Hello,

Are we still trying to find the energy lost in the wire?

For two batteries connected together so that one charges the other, if we consider the voltages to be constant (which they arent as anyone who works with batteries on a daily basis knows) then we have a simple circuit with two voltage sources V1 and V2 where V1 is higher than V2 so we have a different voltage:
Vd=V1-V2

and if the resistance is say 2 ohms, we have current:
i=Vd/2

or just power:
P=Vd^2/2

The energy is then:
W=T*Vd^2/2

where T is the time to charge.

So it is a little simpler than the caps. It's a little different because we are stopping the charge independent of the voltages.

Now the better example i think would be to have one battery slightly higher voltage than the other, and allow the voltage to change as the both pass current. I havent done that yet though.
Interestingly, i think the circuit then would look like two capacitors with offset voltages not part of the real battery, and the two caps exchange energy as the voltages change slightly over time. Once the two voltages are the same, we're done.

[LATER]
Ok it looks the same. The voltage of the one cap comes up to 100.4975 as before.
The intuition about this might be that the offset voltages act the same as the initial condition generator sources, so they appear in the same place as those.
But also, the loss is based on the differential voltage not the absolute voltages.
The circuit was such that one battery was smaller than the other and the smaller charging the larger and allowed to go down in voltage also.
Now if we try to maintain voltages to nearly the same, then we go back to the constant voltages model.

MrAl, I had some questions very specifically about modifying power output on a microwave. I read a previous post of yours and it seems that you were able to achieve a nonpulse uniform energy drop, which is actually exactly what I am curious about. I am incredibly new to forum settings, but I would very much appreciate some guidance on the subject.
Thank you, N

 

MrAl, I had some questions very specifically about modifying power output on a microwave.

Welcome to AAC.
One rule here is not to hijack a thread for a question unrelated to the thread topic.
It muddles up the thread.
Please start you own thread for that.

 

MOD NOTE: I've split your post off from the thread you hijacked, so now you have your own thread for this topic.

@SeriousWombat : To get a member's attention, precede their username with the '@' symbol. They will get an alert and a link to the post containing the tag.

I'm tagging @MrAl for you, so he will probably be by soon.

 

MrAl, I had some questions very specifically about modifying power output on a microwave. I read a previous post of yours and it seems that you were able to achieve a nonpulse uniform energy drop, which is actually exactly what I am curious about. I am incredibly new to forum settings, but I would very much appreciate some guidance on the subject.
Thank you, N

Hi,

First, thanks to WBahn for fixing the thread.

Wombat, yes i did some experiments with reducing the cooking power of a microwave oven. This is based on the fact that if we have a non constant-power device then reducing the input voltage must reduce the power input which must reduce the power output.

So the only trick is to reduce the input voltage to the oven.
It is very non linear though in that as you reduce voltage by even a small percent like maybe 10 percent, the oven will start to show signs of closer cooking. Going down 20 percent we see more change.
I cant remember now what the lowest i was able to go, but going too low and it stops cooking.
For my 120vac oven, i think i went as low as 85vac input. As i got down lower, the cooking time for a food sample would go up a lot.
I used a variac on the input to adjust the voltage to lower than normal line voltages.

One of the problems i experienced though was that as i went down in voltage, it became harder to stabilize the cooking power. As the oven heats up the characteristics change slightly and so the voltage has to be adjusted over again. I used a watt meter on the input to tell me how much power it was drawing in. The power would change even with constant voltage so it had to be adjusted for constant power in.
So the better way would be to use an AC power supply of some type to control the voltage while using a watt reading as feedback.

It was interesting to do this. Not only did i want to know how this worked for a long time, i got a Panasonic oven that has "inverter technology" and that allows constant power at lower levels rather than pulsing on and off, and i wanted to know if i could get the same effect with a regular oven just by lowering the input power. My test oven was a cheap one from Target, i think it was only 50 bucks but had key pad just like all the other ovens rather than a turn dial to set the time.

 

Welcome to AAC.
One rule here is not to hijack a thread for a question unrelated to the thread topic.
It muddles up the thread.
Please start you own thread for that.

My bad about that, this is my first experience with forum type interaction, and I did not see an option for a direct message. Thank you for the heads up!

 

MOD NOTE: I've split your post off from the thread you hijacked, so now you have your own thread for this topic.

@SeriousWombat : To get a member's attention, precede their username with the '@' symbol. They will get an alert and a link to the post containing the tag.

I'm tagging @MrAl for you, so he will probably be by soon.

@WBhan Thank you for the reorganization! I appreciate the help.

 

Hi,

First, thanks to WBahn for fixing the thread.

Wombat, yes i did some experiments with reducing the cooking power of a microwave oven. This is based on the fact that if we have a non constant-power device then reducing the input voltage must reduce the power input which must reduce the power output.

So the only trick is to reduce the input voltage to the oven.
It is very non linear though in that as you reduce voltage by even a small percent like maybe 10 percent, the oven will start to show signs of closer cooking. Going down 20 percent we see more change.
I cant remember now what the lowest i was able to go, but going too low and it stops cooking.
For my 120vac oven, i think i went as low as 85vac input. As i got down lower, the cooking time for a food sample would go up a lot.
I used a variac on the input to adjust the voltage to lower than normal line voltages.

One of the problems i experienced though was that as i went down in voltage, it became harder to stabilize the cooking power. As the oven heats up the characteristics change slightly and so the voltage has to be adjusted over again. I used a watt meter on the input to tell me how much power it was drawing in. The power would change even with constant voltage so it had to be adjusted for constant power in.
So the better way would be to use an AC power supply of some type to control the voltage while using a watt reading as feedback.

