# Regulated lab power supply with variac?

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#### nzoomed

##### Member
Im wanting to build a lab power supply using a variac to control the voltage going into a stepdown transformer to give me the option of variable AC voltage in addition to DC fed through a rectifier.

I will probably have 50V on the secondary or something near enough from the UPS transformers i have floating around.

I dont know if this is the most practical means of building a lab power supply, but I dont really want to go down the path of using an LM317 which dissipates alot of heat.

I would want to also add current regulation, so dont know how you would go about doing this by means of a variac.

Am I better using a switching regulator? I like the idea of building a variable voltage power supply and a variac makes it easy for AC power especially.
If i want lets say 0-50V and current limiting, with the ability to change between AC and DC, whats the best option?

TIA

##### Well-Known Member
I have worked with several older Variac (auto transformer) type power supplies which were 200 VDC 100 ADC units. The downside to what you are describing is lack of regulation. While I would not discourage building a bench power supply I can buy assorted 0 to 30 and 0 to 50 VDC units providing 3.0 to as high as 10.0 amps in the $50 to$60 USD range. I doubt I could build one that inexpensive. Really depends on what your output specifications are?

Ron

#### schmitt trigger

##### Well-Known Member
In the late 1970s, I saw a high power regulated AC supply which utilized servo motors which would drive, via gears, the Variacs.
Even then, it was old technology. The size of a medium household fridge, weighted close to 700 kilograms.

Also, it was slow responding.
Like S-l-o-w.

##### Well-Known Member
Schmitt Trigger:
In the late 1970s, I saw a high power regulated AC supply which utilized servo motors which would drive, via gears, the Variacs.
Even then, it was old technology. The size of a medium household fridge, weighted close to 700 kilograms.

Also, it was slow responding.
Like S-l-o-w.
Yep, worked with plenty of those. Talk about a brute power supply, those were brutes. When I was retiring I left suggestions for replacements. Motor driven variacs all geared and with limit switches at the extremes. You used a crane to plant them and they didn't move. Exactly, they looked like refrigerators, large refrigerators at that. They were late 1960s design and in their defense worked fine for decades and it was impossible to overload those beast.

Ron

#### nzoomed

##### Member
I have worked with several older Variac (auto transformer) type power supplies which were 200 VDC 100 ADC units. The downside to what you are describing is lack of regulation. While I would not discourage building a bench power supply I can buy assorted 0 to 30 and 0 to 50 VDC units providing 3.0 to as high as 10.0 amps in the $50 to$60 USD range. I doubt I could build one that inexpensive. Really depends on what your output specifications are?

Ron
Yes lack of regulation is the main drawback.
As you say, its cheap to buy them pre assembled.
My friend is buying modules from china off ebay with pre-programmed voltages at the push of a button to build his supply.

Dunno what the quality of this sort of stuff is like from china, but the VARIAC may just be best for AC voltages only and i use a seperate regulated unit for the DC supply.

##### Well-Known Member
That is how I would go about it using a variac for a variable AC supply and then build a regulated DC supply but all things considered, I would buy over build a DC bench supply.

Ron

#### schmitt trigger

##### Well-Known Member
Ron;
at the time, Superior Electric "Powerstats" variacs were the "Rolls-Royce" in the industry.
Built, literally, like tanks. Would last forever.

#### ronsimpson

##### Well-Known Member
In about 1970-something I used one of these and a motor, on a 10,000 watt transmitter. (grid power supply) When the output power is over +102.5% the motor turned down the voltage. When the power is under 95% the motor turned the knob up.

Not my finest design, but it did work for decades.

#### MikeMl

##### Well-Known Member
I still have a 10A 0V to 30V variable linear supply with a Variac ahead of the main transformer. The voltage drop across the pass transistors is metered, so the Variac can be set so that the unregulated voltage upstream of the regulator is just above the drop-out voltage to minimize dissipation in the regulator at any preset output voltage...

I built it in the 1970s...

#### ronsimpson

##### Well-Known Member
I built it in the 1970s...
I used a switch to jump the diodes up/down through taps on the transformer. (High/M/Low) This kept the transistors at a good temperature.

##### Well-Known Member
Ron;
at the time, Superior Electric "Powerstats" variacs were the "Rolls-Royce" in the industry.
Built, literally, like tanks. Would last forever.
Yeah, located up the road here in Cleveland, Ohio. Superiour also made some of the brute supplies we mentioned earlier. Think I have a few of their variacs laying around here somewhere.

Ron

#### tomizett

##### Active Member
nzoomed:

Mikes post here:

I still have a 10A 0V to 30V variable linear supply with a Variac ahead of the main transformer. The voltage drop across the pass transistors is metered, so the Variac can be set so that the unregulated voltage upstream of the regulator is just above the drop-out voltage to minimize dissipation in the regulator at any preset output voltage...
is exactly what I envisaged when I read your initial post. It's not the most practical project in this day-and-age, but if you have the parts and want to use them this would be a fascintaing project.
It's essentially an extension of the technique Ron mentions above for switching taps - which is still used in plenty of modern bench power supplies.

As long as your output transistors can take the full output voltage, you can still have good regulation and full current limiting.
----

Myself, I have an isolated variac unit to which I can couple a rectifier set with adjustible overcurrent cutoff (with steep foldback). I find it very useful for working on higher-voltage stuff like off-line switchers and, in centre-tapped mode, high power amplifier modules.

#### atferrari

##### Well-Known Member
Im wanting to build a lab power supply using a variac to control the voltage going into a stepdown transformer to give me the option of variable AC voltage in addition to DC fed through a rectifier.

I will probably have 50V on the secondary or something near enough from the UPS transformers i have floating around.

I dont know if this is the most practical means of building a lab power supply,
And surely bulky. Maybe the AC availability justifies that?

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