[AMPS] Bridge vs. Doubler

[Steve Thompson]

Original message:

>>There may be other factors too - for example, if your filter cap is a
>>single oil-filled unit, I don't see how you can use a doubler without
>>buying another, identical cap.  On the other hand, the rectifiers in a
>>doubler only carry half the current so this might allow some flexibility in
>>component sizing.
>>
>
I think this isn't true. In FW doubler, each diode conducts on alternate
half cycles. In FW bridge, each diode conducts on alternate half cycles.


Replying to the original, Phil VE3OZZ said

>The rms current in each doubler diode is twice the DC load current, and the
>peak secondary winding current is at least 2.8 times the DC load current.
>This is why a transformer supplying a doubler circuit should be designed
>for it.  It isn't enough to just look at the DC load current or the KVA
>rating of the transformer when selecting one.  Knowing the wire size of
>both windings is also a key piece of information.

The whole piece was very well put but I disagree here. The measurements
I have made to date stack up well with Spice simulations - here's what
is predicted by Spice:

The transformer model I used was chosen because I had transformers wound
to this spec.

1500V secondary, 40 ohms total (inc. primary allowance) eff. series
resistance, bridge, 30uF smoothing and 3750 ohms load (560mA dc in
load).

The dc output is about 2100V average with a bit under 200V ripple. The
peak current in the transformer secondary is 3.8A and the rms current is
about 1.4A - 7 and 2.5 x the dc current respectively.

I don't have an equivalent transformer designed for doubling so I made a
guess for the model:

750V secondary, 20 ohms total eff. series resistance, 2 diodes, 2 x 60uF
in series smoothing, 3750 ohms load.

dc output is 2000V average, a bit over 200V ripple. Peak current in the
secondary is 5.5A, 10 x dc load current AND the conduction angle is
longer by about 20%. I haven't done the rms calculation yet.

>
>Sorry for the diatribe, but there's my 2 cents worth.  Transformers are
>more interesting than they look.

And more difficult and much misunderstood.

Steve

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm