Spyder voltage regulation question

[schweinhundert]

I'm trying out RG Keen's Spyder design using the Weber transformer that he recommends. The transformer's output is about 11VAC (RMS) when plugged into the wall. The output of the bridge rectifier that I'm using (Mouser part 625-3KBP06M-E4) is about 10.3 VDC. I am using STMicro's L7809 regulator (here). Oddly, though, the voltage output of these regulators is, at present, 7VDC according to my meter. I know it's not miscalibrated too bad, since if I read the output of one of my wall warts I get to within 0.1V of its rated output.

The dropout on these regulators is supposed to be less than 1.3V (especially under no load), so I can't understand why I'm only getting 7V out of the regulators. Any ideas? Do I need a low dropout regulator here? I'm very puzzled. I can borrow a high-quality Fluke meter tomorrow and check it more carefully, but it sure looks like I'm missing something.

Thanks

[Steve Conner]

10.3V seems a tad low. If we calculate the peak voltage (11 * 1.4 = 15.4V) and then subtract two of the 1.05V drops listed for that rectifier, we get 13.3V. You've lost 3 volts somewhere, so check your wiring, are things getting hot, capacitors in backwards, is the DC volts range on your meter faulty but the AC volts working, etc...

[schweinhundert]

Thanks, yeah, the problem was the input capacitor was way too low. Put in a 220uF and it works like a charm now.

[guitician]

With a dropout voltage of 2.5v, I would say the input would have to be 11.5vdc. for 9vdc out.

[schweinhundert]

Quite right, but the dropout on the LM7809 isn't nearly that high. Moreover, the 11VAC put out by the transformer is the RMS value; the full-wave rectified curve has a peak at almost 15.5V, and the 220uF capacitor smooths this out to a voltage more than big enough to work for that regulator. I get 9V on the money. Having a 10uF also helps a bit with the transient response, but a much smaller capacitor on the output could work too. I've attached an LTspice file (for 14VRMS input and 12V regulator) if anyone is interested.