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 using 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" To: "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 -- ___________________________________________________________ Sign-up for Ads Free at Mail.com http://promo.mail.com/adsfreejump.htm