Isolated Variac Schematic Review

[zanfar]

I am starting to consider a project to build a combo isolation and variable transformer for bench work. I don't have a pressing need, but I do want to do some tube work in the future and the variable output especially would be convenient. As I'm still in the planning phase, I wanted to see if I could get any feedback on an initial device schematic (attached). It will exclusively be used on North American 115VAC and an approximate 750VA capacity (although probably fused quite a bit lower).



I still have not decided on the switching mechanism or fp indicators: I'm waffling between direct switching (as in the schematic), controls-style contactors, or using a micro and relays. Input is welcome.

I am also unsure if I will need or want forced airflow for cooling.

Thanks in advance,

[Kryptychon]

Hi zanfar,

nice project.

I built an isolated variac myself a couple of months ago, so I can share some thoughts first hand:

1.) I'm sceptical about SW4. Depending on what you're doing with the isolated variac, it can be dangerous if it's not completely clear whether primary ground and seconadry ground are connected or not. In my design, there is no connection between primary ground and secondary ground.

2.) I propose to add another fuse on the secondary side.

3.) In my design I included a self-locking circuit so after a power outage, you must deliberately switch on the output again.

4.) As I'm using my isolated variac for testing and repairing devices, I decided to have an emergency shutdown as well. It's is hooked up with a 3m cable, so I can put it right next to where I'm working.

5.) Regarding cooling, my transformer is rated for 650VA and I rely on passive cooling through the perforated plates of the enclosure. However, using a fan does not hurt.

6.) Depending on the fuses in your electrical installation it might be beneficial to have a soft-start for your transformer. I used an electronical soft start relay from FSM.

7.) On the front panel, I have indicators and gauges for:

- Device powered (white)
- Output on (red)
- Output voltage
- Output current

I attached some images for you.

Bets regards
Kryp

[Phil1977]

I think you know about the PE-issue so let´s not warm it up again 

My other two cents:
- Why don't you want to build in a current-meter? I would even use a combo, an old-style analogue like in Kryptychons wonderful setup. And one 4-digit one - they are cheap and sometimes the small changes in input current give you the crucial hint what´s happening in the DUT.
- For SW2 I´d use a double switch and completely switch off the variac when not needed. Reason is that every variac has some shorted windings where the wiper is, so it´s humming all the time and ageing while not in use
- I´d think about including some current limiter like the famous dim-light-bulb. An output fuse was already mentioned - it´s important, a variac can multiply the input current so that it´s perfectly able to destroy itself with a fused input. I´d use a resettable breaker there.
- Having two input switches is no crime but also not beautiful.

[zanfar]

Thanks, Kryptychon and Phil1977. I have some replies below. A lot of your comments are things that are definitely on my mind; I just haven't decided how to execute them.

* SW4 / PE: The short answer is that for every claim or advice I can find about isolating or not isolating ground, I can find an alternate reason to do the opposite. I can see reasons to want it either way, so I figured a switch would give me flexibility. IMO, there is no piece of test equipment that doesn't require your brain to be engaged, and repairing or developing a circuit is, by definition, "unsafe." If you feel strongly, just believe that I will keep the switch in one position.

* A secondary fuse is an obvious oversight.

* Self-locking/auto-power-down: this was the attractive part of using a contactor or relay system. I may do so, at least on the output switch.

* If I use a relay system for control, I will definitely add an emergency stop. In reality, I don't have room on my bench to get more than arm's length away from the device, but safer is usually worth it.

* The Voltmeter element will almost certainly be a full power meter, just because it's easier. I am also thinking of adding a Voltmeter to the Variac output, independent of the final output, but I'm still balancing that against being able to turn the Variac off when in bypass mode

* I do want to explore a current limit, but I don't really like the dim-bulb approach. I appreciate its simplicity, but it ruins the dimensions, and I don't like using consumables if I don't have to. If I have a current meter, I may just add an internal resistance that I can switch in. If I use a micro, there will definitely be a programmable current limit.

* The first input switch is simply because I have those input modules available, and I really like knowing there is a switch at the cable end. Granted, if I can reach that switch, I should be able to unplug the device. It would also be helpful if the front switches were latched, as they wouldn't be master power switches.

As you can see, I'm still working through many items.

Thanks for the input.

[zanfar]

Depending on the fuses in your electrical installation it might be beneficial to have a soft-start for your transformer. I used an electronical soft start relay from FSM.

I have been trying to determine if, and if so, how to implement a soft-start. Unfortunately, I cannot find any standalone modules specific to transformer soft-starts (like your FSM module is) from any of my distributors.

Do you know if there is any difference between a motor soft-starter and an inductor soft-starter? For example, a generic 110V, single-phase, motor soft-start module looks suspiciously similar to the FSM module in your photos. (https://www.aliexpress.us/item/3256806857356958.html?spm=a2g0o.productlist.main.103.4e35ZptLZptLRo)

Currently, the winning design is to use an inline power resistor and short it out after some delay.

