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!