piclist 2007\03\31\070818a >
Thread: Capacitor-selection
face BY : Russell McMahon email (remove spam text)

>> - 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.


<017601c77374$210bcad0$e701a8c0@y2k> 7bit

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