It was interesting to do this. Not only did i want to know how this worked for a long time, i got a Panasonic oven that has "inverter technology" and that allows constant power at lower levels rather than pulsing on and off, and i wanted to know if i could get the same effect with a regular oven just by lowering the input power. My test oven was a cheap one from Target, i think it was only 50 bucks but had key pad just like all the other ovens rather than a turn dial to set the time.

Hi,

First, thanks to WBahn for fixing the thread.

Wombat, yes i did some experiments with reducing the cooking power of a microwave oven. This is based on the fact that if we have a non constant-power device then reducing the input voltage must reduce the power input which must reduce the power output.

So the only trick is to reduce the input voltage to the oven.
It is very non linear though in that as you reduce voltage by even a small percent like maybe 10 percent, the oven will start to show signs of closer cooking. Going down 20 percent we see more change.
I cant remember now what the lowest i was able to go, but going too low and it stops cooking.
For my 120vac oven, i think i went as low as 85vac input. As i got down lower, the cooking time for a food sample would go up a lot.
I used a variac on the input to adjust the voltage to lower than normal line voltages.

One of the problems i experienced though was that as i went down in voltage, it became harder to stabilize the cooking power. As the oven heats up the characteristics change slightly and so the voltage has to be adjusted over again. I used a watt meter on the input to tell me how much power it was drawing in. The power would change even with constant voltage so it had to be adjusted for constant power in.
So the better way would be to use an AC power supply of some type to control the voltage while using a watt reading as feedback.

It was interesting to do this. Not only did i want to know how this worked for a long time, i got a Panasonic oven that has "inverter technology" and that allows constant power at lower levels rather than pulsing on and off, and i wanted to know if i could get the same effect with a regular oven just by lowering the input power. My test oven was a cheap one from Target, i think it was only 50 bucks but had key pad just like all the other ovens rather than a turn dial to set the time.

@MrAl , Exactly what I am looking for is the modification technique. I am modifying a microwave for use in a chemistry lab, and we have been damping the power in different ways, this electronic solution sounds great!
Is there a way I could see a diagram of the modification?

 

@MrAl , Exactly what I am looking for is the modification technique. I am modifying a microwave for use in a chemistry lab, and we have been damping the power in different ways, this electronic solution sounds great!
Is there a way I could see a diagram of the modification?

Hi,

What modification?
The only thing i did was used a Variac to power the oven rather than plugging it directly into the power line outlet.

To keep it regulated at a given power level you'd have to use an AC power supply and watt sensor as feedback.

 

Hi,

What modification?
The only thing i did was used a Variac to power the oven rather than plugging it directly into the power line outlet.

To keep it regulated at a given power level you'd have to use an AC power supply and watt sensor as feedback.

That is all that was necessary? Wow, so a variac and a watt sensor. And the power output was non liner you said, so the bottom bound is unknown in terms of power damping, thank you I will give this a shot!

 

Hi,

What modification?
The only thing i did was used a Variac to power the oven rather than plugging it directly into the power line outlet.

To keep it regulated at a given power level you'd have to use an AC power supply and watt sensor as feedback.

Any feel for the relationship between the power consumed from the AC source and the power delivered to the chamber? I would be surprised if it is a constant ratio as you adjust the power, especially since it is so nonlinear as you say. The question is whether the ratio is close enough to a constant that it can be assumed to be constant for the purpose at hand.

 

Any feel for the relationship between the power consumed from the AC source and the power delivered to the chamber? I would be surprised if it is a constant ratio as you adjust the power, especially since it is so nonlinear as you say. The question is whether the ratio is close enough to a constant that it can be assumed to be constant for the purpose at hand.

I feel like as long as the power damping results in a sensible relationship it should be pretty evident by running some basic solvent heating tests. is there a brand or other considerations I should make when purchasing the variac?

 

That is all that was necessary? Wow, so a variac and a watt sensor. And the power output was non liner you said, so the bottom bound is unknown in terms of power damping, thank you I will give this a shot!

Hi,

Yes that is all i used:
1. Variac
2. Watt meter

I measure the input power with the watt meter and try to set it for something low for slow cooking, then try to keep it constant by turning the variac up or down as needed.

 

Any feel for the relationship between the power consumed from the AC source and the power delivered to the chamber? I would be surprised if it is a constant ratio as you adjust the power, especially since it is so nonlinear as you say. The question is whether the ratio is close enough to a constant that it can be assumed to be constant for the purpose at hand.

Hi,

Well going by memory i remember something like 60 percent efficiency which went down a little as i went down in power.
I posted results somewhere on the web one of the forums but cant remember which. I cant find my notes on this either.
According to the back markings the input is 1550 watts and output 700 watts, which puts the eff at around 45 percent but i saw better than that.

 

is there a brand or other considerations I should make when purchasing the variac?

Make sure it is rated to handle at least the microwave input power on full power. Allow a good safety margin.

 

Hi,

Well going by memory i remember something like 60 percent efficiency which went down a little as i went down in power.
I posted results somewhere on the web one of the forums but cant remember which. I cant find my notes on this either.
According to the back markings the input is 1550 watts and output 700 watts, which puts the eff at around 45 percent but i saw better than that.

How did you determine the output power quantitatively? I thought all you were measuring was the input power.

 

How did you determine the output power quantitatively? I thought all you were measuring was the input power.

Hi,

Good question

I timed the temperature rise of a sample of plain water from room temperature to boiling and computed the energy needed to do that and thus the power output.
It's not a perfect test, but it's about all that i had besides judging the cooking time of a sample of hamburger meat which seemed approximately commensurate.

 

Make sure it is rated to handle at least the microwave input power on full power. Allow a good safety margin.

Hi,

Yes i forgot to mention that. I used a 20 amp rated Variac.