Bucking Transformer Built In to Chassis

[chinrest]

Has anyone experience putting bucking transformers in the chassis? Any warnings? How critical is placement/orientation? I was thinking about wiring it in between the AC switch and PT (see attached), either inside the chassis or on top.

Thanks in advance for any comments.

[Tom Phillips]

I have used a bucking transformer to lower the mains voltage reaching the PT primary. I mounted the small bucking transformer inside the chassis at the PT end. Reference the attached photo. Worked as intended with no side effects.

One comment: To me it looks like your schematic shows it wired as a boost transformer.

Cheers,
Tom

[chinrest]

One comment: To me it looks like your schematic shows it wired as a boost transformer.

Thanks for your comment and the pic. I'm going for it. I'm fairly certain the schematic is correct. I'm referencing R.G.'s article: Vintage Voltage Adapter

[chinrest]

Also,

The particular transformer I'm using (RadioShack 12v) sings quite a bit when it's bucking (mechanical buzz). I hope it doesn't drive me batty when it's in the cab.

[eschertron]

Thanks for your comment and the pic. I'm going for it. I'm fairly certain the schematic is correct. I'm referencing R.G.'s article: Vintage Voltage Adapter

R.G. says "This either adds or subtracts the 10VAC from 125VAC, making your amp see either 115Vac or 135Vac, depending on how you wire secondary. If you wire it to subtract, the AC power to your amp back to about what it was designed to use." And then goes on to say test the voltage out to verify the coils are phased correctly. So you're OK as long as you verify the voltage out.
I'd rather see the incoming voltage across both windings and the dropped voltage across the original primary. That would be the topology of a standard autotransformer. Tom didn't say explicitly, but he may have been alluding to that.

[chinrest]

Like this?: http://www.jogis-roehrenbude.de/Send...ender/Fig4.jpg

[Edit] Wait that's the same


This is different: http://www-rohan.sdsu.edu/~nash/ARF/auto_xfmr.png

[Tom Phillips]

…The particular transformer I'm using (RadioShack 12v) sings quite a bit when it's bucking (mechanical buzz). I hope it doesn't drive me batty when it's in the cab.

That’s a shame. It indicates substandard workmanship and I’d return the transformer if you have the option. It could buzz along for the whole life of the amp with no other problems but it would be nice to use better parts since you are building with your own labor.

…Tom didn't say explicitly…

I was referring to the phasing. Those dots on the transformer schematic symbol indicate the relative phase of the windings. The phase dots are not always shown on schematic diagrams because the phase is not important in many applications. Attached is a snippet of the schematic for the amp shown in my earlier photo. Note the out-of-phase relationship of the bucking transformer secondary and the original amp PT primary. That produces the bucking voltage reduction. As has already been pointed out it’s easiest to hook it up and do a quick voltage test to figure out which wiring gives you buck vs. boost.

The second link in post #6 shows a single transformer with a secondary winding used to buck the primary voltage on that same transformer. It seems to be the method that eschertron was describing but I don't see the advantage. Also with that method, the primary winding of the small bucking transformer must be capable of passing the full current drawn by the primary of the amp's PT. That would unnecessarily increase the size of the required bucking transformer.
Cheers,
Tom

[eschertron]

This fellow describes what I was suggesting. And discusses full load current. I don't have the math background to verify it, but his arguments can be found about 2/3 of the way down the page (excepted here).

----------------------------------------------

What is commonly overlooked when auto transformers are specified, is that the requirements are actually almost the same as for a bucking transformer. As a result, there is no reason not to connect your bucking tranny as an autotransformer. This means the normal thin primary wire for the majority of the winding, and thick secondary wire only for the high current part of the winding. Current in the lower section of the winding is reduced to 775mA for the 240V version. From this, we can do a bit of lateral thinking and reconfigure the bucking transformer so that it works properly. This increases the output voltage by a small amount - it will be of no consequence in most cases. This is just as cheap and effective as the bucking transformer shown above, but is slightly more efficient.


Figure 4 - Proper Way To Wire A Bucking Transformer
(picture not available - follow link)

You won't see this arrangement described very often (if at all), but it is a far better solution. In Figure 4, I have simply rewired the circuit as an autotransformer, and the equivalent circuit shows that this is indeed the case. The transformer is exactly the same as used in previous examples. The incoming mains connects across the entire winding ... the primary in series with the secondary, wired in phase. The output voltage is taken from the tap - this is identical in every way to a normal autotransformer connection. The output voltage is fractionally higher than with the bucking configuration - the 240V version gives 221.5V RMS output (110.75V RMS for the 120V version). Again, double check all winding polarities before connecting to any equipment.

You can also push this version a little harder than a traditional bucking transformer. The normal output current (based on our initial criteria) is 10A at 220V, but with the arrangement shown in Figure 4 you can have an output current of about 10.8A (a total of 2,400VA) without exceeding the transformer's secondary current rating. That's because the currents are subtracted within the winding itself, because of the transformer action. The main primary runs at a current of about 835mA at the maximum output of 2.4kVA.

[chinrest]

"You won't see this arrangement described very often (if at all), but it is a far better solution."

Bold statement. I like it. I also like that it will buck closer to 11v, rather that 12v. In my particular application -11v puts me closer to ideal.

Thanks.

[Tom Phillips]

I will keep thinking about this but as of now I don't understand why he says he has a "...far better solution."
Cheers,
Tom

[eschertron]

I will keep thinking about this but as of now I don't understand why he he says he has a "...far better solution."
Cheers,
Tom

I agree that 'better' is not easily quantifiable, and I don't pretend to understand all the physics of the transformer. I posted the link purely to show support for what, in my mind, was a 'first guess' design.

[R.G.]

It has to do with the way autotransformers work. And what you mean by both "far" and "better". Autotranformers have some of the current to the output coming directly from the input, and only have to transform the difference in power between the input and output to work properly. In general, auto transformers will be smaller and lighter - hence cheaper - than an isolating transformer providing the same power.

This gets fairly diffuse in the case of a buck/boost transformer for small differences in input/output voltages. The savings are almost entirely in the primary of the buck/boost transformer winding itself.

It's a matter of perspective as to whether it's "better", "far better", or marginally worth worrying about. With a second transformer hooked up as a buck/boost setup, the differences get even smaller as the transformed power is very close to the difference between the power in and out anyway.

The words better, best, worse, etc. all need some metric to measure how much and in what dimension they're better, etc. to be able to make sense.