Use 14.4V NiCad battery through a L7812 regulator?

[John Doe]

I need to power three Cree XLamp XRE LEDs. The standard sealed lead
acid battery is 12 V, but it is heavier than I would like to carry. I
have some spare rechargeable 14.4 V DeWalt tool batteries.

The L7812 regulator says it needs at least 14.5 V. will a 14.4 V
NiCad tool battery be acceptable for that?

Thanks.

 

[Tom Biasi]

I need to power three Cree XLamp XRE LEDs. The standard sealed lead
acid battery is 12 V, but it is heavier than I would like to carry. I
have some spare rechargeable 14.4 V DeWalt tool batteries.

The L7812 regulator says it needs at least 14.5 V. will a 14.4 V
NiCad tool battery be acceptable for that?

Thanks.

Why do you need the 7812?

Tom

 

[ehsjr]

I need to power three Cree XLamp XRE LEDs. The standard sealed lead
acid battery is 12 V, but it is heavier than I would like to carry. I
have some spare rechargeable 14.4 V DeWalt tool batteries.

The L7812 regulator says it needs at least 14.5 V. will a 14.4 V
NiCad tool battery be acceptable for that?

Thanks.

LEDs are best driven by a constant current supply, like the
one drawn below.

-----
+14.4 ---in|LM317|out---+
----- |
adj [R]
| |
+---------+---[LED]-[LED]-[LED]---+
|
Gnd --------------------------------------------+

Choose R for the current you want by R = 1.25/I
According to the LED data sheet, the maximum current is 1 amp,
so R must be no less that 1.25 ohms. The resistor would dissipate
1.25 watts at that value, so 2 watts or higher should be used.

The LM317 will dissipate ~3.3 watts nominal at that current, but
over 6 watts if the battery is at full charge voltage (over 17
volts) and taken immediately off the charger and connected to
the circuit. So, size the heat sink accordingly.

You could use a 7812 in the same configuration with R = 12/I,
but the resistor would need to be about 10 times the wattage
of the resistor used with the LM317.

I would recommend running the thing at well under maximum current
for longer run time, less heat, and longer LED life. If you use
3.5 ohms for R, I will be ~357 mA, R will need to dissipate about
1/2 watt, and the LM317 dissipation will be under 3 watts worst
case, and about 1.6 watts nominal. You'd get more than triple
the run time that way, too.

Ed

 

[Jasen Betts]

I need to power three Cree XLamp XRE LEDs. The standard sealed lead
acid battery is 12 V, but it is heavier than I would like to carry. I
have some spare rechargeable 14.4 V DeWalt tool batteries.

The L7812 regulator says it needs at least 14.5 V. will a 14.4 V
NiCad tool battery be acceptable for that?

what exactly are you doing with with the 12V. - what circuit are you
planning to use for the LEDs ?

In most cases 14.4 from a rechargable battery pack is a good
replacement for 12-14V from from a "12V" lead-acid battery.

 

[Tom Biasi]

In case you haven't figured it out, DO NOT connect a plain-old LED to a
stiff voltage source like a NiCd or lead-acid battery. At least, unless
you want a burnt out LED and an unhappy battery. You can kinda sorta get
away with it if you have an LED that drops 1.5V and a dry cell battery,
but it's still not good practice.

LEDs want constant current sources -- which is what the other posters are
offering up to you.

I was assuming he was using the Cree XLamp used as a replacement bulb
operating at a nominal 12 VDC. They are used routinely in RVs and routinely
operate at 14+ volts while the vehicle is running.

Tom

 

[John Doe]

what exactly are you doing with with the 12V. - what circuit are
you planning to use for the LEDs ?

In most cases 14.4 from a rechargable battery pack is a good
replacement for 12-14V from from a "12V" lead-acid battery.

I just weighed eight 2ah AA NiMH batteries. They weigh less than
one third of the next alternative, so they are looking very good.

 

[Tom Biasi]

I wasn't sure, which was why I qualified my statement with the "plain
old".

And I couldn't remember the nominal DC bus voltage during charge -- I
thought it was more like 13.8?

But, that thing should have some sort of current limiting, if not
outright current regulation.

Yes, those devices are made for such use. The OP shouldn't need to do
anything to use the NiCads.
Vehicle nominal charge voltage is about 13.8 V but trips above 14 are not
uncommon.
Regards,
Tom

 

[John Doe]

LEDs are best driven by a constant current supply,

That looks like a clue... Thanks.

like the one drawn below.

-----
+14.4 ---in|LM317|out---+
----- |
adj [R]
| |
+---------+---[LED]-[LED]-[LED]---+
|
Gnd --------------------------------------------+

Choose R for the current you want by R = 1.25/I
According to the LED data sheet, the maximum current is 1 amp,
so R must be no less that 1.25 ohms. The resistor would dissipate
1.25 watts at that value, so 2 watts or higher should be used.

The LM317 will dissipate ~3.3 watts nominal at that current, but
over 6 watts if the battery is at full charge voltage (over 17
volts) and taken immediately off the charger and connected to
the circuit. So, size the heat sink accordingly.

You could use a 7812 in the same configuration with R = 12/I,
but the resistor would need to be about 10 times the wattage
of the resistor used with the LM317.

