Variac Fuse

[Polybus]

Howdy,

I just got myself a PowerTech Variac SRV-5 500VA - got it second hand in a garage sale for pretty much nothing.

I put plugged it in and put my multimeter on it and it shows good voltage from about 0 to 260v - which pleased me.

Then while I was sitting there congratulating myself on a bargain, I noticed the fuse and fuse holder were missing from the front of the unit.

Hmmmmm -  how can it be working without the fuse I thought?

Maybe its just dodgy design and it wont be able to power anything - but to my surprise, I plugged it in to the wall and plugged in a 60w light bulb to the Variac output and it worked!!! With no fuse in.
 
What gives?? 

I would have thought the fuse was there to protect something.....(anything!!)....but this thing seems to work the same whether the fuse is plugged in or not.

Anyone care to comment - why is this happening??

Is the fuse there for something that I'm not understanding?

At this stage I am assuming someone has gone inside the unit and hacked it somehow to remove the fuse from the circuit - but why on earth would anyone do that?? I've heard these things blow fuses a lot - but surely a slow blow fuse would sort that out (the new units are in fact supplied with a slow blow fuse) - hard wiring the fuse out of the circuit seems a bit silly (if that is what has happened).

I haven't looked inside yet - if someone here is able to explain whats going on, perhaps I will not need to pull it apart - but if someone has got in and hacked it - I may be forced to open it up and put things back where they belong.

Thanks for your thoughts.

Polybus

[Andy Watson]

At this stage I am assuming someone has gone inside the unit and hacked it somehow to remove the fuse from the circuit

I often take new equipment apart, or at least lift the lid before plugging it in for the first time. You have a second hand device of dubious provenance and signs that it's been "got at"! I think an internal inspection is overdue.

[Polybus]

Probably good advice - Thanks Andy.

The only reason I hesitated is because I am not familiar with these devices and thought that perhaps that was how they are supposed to behave.

My electronics knowledge is such that the first thing I always doubt is myself!! 

In the absence of anyone explaining the curious behaviour - I will open it up and have a look......

Thanks Andy.

[Berni]

In that case just post some photos of the insides and people will quickly point out anything remotely unsafe.

[Polybus]

So I've pulled it apart - and I *think* I know why it's displaying the described behaviour.

Its got a capacitor across the fuse and the input leads.

So it allows the AC signal through the capacitor even when the fuse is not in place.

Looking around - it doesn't look like anybody has been playing around in there - not since the factory anyway....

Here are a few photos.

I've tried to show the capacitor I am referring to.

What are your thoughts? Does this explain the behaviour I described in the initial post?


Is that capacitor just supposed to smooth out the AC a little?

It seems a bit dangerous to me if it allows the unit to be used with no fuse.

Polybus

[Andy Watson]

It's not a cap, it's a polyfuse. Can you read the markings on it?

[Polybus]

Markings:

72
XF110

On the other side:
PNFD
CHINA


A PolyFuse - that might explain a few things - from the reading I have been doing it looks like a lot of people have trouble with these things blowing fuses all the time - so perhaps the previous owner got sick of buying tons of fuses and decided to solder in his own resettable fuse.....maybe.....

At least now I'm not so confused as to why its working without the fuse in place.

Polybus.

[Berni]

Yeah variacs tend to be used to power up weird or broken devices and since there is no current limiting that can blow the fuse quickly. Especially if the output is shorted then turning the variac up just a little bit will send tons of current trough the short and blow the fuse.

Id say its a reasonably safe was to do it. The poly fuse looks like its a good distance away from anything else so if it gets really hot in a short circuit it won't melt something or catch something on fire. As long as the voltmeter  is after the poly fuse you will see the voltage fall off when it goes into overload. Just make sure you don't touch things without turning it off first. A tripped poly fuse acts like a few KOhm resistor that just limits the current, so the load might appear its off until the fuse is let to cool off but there is still plenty of current available to give a good shock if the load disappears and stops sinking the residual current.

[james_s]

I'm tempted to do that hack on my variac, I blow the 10A fuses just often enough to be annoying, something that auto-resets would be convenient. I suppose a circuit breaker would be even nicer.

[Hydron]

I've chucked small circuit breakers into a couple of Variacs that I've modded (in the output wire). Makes it much less likely to cook things, though by no means bullet-proof (I blew up a brush once despite having a fuse in series with a large fault current caused by forgetting to turn down the output before throwing the switch).

I've also added panel meters - these ones are nice as they do true-rms current, voltage and power/VA: https://www.ebay.co.uk/itm/5-in-1-Function-Voltmeter-Ammeter-LED-Digital-Volt-Amp-Meter-AC80-300V-100A/253002025813 (not recommending the seller, it's just the first link i found).

The "80-300V AC" type panel meters do suffer from not being able to work when output voltage is below 80V - they use the same connection for both measured voltage and power (via capacitor dropper circuit) and drop out at low voltages. This can be overcome by modding the meter to separate the phase connections for measurement and power - with the latter powered from the variac input supply and the former by the output they seem to measure fine down to 0V input. That's a bit more of an advanced mod than adding a breaker or something though!

[Alex P]

Here are a few photos.

Jolly good ones!

[MarkL]

That Polyfuse is not rated for AC mains protection.  It's only rated for 72V max and it may not perform the overload protection properly (likely, especially at 240VAC) and/or end up being a fire hazard.

1.1A is also the wrong current rating if you intend to get 500VA out of the variac, but that's besides the point.

I would remove the Polyfuse and put in a regular fuse.  Or do the circuit breaker thing; that sounds like a good idea.