally speaking, if
an equipment manufacturer decided to spend more money to
add a part, it is probably a good idea for us to spend it
when we repair the thing.

 

[Asimov]

"ehsjr@bellatlantic.net" bravely wrote to "All" (15 Nov 03 21:11:30)
--- on the heady topic of "Re: What's this surge protection component?"

eh> From: ehsjr@bellatlantic.net

eh> DaveC wrote:

On Fri, 14 Nov 2003 13:03:15 -0800, ehsjr@bellatlantic.net wrote
(in message <3FB542F8.D10AEAD0@bellatlantic.net&gt:

A resistor is not a suitable replacement for one of these
limiters. Also, while using a jumper, as was mentioned in
another reply, can be a good diagnostic effort, I would
recommend against leaving it in as a permanent solution.

Why is it not a good idea to leave it jumpered rather than replacing it with
another NTC varistor? I know the inrush current will be high, and possibly
blow the slow-blow fuse, but if I notch that up another amp or so, what's the
down side? Is this too much current for the rectifier, transformer windings,
etc., in the PS circuit?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

eh> Dave,

eh> The downside is unnecessary stress on the components -
eh> diodes, xformer, caps. You could jumper it and leave
eh> it that way and run it for the next 50 years with no
eh> problems - or you could have premature component failure
eh> and be back inside the amp in the near future replacing
eh> something else. A second reason: generally speaking, if
eh> an equipment manufacturer decided to spend more money to
eh> add a part, it is probably a good idea for us to spend it
eh> when we repair the thing.

In the first amp I designed and built, the full-wave rectifier was one
problem which confronted me. What I finally did was to calculate the
maximum surge through the transformer impedance and use that information
to find a suitable bridge rectifier. Naturally when I turned on the
power to the amp the house lights dimmed noticably. So I then used an
NTC in series with the fuse. Worked like a charm and had no more dimming
of the lights and it used a much cheaper bridge in the final prototype!

.... Resistance Is Futile! (If < 1 ohm)

 

[BWL]

Many TV sets use low value/ high wattage resistors on the AC line as inrush
current limiters. Some of these sets draw 375 watts or more, and usually you'll
see a 0.18 to 1.5 ohm at 15 or 20 watts, depending on mfr. They do get warm.

 

[DarkMatter]

On Fri, 14 Nov 2003 6:05:41 -0800, DaveC <me@privacy.net> Gave us:

Can someone please give me the English name for this, and maybe an on-line
reference to a manufacturer or supplier?

It is called a thermistor in american circles, if that's what you
meant.

They vary in size and functional parameters, so it would be hit and
miss without knowing what was removed or the circuit particulars.

Mouser, and digi-key, among others, have pages where thermistors re
offered.

 

[DaveC]

OK, so the original NTC thermistor is toast. The reason, probably: I upgraded
the capacitors in this amp when the originals went bad. Originals: four
19,000 uF. New ones: four 54,000 uF ("hey, they fit!"). I presume that the
inrush current increased in proportion to this additional turn-on load.

So how do I determine what the new thermistor should be? Can I determine the
specs of the old one (p/n is still partially visible, although the company is
defunct) and just triple that value (based on tripling the capcitance turn-on
load)?

I've put another NTC in there, which is also too small in value, and it's
sparking (quite an effect at turn on to see sparks spit from this thing). But
it continues to work, sparks and all. I measured the inrush current with this
one in the circuit and it is right about 48 amps. But since this is measured
with an unknown value of thermistor in the circuit, I'm not sure if the
measurement is at all useful. The slow-blow 8A fuse has held up through all
this, although I'm not sure that's a good thing...

Is my only choice to install a jumper in place of the the thermistor and
measure the inrush? I fear doing some damage to the rectifiers or something
else in the circuit.

