[sdiy] psu negative too high

[rsdio at audiobanshee.com]

Yikes, no! Never connect an LED to a voltage source without the appropriate current-limiting resistor. You risk burning out the LED. For a -15 V supply, you'd need at least 600 to 700 Ohms.

Of course, by the time you find a resistor to protect the LED, you could just load down your supply with the resistor alone. A 600 Ohm resistor will draw 25 mA from the -15 V supply, but you'd better have a 1/2 Watt resistor because it's going to get warm. Also, don't put anything that will melt near that resistor - like your multimeter test probes.

A 1 kOhm resistor will only draw 15 mA, so you'd only need the standard 1/4 Watt resistor. It still might get hot.

Brian Willoughby
Sound Consulting


On Nov 17, 2015, at 6:01 AM, Carlos Portela <carlosrochaportela at gmail.com> wrote:
> you mean like a led?
> 
> 2015-11-17 15:00 GMT+01:00 Werner Flügel <werner.fluegel at b-tu.de>:
>> Am 17.11.2015 um 14:47 schrieb Carlos Portela:
>>> Hi all,
>>> 
>>> Im having a problem with my psu.
>>> Its a 15v dual psu + 5v. The 15V and the 5V are working; the negative
>>> outputs around 30V.
>> 
>> Put a little load on the negative rail - a multimeter has too high input
>> impedance. Had the same effect recently ;-)