Microwave Transformer Isolation

[Proschuno]

So I have a microwave transformer which uses its own body as one of the conductive paths and hence is not isolated, that I am trying to use to make a 0-1000 volt power supply to power vacuum tubes (I will only need 1000 volts for the really big tubes I might happen to come across; I only use about 100-300 on average), which will be fed by a step down transformer which provides isolation.

My question, granted it be dangerous, is can I theoretically mount the microwave transformer in an insulated non conductive box with just the 4 leads coming out of it for power, as a way to provide isolation? If the body does not conduct to the body of the supply, I figured this wouldn't break isolation. I know this is unsafe but would it work?

 

[duke37]

The microwave transformer that I have seen is an auto transformer with no isolation.

If the core is connected to either input or output, then insulation may not be adequate to connect it differently.

 

[Proschuno]

Even if I used at least 1/4" of wood, or plastic? I'm assuming because the microwave transformer's body (and hence one of the leads) is not conducting to the body, all I would electrically see is just two floating leads which neither are connected to earth. Is what I'm assuming correct? Also, neither of the primary leads conduct to the body, only one of the secondary (High voltage) leads touches the body.

 

[BobK]

If it is an autotransformer, as duke said, the secondary is not isolated from the line. An autotransformer has only one coil, which is tapped. The input is placed from one end to the tap and the output is from the two ends of the coil.

Bob

 

[Proschuno]

But I though auto transformers had all taps on the same coil unless I'm mistaken, so that ruins there isolation, where as what I have the primaries are totally independent of the secondaries.... Unless I'm mistaken I wouldn't think I have and auto transformer

 

[BobK]

Well, you can check it with an Ohmmeter. If there is continuity from the "primary" to the "secondary" is is not isolated.

Bob

 

[Proschuno]

I already have and there is none, that's why I assumed isolation can be achieved by putting it in a big insulating box touching nothing metal

 

[KrisBlueNZ]

I've never heard of a microwave oven transformer being an autotransformer. That would be unsafe and illegal. The anode of the magnetron connects to the microwave oven's metalwork. If the transformer was an autotransformer, there would be a path from mains phase and/or neutral to the case.

 

[BobK]

That makes more sense.

bob

 

[duke37]

I may be wrong about the autotransformer, it has been known.

The live core means that if the core is insulated and allowed to rise well above the input winding, then the insulation may be compromised.

I put the microwave to one side as I did not have the courage to repair it.

 

[davenn]

I agree with Duke its pretty common knowledge that those transformers are much like an auto transformer.
I see this discussed often in other forums

I

already have and there is none, that's why I assumed isolation can be achieved by putting it in a big insulating box touching nothing metal

the idea of needing isolation is to protect the user of the equip from mains voltages
putting it in a insulated box isnt going to do that as you still have unisolated mains coming out of the box.

Dave

 

[Proschuno]

Even if the microwaves transformer's primaries aren't connected to the mains? What's confusing me is that even though the primaries don't conduct to the earth at all, how does the mains happen to get through?

As I explained previously I'm using a regular isolated transformer to step down, then the microwave to step back up, which using the regular transformer's leads are isolated.

I believe isolation can be broken not because I have an autotransformer (which continuity testing proved that I DO NOT have an autotransformer), but because since the core, hence one of its secondaries, is connected to earth, which since my body can be considered grounded that would make the other secondary 1000 volts higher than me, which is scary.

I uploaded a drawing of what I plan on doing, notice the microwave transformer's secondary attached to its own core, the whole transformer sitting in it own heavily insulated box (dotted-line box). Please forgive the terrible handwriting

What is now confusing me is that if the microwave transformer's core IS NOT attached to its primaries, how are its secondaries directly conducting to them which would break isolation?

 

[Proschuno]

And also I understand that this is a crazy and dangerous undertaking, but I figured I have these two hefty transformers lying around so why not make use of them? I also figured that insulating the microwave would make this safe to use as well.

