Current limiting with an visual indicator of overload

[OBones]

Hello,

I'm currently planning the conversion of an AT computer power supply
that I have lying around into a base supply for my hobbyist electronics
projects .
I know that there is a reason a lab supply costs that much, but for the
use I'm planning, a computer supply will be enough.
However, I would like to protect it against overload as I have no idea
if the supply in itself is fitted with such circuit.
I know I can use a fuse, or a PPTC but this type of circuit does not
give me any indication that it is about to shutdown the power.
I have in mind an old model train transformer that had a red light that
progressively lights up as one approaches overload up until it is fully
on and the supply to the system is shutdown
I would thus like to reproduce this kind of behavior but I must admit
that I'm a bit lost as to what starting point I could use.
What resources would you recommend?
Are there any base circuits that I can modify easily to add an indicator
light?

Thanks for your help
OBones

 

[Baron]

Hello,

I'm currently planning the conversion of an AT computer power supply
that I have lying around into a base supply for my hobbyist
electronics projects .
I know that there is a reason a lab supply costs that much, but for
the use I'm planning, a computer supply will be enough.
However, I would like to protect it against overload as I have no idea
if the supply in itself is fitted with such circuit.
I know I can use a fuse, or a PPTC but this type of circuit does not
give me any indication that it is about to shutdown the power.
I have in mind an old model train transformer that had a red light
that progressively lights up as one approaches overload up until it is
fully on and the supply to the system is shutdown
I would thus like to reproduce this kind of behavior but I must admit
that I'm a bit lost as to what starting point I could use.
What resources would you recommend?
Are there any base circuits that I can modify easily to add an
indicator light?

Thanks for your help
OBones

As far as I am aware AT PSU don't have any SC protection but ATX PSU
do !

 

[Phil Allison]

"OBones"

I'm currently planning the conversion of an AT computer power supply that
I have lying around into a base supply for my hobbyist electronics
projects .
I know that there is a reason a lab supply costs that much, but for the
use I'm planning, a computer supply will be enough.
However, I would like to protect it against overload as I have no idea if
the supply in itself is fitted with such circuit.
I know I can use a fuse, or a PPTC but this type of circuit does not give
me any indication that it is about to shutdown the power.
I have in mind an old model train transformer that had a red light that
progressively lights up as one approaches overload up until it is fully on
and the supply to the system is shutdown

** The supply has a lamp ( ie a 12 volt, 21 watt car brake lamp ) in series
with the output.

I would thus like to reproduce this kind of behavior but I must admit that
I'm a bit lost as to what starting point I could use.

** Why not start with a lamp or two ?

6 volt for the 5 volt rail and 12 volt for them.


...... Phil

 

[OBones]

** Why not start with a lamp or two ?

6 volt for the 5 volt rail and 12 volt for them.

Wouldn't the voltage drop modify the output voltage even for low currents?
I mean, the point is indeed to lower the voltage when there is a lot of
current going through, even to the point that there is not enough
voltage anymore for the circuit, but what about normal operating conditions?

 

[Phil Allison]

"OBones"

Wouldn't the voltage drop modify the output voltage even for low currents?

** Nope.

Lamps have a low resitance at low current levels - about 12 times less than
at rated current.

How do you think that lamps in series with model railway boxes work ??

BTW:

You are ***way*** over snipping.

Makes you look like a context shifter.

IE, the lowest scum on usenet.




...... Phil

 

[Ecnerwal]

give me any indication that it is about to shutdown the power.
I have in mind an old model train transformer that had a red light that
progressively lights up as one approaches overload up until it is fully
on and the supply to the system is shutdown
I would thus like to reproduce this kind of behavior

The light (in series with the supply) is the current limiter, in the
system you describe. They work quite well, though of course they do not
have a hard response (ie, if you are limiting to 5 amps at 5 volts,
you're not going to have 4.9 amps at 5 volts - in particular, you won't
have 5 volts out the far side of the lightbulb at 4.9 amps.) But for
typical bench supply use, this is not usually a problem. In fact you can
set up a lamp board where you switch in or out various bulbs based on
what you expect the load to be (and thus where you want to limit the
load to). Old fashioned but still works perfectly well.

Various versions of far more complex "modern" circuits could achieve a
similar function with a harder response.

An ammeter (fairly inexpensive ones are not too hard to find) and a fuse
or circuit breaker work perfectly well, too, without much complexity.

 

[ehsjr]

Why not use a 7812 and 7805 to protect the supply against short-
circuits ?

You need some headroom (Vin > Vout) with those chips.
The 7812 won't work with +12 Vin. You could use the
7805 with Vin of 12 volts, but you make extra heat in the
chip with a 7 volt Vin-Vout difference.

Ed

 

[Jasen Betts]

Hello,

I'm currently planning the conversion of an AT computer power supply
that I have lying around into a base supply for my hobbyist electronics
projects .
I know that there is a reason a lab supply costs that much, but for the
use I'm planning, a computer supply will be enough.
However, I would like to protect it against overload as I have no idea
if the supply in itself is fitted with such circuit.

they typically are, they'll shut down and need to be power-cycled if
you short the +5 or +12

I know I can use a fuse, or a PPTC but this type of circuit does not
give me any indication that it is about to shutdown the power.
I have in mind an old model train transformer that had a red light that
progressively lights up as one approaches overload up until it is fully
on and the supply to the system is shutdown
I would thus like to reproduce this kind of behavior but I must admit
that I'm a bit lost as to what starting point I could use.

use an automotive headlamp in series with the 12V

for the +5V you'll probably want a lower voltage lamp.

you could use nichrome wire instead of lamps, it lights up with high
current too!

 

[OBones]

they typically are, they'll shut down and need to be power-cycled if
you short the +5 or +12


use an automotive headlamp in series with the 12V

for the +5V you'll probably want a lower voltage lamp.

you could use nichrome wire instead of lamps, it lights up with high
current too!

Thanks for the suggestion, I'll look into it.

 

[OBones]

Try this:
http://datasheets.maxim-ic.com/en/ds/MAX4172.pdf

This even gives you an output current. You may have to amplify it depending
on the light indicator you choose.

Thanks, it looks interesting, and the application notes around it
provide a good starting point