> Thanks for the insight, but the filtering does not sound trivial to me. As it > appears, MOT spot welders can also be made without any electronics:
Yeah, you don't need any electronics, you can just control the primary with a foot switch. > How much current is actually flowing in the primary circuit? As it happens, MOTs don't really have sufficient inductance to current limit on their rated line voltage. If you remove both secondaries entirely, and hook up the primary, the transformer will sit there and draw 400-500 watts (I'm not giving the primary current, as that depends on the mains voltage in your country). This is because they're optimized for cheapness, not energy efficency, and run at the edge of saturation: http://wiki.4hv.org/index.php/Microwave_oven_transformer This will vary depending on whether the magnetic shunts are removed (general wisdom seems to be to leave them in). When you're driving a secondary load, primary current will tend to rise, as you're dissipating more power. Most MOTs can deliver 700-1200 watts into the magnetron, so figure a primary current equivalent to 1800 watts or so max, possibly more as it's not a resistive load, so VA can be higher than the wattage, due to phase shift (one of the functions of the magnetic shunts is to try to reduce the phase shift, when the MOT is operating into its original load - with a spot welder load, things will be different). Many people will have some sort of lashup to reduce the primary current. These are generally one of five varieties - adding a ballast resistance (like a toaster or space heater) in series, adding some inductance (such as another MOT, often with a shorted secondary), using a bucking transformer to reduce the incoming mains voltage, using a bucking winding on the MOT itself to do the same thing, or using a 220V MOT on 120V mains. My favourite is the bucking winding on the same transformer (or, you can think of it as adding some turns to the primary). It is my favourite approach because it doesn't require additional parts, just more wire. Simply add several turns of wire sufficent to handle the primary current to the core, and wire it in series with the primary, observing phasing. I'd add the additional bucking winding first, keeping it compact, then wind the heavy secondary turns. Then I'd hook up the bucking winding and primary in series, and drive the secondary with a few volts. Then swap the leads to the bucking winding and try again. Use the hookup that gives the most volts out of the primary + bucking windings while driving the secondary. This will nicely reduce your power consumption, and keep your core from getting quite so hot. I realized I answered your one-sentence question with several paragraphs of blather, but when working with hand built mains operated equipment, I really want people to understand what they're getting in to. - John -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
