Foam Cutter Variac w/ Isolation?

Has anyone ever heard of a variac with built-in isolation (I assume just a 1:1 output transformer)?

RP Electronics - Test Instruments , Power Supplies , Inverters , Electronic Project Kits and more!

I just find it a little peculiar since it is the only one of the 4 here that you can find at various places around the web:
RP Electronics - Test Instruments , Power Supplies , Inverters , Electronic Project Kits and more!

And nowhere else does it talk about isolation for any other variac, anywhere else.

THere also seems to be a doubling of price for the 20A variac too. The price of ~$200 is what most places seem to have it for, but for some reason this one place has it for half as much. I am not sure what is up with that.
**broken link removed**

The nearest I have seen is a model train power transformer, at least an antique one; being isolated and with a 'speed' wiper on the secondary windings.

Seems about right to me, for price that is for the 2KV one.
MPJA - Power Supply, Power Supplies, Security Cameras, LCD, Fans, Etc. sells a unit that looks remarkarbly like that for the same price. The 300VA units are either overly marked up, overbuilt (or properly built for abuse) or contain more components not made in China.

Doubt you're going to find one with a built in isolation transformer it wouldn't be a Variac then. Using a 1:1 transformer is going to just about double the price.

Why are you trying to use a variac for a foam cutter anyways, those are easier to do using straight DC and a simple PWM module.
The types of variacs listed are general purpose devices that have a wide range of uses.

Sceadwian said:

Seems about right to me, for price that is for the 2KV one.
MPJA - Power Supply, Power Supplies, Security Cameras, LCD, Fans, Etc. sells a unit that looks remarkarbly like that for the same price. The 300VA units are either overly marked up, overbuilt (or properly built for abuse) or contain more components not made in China.

Doubt you're going to find one with a built in isolation transformer it wouldn't be a Variac then. Using a 1:1 transformer is going to just about double the price.

Why are you trying to use a variac for a foam cutter anyways, those are easier to do using straight DC and a simple PWM module.
The types of variacs listed are general purpose devices that have a wide range of uses.

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I had figured that with a variac I could find other uses for it other than just the foam cutter. I was roaming the web, however, and was about thinking of finally getting around to getting a variable DC power supply to run the foam cutter and for typical bench uses since that seems most cost effective with everything rolled up into one supply.

But I started thinking and went downstairs to look at the one power supply I do own. It's a high current AC-DC supply I use to run my battery charger. I stared it at for a bit before getting suspicous that it was actually a linear supply from it's appearance (I had always assumed it was a switching supply, but I wasn't as knowledgable about power supplies when I bought it as I am now). To my suprise and horror, the manual actually provides a schematic confirming it's a linear supply. Really wasteful for a high current supply that does nothing but charge batteries. I don't do radio work so I don't need a high current linear supply.

I guess I need to find a variable supply for bench testing and foam cutting, and a high current switching supply for the battery charger or a single variable high current supply. Anyone in the market for a used 13.8V, 18A linear supply?

There's nothing wrong with linear supplies they're much lower noise than switchmodes, would be good for any audio apps, and the voltage is nearly perfect for simulating a cars electrical systems (sans noise) If the transformers downrated voltage is designed properly to be just above the dropout of the linear regulator then it's not going to be as wasteful as you might think. Maybe another 10-20% loss, I would hardly call that horrible for a nice supply. You're only talking about 10-20 watts or so max wasted, for an extra decimal place in linearity and exponential drop in noise.

lol I was actually hoping for a noisy power supply since I'd rather have noise problems show up on the bench rather than, say, in the air. There is a point I don't quite understand, though, however. In the suprisingly short but detaield manual it says efficiency is a minimum of 60% at max load which isn't so bad I guess. How efficient are bench switching supplies? that 80%-90% figure seems to be what small switching supplies (like those in embedded systems) like to linger around at. If that still holds true for large switching bench supplies then that's aboug 50W-75W difference at max load.

I am curious why the minimum efficiency is not significantly higher than 60% though, because they do use a transformer ahead of the linear regulator. You'd think they would be able to size it a lot closer knowing mains voltage and it's tolerance, as well as the output voltage since it's fixed. Unless I'm missing something of course.

I was looking at this one. Too bad we cant see into the future or else I would have just bought this one right off the bat:
**broken link removed**
It would have futureproofed it for higher capacity battery chargers, and allowed it to be used for PCB testing, foam cutting, and whatever else... It was probably a case of "I'd save money in the long run if I spent a lot of money now, but I don't have that money now!"

I also wouldn't have bought that ridicuously expensive Neumotor so I could buy this thing more easily. Hmmm...what to do. I am very long overdue for a low voltage variable bench supply. I've been using batteries since that's what my stuff is supposed to run off anyways.

