Variable current power supply

[daniel]

is there a way of making a variable current supply with an output anywhere
form 0.1A to the maximum of the transformer but using a fixed voltage e.g
12V.

thanks

 

[happyhobit]

Hi Daniel,

An adjustable voltage regulator like the LM317 can be configured for current
or voltage regulation. The LM317 wastes 'up to' 3 volts each and outputs 1.5
amps. The LM1086 wastes up to 1.3 volts and will deliver 1.5 amps, the
LM1084 will deliver 5 amps. You can configuring one as a current limiter and
a second as a voltage regulator to get what you want.

You will also need a rectifier (bridge, half or full wave), a couple
capacitors and maybe a choke to get the DC to start with..

NOTE: Nothing is 100% efficient. Every component wastes some current and
voltage. You will not get 'the maximum of the transformer'.

Jay

 

[Wilson]

National LM317 or similar regulator can be used. Data sheet below

http://www.national.com/pf/LM/LM317.html

http://cache.national.com/ds/LM/LM117.pdf

Look on page 16 for a 1 amp current regulator circuit. Page 17 and 19 have
similar circuits for lower current. The "precision" circuit on p17 shows how
to make it adjustable. Remember that your load current flows through the
variable resistor, so make sure it has high enough power rating.

These circuits all assume that the load resistance is low enough that it
takes less than (Vin - Vdrop of Regulator - Vref ) volts across the load to
get the desired current. The LM317 needs about a 3V drop, and has a 1.2V
Reference voltage (which is dropped across the resistor). That leaves you
about 7.8V available at the load. Lower dropout regulators are available,
and most data sheets will show similar circuits.

Wilson

 

[Luhan Monat]

is there a way of making a variable current supply with an output anywhere
form 0.1A to the maximum of the transformer but using a fixed voltage e.g
12V.

thanks

Hi,

Thats a theoretical impossiblility. The only way to make a supply with
a specific current is by letting the voltage vary. You cant both
specify the voltage and current with anything but the R=E/I (resistive)
load value.

Luhan

 

[Geoff Hackett]

I think you are confused.

What are you trying to do here.

Geoff.

 

[happyhobit]

Well, I was proposing a 'Voltage Regulated, Current Limited' power supply.
Basically two LM317's, one configured as a 'current limiter' and the other
as a 'voltage regulator'.

O.K.?
Jay

 

[[email protected]]

I sugest you use de L200. It's an voltage and current regulator. Wich
opperates from 2,6 to 36V and has an max current of 1.5
De application schematics will help you to complete the job

 

[Geoff Hackett]

Sorry happyhobit I was referring to the original post from Danial, (where
has he gone), your idea's are fine. The post from Luhan Monat is right
however, It is not possible to have variable current at a fixed voltage with
a fixed load. When a current sense network kicks in it must reduce the
voltage into the load, (at least for DC.) Or for a variable voltage the
current must change. The circuit can also incorporate some form of tripping
device that requires resetting in the event or a fault, or this can happen
automatically.A indication of the degree current limit applied is useful
also.

I still am not sure what you are trying to do?

Geoff Hackett.

 

[stoj]

i have a 30V AC to AC transformer which has an output current of 2A and i am
trying to get a lower current output around 0.5A which can be adjusted to a
greater current like 1.5 or 2A