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