Reduce power of a microwave oven?

[mike]

Or use the defrost cycle!

When I ask a question on line, I go to great lengths to describe the issue
in detail to keep the discussion from running off in all directions.
It rarely works, but I keep hoping that people will actually read the
info before shooting from the hip.

With this, and every other low-end non-inverter microwave oven I've ever
encountered, the power is adjusted by pulsing the magnetron in bursts
of approximately 15 seconds followed by an off-time required to get the
average power you programmed.
The RELEVANT number is the 15 second MINIMUM on-time. Doesn't matter
what buttons you push, you can't get an on-time less than 15 seconds unless
you program a single burst that's shorter.
If 15 seconds is long enough to boil the liquid in part of the payload,
defrosting can make a mess of things.

In this case, the "defrost" button has some algorithm that they don't
disclose
and I've been too lazy to reverse-engineer, but the minimum on-time
is still about 15 seconds.
It's a fundamental property of the design.

 

[Lab1]

It is. No one is claiming it's always absolutely perfect.

Reminds me of this interesting video showing how microwave energy is
distributed inside a standard microwave...

 

[mike]

Mike-

Does the new microwave have a Defrost option? If so, does it also cook
for 15 seconds at full power?

I had a small microwave back in the 70s that had a low power setting.
My memory is a little hazy, but I think the low power setting switched a
capacitor in series with the high voltage transformer primary. It acted
as a ballast to reduce magnetron voltage.

Fred

Well, that's what I was asking about.
I don't think there's anything you can do in the primary circuit.
The magnetron filament runs off the same transformer as the high voltage.
If you reduce the average input, you also reduce the average filament
voltage/temperature.
There are two basic issues that I can think of.
1) It takes time for the filament to come up to temperature.
2) The more you thermal-cycle the filament, the shorter the life.
The 15-seconds is a compromise that mostly works.

If you want to pulse the anode, you have to have a separate transformer for
the filament to keep it up to temperature while you pulse the anode.
Conceptually trivial, but it adds to the cost.

As I mentioned in the original posting, you probably can change the value
of the secondary cap, but that takes a high-voltage switch.
I'm interested to learn if anybody's done that. Or if there's
any theoretical basis for concluding that it's a bad idea.

 

[mike]

Huh? Virtually all the ones I've seen do just that: run a 10 to 15
second on/off time cycle. The magnetron is turned on with a relay, so
rapid cycling just begs to blow that relay.

It's worse than that.
A triac can fix the relay issue. The problem is the thermal
time constant of the magnetron filament.

 

[mike]

Put in a smaller high voltage capacitor?

Thanks for repeating one of my suggested options.
Can you be a little more theoretical?
One reference suggested that the cap is actually
sized to resonate with the transformer. That would make
the change of cap value much more sensitive than just a power
ratio.
Relevant input?
Thanks, mike

Get an oven with inverter power control, like a Panasonic?

Simple matter of price ;-)

 

[William Sommerwerck]

The filament winding is on the same transformer as the HV,
so the filament turns off when the transformer isn't powered;
and magnetrons are a diode, it ONLY has the HV electrode
available to control the power.

Ineed, the patent I referred to shows the filament secondary as part of the
main transformer. But the system presumably works.

If there were separate filament and HV transformers, the price
would reflect this 'feature' in unpleasant ways.

I can't imagine a separate filament transformer being /that/ expensive.

 

[David Nebenzahl]

It's worse than that. A triac can fix the relay issue. The problem
is the thermal time constant of the magnetron filament.

I guess I'm showing my ignorance here, as I don't know much about
magnetrons, but why can't you just let the filament burn while you cycle
the HV, like you'd do with any other ordinary tube? I mean, with other
tubes, it's OK to apply power to the heater without any anode voltage,
right?


--
Comment on quaint Usenet customs, from Usenet:

To me, the *plonk...* reminds me of the old man at the public hearing
who stands to make his point, then removes his hearing aid as a sign
that he is not going to hear any rebuttals.

 

[mike]

I thought you meant adding another capacitor and switching between it
and the original.

