Re: 116 vs. 716 and more (part III - conclusions)

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Thu, 19 Dec 1996 17:37:56 -0600
>From: P3D Steve Spicer  <s.spicer@xxxxxxxxx>
>Subject: Re: 116 vs. 716 and more (part III - conclusions)

>Another option is to place the transformers outside the unit. For the USA,
>it may be economic to use an auto-transformer, which is cheaper and
>smaller. These  only work out well if the step-down ratio is between about
>0.5 and 1. So with 110V line volatge, and with 2 lamps in series, 48Volts
>would be required for the two lamps (or 72V for the 400W units).

>John Bercovitz has has previously raised the issue of "filament notching"
>when operating the two lamps in series. To circumvent that, I would put a
>tap at 24Volts on the auto-transformer, and feed that to the junction point
>of the two lamps in series. (Then the inputs are: 0 and 110 Volts. And the
>outputs are: 0, 24V & 48V). The 24V tap carries zero current if the lamps
>are matched, and very small correction current if the lamps become
>un-matched, or full load current if one lamp fails.

Incidentally, what are the safety concerns for working with 24V and 48V?
I would imagine that a certain percentage of the users would treat it more
casually than 110V "because it's low voltage", so if there is a potential for
injury it might be useful for the designer to take a few extra precautions -
for instance if an autotransformer is used, making sure that the polarized
plug is hooked up correctly (as would be done for 110V lamps). If 24V is
significantly safer than 48V, that might be an incentive to use a floating-
output (center-tapped or otherwise) transformer rather than an autotransformer.

John R.


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