Suggestions?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

 

[John Woodgate]

I read in sci.electronics.design that DaveC <me@privacy.net> wrote (in
<0001HW.BBDDAAAB000B5840F0080600@news.individual.net&gt about 'OK, so how
do I choose a replacement thermistor?', on Sun, 16 Nov 2003:

OK, so the original NTC thermistor is toast. The reason, probably: I
upgraded the capacitors in this amp when the originals went bad.
Originals: four 19,000 uF. New ones: four 54,000 uF ("hey, they fit!").
I presume that the inrush current increased in proportion to this
additional turn-on load.

No. The peak current is limited by the resistance in the circuit. This
may have gone down a bit if the new caps have lower ESR (Effective
Series Resistance) than the old ones, but probably not by much. However,
the *duration* of the current pulse will have increased. What matters is
the RMS value of the current pulse, and it's not possible to determine
how much it has gone up. It is most unlikely to be three times what it
was.

So how do I determine what the new thermistor should be? Can I determine
the specs of the old one (p/n is still partially visible, although the
company is defunct) and just triple that value (based on tripling the
capcitance turn-on load)?

No, whatever you mean by 'triple that value'.

I've put another NTC in there, which is also too small in value, and
it's sparking (quite an effect at turn on to see sparks spit from this
thing). But it continues to work, sparks and all. I measured the inrush
current with this one in the circuit and it is right about 48 amps.

How did you measure it? It's not by any means an easy thing to measure.
But if you can measure it meaningfully, it will enable you to pick a
replacement NTC thermistor. However, even translating the measured
current into an NTC sec is not simple.

But
since this is measured with an unknown value of thermistor in the
circuit, I'm not sure if the measurement is at all useful. The slow-blow
8A fuse has held up through all this, although I'm not sure that's a
good thing...

Is my only choice to install a jumper in place of the the thermistor and
measure the inrush? I fear doing some damage to the rectifiers or
something else in the circuit.

No, don't do that. You might explode the new caps, which you wouldn't
enjoy at all.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

 

[DaveC]

On Sun, 16 Nov 2003 23:53:48 -0800, John Woodgate wrote
(in message <yVYJz5AM6Hu$EwD6@jmwa.demon.co.uk&gt:

So how do I determine what the new thermistor should be? Can I determine
the specs of the old one (p/n is still partially visible, although the
company is defunct) and just triple that value (based on tripling the
capcitance turn-on load)?

No, whatever you mean by 'triple that value'.

Thanks, John.

New caps are approx triple value of old (54K vs 19K uF). I was going on the
presumption that this triples the inrush current, but you say not so...

I've put another NTC in there, which is also too small in value, and
it's sparking (quite an effect at turn on to see sparks spit from this
thing). But it continues to work, sparks and all. I measured the inrush
current with this one in the circuit and it is right about 48 amps.

How did you measure it? It's not by any means an easy thing to measure.
But if you can measure it meaningfully, it will enable you to pick a
replacement NTC thermistor. However, even translating the measured
current into an NTC sec is not simple.

Fluke model 43 Power Quality Monitor. It has inrush current setting which
measures current flow (using current clamp) over time. I get a nice waveform
that peaks at turn-on and degrades over milliseconds' (?) time. Max is shown
as 48 A.

And so here I am, with this measured inrush current value (with an unknown
thermistor installed) so what to do with this knowledge? I agree, from what I
see in the application notes of some NTC manufacturers, selection is indeed
not a simple "look-up table" task.

Suggestions?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

 

[Asimov]

"DaveC" bravely wrote to "All" (17 Nov 03 04:50:52)
--- on the heady topic of "Re: OK, so how do I choose a replacement thermistor?"

Da> From: DaveC <me@privacy.net>
Da> And so here I am, with this measured inrush current value (with an
Da> unknown thermistor installed) so what to do with this knowledge? I
Da> agree, from what I see in the application notes of some NTC
Da> manufacturers, selection is indeed not a simple "look-up table" task.

Da> Suggestions?

