450ma led backlite question
 

Not sure why this one is elluding me, but , I have a graphics display with
an led backlight drawing about 450ma @ 5v. While a large 7805 is rated for
an amp it's getting pretty hot even with a nice sized heat sink.

Any thoughts on another way to drive this backlight? I was thinking of
running it at 12v (it's going in a radio) and a dropping resistor. What say?

ddz

In article , "Tim" letter T letter
H @lakeoriongroup.com says...

Not sure why this one is elluding me, but , I have a graphics display with
an led backlight drawing about 450ma @ 5v. While a large 7805 is rated for
an amp it's getting pretty hot even with a nice sized heat sink.

What voltage are you feeding IN to the 7805?

Example: Say you feed 20VDC to the input. That means the regulator
gets to waste 15V of that as heat. Although current in a series circuit
is constant, one must take into account the current used by the
regulator itself to operate. So, in addition to your 450mA for the
backlight, you're going to be drawing up to 8mA more for the regulator.

Given (Ohm's Law) P=IE: You have up to 458mA times 15V giving 6.87
watts worth of power dissipated as heat. That's a lot of heat, so yes,
your regulator getting warm is very normal. It gets worse when the input
voltage goes up, or when the current draw increases.

The 7805 was a fine part in its time, but there are other more
modern (switching) regulators that do a much more efficient job.
Although implementing such may take a few more parts, I would look
seriously at using something from, say, Linear Technology.

Any thoughts on another way to drive this backlight? I was thinking of
running it at 12v (it's going in a radio) and a dropping resistor. What say?

Even worse. You'll need a plenty big power resistor, it'll waste a
lot of power as heat, and you'll have practically no immunity from
spikes and surges. Try the switching regulator idea first.


--
Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute
(Known to some as Bruce Lane, KC7GR)
kyrrin a/t bluefeathertech d-o=t c&o&m
Motorola Radio Programming & Service Available -
http://www.bluefeathertech.com/rf.html
"Quando Omni Flunkus Moritati" (Red Green)

In article , "Tim" letter T letter
H @lakeoriongroup.com says...

Not sure why this one is elluding me, but , I have a graphics display with
an led backlight drawing about 450ma @ 5v. While a large 7805 is rated for
an amp it's getting pretty hot even with a nice sized heat sink.

What voltage are you feeding IN to the 7805?

Example: Say you feed 20VDC to the input. That means the regulator
gets to waste 15V of that as heat. Although current in a series circuit
is constant, one must take into account the current used by the
regulator itself to operate. So, in addition to your 450mA for the
backlight, you're going to be drawing up to 8mA more for the regulator.

Given (Ohm's Law) P=IE: You have up to 458mA times 15V giving 6.87
watts worth of power dissipated as heat. That's a lot of heat, so yes,
your regulator getting warm is very normal. It gets worse when the input
voltage goes up, or when the current draw increases.

The 7805 was a fine part in its time, but there are other more
modern (switching) regulators that do a much more efficient job.
Although implementing such may take a few more parts, I would look
seriously at using something from, say, Linear Technology.

Any thoughts on another way to drive this backlight? I was thinking of
running it at 12v (it's going in a radio) and a dropping resistor. What say?

Even worse. You'll need a plenty big power resistor, it'll waste a
lot of power as heat, and you'll have practically no immunity from
spikes and surges. Try the switching regulator idea first.


--
Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute
(Known to some as Bruce Lane, KC7GR)
kyrrin a/t bluefeathertech d-o=t c&o&m
Motorola Radio Programming & Service Available -
http://www.bluefeathertech.com/rf.html
"Quando Omni Flunkus Moritati" (Red Green)

Not sure why this one is elluding me, but , I have a graphics display with
an led backlight drawing about 450ma @ 5v. While a large 7805 is rated for
an amp it's getting pretty hot even with a nice sized heat sink.

Any thoughts on another way to drive this backlight? I was thinking of
running it at 12v (it's going in a radio) and a dropping resistor. What
say?
==============================
If your supply voltage is 12 V and need 5 V @ 450 mA , you have to get rid
of 7 V @ 450 mA or 3.15 Watts
in whatever way you do that . Suggest you stick with the 7805 , and
increase the regulator's heat sink if that makes you feel happier .
You can also spread the heat by adding a resistor 'upstream' of the voltage
regulator.
Say you drop 4.5 V across the resistor which @ 450 mA would have to be 10
Ohms ,2 W (take a 3 or 4 W resistor)
Locate the resistor off any PCB (preferably using ceramic ferrules) in a
relatively open area ,not too close to other components
The voltage regulator would then only have to dissipate 1.15 Watt .

