www.piclist.com/techref/index.htm?key=capacitor+selection

>> - an unloaded small 9V transformer can give much more than 9V

>> (measure

>> or find the specs), calculate with at least 12V, maybe even 18V

> Using my Fluke 179 (RMS-DMM), it said 9.6VAC for the unloaded

> transformer (no bridge). Measuring over "+" and "-" on the bridge,

> it

> said 9.5VDC. Adding a load (100k) over the bridge yielded no change

> for the voltage (9.5VDC still).

>

> Could it be the transformer is marked with its effective voltage?

> Else

> I should have seen sqrt(2)*9 on my DMM, no?

With no filter capacitor you tend to see a full wave rectified sine

wave - ie the negative half cycles are inverted so you get a series of

positive half sine cycles. The mean DC level of this is less than the

steady DC you'd get with a filter and what your meter reads on a DC

range depends on how it responds to the unexpected AC fluctuations.

There will be some stray capacitance and this plus meter load will

lead to an unknowable end reading. Add even a 10 uF cap and the result

will change.

Aluminium electrolytics should ideally be run at near but below their

rated voltage. A 10 volt capacitor rating is too low in the

application you describe.

Russell

See also: www.piclist.com/techref/index.htm?key=capacitor+selection