Sure check the graph of temperature vs resistance and select a ballpark
temperature for the thermistor when the amp is running at maximum power.
Cover the NTC with a glass fiber sleeve to prevent fires if the worst
case temperature is really high. The rms power consumed from the a.c.
line is the value that should be referred to for the NTC current
calculation. Select the NTC to have the smallest a.c. loss practical.
i.e. less loss = less heat BUT also less surge protection

Another option to a NTC is to use a delayed turn-on circuit. Basically
the capacitors are initially charged using a relatively high resistance
and after a delay the circuit shorts out the resistors to make a direct
connection. A mechanical relay or a triac could be used on the ac side.

Asimov
******

.... I worked hard to attach the electrodes to it.

 

[John Fields]

On Mon, 17 Nov 2003 4:50:52 -0800, DaveC <me@privacy.net> wrote:

And so here I am, with this measured inrush current value (with an unknown
thermistor installed) so what to do with this knowledge? I agree, from what I
see in the application notes of some NTC manufacturers, selection is indeed
not a simple "look-up table" task.

Suggestions?

---
Use a BF series resistor with the open contacts of a SPSTNO relay in
parallel with it to limit the inrush current, then when the voltage
across the rails rises to, say, 80% of nominal, close the contacts and
hold them closed until the unit is unswitched or disconnected from the
mains.

--
John Fields

 

[John Woodgate]

I read in sci.electronics.design that DaveC <me@privacy.net> wrote (in
<0001HW.BBDE042C000DD59CF0080600@news.individual.net&gt about 'OK, so how
do I choose a replacement thermistor?', on Mon, 17 Nov 2003:

And so here I am, with this measured inrush current value (with an
unknown thermistor installed) so what to do with this knowledge? I
agree, from what I see in the application notes of some NTC
manufacturers, selection is indeed not a simple "look-up table" task.

Suggestions?

Ask the NTC manufacturers if they have any design guides? I've only
studied this purely theoretically, for IEC/EN 61000-3-3, and haven't had
to do a specific design job, especially without full data on the total
resiance in the circuit. I don't even know what (if any) transformer you
have there.

The maximum permissible surge current of the rectifier diodes and the
maximum permissible surge current of the capacitors may give you a lead
as to what maximum surge current you can allow. 24 A on a 230 V supply
is not untypical of a high-power unit (and meets the standard), so I
think your 48 A on 120 V supply is in the right ball-park. This current
is equal to the 120 V divided by the total resistance in the charging
circuit. I don't have enough data to even estimate that.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

 

[default]

On Sun, 16 Nov 2003 22:28:59 -0800, DaveC <me@privacy.net> wrote:

OK, so the original NTC thermistor is toast. The reason, probably: I upgraded
the capacitors in this amp when the originals went bad. Originals: four
19,000 uF. New ones: four 54,000 uF ("hey, they fit!"). I presume that the
inrush current increased in proportion to this additional turn-on load.

So how do I determine what the new thermistor should be? Can I determine the
specs of the old one (p/n is still partially visible, although the company is
defunct) and just triple that value (based on tripling the capcitance turn-on
load)?

I've put another NTC in there, which is also too small in value, and it's
sparking (quite an effect at turn on to see sparks spit from this thing). But
it continues to work, sparks and all. I measured the inrush current with this
one in the circuit and it is right about 48 amps. But since this is measured
with an unknown value of thermistor in the circuit, I'm not sure if the
measurement is at all useful. The slow-blow 8A fuse has held up through all
this, although I'm not sure that's a good thing...

Is my only choice to install a jumper in place of the the thermistor and
measure the inrush? I fear doing some damage to the rectifiers or something
else in the circuit.

Suggestions?
If you have a "Variac" (variable autotransformer) Try creeping up on

the current while measuring it with an ammeter.

If the NTC thermistor is to limit inrush current you might just need
one with higher dissipation. Thermistors are rated for power
dissipation as well as Delta R.

You don't say what the resistance of the original thermistor is. From
what I understand. Thermistors are rated in delta R for delta T, and
dissipation in watts.

You power up the amp and the thermistor has a relatively high
resistance (10 ohms?), the power dissipated in the thermister(it heats
up) causes it to drop in resistance (.1 ohm?) thereby letting full
power (minus whatever normal HOT state of the thermistor is), into the
power supply.

