Power supply current rating question (simple if you know pow

[Jon B]

I am using a power supply that is already built. (I want to say wall
wart, but it's the kind that sits on the floor, with a little cord.
Otherwise, same thing.) Anyway, it is regulated and puts out +15,
-15, and ground. it uses a center tapped transformer, rectifier
diodes and 7815/7915 regulators, with filter caps. the fuse is rated
for 400 mA/250 V. My line is 120 VAC. About how much current can i
safely draw from the 15 volt side?

If the answer was, let's pretend, 3 amps, does that then mean i could
connect a 10 ohm resistor between 15 and -15 volt lines? or does that
mean 3 amps per side?

 

[Tim Wescott]

Jon B wrote:

I am using a power supply that is already built. (I want to say wall
wart, but it's the kind that sits on the floor, with a little cord.
Otherwise, same thing.) Anyway, it is regulated and puts out +15,
-15, and ground. it uses a center tapped transformer, rectifier
diodes and 7815/7915 regulators, with filter caps. the fuse is rated
for 400 mA/250 V. My line is 120 VAC. About how much current can i
safely draw from the 15 volt side?

If the answer was, let's pretend, 3 amps, does that then mean i could
connect a 10 ohm resistor between 15 and -15 volt lines? or does that
mean 3 amps per side?

Well, gee -- how many people can ride in a car?

Depending on how the supply is built the output stages may each be
capable of delivering as much (or more) as the transformer is capable of
delivering, or they may each be designed to handle exactly half the
total, etc. If the two regulators have the same heat sinking they can
probably each handle the same amount of current.

The upper limit to the power input is 48W -- and that's assuming no
overhead on the fuse and a power factor of 1 which is absurd.

So assume that the input current is 300mA RMS (still on the risky side
for that fuse). If the output side has a 20VDC rail ahead of the
regulators you're probably delivering about 500mADC at best -- and if
they're the TO-220 packaged 78M15s, that's about right.

If you really have to know, since you have the thing open, run it for a
while at 500mA. Check the temperature of the regulators and the
transformer -- if you can't hold your finger on it, it's getting too hot
and the current's too high. It'll take a few hours for the transformer
to come to temperature. If the transformer's warm and the regulators
are _really_ cool then you can probably go higher on each output.

---------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Jon B]

Basically the same as this:

http://www.elfrad.net/Pics/pwrsup.gif

but with 15 volt regulators instead.

 

[Ban]

Jon B wrote:

I am using a power supply that is already built. (I want to say wall
wart, but it's the kind that sits on the floor, with a little cord.
Otherwise, same thing.) Anyway, it is regulated and puts out +15,
-15, and ground. it uses a center tapped transformer, rectifier
diodes and 7815/7915 regulators, with filter caps. the fuse is rated
for 400 mA/250 V. My line is 120 VAC. About how much current can i
safely draw from the 15 volt side?

If the answer was, let's pretend, 3 amps, does that then mean i could
connect a 10 ohm resistor between 15 and -15 volt lines? or does that
mean 3 amps per side?

Jon,
since the regulators are specified for a maximum current of 1A, you cannot
take out more current. But I doubt this is even possible. It depends on the
size of your transformer. If it is square, measure one side.
55mm will give 15W- 30% for regulation makes 10W or 330mA
65mm 33W- 30% =22W or 700mA
74mm 55W- 30% = 37W sufficient for 1.2A
Once you have determined this maximal power handling, you can analyze the
continuous current capability, which is determined by the size of the
heat-sink. Without heat-sink each regulator can at the most dissipate 2.5W,
which would correspond to roughly 400mA output current.
The size of the primary fuse is not an indication of power capability, it
will blow only in case of a transformer failure and is usually rated at
least at twice the max. power.
--
ciao Ban
Bordighera, Italy

 

[mikem]

Jon B wrote:

I am using a power supply that is already built. (I want to say wall
wart, but it's the kind that sits on the floor, with a little cord.
Otherwise, same thing.) Anyway, it is regulated and puts out +15,
-15, and ground. it uses a center tapped transformer, rectifier
diodes and 7815/7915 regulators, with filter caps. the fuse is rated
for 400 mA/250 V. My line is 120 VAC. About how much current can i
safely draw from the 15 volt side?

If the answer was, let's pretend, 3 amps, does that then mean i could
connect a 10 ohm resistor between 15 and -15 volt lines? or does that
mean 3 amps per side?

No, because each regulator is only rated at 1A, the max
short term current you can draw is 1A per side. The lowest
value resistor that you can connect between 15/-15 without
losing voltage regulation would be 30V/1A=30Ohms, or 30W.

The long-term total power available will be much less than
30W, probably more like only 15W. This has to do with heat
dissipation inside the box, which comes from the transformer,
the rectifiers and the two regulators. Heat has to flow through
the sides of the plastic box.... It will take a while
(several miniutes) for the box to reach thermal equilibrium.

The regulators contain a thermal shutdown self-protection circuit.
The two regulators share a heat sink; if either regulator is heavily
loaded, then its heat load raises the temperature of the heat sink,
possibly effecting its neighbor. When the temperature inside the
box rises to ~100deg, either/both of the regulators will go into
thermal shutdown, and the output voltage(s) will drop precipitously.

See the 7815 spec sheet at:

http://www.hep.upenn.edu/SNO/daq/parts/lm7815.pdf

MikeM