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.
