Lower voltage diode the other side of a step up transformer?

I thought current always flowed the same way in primary and secondary.... You know, phases and all that.

Assume a loaded bifilar transformer, both dots up, and draw the winding currents.

The reason that loading a transformer doesn't make it saturate is because the primary excitation flux and the flux from the secondary load current cancel. Loading a transformer reduces the core flux.

No exactly. The core flux has to keep increasing to generate the voltage the drives the current into the external load. Because you drive transformers with AC the voltage eventually reverses, ideally before you saturate the core.

You clearly shouldn't have anything to do with transformers and diodes.

He clearly doesn't like them, and refuses to design his own - if he uses a transformer it has to be a standard part that he can buy of the shelf.

He has to buy a lot of them, because his approach to circuit "design" - actually evolution - is to hook them up at random until he find a circuit that doesn't actually blow up.

Margaret Thatcher did have a lot of silly ideas. The one thing she did achieve was to make the UK more unequal.

" Britain’s gini score went up from 0.253 to 0.339 by the time Thatcher resigned." She probably thought that it was good thing.

I was actually talking about a stepup transformer. 120V to 6000V. Yes it's American, I got it cheap. To avoid finding a 6000V diode, I thought I could use a 120V diode. From all the answers it appears I was wrong to assume because the current is going one way through the primary it must be going the same way through the secondary. Actually the DC component is lost and the AC is passed, so I just get squint AC.

Or should learn more. Spice is a good tool for things like this. As Mike E says, the main function of LT Spice is to train your instincts.

The penny drops.

Trying to make a high voltage DC supply? You can get 1KV 1 amp diodes for a few cents each. Those can be used in series.

When I was a kid, I used old neon sign transformers and 1B3 rectifier tubes to make high voltage DC supplies. I couldn't afford a Variac so adjusted the filament voltage to regulate the output voltage. I used a D-cell battery and a rheostat, and somehow I'm still alive.

Digikey has a 16 KV diode for 33 cents.

Sadly, an instinct is an "an innate, typically fixed pattern of behaviour in animals in response to certain stimuli."the homing instinct" "

You can't train an innate response. You can build up a personal data base of how simulated circuits respond to particular sorts of stimuli.

John Larkin has been known to post LTSpice .asc files that don't run for long enough to let you see what's actually going on, so his personal data base may be built up out of simulations that showed him what he wanted to see.

Bill, I think this assumes that the voltage is regulated with feedback.

Driving the transformer with, say, just grid voltage would do what JL is talking about because the IR drop when loaded, reduces the volt-seconds imposed on the core which is what causes saturation.

Pretty sure that's it anyway.

boB

It doesn't. It's a simple statement of the physics involved.

Wrong. What causes saturation is the net current circulating around the core. The current coming out secondary cancels some of the current being fed into the primary, but not all of it, because some of the current being fed into the primary is being fed in to increase net current circulating around the core to produce the voltage across the secondary outputs to drive the output current through the load. The IR drop inside the secondary is just part of the voltage that has to be generated to.

Then you need to revisit your assumptions.,

Not totally. IIRC correctly the current in the secondary is 'backwards'; on terms of its ability to generate back EMF.

Furthermore the impedance of the transformer to rectified AC with a DC component will now be so low that the primary will burn out.

You should generally be careful doing that. If one is leaky it will put a far higher voltage across the others. In general you need to select for reverse leakage current OR put high value resistsiors across them to guarantee a predictable 'leakiness'

Modern diodes are almost all controlled-avalanche. They zener without a negative-resistance snap effect, so are safe to run in series; they self-adjust. Silicon HV diodes are a stack of lower-voltage junctions.

You can Spice a series string of various-voltage zeners and see what happens.

Two items of folk wisdom "don't put diodes in series" and "don't put diodes in parallel" are mostly wrong.

Yes. Shorting the secondary will drop the core flux about in half.

Sloman has lectured us hundreds of times about how we should design and not buy transformers, so it's hilarious that he doesn't understand the basics.