# Halogen bulb/LED cluster swap

Discussion in 'The Projects Forum' started by doug3460, Jan 26, 2009.

1. ### doug3460 Thread Starter Active Member

Oct 19, 2008
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0
Hi all. Have been gone a bit but am back & still trying to blow myself up!

Converting a 12VAC 10VA line to run LED cluster. Original had an M11 size 5ω halogen bulb. Should be no problem, right? 4 diodes & a capacitor (see attachments).

Question: is there a good rule of thumb for determining an appropriate capacitor in simple circuits? For this one I chose a value based on research. I'd prefer to be able to compute a value without necessarily using tangent & cosine equations .

Suggestions?

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2. ### Audioguru Expert

Dec 20, 2007
10,789
1,212
The capacitor will quickly charge to 16.8V.
the value of the capacitor depends on how much ripple your circuit can tolerate.

3. ### mik3 Senior Member

Feb 4, 2008
4,846
69
If Vm is the peak voltage of the secondary and Vr is the ripple voltage then Vdc will equal:

Vdc=Vm-Vr/2

and

Vr=Idc/(2*f*C) or Vr=Vdc/(2*f*C*R)

Vr=peak-peak ripple voltage
Idc=dc current the load draws
Vdc=dc voltage across the load
f=mains frequency
C=filter capacitor value
R=resistance of the load

Note that the above equations give approximations of the voltages but close enough.