[Amps] Buck-Boost Transformer Selection

Thu Jun 26 00:42:10 EDT 2014

Hi Dick,

I believe you are misinterpreting the KVA rating in the table. The KVA refers to the power dissipated in the load (i.e., your amplfier) and not the KVA rating of the buck-boost transformer. The actual power rating of the transformer is just the current through the secondary times the number of bucking volts supplied. 

For example, let's say your actual line voltage is 250V and the amplifier draws 14 Amps, which correspnds to a power rating for the amplifier of 3.5KVA. (That, by the way, is the peak power. The average power is probably about half that.)  Now suppose you want to reduce the line voltage to, say, 238V, and we'll assume the amp still draws 14 Amps at this lower line voltage.  You are bucking the voltage by 250V-238V = 12V. The bucking transformer will then have to carry 14 Amps of current, so its power rating will be 12Vx14A=168W. An ordinary filament transformer with a 240V primary and a 12V secondary rated at 14A would be perfect for this application.  I wouldn't buy the Hammond buck-boost transformer, because you'll be spending a lot of money unnecessarily. The Hammond transformers are designed for industrial, continuous duty application at full rated load. You'd be paying for a lot of capability which you don't need. Personally, I'd think a 12V filament transformer rated at 10A or more would be adquate for your amplifier.

There is one more consideration, which may complicate things slightly. If your amplifier is designed for the US market, it may require a neutral line which in the US is at 0V, with the L1 and L2 lines each being 120VAC (out of phase with each other). Most likely, that's not the case with a Quadra, but you should check to be sure. If you do require a neutral line, then you'd need two filament transformers, each having half the required voltage rating ( e.g. 6V instead of 12V). It's a minor complication.

In principle, you could also use a Variac,which is an autotranformer, to get the reduced line voltage, although I don't recommend it. Unlike an ordinary transformer, which has a primary and secondary winding, an autotransformer has only one winding. A slider on the winding (or a fixed tap) picks off the desired voltage. You do not want to use a Variac for your situation, because the full line voltage appears across the variac, which means you'd need a variac rated at 3.5 KA. That would probably weigh about a hundred pounds and would be very expensive. A variac would be gross overkill merely for reducing the line voltage for your amplifier.

I get the impression, Dick, you are making this a harder problem than it really is. All you need is a filament transformer whose secondary voltage will drop your line voltage by a few volts, to get it into the recommended range for your amplifier.  You can probably find a dozen or more suitable transformers on eBay or QTH.com for well under $50. 

73,

Jim W8ZR

From: Richard Solomon [mailto:dickw1ksz at gmail.com] 
Sent: Wednesday, June 25, 2014 9:37 PM
To: Jim Garland
Subject: Re: [Amps] Buck-Boost Transformer Selection

I found this web site while looking up how to connect the transformer.

http://www.hammondpowersolutions.com/files/HPS_Catalog_BuckBoost_Section2.pdf

According to it, using it with the Quadra (14 Amp @ 240 vac), I need a 3.5 KVA Transformer.

That's quite a bit larger than what you say I need.

Still confused ... 

73, Dick, W1KSZ

On Wed, Jun 25, 2014 at 10:17 AM, Jim Garland <4cx250b at miamioh.edu> wrote:

Hi Dick,
You don't need a 1.5KV rating. Let's say you want to drop the line voltage
by 12V and suppose  the maximum current drawn from the AC line by the
amplifier is 15 Amps. You'd thus  need only a 12Vrms/15A transformer (180W).
Because of the limited duty cycle of the Quadra, probably a 10-12A
tranformer would work fine.
73,
Jim W8ZR.

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Richard
Solomon
> Sent: Wednesday, June 25, 2014 11:06 AM
> To: amps at contesting.com
> Subject: [Amps] Buck-Boost Transformer Selection
>
> I have a lead on a late production Quadra so I guess I better
> invest in a B-B Xfmr to get my sometimes 250 volt line down
> to something acceptable to my new little friend.
>
> Question is on the rating for the Xfmr ... I expect I should get
> something rated for the load, say 1.5 KVA (considering the
> rating is for CCS use not ICAS).
>
> Now I find lots of them over on the evil empire, but looking at
> the specs on the label is confusing:
>
> One I found says:
>
> Max Volts 264   Max Amps 62.5  1.5KVA
>
> The current rating does not make sense. How can I sort this out ??
>
> Thanks, Dick, W1KSZ

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