With a switch mode regulator you might introduce RF problems.

Frank GM0CSZ / KN6WH

Not sure why this one is elluding me, but , I have a graphics display with
an led backlight drawing about 450ma @ 5v. While a large 7805 is rated for
an amp it's getting pretty hot even with a nice sized heat sink.

Any thoughts on another way to drive this backlight? I was thinking of
running it at 12v (it's going in a radio) and a dropping resistor. What
say?
==============================
If your supply voltage is 12 V and need 5 V @ 450 mA , you have to get rid
of 7 V @ 450 mA or 3.15 Watts
in whatever way you do that . Suggest you stick with the 7805 , and
increase the regulator's heat sink if that makes you feel happier .
You can also spread the heat by adding a resistor 'upstream' of the voltage
regulator.
Say you drop 4.5 V across the resistor which @ 450 mA would have to be 10
Ohms ,2 W (take a 3 or 4 W resistor)
Locate the resistor off any PCB (preferably using ceramic ferrules) in a
relatively open area ,not too close to other components
The voltage regulator would then only have to dissipate 1.15 Watt .

With a switch mode regulator you might introduce RF problems.

Frank GM0CSZ / KN6WH

Went with a KA3406 DC/DC converter pulled off an old motherboard, seems to
be working fine, had to play with the charge caps a bit to get what I wanted
but lost the heat for sure!

Thanks

Tim

"Tim" letter T letter H @lakeoriongroup.com wrote in message
...
Not sure why this one is elluding me, but , I have a graphics display with
an led backlight drawing about 450ma @ 5v. While a large 7805 is rated for
an amp it's getting pretty hot even with a nice sized heat sink.

Any thoughts on another way to drive this backlight? I was thinking of
running it at 12v (it's going in a radio) and a dropping resistor. What
say?

ddz

Went with a KA3406 DC/DC converter pulled off an old motherboard, seems to
be working fine, had to play with the charge caps a bit to get what I wanted
but lost the heat for sure!

Thanks

Tim

"Tim" letter T letter H @lakeoriongroup.com wrote in message
...
Not sure why this one is elluding me, but , I have a graphics display with
an led backlight drawing about 450ma @ 5v. While a large 7805 is rated for
an amp it's getting pretty hot even with a nice sized heat sink.

Any thoughts on another way to drive this backlight? I was thinking of
running it at 12v (it's going in a radio) and a dropping resistor. What
say?

ddz

Use a switching regulator -- you can even get an LM317 to "oscillate" by
putting an RC network in the feedback network -- and use it as a buck
converter -- the schematic is on the Nat Semi website -- just search under
LM117.

"Tim" letter T letter H @lakeoriongroup.com wrote in message
...
Went with a KA3406 DC/DC converter pulled off an old motherboard, seems to
be working fine, had to play with the charge caps a bit to get what I
wanted
but lost the heat for sure!

Thanks

Tim

"Tim" letter T letter H @lakeoriongroup.com wrote in message
...
Not sure why this one is elluding me, but , I have a graphics display
with
an led backlight drawing about 450ma @ 5v. While a large 7805 is rated
for
an amp it's getting pretty hot even with a nice sized heat sink.

Any thoughts on another way to drive this backlight? I was thinking of
running it at 12v (it's going in a radio) and a dropping resistor. What
say?

ddz

Use a switching regulator -- you can even get an LM317 to "oscillate" by
putting an RC network in the feedback network -- and use it as a buck
converter -- the schematic is on the Nat Semi website -- just search under
LM117.

"Tim" letter T letter H @lakeoriongroup.com wrote in message
...
Went with a KA3406 DC/DC converter pulled off an old motherboard, seems to
be working fine, had to play with the charge caps a bit to get what I
wanted
but lost the heat for sure!

Thanks

Tim

"Tim" letter T letter H @lakeoriongroup.com wrote in message
...
Not sure why this one is elluding me, but , I have a graphics display
with
an led backlight drawing about 450ma @ 5v. While a large 7805 is rated
for
an amp it's getting pretty hot even with a nice sized heat sink.

Any thoughts on another way to drive this backlight? I was thinking of
running it at 12v (it's going in a radio) and a dropping resistor. What
say?

ddz