If your thermistor can't handle the power it opens (smoke and sparks)

You can parallel thermistors - to some extent, but one with a high
enough dissipation will work better.

I have a stereo amp that has two switches - one or the other - but not
both. Switch on one amp, wait five seconds switch on the other.
Toroid power transformers, and .5 farad capacitors in the power
supply. Switch on both at once and the computer dies, the microwave
dies, the TV dies . . .



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
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DaveC wrote:

On Sun, 16 Nov 2003 23:53:48 -0800, John Woodgate wrote
(in message <yVYJz5AM6Hu$EwD6@jmwa.demon.co.uk&gt:

So how do I determine what the new thermistor should be? Can I determine
the specs of the old one (p/n is still partially visible, although the
company is defunct) and just triple that value (based on tripling the
capcitance turn-on load)?

No, whatever you mean by 'triple that value'.

Thanks, John.

New caps are approx triple value of old (54K vs 19K uF). I was going on the
presumption that this triples the inrush current, but you say not so...

I've put another NTC in there, which is also too small in value, and
it's sparking (quite an effect at turn on to see sparks spit from this
thing). But it continues to work, sparks and all. I measured the inrush
current with this one in the circuit and it is right about 48 amps.

How did you measure it? It's not by any means an easy thing to measure.
But if you can measure it meaningfully, it will enable you to pick a
replacement NTC thermistor. However, even translating the measured
current into an NTC sec is not simple.

Fluke model 43 Power Quality Monitor. It has inrush current setting which
measures current flow (using current clamp) over time. I get a nice waveform
that peaks at turn-on and degrades over milliseconds' (?) time. Max is shown
as 48 A.

And so here I am, with this measured inrush current value (with an unknown
thermistor installed) so what to do with this knowledge? I agree, from what I
see in the application notes of some NTC manufacturers, selection is indeed
not a simple "look-up table" task.

Suggestions?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

When you are flying by the seat of your pants (don't know
the max permissible inrush) you can try to get around that
by using 4 NTC thermistors. Pick thermistors that can handle
at least 125% of the steady state current. Wire them in
series/parallel (put 2 in series, put the other 2 in series,
and then put the two series strings in parallel).

That's better than a jumper, and better than a single
thermistor that sparks. No way to tell if it limits the
inrush to the maximum permissible, since we don't have
that spec, but it's a step you can take while you're
doing more research.

 

[R.Legg]

default <R75/5@defaulter.net> wrote in message news:<tp7irvoqfr52v32dm3rk9ta9j9oi76c7dp@4ax.com>...

On Sun, 16 Nov 2003 22:28:59 -0800, DaveC <me@privacy.net> wrote:

OK, so the original NTC thermistor is toast. The reason, probably: I upgraded
the capacitors in this amp when the originals went bad. Originals: four
19,000 uF. New ones: four 54,000 uF ("hey, they fit!"). I presume that the
inrush current increased in proportion to this additional turn-on load.

So how do I determine what the new thermistor should be? Can I determine the
specs of the old one (p/n is still partially visible, although the company is
defunct) and just triple that value (based on tripling the capcitance turn-on
load)?

I've put another NTC in there, which is also too small in value, and it's
sparking (quite an effect at turn on to see sparks spit from this thing). But
it continues to work, sparks and all. I measured the inrush current with this
one in the circuit and it is right about 48 amps. But since this is measured
with an unknown value of thermistor in the circuit, I'm not sure if the
measurement is at all useful. The slow-blow 8A fuse has held up through all
this, although I'm not sure that's a good thing...

Is my only choice to install a jumper in place of the the thermistor and
measure the inrush? I fear doing some damage to the rectifiers or something
else in the circuit.

Suggestions?
If you have a "Variac" (variable autotransformer) Try creeping up on
the current while measuring it with an ammeter.

If the NTC thermistor is to limit inrush current you might just need
one with higher dissipation. Thermistors are rated for power
dissipation as well as Delta R.

