John,
David is right on this and I forgot to mention the variac. One thing though, it
seems I remember something about those variacs having a set duty cycle that
might be on a tag on them, I cant remember. The thing is, the heater supply in
an amp is the only supply that is really rated at CCS because it is under load
100% of the time that the amp is on (unless this will be a comercial amp, etc.
broadcasting 100% of the time). If your controlling this with a variac, you'd
need one with a 100% duty cycle. Lets say you had a 5 amp variac with a 50%
duty cycle, you most likely then would have 2 amps at a 110% duty cycle because
your cutting the power rating by a little more than 1/2. Using a 2 amp variac
for this might damage the variac over time, I'm not sure. Some on here may have
a little more experience on using variacs than me and could tell you. A lot of
manufacturers kind of use duty cycle as a sales gimmik to make the output power
sound more than it really is or at least for u
sing one 100% of the time. One thing I'd make sure and do is fuse the variac
at the operating current level. I have seen these go bad using them as a motor
speed control and it dont take long to smoke one.
Best,
Will
----- Original Message -----
From: "David Kirkby" <david.kirkby@onetel.net>
To: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] filament transformer
Date: Fri, 08 Jul 2005 21:15:53 +0100
>
> John Irwin wrote:
> > hi
> > I have a transformer 10vac at 20 amps. 110vac primary. Does
> > anyone know how to calculate the primary ac line draw. I want to
> > use a variac to put it on the money . Need to get the right auto
> > transformer.
> >
> > thanks
> > john kb9tc
>
> The primary current will depend on the secondary current. With no load,
> the primary will draw very little (just the magnetising current). So
> unless you state the heater voltage and current, one can not give you a
> value, but I can tell you how to calculate it.
>
> At full load the transformer will be somewhere around 90% efficient -
> I'm sure Will could give you a better idea, but the exact value does not
> make a lot of difference here. So you must allow 10% more for the losses.
>
> 20 A at 10 V is 200W. Plus another 10% (20W) and that is 220W.
>
> Current = power / voltage
> = 220/110
> =2A.
>
> So a 2A variac will *just* be OK if you need the full 10 V at the full
> 20A. But personally I would go for something a little bigger (the next
> size up) *if* your heaters really are going to be 10V at 20A, since I
> don't like running any component at its maximum ratings.
>
> If the heaters don't need the full 200W the secondary can provide (i.e.
> they need less than 10V, or less than 20A), then 2A would be fine, but
> it is a bit tight if you really need the full 200W on the secondary.
>
> --
> David Kirkby,
> G8WRB
>
> Please check out http://www.g8wrb.org/
> of if you live in Essex http://www.southminster-branch-line.org.uk/
>
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
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