Modified microwave oven transformer

[boylesg]

Thanks to those who were helping me out with this. But this is what I have so far. This was just a quick test to make sure it didn't blow any fuses and it didn't.
I have wired those two primaries in series with that safety switch like device (8Amps) between them. I will have to secure all those loose wires and devise some sort of cover for it when it is in use.

The coloured insulated wires were from the power cords of the microwave ovens I extracted the transformers from so I should be OK. I plan to draw perhaps up to 5Amps at 84V AC from the secondaries and, judging by what others have said about this, that seems about right re power. I will stick some fuses on the secondary side as well to make sure that any slip ups don't result in a flaming transformer.

 

[betwixt]

It would make more sense to wire the MCB in the live feed rather than between the transformers. Electrically it works the same but the whole circuit goes dead if a fault occurs, at the moment, half of it is still live.

What are you trying to achieve here, the secondary windings of microwave oven transformers are generally in the KV region, not 84V.

Brian.

 

[boylesg]

I removed the HV secondaries from those MOTs and added my own secondaries with less loops than the existing primaries.

I made my modified secondaries from the primaries of two other MOTs from which I removed enough loops to give me 84V AC, with each operating briefly on its own.

I presume that if I also wire the two secondaries in series then I will get 42V + 42V = 84V (or close enough to this)

I understand what you mean by half setup remaining live if the circuit breaker goes. But the same thing would happen in the household fuse box if one of them blows? The idea of them is to simply break the circuit and eliminate the short.

If my added breaker blows then I would simply unplug the device and then none of it will be live.

Not familiar with an MCB but will google it - always like to learn new tricks.

 

[betwixt]

MCB (Miniature Circuit Breaker) is the resettable fuse you are already using!

Yes, you can add the transformer secondary voltages together as long as they are in phase with each other. If you reverse one of the windings the voltages subtract instead of adding so you get almost no voltage out.

Brian.

 

[boylesg]

Thanks Brian.....yes that did cross my mind last night about connecting the winding the right way around relative to each other. But no big deal - if I have it wrong then I can just reverse the connections on one the the primaries.

I guess if there is almost 0V on the primaries then I will get almost 0V on the secondaries, no matter how they are connected.

But I am reluctant to put my multiimeter across the mains voltage - I feel more comfortable puting it on the lower secondary voltage.

What exactly is the difference between these MCBs and the safety switches that are being used in new houses these days?

 

[betwixt]

What exactly is the difference between these MCBs and the safety switches that are being used in new houses these days?

They are different names for the same device. Modern buildings also incorporate RCCB (or ELCB) trips as well. An MCB opens when too much current flows through it, the other types look for differece between the line and neutral currents which would indicate a leakage path to some other circuit or ground.

I guess if there is almost 0V on the primaries then I will get almost 0V on the secondaries, no matter how they are connected.

Nothing in = nothing out. Hower, it doesn't matter whether you swap the primary wires or secondary wires on one of the transformers, it will have the same effect of reversing the phase. Don't swap primary AND secondary as that doesn't reverse the phase.

Brian.

 

[boylesg]

Oh - I am thinking of DC rather than AC. I mean I will have to open up one of the cores and flip the primary.

 

[d123]

Hi,

Unsure of reason(s) why, about mains voltage and multimeter, self or multimeter...? I see that as a laudable approach, don't get me wrong. Things I prefer not to touch, just in case, the multimeter gets attached on highest/relevant setting before turning on/plugging in whatever's being measured, that way a) I don't have to touch it, or accidentally short something placing the probes which slip or go to the wrong places and inadvertently kill another tester and component..., b) no need to stand on a rubber mat with one hand in the pocket (joke, but the hand method is important to know). Those hook-on tips are really useful, failing those, the terminal strip in the photo could be extended and the wires of interest jumpered to another set of terminals where you screw the tester probes in.

Is this for the SSTC?

Oh yeah, I think your question about the calculations, perhaps to a great degree, can be solved by applying nothing much more complex than delightful Ohm's Law principles to the different sections considering them in steady state, I think that's maybe in part but not entirely what CataM was explaining. And perhaps, but maybe not, SMPS formulas might provide the kind of formulas you are looking to understand. This is one: Power Stage Topology Reference Guide (slyu036), but you have to sign in for it, so I suppose I'm not meant to upload it here... Short one is **broken link removed**

 

[betwixt]

Oh - I am thinking of DC rather than AC. I mean I will have to open up one of the cores and flip the primary.

No, just swap either the primary wires OR the secondary wires (but not both!) of ONE transformer. They produce AC of course but you want the secondary windings to add rather than subtract from each other. If one goes positive while the other goes negative, the net effect is to get zero out, if they have the same polarity as each other the voltages add which is what you want.

Brian.

 

[boylesg]

The mulltimeter has a range up to 750V AC so it clearly can take 250V AC.

But I still feel nervous about puting the probes across the full mains voltage.

84V AC would probably kill me too if I got careless.....over 40V (or something like that) DC or AC is capable of killing people I believe.

Oh well a little fear with this sort of thing tends to keep one wary and safe so I will continue 'listening' to it.

As for my other post re square wave current calculations.....I got an answer here https://4hv.org/e107_plugins/forum/forum_viewtopic.php?179357.last with one of the Tesla coilers who are familiar with gate drive transformers and gate drivers IC's
The square wave is irrelevant, C6 is effectively 0 ohms and it is a simple ohms law proposition through the gate drive transformer.

I calculate the current going into the FET gates in Steve Ward's original schematic at 1.2A but that his FETs require only several tens of milliamps to overcome the gate capacitance and turn on. The FETs I have chose to use are about the same. So my replacement of UCC37421/22 with UCC27425 should be just fine with a possible max current of just under 1A.

- - - Updated - - -

No, just swap either the primary wires OR the secondary wires (but not both!) of ONE transformer. They produce AC of course but you want the secondary windings to add rather than subtract from each other. If one goes positive while the other goes negative, the net effect is to get zero out, if they have the same polarity as each other the voltages add which is what you want.

Brian.

I had put the terminals of one of the secondaries on the same side of the core as the terminals of the primary, and that was bothering me anyway.

So I unglued the cores, turned that secondary around so that its terminals were on the opposite side of the core and, while I was at it, made sure that all the windings were all around the correct way.