You don't say what the resistance of the original thermistor is. From
what I understand. Thermistors are rated in delta R for delta T, and
dissipation in watts.

You power up the amp and the thermistor has a relatively high
resistance (10 ohms?), the power dissipated in the thermister(it heats
up) causes it to drop in resistance (.1 ohm?) thereby letting full
power (minus whatever normal HOT state of the thermistor is), into the
power supply.

If your thermistor can't handle the power it opens (smoke and sparks)

You can parallel thermistors - to some extent, but one with a high
enough dissipation will work better.

NTC devices will not work in parallel. They are effective in series,

however.

Thermistors have an energy surge limit. This is related to their
material quality, rather than their body size - however for a constant
material quality, larger parts with a larger continuous power rating
(and larger self-regulated constant power loss) will have higher surge
ratings.

By more than doubling the capacitors in an otherwise unchanged circuit
he has increased the input surge power level by the same factor, if R
is constant. If R was reduced, the surge energy will have been
increased further as it is related to the square of the peak current.

A larger capacitance will also increase the RMS input current, if no
other changes are made, due to the increased peak-to-average ratio of
the rectified input current. NTC inrush limiters have a maximum safe
continuous current rating.

Check out typical ratings for more common parts before determining the
part suitable to the new stress levels.

http://www.ametherm.com/

The original poster did not indicate the location of the original
thermistor, however as one alone is used, it is likely in series with
the line. Considering the size of the capacitors mentioned, these are
likely on the secondary-side, after a rectifier that is 'likely' more
complicated than a single full-wave circuit. He will have to reflect
the equivalent capacitance back to the primary, in order to evaluate
the equivalent capacitance seen by the limiting circuitry.

RL

 

[Bruce Rowen]

ehsjr@bellatlantic.net wrote:

DaveC wrote:

On Sun, 16 Nov 2003 23:53:48 -0800, John Woodgate wrote
(in message <yVYJz5AM6Hu$EwD6@jmwa.demon.co.uk&gt:

So how do I determine what the new thermistor should be? Can I determine
the specs of the old one (p/n is still partially visible, although the
company is defunct) and just triple that value (based on tripling the
capcitance turn-on load)?

No, whatever you mean by 'triple that value'.

Thanks, John.

New caps are approx triple value of old (54K vs 19K uF). I was going on the
presumption that this triples the inrush current, but you say not so...

I've put another NTC in there, which is also too small in value, and
it's sparking (quite an effect at turn on to see sparks spit from this
thing). But it continues to work, sparks and all. I measured the inrush
current with this one in the circuit and it is right about 48 amps.

How did you measure it? It's not by any means an easy thing to measure.
But if you can measure it meaningfully, it will enable you to pick a
replacement NTC thermistor. However, even translating the measured
current into an NTC sec is not simple.

Fluke model 43 Power Quality Monitor. It has inrush current setting which
measures current flow (using current clamp) over time. I get a nice waveform
that peaks at turn-on and degrades over milliseconds' (?) time. Max is shown
as 48 A.

And so here I am, with this measured inrush current value (with an unknown
thermistor installed) so what to do with this knowledge? I agree, from what I
see in the application notes of some NTC manufacturers, selection is indeed
not a simple "look-up table" task.

Suggestions?

Thanks,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

When you are flying by the seat of your pants (don't know
the max permissible inrush) you can try to get around that
by using 4 NTC thermistors. Pick thermistors that can handle
at least 125% of the steady state current. Wire them in
series/parallel (put 2 in series, put the other 2 in series,
and then put the two series strings in parallel).

That's better than a jumper, and better than a single
thermistor that sparks. No way to tell if it limits the
inrush to the maximum permissible, since we don't have
that spec, but it's a step you can take while you're
doing more research.