And the reason I don't believe that it conducts to mains is that I have attached a big resistor from the microwave to the ground when I set up the circuit mentioned in the drawing, while it was sitting on the carpet of my house, and found no current at all; my meter literally read 0.00!!!! So i assumed that this meant that the microwave is isolated from the mains. This was safely done with the step down transformer feeding 4VAC into the microwave transformer, which only gave like 80 VAC at the microwave's secondaries. I found not conduction from the body or the other secondary lead

I should have mentioned this before, along with the step down and schematic, 15 posts later, but I forgot, as my mind has been here, there, and everywhere! I apologize for any 'typing' wasted

 

[davenn]

sorry I missed that comment about the stepdown transformer in your first post
seeing your drawing .... no prob, mains is isolated from the rest of the PSU

yup a drawing at the start of the thread would have made it clear
do worry too much, you aren't the first to get so far through a thread before finally posting a circuit and you wont be the last
just make it your last time hahaha

cheers
Dave

 

[Proschuno]

Lol.. In fact all this posting and guess what? I discovered on the primary side one of the secondary taps is screwed onto the body; unscrewing removes the secondary from the core, so now it's just like a regular transformer..... So I literally wasted 15 posts for NO REASON WHATSOEVER!! Remind me to slit my wrists later

 

[TedA]

Most consumer microwave oven transformers follow the same plan.

The primary is completely isolated from the secondary and the core. They are not autotransformers. The primary winding is a physically separate winding, with a considerable space between it and the secondary winding.

The high voltage secondary has one end connected to the core. This connection may or may not be accessible. Even if you can disconnect the high voltage secondary from the core, that end of the secondary winding may not be insulated for the high voltage. So, if you ground the other end of the high voltage winding, or use a full wave rectifier, etc, the transformer insulation may break down. Or it may be fine.

Insulating the entire transformer may work, but bear in mind that the primary to core insulation was not designed to operate with a high voltage on the core.

Another consideration is that these transformers are designed to have a high secondary impedance. The impedance matches-up with the magnetron characteristics to get about the correct power over variations in the line voltage, temperature, and components. This is often not what you want for a DC supply that's not powering a magnetron.

In the space between the primary and secondary windings, the core has a couple of magnetic shunts that greatly increase the leakage inductance. These shunts can usually be removed to get better secondary voltage regulation.

The transformer is designed for a full wave load, so it would be best not to use a half wave rectifier. In the microwave oven, the load is a sort of voltage doubler, where the tube serves as one of the diodes.

All of this information can be found online, but I guess some of what you find may not be exactly correct.

Ted

 

[KrisBlueNZ]

Nice to see you back again, Ted! You don't visit here often, but when you do, your posts are always very helpful

 

[Electrobrains]

I have picked out several microwave transformers and oil burner transformers from used apparatuses. They have always been isolated: Primary winding totally isolated and the inner side of the secondary winding connected to earth= body of transformer and earth of supply cable.

It seems Ted knows these transformers well. My question is only why they wouldn't work well with half wave rectification?
As far as I can determine, they are normal line frequency transformers, with a large secondary winding. They just need to be well dimensioned, or not?

In fact all normal line frequency transformers are designed (labeled) for full wave (resistive!) loads. To use a transformer differently, would always demand derating.
The formula I use for half wave rectification with capacitive load, specifies an over dimensioning of the transformer power (VA) with an approximate factor of 2.8!

I am right now planning to build a high voltage power supply + test unit for isolation tests of different circuits. One transformer I have here is from a micro wave unit and has a secondary voltage of 1.9kV. (It would give a rectified DC voltage of almost 2.7kV).

To make it quick and easy, I plan not to change anything on the transformer. I keep the body grounded and for DC output I half wave rectify the voltage from the hot secondary wire. If I smooth it enough, I suppose it will be ok for my DC test requirements.
I will simply place a variac (adjustable transformer) on the primary side and if really necessary, build an active ripple killer on the output DC side. Some type of resettable fuse would also be included (placed on the primary side).

 

[Proschuno]

So a new question, one that I've posted to another forum also; for powering tubes, and these transformers can provide more than necessary current, and I figured I would have two different fuses for the 1000 Volt setting, then another fuse for anything below 500 volts or so.

So what value fuses should I have? What's the best compromise between "safe" (in quotations yes!!!) and useful for these ranges?

 

[duke37]

The fuse ahould have a current rating above the normal load current but not too much. If charging big capacitors there may be a problem on switch-on and a soft start circuit would help.
Most fuses are 250V, these would not be safe above this voltage. Many years ago I operated some equipment with fuses 2ft (600mm) long.

Fusing the HT to the anode of a tube separately to the screen may burn out the screen.

A transformer with a large leakage inductance will drop a lot of voltage when current is drawn.