Thanks again,
Z

[zanfar]

I've made some updates to the schematic, taking into account many suggestions, corrections, and general progress on the design.

Any further input is appreciated.



Specifically:

* All input and output switches are now latched so they automatically disable in the event of a power loss
* Added emergency stop
* Added secondary-side fusing
* Added indicator lamps
* Soft-start and current-limit options are included
* Variac is disabled in pass-thru mode
* Added hardware details in notes
* I believe, all the switches are now in their "default" or "steady-state" configurations
* Generally, made things more explicit

Thanks,
Z

[bdunham7]

I'd recommend keeping it simple.  Here's a few recommendations:

Put the Variac first.

You need a separate output fuse or other protection, input protection is not enough with a Variac.

If your isolation transformer is a reasonably sized EI, then you likely don't need any inrush protection.  If your isolation transformer is a toroid or a very large EI, then a simple appropriately sized NTC thermistor would be sufficient.  Adding a timed relay would be nice but not necessary.

[zanfar]

Put the Variac first.

Why? How does this keep it simpler?

My reasons for having it follow the isolation transformer:

* Putting the Variac first would allow the isolation transformer to be operated outside its rating unless the primary is reconfigured for 230V operation, which has cascading effects
* Putting the Variac first means there is no longer a reliable 12VAC or 5VAC secondary output

You need a separate output fuse or other protection, input protection is not enough with a Variac.

Like F2? Otherwise, I'm not sure what you're talking about.

If your isolation transformer is a reasonably sized EI, then you likely don't need any inrush protection.  If your isolation transformer is a toroid or a very large EI, then a simple appropriately sized NTC thermistor would be sufficient.  Adding a timed relay would be nice but not necessary.

The isolation transformer IS a toroid, hence the soft start.

I avoided the NTC as it requires a minimum off-time between restarts, and although minor, represents a constant power loss. I'm also unaware of any suitable parts that aren't THT or SMD, meaning that their addition would require a board of some sort and additional wire-to-board connectors. Given everything is freestanding or DIN mount, I'm not sure substituting a single DIN relay for a board and three parts is much simpler. I may add LV components on a board eventually, but I was never expecting it to handle mains-level voltage or the full operating current.

Nice is, well, nice.

[bdunham7]

Why? How does this keep it simpler?

If your isolation transformer or any component after it has a fault or leakage to ground, your output becomes ground-referenced and potentially hazardous.  Variacs have greater potential for developing faults like that.  By putting the isolation transformer last you are minimizing the ways in which things can go wrong.

My reasons for having it follow the isolation transformer:

* Putting the Variac first would allow the isolation transformer to be operated outside its rating unless the primary is reconfigured for 230V operation, which has cascading effects
* Putting the Variac first means there is no longer a reliable 12VAC or 5VAC secondary output

You could experiment with the two components to make sure that the isolation transformer doesn't have excessive magnetizing current when the Variac is turned all the way up.  As for the auxiliary outputs, I didn't see that you were using the 5VAC and 12VAC outputs.  If you have a specific use for fixed outputs then clearly you'd need that transformer to go first, but if you don't then consider whether variable low-voltage outputs might be useful.

Like F2? Otherwise, I'm not sure what you're talking about.

Yes, like you have in the updated schematic.  Use a slow-blow fuse like an MDL-5.

I avoided the NTC as it requires a minimum off-time between restarts, and although minor, represents a constant power loss. I'm also unaware of any suitable parts that aren't THT or SMD, meaning that their addition would require a board of some sort and additional wire-to-board connectors. Given everything is freestanding or DIN mount, I'm not sure substituting a single DIN relay for a board and three parts is much simpler.

IMO people overthink the NTC and inrush issues.  The NTC doesn't need to fully cool off to succeed in limiting inrush current and it will partially cool very quickly if they are fully heated.  However, they will only be fully heated in the first place if the system is running at 100% of its rated power, so it really isn't even an issue except in corner cases.  If you use a 5A rated NTC which may have a cold resistance of 20R, this will limit your inrush to 6-8A.  If it is only partially cooled, such as after a quick power cycle while running at full power, it might have a resistance of only 2R.  This would allow a higher surge but still less than 60A (the remainder of the circuit is not a dead short...) and 60A is still not going to trip your circuit breakers.  You could experiment with this as well to see what it would take to actually cause an inrush surge that large.  Keep in mind that even with a relay/resistor setup, your relay system is going to have a holdup time so that a power glitch that is a fraction of a second could occur without the relay disengaging.   

I've seen NTC installations that don't use boards, you can just connect the wire leads to a terminal strip.  If you are using a relay to bypass the NTC or resistor, then the NTC has the advantage that it won't go up in flames if the relay fails to operate.  A resistor would have to be huge to avoid that problem.