I would recommend running the thing at well under maximum current
for longer run time, less heat, and longer LED life. If you use
3.5 ohms for R, I will be ~357 mA, R will need to dissipate about
1/2 watt, and the LM317 dissipation will be under 3 watts worst
case, and about 1.6 watts nominal. You'd get more than triple
the run time that way, too.

What happens when only a current limiting resistor is used, provided
that current is kept under the maximum safe current for the LED?

http://led.linear1.org/1led.wiz

Should that current limiting resistor calculator be used?

Thanks.

 

[ehsjr]

LEDs are best driven by a constant current supply,

That looks like a clue... Thanks.

like the one drawn below.

-----
+14.4 ---in|LM317|out---+
----- |
adj [R]
| |
+---------+---[LED]-[LED]-[LED]---+
|
Gnd --------------------------------------------+

Choose R for the current you want by R = 1.25/I
According to the LED data sheet, the maximum current is 1 amp,
so R must be no less that 1.25 ohms. The resistor would dissipate
1.25 watts at that value, so 2 watts or higher should be used.

The LM317 will dissipate ~3.3 watts nominal at that current, but
over 6 watts if the battery is at full charge voltage (over 17
volts) and taken immediately off the charger and connected to
the circuit. So, size the heat sink accordingly.

You could use a 7812 in the same configuration with R = 12/I,
but the resistor would need to be about 10 times the wattage
of the resistor used with the LM317.

I would recommend running the thing at well under maximum current
for longer run time, less heat, and longer LED life. If you use
3.5 ohms for R, I will be ~357 mA, R will need to dissipate about
1/2 watt, and the LM317 dissipation will be under 3 watts worst
case, and about 1.6 watts nominal. You'd get more than triple
the run time that way, too.

What happens when only a current limiting resistor is used, provided
that current is kept under the maximum safe current for the LED?

The current through the LEDs changes as the battery voltage decreases.
If that produces acceptable results, it is the simplest way to go.
Note that the LED brightness will decrease as the voltage drops,
which may make it unacceptable.

Using a constant current source keeps the brightness of the LEDs
at the same level - until the battery discharges too low to
maintain the current.

http://led.linear1.org/1led.wiz

Should that current limiting resistor calculator be used?

No, it does not give the proper wattage resistor for worst case
for your design. Also, you need to make some assumptions
when using a current limiting resistor, and the site does
not mention that. Finally, the forward voltage for your
LEDs is not constant - it varies, depending on how much
current is drawn.

Ed

 

[Jasen Betts]

If weight is very important, you may want to take a look at LiPo
batteries. They give you specific charge density at least double of
what you get from even the best NiMH stuff.

They're more expensive, and you need a new charger, so cost might
become an issue. Radio control plane and helicopter modelers don't
even consider anything else these days. They are, of course, very
picky when it comes to weight.

It's energy density is better than nitromethane?

 

[Tom Biasi]

The ones I see are replacement bulbs. I gues the regulation is built in.
Tom

 

[John Doe]

It's energy density is better than nitromethane?

At least here, we have already passed the point of diminishing
returns...

 

[John Doe]

John Doe wrote:
....


The current through the LEDs changes as the battery voltage
decreases. If that produces acceptable results, it is the
simplest way to go. Note that the LED brightness will decrease
as the voltage drops, which may make it unacceptable.

Using a constant current source keeps the brightness of the LEDs
at the same level - until the battery discharges too low to
maintain the current.

At the moment, I am using the circuit that came with the Coleman
LED spotlight, minus the sealed lead acid battery charger and
momentary switch circuits. Using eight AA NiMH batteries... The
OFF current is about 3 microamps, so I guess the wiring is
correct. After charging, the ON current starts out at about 910
milliamps and quickly drops to about 700 milliamps. The current
drops slowly to about 600 milliamps and then slows to a crawl.
Apparently it is not a current limiting circuit, but maybe
efficient enough.

 

[ehsjr]

At the moment, I am using the circuit that came with the Coleman
LED spotlight, minus the sealed lead acid battery charger and
momentary switch circuits. Using eight AA NiMH batteries... The
OFF current is about 3 microamps, so I guess the wiring is
correct. After charging, the ON current starts out at about 910
milliamps and quickly drops to about 700 milliamps. The current
drops slowly to about 600 milliamps and then slows to a crawl.
Apparently it is not a current limiting circuit, but maybe
efficient enough.

The key is that it uses 8 NiMh. At full charge, they will provide
~11.44 volts which is far lower than you would get with your
14.4V tool battery, which can provid over 17 volts immediately
after being taken off the charger. If you go with the 14.4V
battery, use the LM317 constant current circuit. If you don't,
and use the circuit in the Coleman, you'll fry either the circuit,
or the LEDs or both. My guess is that the Coleman current
limiting circuit is nothing more than a 1 ohm resistor, which
is fine for a 9.6V battery & those LEDs. Heck, they might not
even use the resistor.

Ed

 

[John Doe]

No body cares about this shit again, time after time. He always
asked dumb question first! What's wrong with you huh Punk?

Was/is "EHWollmann aol.com" involved in a fringe USENET war? I
recall stumbling over that while doing some research many moons
ago. Are you the real one, or an antagonist?

Just curious.
--