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Re: S-38B Power Consumption

 

Not sure that a higher VA rating is really necessary for this small of an isolation transformer.

"It is possible to estimate the output voltage at intermediate loads since load regulation varies in an almost linear manner. For example, the 241-8-16 has a full-load rating of 16 V @ 6.25 A and a regulation of 10%. Its no-load output would be 10% more than 16 or 17.6 V. At half-load (3.12 A) its output would be 5% more than 16 or 16.8 V. Similar estimates can be made for any % load.

Another factor to bear in mind is that it is possible to safely exceed the VA rating of many small power transformers. If the added regulation (drop in output voltage) is acceptable, an “overload” may be permissible because the design is regulation-limited rather than heat rise-limited."? This is from:?

In other words, if a lower output voltage is tolerable say 95% worse case of 120 vac or some 114 vac then a 25 VA transformer operated at 100% of it's VA rating does no harm.? I doubt that the isolation transformer will be too warm to touch.
Jim


Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy


On Friday, April 11, 2025 at 01:14:48 PM CDT, Don VE3IDS via groups.io <ve3ids.don@...> wrote:


Ron,?

I just measured my s38b and it draws 208 mA with a supply voltage of 121 VAC. I think I would use a transformer with a good safety headroom. Maybe 50 VA so it doesn't get hot.?

73 Don ve3ids?


On Fri., Apr. 11, 2025, 2:31 a.m. Ron Sharp via , <vk2irs=[email protected]> wrote:
Hi folks,
I am restoring a Hallicrafters S38-B receiver and want to add an isolation transformer. So that I can select the correct transformer, I need to know the actual current the S38-B draws. I note in a PhotoFact Folder on the first model (the S-38) the current drain is shown 0.245 Amps @ 117V AC. This is cutting it fine with the transformer that I have (30VA), but as there is no BFO tube in the S-38B, I figure there may be less current drain overall. I recognise that the heater chain will still draw 150mA, but there should be a reduction in the DC (B supply) due to one less tube.
?
Would someone please advise the actual current drawn by the S38-B operating with an AC input?
?
Cheers
Ron
VK2IRS
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