safest way to rectify and regulate 30VAC to 9VDC?

[gutsofgold]

I am using a DB102 rectifier ---> 100uF cap ---> into a 10k/10k voltage divider ---> 78L09 regulator...

I only need this 9VDC to power seven 5mm LEDs at standard brightness. I start with 36VAC, which ends up being 45VDC after the rectifier. The voltage divider for some reason is knocking the 45VDC down to 10.5VDC with all the LEDs off () and the regulator is putting out 9VDC as expected then.

With all seven illuminated the voltage reads 7VDC before the regulator and 6.5VDC after. I guess this is a loading effect due to the simple resistive voltage divider? The LEDs don't seem to dim or flicker with all of them on however.

My questions are... One, how is it the voltage divider is knocking the 45 volts down to 10 volts when all the LEDs are off? Could the circuit still be drawing power and loading down? Two, if the 36VAC is coming from a power transformer, and I am loading down my voltage rectifier/regulator circuit... could I damage this power transformer? Again, only using seven 5mm standard LEDs at reasonable brightness. Three, is there a better way to do this sans transformer?

Thanks!

[R.G.]

My questions are... One, how is it the voltage divider is knocking the 45 volts down to 10 volts when all the LEDs are off? Could the circuit still be drawing power and loading down?

A 10K/10K voltage divider produces 22.5V DC from 25Vdc only when unloaded. In actual operation, 45Vdc through a 10K/10K divider looks to any load like a 22.5V DC source feeding through a 5K resistor. So the voltage out of it drops by 5V per milliampere drawn through it. The 7809 datasheet from Fairchild says it draws typically 5ma, but as much as 8ma for some of them. 5ma times 5K is 25V. The resistance of your voltage divider is so big that your regulator is pulling it down until the regulator stops drawing so much current, that being 6.5-7Vdc.

Two, if the 36VAC is coming from a power transformer, and I am loading down my voltage rectifier/regulator circuit... could I damage this power transformer? Again, only using seven 5mm standard LEDs at reasonable brightness.

No, your 10K/10K divider is too high a resistance. It is very unlikely that you'll damage the power transformer since the 10K/10K is choking the current down so much. At worst, the transformer is driving a 5K resistor.

Seven LEDs at reasonable brightness doesn't tell you much about how much current is being drawn, or the load rating on the transformer, which are the things you really need to know.

Changing to a 1K/1K divider string would be a start. The divider string by itself will draw 45Vdc/2K = 22.5ma, but it will make a 22.5V source that will only drop under load by 0.5V/ma of loading. So the regulator all by itself will load this down to 22.5V - (0.5V/ma*5ma) = 22.5-2.5V = 20V, which will leave your regulator 20V to work from, and it will then regulate. The regulator loading will also load down the DC supply, and when it drops under 11V at the regulator, this will then sag out of regulation. That's a load of 20V-11V/0.5K = 9V/0.5K = 18ma. If you run your LEDs with no more than 2.5ma each, all is well. But that's all you can pull.

The problem is really that 45Vdc is a horribly inefficient raw DC supply to get 9Vdc out of, as you're finding out. Unless you use a switching regulator, you waste a lot of power in getting down to 9V.

Three, is there a better way to do this sans transformer?

Yep. 9V regulated power supply. Or 11Vdc to 18Vdc output DC wall wart into the 7809 regulator.