Powering My New LED 10-Volt 10-watt R.V. Lights

I'll just leave this here

Buck Converters

Nothing wrong with 1960's linear voltage regulators, but I'm just sayin... might be time for something new.

1) i haven't worked with a 7809 in years but always thought the max current was 1A.
2). I never considered it good practice to build a circuit that pushed the specs beyond about 50% ratings.

Given the recently posted new information I would have to withdraw my previous comment that the 7809 would be a poor choice although I'd probably be inclined to use an LM340k instead.

DrewE wrote:
Let's see...20W at 9V is about 2.2A. The LM7809 is usually specified for up to 1A, and has built in current limiting.

I think the regulator would survive the attempt (as in it won't be destroyed), but by shutting itself down. The 780x regulators are pretty hard to kill.

Your heat sinking needs to be able to dump about 7W of power (3ish V * 2.2A). The junction-to-case thermal resistance of the 780x is listed as 5°C / W, so the junction would be about 35°C warmer than the heat sink assuming a very good thermal bond between it and the case. This means that the heat sink must not get warmer than 90°C to keep the device within its maximum rating of 125°C. (These details may vary from manufacturer to manufacturer, in fact; there isn't just one 7809 design.)

One 7809 per panel might work, though it's on the edge of the chips specifications, and sharing a heat sink may be feasible. You still have to remove the same total amount of heat, of course, as the power dissipated by the regulators remains the same.

Another option may be to use a PWM circuit to reduce the average power consumption of the LEDs to something reasonable. You could probably do this with a 555 and a power transistor and a few minor interface bits. That would be more efficient (less heat), though would probably need some sort of filtering to prevent excess RFI.

Wrong.

You need to understand just HOW these LEDs are wired..

I have one of these same LED modules that Mex has so I am very aware of the design and not to mention I have MEASURED current draw of said modules..

These modules are known as COB or chip on board. They are an array of 1W LEDs mounted on a plate.

There is three parallel ROWs of LEDs, each row has THREE WHITE LEDS in series..

Each white LED requires 3.4V to light so each ROW requires about 10.2 V..

Each ROW only consumes about 1W since LEDs in series share the same amount of wattage like a resistor in series)

Each row is 1/3 of the current draw (and wattage) but all the LUMENS are added together..

So technically all three parallel ROWs added together will draw about 3W at 10V if you really wish to be accurate..

The manufacturers kind of skew the actual wattage drawn since they add up the wattage of all the chips on the board.. Technically it is a 9W module but they round it up to 10W for marketing purposes.. But in reality it is drawing not much more than 3W at 10V..

Let's see...20W at 9V is about 2.2A. The LM7809 is usually specified for up to 1A, and has built in current limiting.

I think the regulator would survive the attempt (as in it won't be destroyed), but by shutting itself down. The 780x regulators are pretty hard to kill.

Your heat sinking needs to be able to dump about 7W of power (3ish V * 2.2A). The junction-to-case thermal resistance of the 780x is listed as 5°C / W, so the junction would be about 35°C warmer than the heat sink assuming a very good thermal bond between it and the case. This means that the heat sink must not get warmer than 90°C to keep the device within its maximum rating of 125°C. (These details may vary from manufacturer to manufacturer, in fact; there isn't just one 7809 design.)

One 7809 per panel might work, though it's on the edge of the chips specifications, and sharing a heat sink may be feasible. You still have to remove the same total amount of heat, of course, as the power dissipated by the regulators remains the same.

Another option may be to use a PWM circuit to reduce the average power consumption of the LEDs to something reasonable. You could probably do this with a 555 and a power transistor and a few minor interface bits. That would be more efficient (less heat), though would probably need some sort of filtering to prevent excess RFI.

Ed_Gee wrote:

I would not expect an LM7809 to safely regulate a 10Watt load, if that's any help.

LM 78xx series is good up to 1.5A when properly heatsinked (although 1A is recommended).

Mexs 10W LED is going to draw about .750A or 750ma at 10V.

Also the regulator only needs to drop a few volts to get to the required 10V for this particular LED chip so it won't have to dissipate a lot of heat in the process.. which is well within the reach of a LM78xx regulator.

