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Current limiting with a thermistor?

G

Guy Fawkes

Jan 1, 1970
0
Hi,

I'm trying to protect a battery from overloading when too much current is
drawn from it. Is it possible to do so with a thermistor? Or are there
better ways of doing this?

TIA,

Guy
 
R

Roger Hamlett

Jan 1, 1970
0
Guy Fawkes said:
Hi,

I'm trying to protect a battery from overloading when too much current
is
drawn from it. Is it possible to do so with a thermistor? Or are there
better ways of doing this?
Look at self resetting thermal fuses.
Depending on the actual current levels required you will need such a
larger ratio of resistances from the 'normal' to 'protecting' state, that
something with a sharper cutoff, and a higher ratio between the 'break'
current, and the current then needed to keep the device switched of, is
preferable.
Look at devices like:
<http://www.bourns.com/circuit.aspx?cmsphid=62631736|7413229|8404650>

Best Wishes
 
J

John Fields

Jan 1, 1970
0
Hi,

I'm trying to protect a battery from overloading when too much current is
drawn from it. Is it possible to do so with a thermistor? Or are there
better ways of doing this?

---
There are better ways.

What is the battery voltage, the nominal load current, the current
limit, and do you want the circuit to limit the current into the
load or disconnect the battery if the current limit is exceeded?
 
M

mkaras

Jan 1, 1970
0
Phil said:
"Guy Fawkes"


** The ideal device is a " Polyswitch " .

http://www.circuitprotection.com/polyswitch.asp





....... Phil

Yes a "polyswitch" or a "polyfuse" is a good solution for current
limiting in a battery circuit. But do be aware that these devices are
far from ideal. The most annoying characteristic of these devices are
that their 'trip current level" will change to a higher level with each
successive trip of the fuse device. You may expect to see the actual
trip level change by 2X to 3X through initial trips. This of course
needs to be taken into account when designing the usage senario.
- mkaras
 
P

Phil Allison

Jan 1, 1970
0
** Mad Groper ALERT !!
Yes a "polyswitch" or a "polyfuse" is a good solution for current
limiting in a battery circuit. But do be aware that these devices are
far from ideal. The most annoying characteristic of these devices are
that their 'trip current level" will change to a higher level with each
successive trip of the fuse device. You may expect to see the actual
trip level change by 2X to 3X through initial trips.


** What ABSOLUTE BULLSHIT !!!

Tripping current is strictly related to device temperature.




........ Phil
 
K

Ken Smith

Jan 1, 1970
0
Hi,

I'm trying to protect a battery from overloading when too much current is
drawn from it. Is it possible to do so with a thermistor? Or are there
better ways of doing this?

PTC thermistor devices can work in some applications but not all. It
doesn't take much of a battery to be able to output enough current to
destroy the PTC device if the load side shorts. The ceramic ones have
more resistance and thus hold the current at a lower value while they warm
up.

Making sure that there is enough wiring resistance to protect the PTC is
the easiest way to protect the PTC from over current. Using wiring
resistance spreads the heat over a large amount of material.

The trip point of a plastic PTC moves around depending on its history.
For the Bourns ones at least, the trip point is higher on one that is
fresh out of the box. When it is soldered into the PCB its trip point
tends to decrease slightly. If you take a fresh one and use clip
leads and a power supply to trip it, you will see a reduction in the trip
point after the first trip or perhaps two. After that it settles out to a
final value. You have to be careful to let the thing cool for a long
time. A small difference in temperature makes difference to the trip
point.
 
P

Phil Allison

Jan 1, 1970
0
"Ken Smith"
PTC thermistor devices can work in some applications but not all. It
doesn't take much of a battery to be able to output enough current to
destroy the PTC device if the load side shorts.

The ceramic ones have
more resistance and thus hold the current at a lower value while they warm
up.

Making sure that there is enough wiring resistance to protect the PTC is
the easiest way to protect the PTC from over current. Using wiring
resistance spreads the heat over a large amount of material.


** Raychem suggests that 40 amps is safe for their smaller devices.

If a *dead short* produces more, then a fuse in series is the go.

The trip point of a plastic PTC moves around depending on its history.
For the Bourns ones at least, the trip point is higher on one that is
fresh out of the box. When it is soldered into the PCB its trip point
tends to decrease slightly.


** But then go back up because of the heatsinking effect of the PCB .

If you take a fresh one and use clip
leads and a power supply to trip it, you will see a reduction in the trip
point after the first trip or perhaps two.


** Direct opposite of what "mkaras" just claimed out of thin air.

After that it settles out to a
final value. You have to be careful to let the thing cool for a long
time. A small difference in temperature makes difference to the trip
point.


** The trip point occurs at the same temp for a given device.

The actual (rms) current value to reach that temp obviously depends on the
prevailing ambient temp plus any heatsinking of or air flow round the
device.

The maker's "I trip" spec is for free air and room temp (20 C ?)



