Re: Problem Using a DC Power Supply

On 10 Jun, 15:15, Rodney.Marri...@xxxxxxxxxxxxxx wrote:
On 10 Jun, 14:51, Baron <baron.nos...@xxxxxxxxxxxxxxxxxxxxxxx> wrote:





CWatters inscribed thus:

<frank_lo...@xxxxxxxxx> wrote in message
news:1181466588.531148.57370@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have the following Power Supply
http://www.globalspecialties.com/1302b.html

I can adjust the voltage by rotating the knob but I can't seem to
do the same for the current. When I rotate the knob, the current
stays at zero Ampere. Is the unit broken or am I doing something
wrong here?

Frank

Have you got a load connected? If not no current will flow. Try
connecting a 12v light bulb of around 10 to 20W. Set the output
voltage to 12V and the current limit to max. The bulb should draw
around 800mA(10W) or 1.6A (20W). Then you can wind the current limit
down to see what happens. Note that setting a lower current limit
than the bulb draws will reduce the output voltage. That's how they
limit the current.

Note that you should probably treat the current knob as a safety
device to limit the power in the even of a fault in the load eg When
hooking up a new circuit you have built for the first time. It also
protects the output of the power supply if you accidentally short
circuit the output.

It's not allways possible operate a bench power supply in "current
limit mode" continuously as some power supplies can overheat. Read
the instructions for your model. Some models are ok with this.

I have one! That is exactly how it works. Current limiting is great
for testing small thermister's using self heating.

--
Best Regards:
Baron.- Hide quoted text -

- Show quoted text -

It may be of interest to you, that some portable TV's well some of the
older ones, that were rated 12-24 volts, that as you inclreased the
supply volts, say from 12 to 24, then the current falls. I know this
goes agains ohms law, but they don't work like that! To keep the
wattage about the same, the current has to fall. This is achieved
using a dc-dc converter. If anyone wants to know how these work, just
ask.

Cheers

Rod- Hide quoted text -

- Show quoted text -

Incidentally, current limiting power supplies, use a very low value
resistor, typ 0.5 ohm, which is in series with the load. The volts
drop across this (usually hi-stab) reisitor is proportional to the
current. This is often monitored by an op-amp in a differential amp
mode, whos out put is used to bias a contol transistor, and finally
the main regulator. This control circuit, is usually inside a
dedicated IC. The current limit adjustment would be in the form of a
potential divider, on the inverting or non-inverting input to the op
amp.

Rod

.

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