Yes, that's what I had in mind. All you need is a switch that can
handle that much volts safely...and maybe some means to make sure
that switching a charged cap doesn't explode the diode or the switch.

Some months ago, I had to replace the diode in my old one.
Hooked a resistor to a couple of clip leads and hooked them to the cap.
I never did find any of the resistor pieces. May have gotten swept up
when I cleaned up the puddle I made. ;-)

I doubt the capacitor resonates because years ago someone mentioned a
line of microwaves being identical except for power rating, cavity
size, and the size of the high voltage capacitor, which ranged from
about 0.6uF to 1.0uF.

They're not as expensive as some people implied.

It's all relative. Paid $80 for mine. I did visit a few stores and the
inverter ones seemed to be almost 3x that and they don't go on sale
often. Knowing what I know now, I might have gone with the inverter.
Spilt milk now...

 

[David Nebenzahl]

You exceeded the maximum voltage rating of that resistor.

Gee, ya think?

Another incredibly useful 1-line post from the shoot-from-the-hip master ...


--
Comment on quaint Usenet customs, from Usenet:

To me, the *plonk...* reminds me of the old man at the public hearing
who stands to make his point, then removes his hearing aid as a sign
that he is not going to hear any rebuttals.

 

[PeterD]

I guess I'm showing my ignorance here, as I don't know much about
magnetrons, but why can't you just let the filament burn while you cycle
the HV, like you'd do with any other ordinary tube? I mean, with other
tubes, it's OK to apply power to the heater without any anode voltage,
right?

The filament is a minor current, the main HV voltage is the issue. To
cycle off the transformer rapidly with a relay would likely result in
a relay lifetime of a few weeks at best.

I don't believe there ever was an issue with the filaments.

I agree, a triac could possibly be used, though you are talking about
swtiching as much as 1500 watts, inductive (because of the
transformer). Might be too expensive to find a triac that will take
that kind of abuse and survive!

 

[PeterD]

Not sure about the ones used in uwave ovens, but with some magnetrons,
once they get going, the filament power has to be reduced (sometimes to
zero) because the filament is heated by the impact of accelerated
electrons banging into it.

Neat trick since the filament emitts teh electrons...

 

[David Nebenzahl]

Actually, I think that it is quite a useful comment, as in my experience in
the electronics service business, many techs - particularly modern ones -
have no understanding that a resistor has a maximum working voltage rating,
and have even laughed at the very idea when I have suggested it to them,
until I have shown them in a component catalogue. I'm willing to bet that
prior to this exchange, there were more than a few reading what Michael
said, and doing just that ...

Well, I have to admit that it (maximum resistor voltage ratings) doesn't
often occur to me, simply because I don't work with HV. But the person
Mr. Terrell responded to above was certainly aware of why his resistor
got blown to smithereens. Hence my annoyance.


--
Comment on quaint Usenet customs, from Usenet:

To me, the *plonk...* reminds me of the old man at the public hearing
who stands to make his point, then removes his hearing aid as a sign
that he is not going to hear any rebuttals.

 

[PeterD]

Actually, I think that it is quite a useful comment, as in my experience in
the electronics service business, many techs - particularly modern ones -
have no understanding that a resistor has a maximum working voltage rating,
and have even laughed at the very idea when I have suggested it to them,
until I have shown them in a component catalogue. I'm willing to bet that
prior to this exchange, there were more than a few reading what Michael
said, and doing just that ...

Arfa

Absolutely they do, but I suspect he also exceeded the power rating of
that resistor by a tiny bit as well... <g>

 

[David Nebenzahl]

I see that, but some other less experienced people reading the thread, may
well have learnt something from the comment. Still, all sorted now, and
everyone's up to speed on resistor voltage ratings !

Well, point taken.

While we're on the subject, where would one get HV resistors, anyhow?
Any US suppliers?


--
Comment on quaint Usenet customs, from Usenet:

To me, the *plonk...* reminds me of the old man at the public hearing
who stands to make his point, then removes his hearing aid as a sign
that he is not going to hear any rebuttals.