Lamp limiter plus Variac tip
 

I have three useful devices for working on live gear: isolating transformer, variac and lamp-limiter.
Each has a 13A plug and socket so can be used individually or any combination chained in series.
The tip? If using a variac and a lamp-limiter together, ensure the plugs and sockets are connected so that the lamp-limiter is nearest the device under test.
I got my plugs mixed when powering up a suspect SMPS and had a little puff of smoke from my current meter as I wound up the variac from zero (no limiter glow).
Only 4 volts required to put 200mA through a low resistance test load in that configuration instead 65 volts with the variac -> limiter arrangement.
Obvious?

Re: Lamp limiter plus Variac tip
 

"By George" that's a good tip. I don't think it's been mentioned before but perhaps it's because most people just use one or the other:shrug: [I'm leaving out the Isolating Transformer]. I confess that I haven't yet built a proper Lamp Limiter [tending to just hook up a bulb in series] but I really should!

Dave

Re: Lamp limiter plus Variac tip
 

I have a similar arrangement in my workshop, with isolating transformer feeding variac followed by lamp limiter. But to be honest I leave an old Avometer in circuit switched to the 1-amp AC range, and find this much more sensitive than the lamp. Whilst winding up the variac, it is easy to watch the AC current drawn as the voltage is increased. It's also possible to measure the VA input quite accurately and compare it with the rating plate data for the device under test.

This can be particularly useful when testing devices like SMPSs which don't have a linear voltage-to-current characteristic.

Re: Lamp limiter plus Variac tip
 

If using a Variac, it's definitely worthwhile putting a fuse in series with the OUTPUT!

Driving a faulty, short circuited load, only a few volts will cause umpteen amps to flow, enough to burn out the Variac brush (and maybe a current meter and wiring, as you have found), while still drawing only 100mA or so from the mains, thanks to transformer action.

Re: Lamp limiter plus Variac tip
 

Yes, I can see that. In my case I'm relatively lucky, in that my isolating transformer is actually a 160VA CVT, which saturates at much over half an amp and the output voltage at the isolated secondary winding collapses.

Such devices were commonplace and expensive in the early days of desktop computers. I was fortunate enough to be given mine!

Re: Lamp limiter plus Variac tip
 

Good tip! I always check very carefully before throwing the switch that everything is as it should be, I have an isolating TX in the chain too, which is a good job I do what with some of the lash-ups that I’ve been working on!

My Variac was an uncased one, so I built it into a wooden box with a voltmeter and socket on the front, and I also added fuses to both input and output, I had plenty of panel mount fuse holders so I thought why not!

Regards
Lloyd

Re: Lamp limiter plus Variac tip
 

I use an isolation transformer, variac and lamp limiter.
But the Isolation transformer is connected only to my service equipment and soldering iron.
I prefer to rely on the 30mA earth trip also connected to my "test" socket, where I connect the radio under repair, to safely disconnect the supply should I touch something I shouldn't have in the radio.
The trouble with isolation transformers is they don't protect you if you get a shock from a live point to chassis in an AC/DC set.
I found this out the hard way. The sustained shock I got cauterised my skin so deep it took more than 6 months to heal, I still have the scar.
As per the excellent advice in the OP my lamp limiter is immediately before the "test" socket which can be quickly turned off with a double pole isolation switch. The lamp limiter also has a bypass switch.
I am not pretending this is the best practice, so copy at your own risk.
Maybe if you are a qualified person you may like to comment on the arrangement.
thanks
Mike

Re: Lamp limiter plus Variac tip
 

Completely agree with Mike on this. I feel much happier trying to protect myself with an RCD rather than an isolation transformer when firing up uncertain equipment for the first time. My variac and lamp limiter (switchable) are all in the same box. There are sockets for connecting meters and there is a sensibly rated output fuse. All this just makes it more difficult (but not impossible!) to make a mistake.

