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Current limiter or current booster

C

Claudio Bonavolta

Jan 1, 1970
0
Dear All,

I want to feed the lamp (halogen 12V/75W) of a photographic enlarger
with stabilized DC (voltage variations give exposure/color
variations).
I bought for this a 12V/100W switching supply.
The problem is the cold filament has a much lower resistance than when
hot, this forces the supply to furnish a much higher current than it
is intended and the protection switches the output off and on,
cyclically.

I may, of course, replace the supply by a much oversized model, but
I'd prefer to find a better solution ...

I've tried to put electrolytic capacitors up to 100'000uF but it is
still not sufficient. I may try super capacitors in serial/parallel
configuration but they are not really cheap.

My second option is to put a battery after a diode (to avoid the
discharge through the supply) but I'm not sure it is very good for
battery's life to be connected in such a way.

Another option was to insert serially a choke but their DC resistance
is to high (resulting in heat and voltage drop) and for such currents,
they are pretty heavy and expensive.

Is there a current limiter I can set to, say 8A, with a very low
resistance when not limiting current ?

Any other idea ?


Thanks,
Claudio Bonavolta
http://www.bonavolta.ch
 
R

Rheilly Phoull

Jan 1, 1970
0
Claudio Bonavolta said:
Dear All,

I want to feed the lamp (halogen 12V/75W) of a photographic enlarger
with stabilized DC (voltage variations give exposure/color
variations).
I bought for this a 12V/100W switching supply.
The problem is the cold filament has a much lower resistance than when
hot, this forces the supply to furnish a much higher current than it
is intended and the protection switches the output off and on,
cyclically.

I may, of course, replace the supply by a much oversized model, but
I'd prefer to find a better solution ...

I've tried to put electrolytic capacitors up to 100'000uF but it is
still not sufficient. I may try super capacitors in serial/parallel
configuration but they are not really cheap.

My second option is to put a battery after a diode (to avoid the
discharge through the supply) but I'm not sure it is very good for
battery's life to be connected in such a way.

Another option was to insert serially a choke but their DC resistance
is to high (resulting in heat and voltage drop) and for such currents,
they are pretty heavy and expensive.

Is there a current limiter I can set to, say 8A, with a very low
resistance when not limiting current ?

Any other idea ?


Thanks,
Claudio Bonavolta
http://www.bonavolta.ch
Mebbe you should try a diode or two in series with the lamp, put switch
across them so you could short them out when the lamp is heated up. They
would drop 1.4volts which may well be enough for a soft start. Or maybe a
resistor ??
 
B

Bill Vajk

Jan 1, 1970
0
Rheilly said:
Mebbe you should try a diode or two in series with the lamp, put switch
across them so you could short them out when the lamp is heated up. They
would drop 1.4volts which may well be enough for a soft start. Or maybe a
resistor ??

I would do the startup with an inexpensive unregulated power supply
and switch over to the regulated one after 1 second or so. Use a
mechanical shutter to prevent exposure during the warm up phase.
 
I

Ian Bell

Jan 1, 1970
0
Rheilly said:
Mebbe you should try a diode or two in series with the lamp, put switch
across them so you could short them out when the lamp is heated up. They
would drop 1.4volts which may well be enough for a soft start. Or maybe a
resistor ??

I think a soft start is what you need. very many years ago I worked for a
recording mixer manufactuerer. There were so many capacitors across the
supply in the circuits in it that the off the shelf Coutant power supply
went straight into current limit as they charged up when it was first
turned on. We had to get specially modified power supplies with a soft
start circuit to overcome it.

Ian
 
C

Claudio Bonavolta

Jan 1, 1970
0
Thanks for your comments,

I continued to search the web and found two other solutions:
- there are thermistors intended for this purpose, they limit the
inrush current to a predefined value.
An example:
http://www.ametherm.com/Data Sheets/SL12 1R308.pdf

- the switch I use between the supply and the lamp is a MosFet driven
by a TTL pulse, I didn't realise that I could simply add a RC network
to increase slowly the gate voltage and thus the current flowing
through it. The time constant should be tuned according to the
supply/lamp characteristics.
I found this article:
http://www.reed-electronics.com/ednmag/contents/images/121103di.pdf

Another interesting article on inrush current problems:
http://www.power-one.com/technical/articles/dc-dc_1-app.pdf

Again, thanks to all and best regards,
Claudio Bonavolta
http://www.bonavolta.ch
 
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