[Doctorandus_P]

High current transformers often benefit from a soft start circuit. In audio amplifiers this is often a series resistor, that gets shorted after a few seconds. In audio amplifiers the problem is also made worse because the big capacitors on the secondary side are a near short at startup.

For test equipment power supply it is always nice to have voltage and current meters built into the circuit. With a uC and a relay or contactor, you can also add a (programmable) and resettable "fuse" to the secondary. You can also program I*I*t charateristics in the uC to allow for short surge current peaks.

And for potential lethal voltage equipment, a good indicator lamp whether the secondary circuit is powered or not is an important feature.

[Benta]

Placing the variac first is a no-go.
But you can eliminate the soft-start by wiring the isolation transformer for 230:230 V instead.
You'll have to derate the circuit to 500...600 VA instead of 750 VA, but is that a big issue?
And the transformer will run cooler (significantly lower iron loss) and be more robust.

[bdunham7]

Placing the variac first is a no-go.
But you can eliminate the soft-start by wiring the isolation transformer for 230:230 V instead.
You'll have to derate the circuit to 500...600 VA instead of 750 VA,

Why do you think it is a no-go?  Every commercial product I'm aware of that has this combination (Variac, isolation) puts the Variac first.

I think you're going to have to derate the transformer a lot more than that if you switch from parallel to series operation.

[Shock]

Where is the mains input voltage mentioned? Are you supplying 230V to each 115V winding? As that is what it looks like.

[zanfar]

It will exclusively be used on North American 115VAC

Input is 115VAC, paralleled across the primaries to maintain the 5V and 12V secondary taps.

[Shock]

Ah my mistake I kept seeing German flags while reading and thought wtf. I'm kind of used to seeing 240V ones on youtube and even in the US. My own is 0-280V but can understand it adds a fair bit of expense and you need to source different parts.

[Conrad Hoffman]

I built something similar many years ago. No doubt it suffers from all manner of design flaws, but I did build it with step-up capability. This has proven useful for testing the 220 VAC setting on numerous things, and for testing wide-range input switchers.

[enut11]

Hi @zanfar
Nice worthwhile project. I have 2 suggestions from my own experience with a variac.
1) Digital meters are too slow to react so I reverted back to analog, especially for the ammeter. In fact I have 2 analog ammeters in series - 100mA and 2A with the ability to bypass the 100mA meter.
2) An additional low voltage transformer at the output of the variac. This provides a very fine AC output up to its rated voltage. I made mine switchable with its own output terminals.
enut11

[coppercone2]

with a low voltage transformer you could also use it for electrocleaning metal with a brush

its mega nice since you just need a tray and it could reduce your sanding and stuff requirement down like 500% for metal working

[zanfar]

If you are using a relay to bypass the NTC or resistor, then the NTC has the advantage that it won't go up in flames if the relay fails to operate.  A resistor would have to be huge to avoid that problem.

I hadn't thought about an NTC and the relay--that may be a great compromise.

High current transformers often benefit from a soft start circuit. ...

For test equipment power supply it is always nice to have voltage and current meters built into the circuit. ...

And for potential lethal voltage equipment, a good indicator lamp whether the secondary circuit is powered or not is an important feature.

All of these things are in the updated schematic; can you be more clear about what you think is missing?

Ah my mistake I kept seeing German flags while reading and thought wtf. I'm kind of used to seeing 240V ones on youtube and even in the US. My own is 0-280V but can understand it adds a fair bit of expense and you need to source different parts.

No worries. I would consider it, as the isolation transformer has dual secondaries. It would mean sourcing another Variac, however, and that's the part that was free. I could certainly do that, but I'm not sure I will need 220VAC often enough to justify it. These may be famous last words, but it shouldn't be too hard to retrofit a new variac in the future if necessary. In that case, I might prefer just using/building a separate step-up transformer.

I built something similar many years ago. No doubt it suffers from all manner of design flaws, but I did build it with step-up capability. This has proven useful for testing the 220 VAC setting on numerous things, and for testing wide-range input switchers.

See the reasons above. Tl;dr: it would be nice, but I believe it's more effort and cost than benefit at this point.

[calzap]

Nice project.  I’m currently constructing a variable low-voltage AC power source along the same lines.  Link is  https://www.eevblog.com/forum/projects/low-voltage-ac-power-source-design/msg5561405/#msg5561405 .
It’s 40 VAC max at 6 A output.  I’ll probably never need anything close to 6 A, but that’s the rating of the Triad transformer and Powerstat.

If you’re concerned that something might go wrong inside that would re-reference the output to ground, you could run the output through a GFCI (aka RCD) outlet or breaker.  I converted a Tripp Lite noise isolation transformer to a galvanic isolation transformer.  Among other things, the outlets on the back needed to be changed, so I used a GFCI duplex that was on hand.

Mike