The Impecunious Engineer

[blowlampParticipant @blowlamp]

Ian.

In a similar vein, I was taken aback the other day when I saw that Ty-phoo QT has now been relabled as “Instant black tea with whitener”.

 

Martin.

[Sub MandrelParticipant @submandrel]

> Ty-phoo QT

 

Does that mean it doesn’t work as brazing pickle any more?

 

Good tip i was given for a welder that sets off trips on switch on – use it at the end of a long extension lead, it helps limit the inrush current. Make sure the lead is unwound so it doesn’t overheat. Works every time with my welder.

 

Neil

[John OlsenParticipant @johnolsen79199]

There is an interesting thing with inrush current, the actual size of the current pulse depends on the point in the cycle at which the switch makes contact. This is why you can get the situation where a transformer will just sometimes blow the fuse or trip the breaker on startup. (fuse aging can also affect this of course. )

 

One of our lecturers went through this at university, it is a good exercise in solving differential equations. However, it is now thirty years since I have had to solve one of those, so I will refrain from attempting it here.

regards

John

[Martin WParticipant @martinw]

Hi

 

Another method is to use a ‘Zero Crossing Switch’, these as their name implies switch when the mains voltage passes through or near to 0V which solves the current inrush problem but they are usually semiconductor based devices requiring a low voltage DC voltage to act as the switch so require a power supply to operate. That said I suspect there may well be self contained units on the market by now.

 

Cheers

 

Martin

[ChrisHParticipant @chrish]

Hi Neil,

 

Thanks for the tip on using a long extension lead on the welder – I have a long meaty lead, I will try that.  If it works then that would be a lot cheaper than a new D type MCB!

 

Thinking about it, the lead on the welder is quite short and when used in the past it has always been on an extension lead – now in a new (to us!) house it is the first time I have been able to plug it into a socket direct, and the socket is quite close to the MCB.  

 

This tip should go into the “Hints and Tips” thread! 

Edited By ChrisH on 12/11/2010 10:02:52

[Nicholas FarrParticipant @nicholasfarr14254]

Hi,
    “Zero Crossing Switch” or “Zero Voltage Switch” as it is also refered to, for AC would probaly involve the use of Triacs. Because of the high amperage needed to strike up an arc each time as well as the initial turn on inrush, these would have to have a higher rating to cope with the demands. I would think that such triacs would be much more expensive than a suitable MCB. With my large welder I have to limit the amount of weld current that I use, because the strike up amperage at the top end is as high if not higher than the meter rating. This is served by a 30A fuse rather than an MCB and through a deddicated 32A socket/plug. I’ve had no problem wiyh this. Needless to say I can’t put my cooker on and then go and get on with a bit of welding while it is warming up.

 

Regards Nick.

[Martin WParticipant @martinw]

Hi Nick

 

The units I have looked at run at 30A rms, can withstand a peak of 630A for about 20ms and cost around £40 but of course this doesn’t include a DC supply to control the unit. As you say it is probably cheaper to replace the MCB provided that one can get one that will fit the consumer unit.

 

Cheers

 

 

Martin

[Les Jones 1Participant @lesjones1]

Hi Chris,

                  One thing you could try is to have a suitable value high power resistor in series with the supply to the welder and a high power relay (contactor) with the coil connected in parallel with the welder. Connect the contacts on the relay across the resistor. When first switched on there will be a significant voltage drop across the resistor so the relay will not pull in. When the initial current surge has gone away the relay will have enough voltage across the coil to pull in. The resistor will then be shorted out. The resistor needed would need to be between about 2 an 5 ohms.  You would probably have to make a suitable resistor from a long length of wire coiled up.

Les.

[Sub MandrelParticipant @submandrel]

Money saver alert

 

For us sad old scrotes who still use imperial, Screwfix are selling  a usable set of 1/16″ to 1/4″ bits by 1/64″ half of already low price, about £2.50.

 

The catalogue does not list the reduction, it just came through when I saw the bill, so check first.

 

Useful havinga set or two as spares, even if you have better ones.

 

Neil

[ChrisHParticipant @chrish]

At last, back in the UK and a rare cold visit to the workshop, I tried the trick of using a long meaty extension lead to connect the welder to the socket that kept tripping and bingo, worked every time!  So that is that problem solved and a big thank you to Neil for the tip.

 

Chris 

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