Use of Microwave Diodes

Before the peanut gallery fires: no, they aren't actually microwave, they're microwave oven.

Which suggests a problem: they're probably general purpose, not fast recovery. They probably also have more juction capacitance than the static plates, so they'll never come out of recovery either.

What are you getting 20kHz from, and why doesn't it have a rectifier inside already (i.e. CRT TV flyback since almost forever)?

Also, how do you know (or what makes you think) the ~20kV is actually square? That's not easy to do (within some value of "square"). "Lumpy" is more likely, not that it matters anyway.

Tim

On a sunny day (Sat, 06 Jun 2015 22:00:45 +1000) it happened snipped-for-privacy@netplus.com wrote in :

Those diodes may have have switching times, made for main frequencies, look it up.

The schematics of HV supplies I've seen in the literature use diode strings with each diode loaded by a cap and/or a resistor to keep the reverse-bias equal. Without that one diode will "steal" all the voltage, die short, followed by the others in the string.

I couldn't even begin to recommend values.

Your application doesn't seem to require much power.

Standard circuits using multipliers seem to serve this function without requiring impossible components.

The line-frequency microwave oven rectifier is generally unsuited to higher frequency service.

RL

You'd be better off with a picture-tube color TV HV supply stack. The microwave oven power supply is intended for different current and supply frequency.

If it's an experiment, you might - no kidding - consider using an old color TV rectifier tube, a 1B3 sort of thing. You can run the filament off a C-cell battery. You can even adjust the filament voltage to tweak the HV.

Or, as suggested, rip most of the HV section out of an old teevee.

ebay has surplus HV supplies, out of copiers and such, but probably not 20KV.

I searched ebay for a color TV rectifier or diode, not a single TV rectifier as I remember them.

These will probably work, 100ns Trr, someone else can decide if that is fast enough.

Mikek

Look instead for 'tripler'; a combination of HV diodes and capacitors that can be found in any recent CRT design (recent being 1990 through 2005).

eBay lists hundreds.

Claus,

Modern HV rectifiers are designed to partially avalanche when the reverse voltage begins to exceed their rated PIV. A group of identical HV controlled avalanche rectifiers connected in series will effectively "self-equalize" themselves without the need for external resistors or capacitors. Internally, your existing microwave oven rectifiers actually consist of a number of lower-voltage controlled avalanche rectifiers connected in series. Connecting two (or more) of these rectifiers in series can be used to handle higher voltages.

However, you may need to add a small (500 pF 30 kV ceramic "doorknob" capacitor across your two-plate load to reduce HF ripple.

Bert

But still no HV Rectifiers like the old TV sets had, about 3 inches long, maybe 3/8 in inch diameter with a large ring area at one point.

Mikek

Their still made, EDI and others make HV diodes. You may have to buy more than a 'couple'

Cheers

You would do better making a CockroftWalton multiplier..

With that you can use much lower voltage diodes.

Use 1kV diodes, that way you don't need to get expensive caps and you should be able to get what you need with 22 diode or so.

If you look around I do think you can get Cockroft Waltron kits, you simply need to supply the AC signal.

On Sat, 06 Jun 2015 22:00:45 +1000, snipped-for-privacy@netplus.com Gave us:

An HV diode IS a stack of P/N junctions in itself. Really high voltage examples are upwards of 60 elements seriesed together in one package, and that is why the conduction voltage threshold is so high.

Here is one example:

Note the 16kV version takes 35 volts to begin conduction. There are literally about 20 or 30 or even up to 60 elements in each one package.

You could use a silica powder filled epoxy to act as a thermally conductive bond between the diode and a plate,. yet remain insulated and try to insure that they both keep the same temperature.

We always used Philips, but they stopped making them, IIRC.

The web search appears to show a few companies offering replacements.

If you want to put a diode at 20kV, you need a 30 or 40kV diode as well, BTW.

At those voltages, you want the soldered nodes to be round balls as well. No sharp protrusions like the lead at the end of a fillet type solder joint. "big blobby balls" is best. Absolutely NOT inspectable by a picky QA person.

"philips HV diodes" was the simple search criterion I gave.

On Sat, 6 Jun 2015 07:14:00 -0500, "Tim Williams" Gave us:

I never see square waves that slew to 20kV either. That is a very fast climb rate.

Lumpy is a nice term though.

On Sat, 06 Jun 2015 15:15:38 -0500, amdx Gave us:

They are INSIDE the flyback device. Usually a whole string of 'em, in fact. Non-flyback anode supply designs have them external in multiplier configurations.

On Sat, 6 Jun 2015 21:46:20 -0400, M Philbrook Gave us:

He is not trying to generate voltage, idiot. He wants to rectify an existing signal.

On Sun, 07 Jun 2015 07:43:10 -0500, amdx Gave us:

Absolutely not.

You are in power utility territory at that point.

Vacuum rectifier, mercury arc valve level stuff.

Not discreet PCB mount component stuff, and certainly not anything from a microwave oven.

You are an absolute idiot yourself.. If you can't read between the lines of what someone is attempting then maybe you should shut your trap. Most people in their position may simply assume they need raw HV of desired level to start with and want to use some elementary method of rectifying it. Does it really hurt to suggest a better way peckerhead?

You must be some throwback from the neanderthal days.

Jamie

Resistors. That's how I always did it, a lot. Decades ago and the highest voltage diode I could afford as a student was 1kV so that sometimes meant a lot of soldering.

Nowadays one can easily buy diodes for several kV, like these:

The GP02-40 can be had in singles for 50c or less. Trr looks somewhat ok for a 20kHz job. Depending on how square the square wave really is.

If there is a substantial risk of spikes coming along there may also be a need for a small capacitor across each diode but I've not needed that very often.

It depends on the max spread in reverse current and thus also on the max tempearure they are exposed to. Better to err on the conservative side rather than risk a white-knuckle ride followed by tsssk ... *KAPOW*

Horizontal flyback diodes like Tauno suggested are also an option if t's a low power application. However, due to the advent of flat-screen TVs a long time ago they have become nearly extinct and may no longer be suitable for new designs. For hobby they can be scrapped out of an old set or maybe Ebay.