How easy would it be for me to change the current rating of a constant current supply? One from a CREE LED light fitting, which I think is very roughly 0.5 amps at 7V. I want to lower the current, the voltage is ok.

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On 16/04/14 06:11, Uncle Peter wrote:
How easy would it be for me to change the current rating of a constant
current supply? One from a CREE LED light fitting, which I think is
very roughly 0.5 amps at 7V. I want to lower the current, the voltage
is ok.

My memory of teaching Power Supplies (about 15 years ago), the constant
current was maintained but keeping the voltage developed across a
particular resistor constant, so, in theory, by increasing the value of
the resistor, a lower current would develop the same voltage across the
sensing resistor.

However, you would have to determine which resistor and which value .....!

Daniel

On Wed, 16 Apr 2014 13:01:00 +0100, Daniel wrote:

On 16/04/14 06:11, Uncle Peter wrote:
How easy would it be for me to change the current rating of a constant
current supply? One from a CREE LED light fitting, which I think is
very roughly 0.5 amps at 7V. I want to lower the current, the voltage
is ok.

My memory of teaching Power Supplies (about 15 years ago), the constant
current was maintained but keeping the voltage developed across a
particular resistor constant, so, in theory, by increasing the value of
the resistor, a lower current would develop the same voltage across the
sensing resistor.

However, you would have to determine which resistor and which value .....!

Thanks for the tip, I'll run it on a bench with a dummy load and test some resistor voltages, then try changing one a little.


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On 16/04/14 22:23, Uncle Peter wrote:
On Wed, 16 Apr 2014 13:01:00 +0100, Daniel
wrote:

On 16/04/14 06:11, Uncle Peter wrote:
How easy would it be for me to change the current rating of a constant
current supply? One from a CREE LED light fitting, which I think is
very roughly 0.5 amps at 7V. I want to lower the current, the voltage
is ok.

My memory of teaching Power Supplies (about 15 years ago), the constant
current was maintained but keeping the voltage developed across a
particular resistor constant, so, in theory, by increasing the value of
the resistor, a lower current would develop the same voltage across the
sensing resistor.

However, you would have to determine which resistor and which value
.....!

Thanks for the tip, I'll run it on a bench with a dummy load and test
some resistor voltages, then try changing one a little.

If you've got a circuit diagram of your power supply, and you can
determine which resister is the "sensing" resister, replace it (for the
time being) with a variable resistor/rheostat in the same ohmic range as
the sensing resistor until you can determine what value resistor you
want to stick in.

Daniel

On Thu, 17 Apr 2014 13:42:41 +0100, Daniel wrote:

On 16/04/14 22:23, Uncle Peter wrote:
On Wed, 16 Apr 2014 13:01:00 +0100, Daniel
wrote:

On 16/04/14 06:11, Uncle Peter wrote:
How easy would it be for me to change the current rating of a constant
current supply? One from a CREE LED light fitting, which I think is
very roughly 0.5 amps at 7V. I want to lower the current, the voltage
is ok.

My memory of teaching Power Supplies (about 15 years ago), the constant
current was maintained but keeping the voltage developed across a
particular resistor constant, so, in theory, by increasing the value of
the resistor, a lower current would develop the same voltage across the
sensing resistor.

However, you would have to determine which resistor and which value
.....!

Thanks for the tip, I'll run it on a bench with a dummy load and test
some resistor voltages, then try changing one a little.

If you've got a circuit diagram of your power supply, and you can
determine which resister is the "sensing" resister, replace it (for the
time being) with a variable resistor/rheostat in the same ohmic range as
the sensing resistor until you can determine what value resistor you
want to stick in.

I've got one of those multi-resistor-bank boxes with dials on the top for units, tens, hundreds, etc, so I can select any resistance with it. Very handy!

--
Some "chinese english" instructions for an automatic light switch, needless to say I did not attempt to follow them during the installation....

The surface design is facility, comely but not losing generosity, it will not have accidented feeling after installation.
Wide working voltage: you will not be worried when you go all over Europe carrying it.
You could fix the sensor with two screws on the junction box in circular one, also fix it with special installation shelf.
In a word, whether the junction box installation orientation is true, it makes the installation flatly.
The lamp will be on automatically when you knock at the door or say "I am coming back".
It will make your home warmer and more romantic.
Penetrate the setscrew into installation hole, block on radiator to aim at the installation hole on connection box.
Let electrician or experienced human install it.
The unrest objects can't be regarded as the installation basis-face.
Don't open the case for your safety if you find the hitch after installation.
If there is any difference between instruction and products, please give priority to product, sorry not to inform you again.