Inrush limiting is in almost all cases designed to protect the
rectifiers. If you
can look up their specs for 1/2 cycle current peak, you have an upper
bound on peak
current for your system

-Bruce


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"R.Legg" wrote:

default <R75/5@defaulter.net> wrote in message news:<tp7irvoqfr52v32dm3rk9ta9j9oi76c7dp@4ax.com>...
On Sun, 16 Nov 2003 22:28:59 -0800, DaveC <me@privacy.net> wrote:

OK, so the original NTC thermistor is toast. The reason, probably: I upgraded
the capacitors in this amp when the originals went bad. Originals: four
19,000 uF. New ones: four 54,000 uF ("hey, they fit!"). I presume that the
inrush current increased in proportion to this additional turn-on load.

So how do I determine what the new thermistor should be? Can I determine the
specs of the old one (p/n is still partially visible, although the company is
defunct) and just triple that value (based on tripling the capcitance turn-on
load)?

I've put another NTC in there, which is also too small in value, and it's
sparking (quite an effect at turn on to see sparks spit from this thing). But
it continues to work, sparks and all. I measured the inrush current with this
one in the circuit and it is right about 48 amps. But since this is measured
with an unknown value of thermistor in the circuit, I'm not sure if the
measurement is at all useful. The slow-blow 8A fuse has held up through all
this, although I'm not sure that's a good thing...

Is my only choice to install a jumper in place of the the thermistor and
measure the inrush? I fear doing some damage to the rectifiers or something
else in the circuit.

Suggestions?
If you have a "Variac" (variable autotransformer) Try creeping up on
the current while measuring it with an ammeter.

If the NTC thermistor is to limit inrush current you might just need
one with higher dissipation. Thermistors are rated for power
dissipation as well as Delta R.

You don't say what the resistance of the original thermistor is. From
what I understand. Thermistors are rated in delta R for delta T, and
dissipation in watts.

You power up the amp and the thermistor has a relatively high
resistance (10 ohms?), the power dissipated in the thermister(it heats
up) causes it to drop in resistance (.1 ohm?) thereby letting full
power (minus whatever normal HOT state of the thermistor is), into the
power supply.

If your thermistor can't handle the power it opens (smoke and sparks)

You can parallel thermistors - to some extent, but one with a high
enough dissipation will work better.

NTC devices will not work in parallel.

Can you go into a little more detail on this?
"Will not work" could mean many things. For example:
Say you add an identical NTC in parallel with an existing
one. Do you degrade the function, improve the function, or
have no effect on the function of the NTC?

I ask because I don't know for sure. But the way I would
describe "will not work" in this context is as follows:
It seems to me that if you parallel 2 equal limiters you'll
increase the inrush current over what it would be with just
one, by halving the limiting R. That would hurt, rather than
help the protected circuit during inrush. However, the NTC's
would have similar delta R as they heated, so they would very
likely both reach their low R points. Thus there would be an
improvement in steady state current. It seems to me that the
improvement in steady current handling is not a factor of
interest - you should choose an NTC that can handle the steady
state current with the proper hot R.

To that end, I recommend that, if you want to parallel
2 NTC thermistors, use 4 in series/parallel. The series
part doubles the inrush limiting, and the parallel part
restores the steady state hot R. Obviously, the better
and proper approach is sizing the NTC to the max inrush
and steady state specs of the device, but when you're
operating in the dark, the series/parallel approach is
better than strictly parallel, which will do more harm
than good.




They are effective in series,

however.

Thermistors have an energy surge limit. This is related to their
material quality, rather than their body size - however for a constant
material quality, larger parts with a larger continuous power rating
(and larger self-regulated constant power loss) will have higher surge
ratings.

By more than doubling the capacitors in an otherwise unchanged circuit
he has increased the input surge power level by the same factor, if R
is constant. If R was reduced, the surge energy will have been
increased further as it is related to the square of the peak current.

A larger capacitance will also increase the RMS input current, if no
other changes are made, due to the increased peak-to-average ratio of
the rectified input current. NTC inrush limiters have a maximum safe
continuous current rating.