........ Phil
 
D

Don Foreman

Jan 1, 1970
0
PTC thermistor devices can work in some applications but not all. It
doesn't take much of a battery to be able to output enough current to
destroy the PTC device if the load side shorts. The ceramic ones have
more resistance and thus hold the current at a lower value while they warm
up.

Making sure that there is enough wiring resistance to protect the PTC is
the easiest way to protect the PTC from over current. Using wiring
resistance spreads the heat over a large amount of material.

The trip point of a plastic PTC moves around depending on its history.
For the Bourns ones at least, the trip point is higher on one that is
fresh out of the box. When it is soldered into the PCB its trip point
tends to decrease slightly. If you take a fresh one and use clip
leads and a power supply to trip it, you will see a reduction in the trip
point after the first trip or perhaps two. After that it settles out to a
final value. You have to be careful to let the thing cool for a long
time. A small difference in temperature makes difference to the trip
point.

Fuses aren't exactly precision devices either ... and of course they
vary quite a lot between first trip and subsequent trips!
 
D

Don Foreman

Jan 1, 1970
0
** Direct opposite of what "mkaras" just claimed out of thin air.

Mkaras didn't say which way it varied, nor what cooling time he
allowed between trips.
 
P

Phil Allison

Jan 1, 1970
0
"Don Foreman"
Mkaras didn't say which way it varied,


** Yes he did - the direction and the amount too.

Try learning to read sometime.

nor what cooling time he allowed between trips.


** Irrelevant.




........ Phil


......... Phil
 
D

Don Foreman

Jan 1, 1970
0
"Don Foreman"



** Yes he did - the direction and the amount too.

Try learning to read sometime.

Yah, you're right, he did say increasing. Back to the ABC blocks for
me re reading skills.
** Irrelevant.

Wrong. Perhaps I write no better than I read so I'll not expound
further. I've used and tested these devices. I seem to be able to
read lab instruments well enough to get things right most days.
 
K

Ken Smith

Jan 1, 1970
0
"Ken Smith"
[...]
Making sure that there is enough wiring resistance to protect the PTC is
the easiest way to protect the PTC from over current. Using wiring
resistance spreads the heat over a large amount of material.


** Raychem suggests that 40 amps is safe for their smaller devices.

If you take a look at the Bourns products, you will find some with a limit
of only 10A.
If a *dead short* produces more, then a fuse in series is the go.

...or adding resistance to if that can be done. This is often a better way
to go than a fuse. In many situations, a blown fuse is "broken".

[....]
** Direct opposite of what "mkaras" just claimed out of thin air.

Yes, but I only really can say for sure about Bourns parts. They are the
ones I use. If the ones "mkaras" uses are made in someones garage, all
bets are off.

** The trip point occurs at the same temp for a given device.

The actual (rms) current value to reach that temp obviously depends on the
prevailing ambient temp plus any heatsinking of or air flow round the
device.

It also depends on the resistance of the device. This is the one internal
thing that varies. The material undergoes a phase change at a specific
temperature. It does not however perfectly recover when it is cooled.
 
K

Ken Smith

Jan 1, 1970
0
Don Foreman said:
Fuses aren't exactly precision devices either ... and of course they
vary quite a lot between first trip and subsequent trips!

Yes and if you need a 7A fuse you can't parallel a 2A with a 5A to get it.

BTW: With fuses rated for about 30V and operating at about 300V, the
second trip can be to the stock room to get a new holder.
 
P

Phil Allison

Jan 1, 1970
0
"Ken Smith"
If you take a look at the Bourns products, you will find some with a limit
of only 10A.


** The Bourns ones I use are all rated at 40A .

http://www.farnell.com/datasheets/36769.pdf


Yes, but I only really can say for sure about Bourns parts.


** I was not complaining about *you* !

If the ones "mkaras" uses are made in someones garage, all bets are off.


** His absurd libel was against one and all such devices.


It also depends on the resistance of the device. This is the one internal
thing that varies. The material undergoes a phase change at a specific
temperature. It does not however perfectly recover when it is cooled.


** Not done enough test cycles to have observed a significant change in the
apps I have used them in.

( Mostly low voltage transformer winding protection)




........ Phil
 
G

Guy Fawkes

Jan 1, 1970
0
John Fields said:
---
There are better ways.

What is the battery voltage, the nominal load current, the current
limit, and do you want the circuit to limit the current into the
load or disconnect the battery if the current limit is exceeded?

Voltage = 12V
Current = nominal 200mA, transient 1.5A (50ms)
Current Limit = 300mA
Current Limit (no disconnect)
 
J

John Fields

Jan 1, 1970
0
Voltage = 12V
Current = nominal 200mA, transient 1.5A (50ms)
Current Limit = 300mA
Current Limit (no disconnect)

---
I've posted a circuit for you on alt.binaries.schematics.electronic,
but it occurs to me that you may need that transient to get through
for some reason, and my circuit will squish it.

Can you provide a little more detail about your application?
 
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