Alan

Re: Lamp limiter plus Variac tip
 

Where's the live on the secondary of an isolating transformer? If you connect yourself between HT and chassis when the device is powered by via an isolating transformer you'll get a shock, likewise if its powered via an RCD only, an RCD is designed to trip if there's a sufficient difference in current flow between the live and neutral downstream of it such as that provided by a current path to earth.

Lawrence.

Re: Lamp limiter plus Variac tip
 

They are actually both live coming out of an isolation transformer, ie. you don't have a Neutral ;)
If one (either) side happens to be connected to chassis you will always get a belt if you happen to simultaneously contact both chassis and the supply side, hence the one hand in your pocket rule.

Ps. with an isolation transformer you can't get a L to E shock, unless you inadvertently happen to Earth the chassis,

Re: Lamp limiter plus Variac tip
 

Even the 'one hand rule' wouldn't have saved Mike from a nasty burn as his hand must have made contact with two nearby points at different potentials. Furthermore, using an isolation transformer negates the ability of an RCD to detect earth leakage. After all this an RCD feature which can and does save lives.


I'd recommend this article by Rod Elliott of ESP to anyone who might be unfamiliar with the pros and cons of using an isolation transformer:


http://sound.whsites.net/articles/iso-xfmr.htm


Alan

Re: Lamp limiter plus Variac tip
 

Hi Alan, Electrical separation (isolation transformer) and RCD are two different protective measures, there is little point in using an RCD with a fully floating supply as it won't detect an earth fault because there is no potential difference wrt to Earth.
If you happen to come into contact with two current carrying conductors at differing potentials from the same source, or commonly connected whether via earth, chassis or other means then you will always get a belt, regardless of *any * protective measure applied.

I am going to leave it at that because we have had many, many, sometimes heated discussions over the years with the inevitable thread closure that usually follows. I will respectfully bow out now.:thumbsup:

Re: Lamp limiter plus Variac tip
 

From that article:

"The only time you will get an electric shock is if you make contact with the live mains - everything else is 'safe'. Should that happen, the safety switch operates and saves you from further harm"

??

Lawrence.

Re: Lamp limiter plus Variac tip
 

Exactly the point and so important to understand the distinctions.

Alan

PS This comment refers to Red to black's most recent post.

Re: Lamp limiter plus Variac tip
 

All a question of context I think Lawrence.


Alan

Re: Lamp limiter plus Variac tip
 

A thought following from that. Choose a mains isolating transformer with 115-v. / 0v / 115v. secondary. Route the two phases of the 115-v. through an RCD to a 3-pin socket (one phase to 'L', one phase to 'N',) with the 0v. to its 'E' pin. That 0v. is not connected to supply earth. That 'earth' wire connects to metalwork of the load. We then have a balanced bi-phase supply of 230-v. for the load with RCD protection.

Al.

Re: Lamp limiter plus Variac tip
 

But would 115V drive sufficient current through you (under normal, dry conditions) as to trip the RCD?

Re: Lamp limiter plus Variac tip
 

On the basis that 'It's the volts that jolts but it's the mills that kills!", if the current is high enough, then the RCD is doing its job. OTOH, if the current isn't high enough, then the current through you - although unpleasant - will not be of a sufficient magnitude to kill you - since the ultimate function of an RCD is to prevent that result.
Or have I over-looked something in all that? :shrug:

Perhaps I'll do an experimental lash-up to investigate . . . .

Al.

Re: Lamp limiter plus Variac tip
 

Al! Don't do it! The Forum is important, but not that important! We'd all love to know if it works, but we'd hate to lose you in the process...

The authorities have decreed that currents under 30mA for a few milliseconds are sufficiently unlikely to be fatal as to be not worth worrying about. So I would agree with the notion that if it's too low to kill you, it won't trip, and vice versa.

Re: Lamp limiter plus Variac tip
 

Don't do it using wet fingers! :o