On Thu, 17 Apr 2014 13:42:41 +0100, Daniel wrote:

On 16/04/14 22:23, Uncle Peter wrote:
On Wed, 16 Apr 2014 13:01:00 +0100, Daniel
wrote:

On 16/04/14 06:11, Uncle Peter wrote:
How easy would it be for me to change the current rating of a constant
current supply? One from a CREE LED light fitting, which I think is
very roughly 0.5 amps at 7V. I want to lower the current, the voltage
is ok.

My memory of teaching Power Supplies (about 15 years ago), the constant
current was maintained but keeping the voltage developed across a
particular resistor constant, so, in theory, by increasing the value of
the resistor, a lower current would develop the same voltage across the
sensing resistor.

However, you would have to determine which resistor and which value
.....!

Thanks for the tip, I'll run it on a bench with a dummy load and test
some resistor voltages, then try changing one a little.

If you've got a circuit diagram of your power supply, and you can
determine which resister is the "sensing" resister, replace it (for the
time being) with a variable resistor/rheostat in the same ohmic range as
the sensing resistor until you can determine what value resistor you
want to stick in.

This is a partial drawing of the circuit diagram of the power supply. I can't see half of the circuit connections as they go underneath the unknown microchip!
http://petersphotos.com/temp/PSU.jpg

--
Some "chinese english" instructions for an automatic light switch, needless to say I did not attempt to follow them during the installation....

The surface design is facility, comely but not losing generosity, it will not have accidented feeling after installation.
Wide working voltage: you will not be worried when you go all over Europe carrying it.
You could fix the sensor with two screws on the junction box in circular one, also fix it with special installation shelf.
In a word, whether the junction box installation orientation is true, it makes the installation flatly.
The lamp will be on automatically when you knock at the door or say "I am coming back".
It will make your home warmer and more romantic.
Penetrate the setscrew into installation hole, block on radiator to aim at the installation hole on connection box.
Let electrician or experienced human install it.
The unrest objects can't be regarded as the installation basis-face.
Don't open the case for your safety if you find the hitch after installation.
If there is any difference between instruction and products, please give priority to product, sorry not to inform you again.

On 23/04/14 08:02, Uncle Peter wrote:
On Thu, 17 Apr 2014 13:42:41 +0100, Daniel
wrote:

On 16/04/14 22:23, Uncle Peter wrote:
On Wed, 16 Apr 2014 13:01:00 +0100, Daniel
wrote:

On 16/04/14 06:11, Uncle Peter wrote:
How easy would it be for me to change the current rating of a constant
current supply? One from a CREE LED light fitting, which I think is
very roughly 0.5 amps at 7V. I want to lower the current, the voltage
is ok.

My memory of teaching Power Supplies (about 15 years ago), the constant
current was maintained but keeping the voltage developed across a
particular resistor constant, so, in theory, by increasing the value of
the resistor, a lower current would develop the same voltage across the
sensing resistor.

However, you would have to determine which resistor and which value
.....!

Thanks for the tip, I'll run it on a bench with a dummy load and test
some resistor voltages, then try changing one a little.

If you've got a circuit diagram of your power supply, and you can
determine which resister is the "sensing" resister, replace it (for the
time being) with a variable resistor/rheostat in the same ohmic range as
the sensing resistor until you can determine what value resistor you
want to stick in.

This is a partial drawing of the circuit diagram of the power supply. I
can't see half of the circuit connections as they go underneath the
unknown microchip!
http://petersphotos.com/temp/PSU.jpg

O.K., that's more than I expected, thanks.

(I'm really guessing that the 280ohms you measured is the 282ohm
resistor next to it, but that's by-the-by)

The NPN transistor that you question mark as "Low Freq" ... I would be
expecting it to be as High Freq Switching Tranny, which would, somehow,
by "driven On/Off/On/Off by the IC in middle of the diag, causing an
On/Off/On/Off current to flow through the coil/Transformer, and these
pulses are transformer coupled across the coil/Transformer, to be
rectified by the diode and filtered by the capacitor and 4.7kohm
resistor across the output.

Are you sure this is a "constant current supply"?? Depending on the
arrangement of the pins of the coil, this could just be a fairly regular
variable voltage PS.

Do you have access to an Ohmmeter?? If so, with the device switched off,
and unplugged, can you measure the resistance from each connected
terminal of the coil to each other terminal?? I'm really only interested
in connection/no connection, not the actual ohmic values. Are you right
in calling it a coil rather than me calling it a transformer!!

Daniel