Switch on surge.

There may be an easier way..

Use a relay in series with a diode and a resistor, and with a fat electrolytic across the coil.

In fact, I suspect something like a triac would work in that mode, too.

But I still like the thermistor the best.

Less components, and possible to use a really big fat one!

The CL-80 from here looks about right

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current would be limited to about 5A, and at 2A resistance has dropped from a cold value of 47ohms to less than an ohm. The only issue is that these things will get very hot, and may be running at mains potential, so you need something like an earthed mesh box to pop it into.

I think I would be tempted to put it in the actual backing box of the switch itself.

One end into the switched live terminal, and the other crimped to the switche live wire. Then glue some insulation to any nearby part of the backing box in case it touches that.

Or, if it was not appropiate to use the backing box, use another backing box with a 4 terminal block in it, and mount that where it can get hot in peace!

Reply to
The Natural Philosopher
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I love your idea of moderate sized transmitters John ;-)

I remember reading in "Radio Communication" decades ago about a fellow Amateur whos transmitters' mains fuse would blow every night, but only if he was transmitting at the time. The thing he noticed was it blew with a strange regularity, each night a short time later than the previous night. He could draw a graph of his observations. I'll stop there and ask if you can see where I am going with this...

Reply to
Graham.

My pal actually found the installation instructions - and they suggest it might well need an MCB changed to a C type.

Have also had it confirmed the problem started when changing the CU from the original fuse one to one with MCBs. By looking at next door's CU - it's an estate which was built over a short period of time.

Reply to
Dave Plowman (News)

I have something similar made up to delay the switch-off of a heating pump, with an older boiler that doesn't sensibly control it - but I had enough space for small transformer. I wonder if you could use a high voltage DC relay, with directly rectified mains, a resistor and C - to just slow the relay closure down enough. Would all be rather nasty high voltage stuff though....

Charles F

Reply to
Charles Fearnley

I'd vote for the simple relay timer over the electronics too. A ordinary mains coil relay is fine on dc, just pick the series resistor (on the relay coil) to act as a suitable dropper for it, then add the C to give delay.

NT

Reply to
meow2222

Well you had to put this into context - the smaller power amps were 1 and 2kW models in 19" rack cases (that included things like antenna tuning units and matching units etc), and the large ones were things similar to the Voice of America 1MW HF transmitter that was sat 15 yards away from the one I was working on. That was quite a beast - the auto pre amp that drove it has a 250kW output and the coax feeder on the output was approx 1' in diameter!

For reference it looked like:

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with the covers off:

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pictured one is an old model prior to the control system upgrade I was working on. The basic amp was a 40 odd year old design. The lump glued to the top was a state of the art DSP based digital "drive" (posh HF transceiver)

Sounds like he had something that kicked in on a time switch, which combined with the Tx load was enough to blow the fuse. Each blow of the fuse resulting in the clock stopping, and hence subsequent switchings on of said load would be slightly later each time.

Reply to
John Rumm

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fuses are fine and can have advantages from time to time. Re-wireable are allowed, but there is no sane reason for fitting one.

Reply to
John Rumm

I only really looked for Hager - that's the CU fitted.

Reply to
Dave Plowman (News)

Oddly Hager was always my first choice for HRC carriers - however when I looked this time I could not see them.

I have one here feeding my garage/workshop. The existing cable was a bit smaller than I would have liked (but a relatively major pain to upgrade), so to get best usage on it, I put a 32A HRC at the head and use a 32A MCB on the power circuit for the workshop - which should hopefully discriminate on a fault.

Reply to
John Rumm

Dunno we used to work on 40 kW TV ones some years ago;!..

Who was that for?..

Humm dunno that they should be allowed so much "input power"

Reply to
tony sayer

No. It's a long time since I read the article but AFAICR it was something the electricity board or CEGB were putting on the supply to command street lighting (solar timing). They did it by somehow superimposing DC on the mains for a short time. Does that sound feasible? I'm sure it was something like, that and it took a lot of determined investigation to get an admission of what was going on. Apparently the fuse blew because the tranny was operating near saturation and the dc component was the last straw. He noticed increse in vibration from the transformer just before it blew its fuse.

I take Tony's point that he may have been pushing things too.

Reply to
Graham.

I think I follow, but it seems a bit bizarre. You are adding 12v to the mains by using a transformer. Primary connected to the mains conventionally, and secondary in series, and in phase with the feed to torroid via a reactive dropper? The configuration will certainly add 12v to the mains but I would hardly call your transformer a step-up transformer. I am wondering what make of transformer is speced to have its secondary floating at mains potential in a domestic installation?

Reply to
Graham.

The one in the picture - no idea - it was probably a lab model. The Navy use quite a number of them though (often the smaller 1 and 2kW jobbies on ship and those sort of things in shore stations)

They can be used anywhere you want a tunable narrowband transmitter[1] basically. Marconi sold them all over the place for a variety of applications. They were even used for some early trials in RADAR applications for the JORN system:

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actual JORN amp was 20kW and solid state - and used in banks of 28!)

[1] no use for frequency agile apps though - they take several seconds to tune.
Reply to
John Rumm

They still show on Hagers web site

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Reply to
ARWadsworth

yup

not sure it makes much difference what you call it :)

Its rare that they dont have sufficient insulation for that. Microwave tranformers dont, and probably some very old bobbinless ones.

NT

Reply to
meow2222

Ah, had a similar problem. I fitted a dimmer specified for use with transformers, and the problem went away...

Thomas Prufer

Reply to
Thomas Prufer

Snag is it's on a landing and two way switched - and each of the two way switches are 2 gang.

Reply to
Dave Plowman (News)

ARWadsworth coughed up some electrons that declared:

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Might be special order time down at TLC, who do have an account with Hager directly (almost obviously) and can, if asked nicely, actually get

*anything* from Hager's catalogue (I got them to get me some double pole RCBOs - try B&Q for that!)

Personally, I would try a type C breaker first - they're easy enough to get for Hager boards (I have a Hager, specifically because the massive choice of MCBs and RCBOs is excellent).

Cheers

Tim

Reply to
Tim S

Dave Plowman (News) coughed up some electrons that declared:

GET touch dimmers are OK with transformers (and CFLs)[1] that are designed to be dimmed - and have both soft start and remote control (n-way switching). Wiring wise, you need to present supply live and one remote control wire at all secondary points back to where the actual switch is. Is that an option?

Homebase sell the GET dimmers I'm talking about for less than most places if you wanted to try one.

Cheers

Tim

[1] Well, strictly, GET do another range of knob dimmers called "LV" - but the bloke at GET told me that the touch dimmers produce roughly the same dimming pattern on the mains in terms of how they chop the sine wave up as any other dimmer, so whilst they would not guarantee the touch dimmer with dimmable CFLs, in actual fact they seem fine when I tested them.
Reply to
Tim S

If it were my place I'd examine all the options - but the owners only want the fitting to work as it did before the CU was changed. And would, I think, had a deal of resistance to having to have a dimmer where they don't actually want one.

I'll try a C type MCB first - and if that doesn't work replace it with a fuse.

Reply to
Dave Plowman (News)

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