> From: K8MLM@aol.com
> My question is this. Can I use a auto transformer to step up the transformer
> input voltage to get a higher DC voltage output.... would like to operate at
> between 4,500-5,000 volts? The auto transformer I have is rated for 240 vac
> in, 0-280 vac out at 28 amps
Hi Bob,
You can run into several problems.
First, all steel cores saturate at some flux level. When that
happens, any additional primary voltage no longer adds core flux at
the same rate.
The result is the primary current skyrockets, because it's impedance
drops at voltage peaks.
You can find this point by measuring the primary AC current as you
increase transformer primary voltage. When the current suddenly
starts to climb, the transformer is saturated.
The only ill effect of transformer saturation is the transformer
runs hotter, and the secondary peak voltage stops increasing
with more primary voltage.
(By the way NONE of this has anything to do with secondary
load current)
Another problem is the auto-transformer adds needless ESR to the
transformer. The percentage of voltage sag will increase when you
load the transformer down. With a capacitor input supply, the VAR
power is much higher than the dc power you see on the meters, so a
ting bit of ESR can ruin the regulation.
You also have to watch component voltage ratings.
A final problem is the tube may become more unstable with higher
anode voltage. 4-1000A's like to oscillate near the FM broadcast
band, and that means it's a strain to suppress the parasitics without
overheating the suppressor while using the PA on 21 or 28 MHz.
73, Tom W8JI
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