wall wart and leds

M

medusa

Guest
I am trying hookup a 10mm superbright led to a wall wart that is rated
output 9v, 300ma. I used a 470 ohm resistor for this connection
but .the wire feel a little warm to me. When working with wall
warts how does one compute the resistors need for the wall wart
strength. Do we go solely by the output info or what? thanks in
advance for your help. BTW if I add more leds later is there a simple
design for a brightness adjustment...i'm trying to make a sub stage
microscope light source.
While one bulb works well on the lower power I might need to add more
in order to view the higher magnifications and I'd like to be able to
adjust the led brightness.
 
"medusa" <medusa569@verizon.net> wrote in message
news:b3bb265f-9da6-4f1a-8300-98eb503d10a8@k4g2000yqb.googlegroups.com...
I am trying hookup a 10mm superbright led to a wall wart that is rated
output 9v, 300ma. I used a 470 ohm resistor for this connection
but .the wire feel a little warm to me. When working with wall
warts how does one compute the resistors need for the wall wart
strength. Do we go solely by the output info or what? thanks in
advance for your help. BTW if I add more leds later is there a simple
design for a brightness adjustment...i'm trying to make a sub stage
microscope light source.
While one bulb works well on the lower power I might need to add more
in order to view the higher magnifications and I'd like to be able to
adjust the led brightness.
Click to expand...
You compute the resistor needed just like you do any other time. It seems
that you did that.
With wall warts the specified outputs may only be true when powering the
intended device.
I would suggest that you measure the output voltage in actual operation with
your circuit.
You didn't say if it was an AC or DC wall wart. If its an AC wall wart this
could be why your wires feel warm.
An LED is not a bulb.
Tom
 
The wall wart says input...120vac 60hz 6 w, output 9vdc 300ma.
I used the 470ohms not because I computed it but elsewhere it said
for a 12 volt power source a 470 resistor would be adequate.I got the
impression there would be no sensation of heat anywhere.



You compute the resistor needed just like you do any other time. It seems
that you did that.
With wall warts the specified outputs may only be true when powering the
intended device.
I would suggest that you measure the output voltage in actual operation with
your circuit.
You didn't say if it was an AC or DC wall wart. If its an AC wall wart this
could be why your wires feel warm.
An LED is not a bulb.
Tom
Click to expand...
 
medusa wrote:
I am trying hookup a 10mm superbright led to a wall wart that is rated
output 9v, 300ma. I used a 470 ohm resistor for this connection
but .the wire feel a little warm to me. When working with wall
warts how does one compute the resistors need for the wall wart
strength. Do we go solely by the output info or what? thanks in
advance for your help. BTW if I add more leds later is there a simple
design for a brightness adjustment...i'm trying to make a sub stage
microscope light source.
While one bulb works well on the lower power I might need to add more
in order to view the higher magnifications and I'd like to be able to
adjust the led brightness.
Click to expand...
The circuit below regulates the current delivered to the LED,
regardless of the voltage provided by your 9V wall wart.
Connecting R between the output pin and the adjust pin
sets the current that will be available to the load, which
is connected to the adjust pin along with the resistor.

+----in[LM317]out---+
| adj |
------- | | [R]
| Wall +|---+ | |
| 9V | +---------+
| Wart -|---+ |
------- | [LED]
| |
+---------+

Compute R as follows: R = 1.25/Iled
You are looking for Iled ~20 mA with your 470 ohm resistor in your
original circuit, and that 20 mA computes to 62.5 ohms in this
circuit (1.25/.02). A standard 62 ohm resistor would be fine. You
could also go a little lower if you want, say a 56 or a 51 ohm resistor.
For example, a standard 51 ohm resistor would yield ~24.5 mA
You can add a 1 or 2 watt pot in series with the 51 ohm resistor
to adjust the brightness. You can add an LED or two in series with
the LED in the diagram.

This is not the only way, but it is simple, requiring only the
LM317 IC and the resistor for the basic circuit.

Ed
 
ehsjr wrote:
medusa wrote:

I am trying hookup a 10mm superbright led to a wall wart that is rated
output 9v, 300ma. I used a 470 ohm resistor for this connection
but .the wire feel a little warm to me. When working with wall
warts how does one compute the resistors need for the wall wart
strength. Do we go solely by the output info or what? thanks in
advance for your help. BTW if I add more leds later is there a simple
design for a brightness adjustment...i'm trying to make a sub stage
microscope light source.
While one bulb works well on the lower power I might need to add more
in order to view the higher magnifications and I'd like to be able to
adjust the led brightness.


The circuit below regulates the current delivered to the LED,
regardless of the voltage provided by your 9V wall wart.
Connecting R between the output pin and the adjust pin
sets the current that will be available to the load, which
is connected to the adjust pin along with the resistor.

+----in[LM317]out---+
| adj |
------- | | [R]
| Wall +|---+ | |
| 9V | +---------+
| Wart -|---+ |
------- | [LED]
| |
+---------+

Compute R as follows: R = 1.25/Iled
You are looking for Iled ~20 mA with your 470 ohm resistor in your
original circuit, and that 20 mA computes to 62.5 ohms in this
circuit (1.25/.02). A standard 62 ohm resistor would be fine. You
could also go a little lower if you want, say a 56 or a 51 ohm resistor.
For example, a standard 51 ohm resistor would yield ~24.5 mA
You can add a 1 or 2 watt pot in series with the 51 ohm resistor
^
Click to expand...
1/2 was what I meant to say (not that 2 is wrong)
to adjust the brightness. You can add an LED or two in series with
the LED in the diagram.

This is not the only way, but it is simple, requiring only the
LM317 IC and the resistor for the basic circuit.

Ed
Click to expand...
 
In <077f4450-0a40-4de5-bc0a-b36134641c3e@a7g2000yqo.googlegroups.com>,
medusa wrote:

The wall wart says input...120vac 60hz 6 w, output 9vdc 300ma.
I used the 470ohms not because I computed it but elsewhere it said
for a 12 volt power source a 470 resistor would be adequate.I got the
impression there would be no sensation of heat anywhere.
Click to expand...
A 9V 300 mA DC wallwart will likely produce 12 volts when loaded with a
light load.

What wires got hot?

If the LED is a white one, then it probably has a voltage drop of
3.2-3.5 volts at a usual amount of current. With 12 volts supply, that
leaves a little less than or around 9 volts across the 470ohm resistor.
That means current a little less than 20 milliamps flowing through the
resistor and the LED.

This means about 70 milliwatts being dissipated in the LED and a little
less than .18 watt being dissipated in the resistor. The resistor will
probably feel a little warm.

- Don Klipstein (don@misty.com)
 
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