Spot Welder

The latest MEW boasts a design for a spot welder. I haven't got a copy of the magazine yet, but I'm guessing this might be using a microwave oven transformer with the HV secondary removed, and replaced with a couple of turns of heavy duty cable to make a low voltage, high current output.

What size microwave are we talking, 700W? Or would an 800W one be needed?

I should state here that there is a great danger lurking inside every microwave. The magnetron is driven by High DC Voltage, and there is a high voltage capacitor that is charged to something in th eregion of 150kV. This will take no prisoners. It will not just hurt, it will kill. This has to be safely discharged before venturing any further, even if the microwave has been turned off for a long time.

There are various procedures for doing this online. The main thing to note is to have a good stout pair of rubber gloves, insulated tools and a pair of goggles.

Hello Carl,

The magazine article uses the transformer from an 800W microwave and 5 turns of 25 sq. mm cable.

Your caution over the HV capacitor is timely but it might be helpful to tell those of us who haven't seen one what it looks like and how to discharge it. I have blown lumps out of screwdrivers discharging ordinary motor capacitors, this beast sounds rather more important.

Regards

Brian

I think that the voltage from the transformer and on the capacitor is more in the region 1.5 kV not 150 kV. This is still very much a lethal voltage so care is required. After a few days of not being used the capacitor will be discharged in my experience but I still wear thick gloves and short out the capacitor before dismantling, just to be sure.

Mike

Google 'microwave oven magnetron capacitor' for plenty of pictures; generally around 1µF and the operating voltage is more like 2kV AC. Still enough to give you a belt though!

Andrew

Why do people so often use the H&S to frighten people, high voltage caps have been around for years. Since 1972 I've involved with amateur built high voltage linear amplifiers and have never heard of any one receiving a fatal shock from the high voltage power supplier usually working in the 2 KV area similar to the voltage of a micro wave cooker. Not to say some haven't had a shock and are still with us. How did the TV repair men go on when working in the area of the CRO tube ? , as they said we got a "belt" and used good sense also many of these high voltage caps have built in discharge resistors. Good sense should prevail at all times,. If in doubt as some one has said leave the caps for a week or so, then short out the terminals.One can easily dismantle a micro wave without toughing the caps at all snip one wire at a time and leave on the bench, then remove the transformer.Ted

can you buy single digital copies as wouldn't  mind having article.

oh and dismantled loads of microves mainly for the magnets to use when welding with no probs

Edited By naughtyboy on 29/05/2015 19:32:40

Hands up straight away, I forgot the decimal point. 1.5kV it is. Thanks for that Michael. Thank you too Brian, 800W.

"Why do people so often use the H&S to frighten people, high voltage caps have been around for years. Since 1972 I've involved with amateur built high voltage linear amplifiers and have never heard of any one receiving a fatal shock from the high voltage power supplier usually working in the 2 KV area similar to the voltage of a micro wave cooker. Not to say some haven't had a shock and are still with us. How did the TV repair men go on when working in the area of the CRO tube ? , as they said we got a "belt" and used good sense also many of these high voltage caps have built in discharge resistors. Good sense should prevail at all times,. If in doubt as some one has said leave the caps for a week or so, then short out the terminals.One can easily dismantle a micro wave without toughing the caps at all snip one wire at a time and leave on the bench, then remove the transformer.Ted"

Right. This is why I'm always loath to post here. I have the odd lapse every so often though, and think that maybe the human race ain't so bad after all. Unfortunately someone here always proves me wrong. I was not trying to use H&S to frighten anybody. In my job, I regularly work with 3kV AC and 600VDC driving large electric motors. I have a healthy respect for these potentials. I was merely pointing out the degree of hazard, as I think anyone with any sort of a responsible attitude should. I most certainly would not be blase about the possibility of obtaining a "belt" from any piece of equipment and I would not write so irresponsibly on a public forum, where there are likely to be inexperienced or possibly young readers, either.

There are plenty of tutorials online that show how to safely discharge these capacitors and show what they look like. I was merely pointing out that the hazard exists and therefore needs to be treated with an appropriate level of respect.

Add Bosch battery lawnmower chargers to the list. Open them up, and there is a capacitor just waiting to discharge through the unwary. It burnt the end off my screwdriver - perhaps that is why they say "No serviceable parts inside" !!

Where's the spot welder?

Ian S C

Damn 18V Makita drill took the tip off one of my multimeter probes the other day... wasn't even my drill.

.

Here

"It's the Volts that jolts, but the Mils that kills"

Prewar TVs used a separate mains power supply for the EHT to the tube. VERY dangerous! I have had the odd belt from a CRT, but since these used flyback EHT, very little energy was available. (Ditto for OLD coil ignition systems on engines, BUT beware of the more modern ones. They do pack a dangerous punch).

The current available is driven by the voltage and resistance into which it is delivered, and by the power output of the supply. The TV was low power, but the same can't be said of a 400Mw Power station which ultimately feeds the mains. I have survived a 240 volt shock, fortunately standing on a 5Kv proof floor! If it had been a puddle, you might not have been bored by reading this.

In the Electronics and Vibration Lab we used to check our body resistance, every Monday with the AVO 7. If we were under the weather, with say, a cold, we read 120K instead of 140 - 150Kohms

A 12 volt car battery will not kill you, (Your body resistance of over 100K Ohms is high enough to limit the current to less than 1 MilliAmp), but WILL burn hole through a screwdriver, or lumps out of a spanner, because it can deliver 300 - 400 Amps when short circuited!

Treat Capacitors with similar respect. They can hold a high voltage charge for some time, and deliver, for a short time, a high current. Those Coulombs (Amps x Milliseconds) can be enough to kill!

An electronic flash for a camera can deliver 100 watt seconds of energy, in 1/1000 of second. The tube needs 500 volts to strike.so the current is about 200 MilliAmps, delivered in 1 Millisecond, or less. 200 Coulombs is LOT of energy into a human body. At worst, lethal, but if survivable certainly VERY unpleasant, and definitely non preferred.

Preferably, discharge the Capacitor with a high value resistor, say 1 Mohm, or more, to minimise the current, and as advised, wear rubber gloves and use insulated tools, in a dry location.

H T H

Howard

I'v seen the results of a spanner, a wedding ring, and a short across the termanals of a 12V battery, ring welded to spanner, one burnt finger, the bang of the contact drowned out by rather loud 4 letter exclamation.

Ian S C

.

Was it 'ouch' ?

MichaelG.

I just made a spot welder and had trouble making good welds, I tried more turns and then used two microwave transformers
with the primary's in parallel and even more turns on the secondary's with only a little better welds. One of my main problems was I was trying to
use some smaller diameter wire i got for free in the secondary and was trying to increase voltage to get more power. I ended up buying the biggest diameter wire that I could fit into the transformer. with a total of 3turns on the 2 transformers I then had a new problem, I had to much power I made a little timer circuit to set the on time of the spot welder and now I can weld 0.2mm nickle to the POS side of the A123 battery in just under 100mS or less and the welds are bloody strong. I had a quick play using Tungsten for the electrodes but they just lighted up like a light bulb so went back to copper.