>> 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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