Check out typical ratings for more common parts before determining the
part suitable to the new stress levels.

http://www.ametherm.com/

The original poster did not indicate the location of the original
thermistor, however as one alone is used, it is likely in series with
the line. Considering the size of the capacitors mentioned, these are
likely on the secondary-side, after a rectifier that is 'likely' more
complicated than a single full-wave circuit. He will have to reflect
the equivalent capacitance back to the primary, in order to evaluate
the equivalent capacitance seen by the limiting circuitry.

RL

 

[DaveC]

On Mon, 17 Nov 2003 17:06:47 -0800, ehsjr@bellatlantic.net wrote
(in message <3FB97087.F335B6B4@bellatlantic.net&gt:

To that end, I recommend that, if you want to parallel
2 NTC thermistors, use 4 in series/parallel. The series
part doubles the inrush limiting, and the parallel part
restores the steady state hot R. Obviously, the better
and proper approach is sizing the NTC to the max inrush
and steady state specs of the device, but when you're
operating in the dark, the series/parallel approach is
better than strictly parallel, which will do more harm
than good.

E,
Your logic sounds... well, sound, to me. Let's see how L. Legg responds.
--
DaveC
me@privacy.net
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[John Woodgate]

I read in sci.electronics.design that ehsjr@bellatlantic.net wrote (in
<3FB97087.F335B6B4@bellatlantic.net&gt about 'OK, so how do I choose a
replacement thermistor?', on Tue, 18 Nov 2003:

Can you go into a little more detail on this? "Will not work" could mean
many things. For example: Say you add an identical NTC in parallel with
an existing one. Do you degrade the function, improve the function, or
have no effect on the function of the NTC?

The one with the lower initial resistance, or the one with the steeper
fall with temperature, will hog nearly all the current.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
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PLEASE do NOT copy news posts to me by E-MAIL!

 

[DaveC]

On Mon, 17 Nov 2003 9:17:01 -0800, John Woodgate wrote
(in message <6SY7yZDNKQu$EwyI@jmwa.demon.co.uk&gt:

Ask the NTC manufacturers if they have any design guides? I've only
studied this purely theoretically, for IEC/EN 61000-3-3, and haven't had
to do a specific design job, especially without full data on the total
resiance in the circuit. I don't even know what (if any) transformer you
have there.

Thermometrics' applications engineer, on the phone, was quite taken aback by
my requirements. Their largest NTC thermistor can handle a maximum
capacitance load of 6000 uF (while this is not an absolute limit, "it is an
experimentally determined value beyond which there may be some reduction
in the life of the inrush current limiter" -- maybe that's why my thermistors
have been sparking and fragmenting...). He was surprised that with the
original total capacitance (<80,000 uF) that the original thermistor worked
for all those years. He was speechless when I told him the requirement now
has almost tripled. He said that I'd need 9 of these things in series to
handle the inrush current.

Thermometrics data sheets and app. notes:
http://www.thermometrics.com/assets/images/cl.pdf
http://www.thermometrics.com/assets/images/tp.pdf

The maximum permissible surge current of the rectifier diodes and the
maximum permissible surge current of the capacitors may give you a lead
as to what maximum surge current you can allow. 24 A on a 230 V supply
is not untypical of a high-power unit (and meets the standard), so I
think your 48 A on 120 V supply is in the right ball-park. This current
is equal to the 120 V divided by the total resistance in the charging
circuit. I don't have enough data to even estimate that.

It looks like the NTC thermistor is not capable of handling this large an
inrush current. I do think I'm going to have to use a light bulb or the
resistor-and-cutout relay solution, now.

As always, other observations and suggestions welcome.
--
DaveC
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[John Fields]

On Tue, 18 Nov 2003 8:22:23 -0800, DaveC <me@privacy.net> wrote:

It looks like the NTC thermistor is not capable of handling this large an
inrush current. I do think I'm going to have to use a light bulb or the
resistor-and-cutout relay solution, now.

As always, other observations and suggestions welcome.
---

Don't use a light bulb since it will do exactly the opposite of what you
want!

Want a schematic for the resistor and relay solution?

--
John Fields