Buck-Boost-Isolation Transformers

[W9RAN]

The topic come up fairly regularly about how to deal with today's line voltages that are  generally higher than our boatanchors were designed for.     The usual suggestion is to wire up a filament transformer as a bucking transformer, and that's what many of us have done.   But now there's also a readily available commercial solution available through Automationdirect.com, which has acquired a good reputation for competitive pricing in the industrial control arena.   They now offer NEMA-rated Buck-Boost transformers made by Jefferson Electric:

http://www.automationdirect.com/adc/Overview/Catalog/Power_Products_%28Electrical%29/Power_Transformers/Buck-Boost_Transformers_%28NEMA_Rated%29

Not only are these transformers new, agency-approved to meet applicable standards and section 210.9 of the NEC, and attractively priced,  but they're available in sizes that can handle an entire 20A branch circuit.     For those interested, there are two good references at the link above - a PDF file that provides detailed technical information on buck-boost transformers, and an Excel worksheet that makes it easy to select the right unit.

Any transformer can also be wired as an isolation transformer, or as a conventional step-down or step-up transformer.  See the documentation for wiring details.

Prices start at $53, and the 500VA model that would reduce a high line voltage of 127 to 115 volts and capable of handling a complete 20A branch circuit is $98.  This transformer could also be used in step-down configuration to get 40A of 12 volts or 20A of 24VAC.

No connection to ADC except as a satisfied customer.

73, Bob W9RAN

[KE6DF]

One problem I have is our line voltage runs around 121 v all the time.

Most modern buck/boost units are 12 v or more.

A 12 volt reduction is a bit much.

A 6.3 V filament tranny is just right. And 20 amp 6.3 volt filament transformers are easy to come by.

I also scored a 1000VA stancor isolation transformer on ebay for a good price.

It has primary taps for 105, 115, and 125 for a nominal output of 115.

It too is a bit more stepdown than I need, but when lightly loaded the output is pretty close to 115 with 121 input to the 125 primary tap.

I have it hooked up to a separate plug strip at my bench.

Dave

[K5WLF]

Neat site,Bob. Thanks for posting. I saved the PDF on the buck/boost xfmrs, but even better I found a potentially good source for stepper motors for a radio-telescope project I'll be working on this summer at the university I work for. Thanks again.

[Opcom]

For those who find the bucking voltage selection too limited or imprecise due to common voltages like 6.3V or 12.6V etc., there is nothing wrong with putting a variable autotransformer on the primary of the buck transformer and adjusting it to suit.

It need not be large, the beauty of it is that the little 1-3A ones are perfect for typical loads. How to calculate the " variac's " requirement?

how many amps drawn by your load?
How many volts to buck (or boost)?
A * V(buck) = watts.
watts / 120 = variac amp rating.

121V buck to 117V = 4V
117V to a few receivers.. say the load is 4A
get a 6.3V 4A (or more) transformer
4A * 4V = 16 watts
16W / 120V = 133mA - so there's the slot for that 1.25A variac.

125V buck to 110V = 15V
110V load is a small transmitter and receiver and the load is 16A
get a big ol 20-24V 16A transformer from a battery charger
16A * 15V = 240W
240W / 120V = 2A - so there the 2.5A one or 3A 'variac' is good.

This works well with a 'variac' because it's an autotransformer and all it's doing is adjusting the low-power buck transformer's input voltage, not the whole 0-120V output load.
It won't work right other means of feeding the buck transformer such as resistors, reactances, or solid state dimmers.

Tips
- the regulation will be fine if the variac is clean and the transformer is good quality.
- the bigger the variac current rating the better the regulation, 2X is more than enough.
- the bigger the transformer current rating the better the regulation, 2X is more than enough.
- This can all be put in a project box and the I/O can be an extension cord you cut in 'half' so you got a plug and a socket. Save $!
- It can also be racked by mounting the variac and transf. on a panel, and put this in the lower part of the rack out of sight, or up top and include an output voltage meter.
- Try for an expanded scale meter. If one is not lying around, they are easy to make up.
- Don't forget to put a fuse on the (low current) variac input as well as a fuse for the 120V mains input.
- A bigger knob makes a more accurate adjustment but the range is only as much as your transformer secondary voltage so it's not hard to adjust close.

Just suggesting ways to be more precise without spending $!

There's also a similar commercial item I use for the receiver racks that uses a motorized 'variac' to buck-boost a 115V load and there are 12 receivers on that output. I don't ever have them all on so I don't need as much power as it can regulate, but the thing was $25 in a surplus shop. That is like free.
A control board drives a motor on the 'variac' and it powers a low voltage high current transformer rated like the ones shown on that buck-boost website. It's a rack-mount Stabiline EMT41060 unit by Superior, that can be set from about 110-120V output, and hold the set voltage from 95-135 input and run 6.6KVA. Just saying you can find automatic ones on a deal from time to time and then you don't have to worry or adjust. And, you can turn off the motor so it won't hunt or consume power, if you are happy with the voltage being within 2-3V of your target. Otherwise it can keep the voltage to within a volt or so. Mine must be 40 years old and still works like new.

- A tip on those things - if the load is very light, a big one like the 6.6KVA one won't track as accurately as when it is loaded to 500W or more. I don't know why that is. So a 6.6KVA one is not so great for a 200W load. Bigger is not always better. Maybe newer ones are better but they are expensive. Oddly there is one for sale here:
http://www.buckspmt.com/bucks_precision_machine_tools_alabama_246.htm but like I mentioned, it is much too big for typical use. And the price is too much for hobby users but is in line with the people he is selling to. There are several smaller ones on ebay.

Don't confuse them with other 'boxes' that don't have motor controlled 'variacs' but instead use ferroresonant transformers and no moving parts. Those items distort the AC waveform.

Saturable reactor regulators can also be found, there's a tube-type on on there now.

Happy bucking!

[KM1H]

One problem I have is our line voltage runs around 121 v all the time.

Most modern buck/boost units are 12 v or more.

A 12 volt reduction is a bit much.

A bit too much for what? A radio that wont work as specified at 109V isnt worth owning or it has problems. The heat reduction alone is worth it.

A 6.3 V filament tranny is just right. And 20 amp 6.3 volt filament transformers are easy to come by.

And usually a lot more expensive than oddball voltages in the 8-13V range.

Prices start at $53, and the 500VA model that would reduce a high line voltage of 127 to 115 volts and capable of handling a complete 20A branch circuit is $98.

That may be fine for those not near hamfests or industrial surplus stores. My full bench iron is in the 8-13V range at 20A or more and I havent paid over $10 for any.  One $50 exception that is not in use is a 240V to 8V at 100A as marked that I suspect is really a 7 or 7.5V piece for a tube with handles or water jackets. I need to confirm that before either putting on fleabay for the CBers or using to boost the voltage out to the far outbuildings; maybe JJ or 1KW can use it  

[k4kyv]

Actually, the low-voltage boost/buck method is to be preferred over the 1+ KVA isolation or autotransformer.  The large transformers tend to have a huge inrush of core excitation current, and sometimes pop breakers or blow fuses when the switch happens to be turned on right at the peak of a cycle.  OTOH, a 6.3v or other low voltage high current transformer needs only be rated for 100 watts or so, and the inrush current is negligible.  And don't forget, a transformer is not 100% efficient. Running a fully loaded 100 V-A transformer at 90% efficiency is a lot less wasteful than running a 1000 V-A auto- or isolation transformer fully loaded at 90% efficiency.