At 13V input the regulator needs to drop 3V at 750ma, that is about 2.25W of heat..

At 12V drop is 2V at 750ma for 1.5W of heat..

All well within the allowable specs for this chip..

Verify manually VA supply versus lumen output. The chips are sold in China. They are advertised as 10-watt. If they are driven at `12.00 volts they consume NINETEEN WATTS. 19.00 va.

When heat-sinked correctly to a 90 mm star shaped heatsink (round with rays) the temperature of the chip rises to 90c in two minutes and accelerating temp rise time. The heat sink measures 70c. This spells fast and certain doom for that chip.

Less than 12.00 volts is required. I -could- use a constant current LED power supply for a 10 watt chip but the driver will not fit behind the heat sink. There is not that much room above the lamp.

The chips are immeasurably brighter than common LED retrofit bulbs.

They are also pure white. Absolutely no blue or yellow no matter what angle they are viewed from. Their beam spread is also around 140 degrees of angle.

10 watts at 10 volts over-drives a 1.5 amp rated LM7809 ? I guess an ampere isn't an ampere.

Guess I'd better wait for the power supply. And LT 783 7.5 ampere voltage regulators. They need a circuit board. Meaning they cannot be mounted next to the lamp. The wiring in between is going to act like a transmitting antenna for the device switching.

Such is life.

Mexi it's talking about discreet LED chips, not RV light replacements
Possible they came from Marlin P Jones & Associates inc or eBay

MPJA is a well know supply house for engineers and hobbyists

MEXICOWANDERER wrote:
12 Volts over-drives the chips by a bunch.

Three diodes in series in a PITA. I have a bunch of LM7809 regulators. With a silicon rectifier biasing the negative...or not.

Can this regulator survive driving a pair of 10-watt LED chips? If I do not bias the regulator it would mean driving the chips at 9 volts.

I am purchasing a 5 amp 90-volt power supply tomorrow. But I need heat sinks right away. If I can double up on the chips per heat sink, and use a fan for cooling, then I will save valuable mounting space. It isn't the 7809 it is the extra 7809 heat sink per the added 10-watt chip that's a killer to my plans.

20 watt chips use a weird 28 volt supply.

A single 10 watt chip is not enough light and at twelve dollars a pop the fan+heat sinks add up fast.

Yes I am getting old and I need "blindingly bright" light to see well.

Forum members familiar with the LM7809 may also be familiar with what I can get away with. And what I cannot.

Thank You

Yes, you can use a diode on the ground pin of the 78xx regulators, this will bring the 9V up to about 9.7 ish volts. However be aware that it is not really a recommended way of lifting a fixed regulator voltage (but it does work, I have done it)..

The correct way is to provide a 400-500 ohm resistor from the output to the ground pin. Then using a 500 ohm to 5K ohm pot with the wiper tied to one end of the pot from the ground pin to actual ground.

This now gives you an adjustable regulator from the fixed voltage up to about 1V below the input voltage.

You can change the pot to a fixed resistor once you get the regulator voltage set by taking an ohm reading of the pot..

If you don't mind doing the math you can also use the equations provided by the manufacturers documents for any of the adjustable regulators to determine the resistor values..

Do also be aware, the metal tab of the T-220 case of the regulator is also at ground potential.. In lifting the ground pin you MUST ensure the T-220 heatsink tab is ISOLATED from your power supply ground or the entire heatsink must be isolated from the power supply ground.

If you would better describe what those " 10 Watt LED chips " are, it might be easier to figure out just what you are trying to do or work with. Most RV LED modules I have worked with are plug and play....no extra circuitry needed. And arrays of LEDs generally work right off the RV 12VDC supply. I have 35 years as a Sr. Telecomm. Technician yet can't figure out quite what you want or have there.

I would not expect an LM7809 to safely regulate a 10Watt load, if that's any help. Also, what the heck is your " 5 amp 90 volt power supply " supposed to do???

edit. After re-reading the original post it appears you got some non-RV specific LED arrays that work off 10 volts. Given the efforts to work around the 10 volt limitation, wouldn't it just be more cost effective to obtain a couple high Lumen output LEDs that were designed for 12Volt RV use instead of acquiring and building all the necessary power regulation to get around that?