Nothing wrong with that unit, except the 300+ dollar cost, you'll prolly pay another 20-50 for shipping. You can lament all you want about stuff you have now you bought with a differen purpose in mind but now know better about, all you can do is live and learn =>
I wouldn't mind the supply you have, but the shipping cost is going to be more than it's actually worth to me =)

dknguyen said:

lol I was actually hoping for a noisy power supply since I'd rather have noise problems show up on the bench rather than, say, in the air. There is a point I don't quite understand, though, however. In the suprisingly short but detaield manual it says efficiency is a minimum of 60% at max load which isn't so bad I guess. .

I am curious why the minimum efficiency is not significantly higher than 60% though, because they do use a transformer ahead of the linear regulator. You'd think they would be able to size it a lot closer knowing mains voltage and it's tolerance, as well as the output voltage since it's fixed. Unless I'm missing something of course.

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The hint might be that figure is given at full load. The power supply might be fixed voltage, but for over a wide current load. To insure sufficient voltage at maximum current, the PS will use higher head voltage than if it were only a low current one. For example, if your supply uses 4V head volage @ 10 amps, that's 40W that goes into thin air. If it delivers 65W full load, then the effeciency would be about right.

Sceadwian said:

Nothing wrong with that unit, except the 300+ dollar cost, you'll prolly pay another 20-50 for shipping. You can lament all you want about stuff you have now you bought with a differen purpose in mind but now know better about, all you can do is live and learn =>
I wouldn't mind the supply you have, but the shipping cost is going to be more than it's actually worth to me =)

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Yeah. I can get it locally for $350CAD. I don't think the one online place that has it for $279USD can beat that by very much unless shipping was free (which they say it is, but I'm not in the US, but it also doesn't say if that free shippign only applies to the US but it probably does lol).

Yeh, I got so much unused crap at home lol. THe worst are things that have specialized uses or things you bought for use with other things that you changed your mind about using.

If you want it cheap, use a dimmer to power a Weller 'pistol' soldering gun and use the heating tip terminals for the foam cutting hot wire. You may already have all among your things... $0?

Be aware that isolation is not always isolation. Clicky.

Mike.

dk, take a look at www.mpja.com's bench supplies, they have a decent range of variable bench supplies, the one you linked is only 3-15 volts, I have a feeling that you're going to outgrow that rather quickly. I saw a programmable one (constant voltage current or power) that's 0-40 volts (decent range) at 5 amps for 279.00 They ship to Canada but you have to phone the order in. The current your 3-15v one supplies is nice but generally variable bench supplies don't have a monstrous current rating because they're for testing voltage not really supplying huge amounts of power. That being said 40 volts at 5 amps is 200 watts of power into an 8ohm load, more than enough for just about any bench need.

My foam cutter uses a 15vdc 2amp big old stacked-plate plastic cased "wall wart" appliance adapter along with a light dimmer switch both very common items at goodwill for about a buck each. I don't do elaborate stuff with it. I set the arbor up to use the thinnest gauge steel braid fishing leaders from wall mart since they seem to last forever and handle heat well without stretching/breaking. They're also standard sized with looped ends, so they're easy to replace. I redial the dimmer to suiit a few experimental scrap cuts in the thickness of foam I'm cutting.

Terribly cheap, but effective. Lots of one dollar laptop/printer smps at goodwill too. My box--o-transformers is so big now that I just stopped buying anymore.

Yeah, I'm aware of the rarity of high current power supplies with variable voltage. The reason I was searching for one was so it could also double for my battery charger to replace my current one (and those do draw 20-40A at lead acid voltages.)

THe only thing missing from the high current supply I was looking at was current limiting which is useful for regular electronics testing (not so useful for charging batteries). But Ill have a look around MPJA. THey seem to be well priced.

Or perhaps I'm not seeing something. What particular aspect do you think I would outgrow? The max voltage range? All of it is under 15V with the exception of motor drives which can exceed that. But those systems also draw much more more current than those higher voltage power supplies can provide anyways (definately much more than 5A). Almost always hovering around 10-20A. ANd on propeller drives, easily exceeding 30A or 40A. Best just to use batteries there.

Of course, there's always this thing for hot wire cutting on the website you provided:
https://www.mpja.com/prodinfo.asp?number=9902+PS
And get the BK-1692 to power my battery charger, hot wire cutter, and bench testing at a later date when I'm more financially sound.

I guess it depends on what you're working with dk, printers use 24 volts frequently never more than an amp or so for motors.

I cut lots of nylon rope for rigging a sailboat. I use a hot wire stretched across the terminals of my pistol type soldering iron, where the heater element usually goes. I just stick a couple copper rods in the iron, and braze the wire to those rods. I also bend the copper rods to open up the ends to a couple inches. It works great, and I imagine it would work for small foam parts.