Do I need a step down adapter?

[mau]

Hello,
I'm far from being an expert, so sorry if I the question is very simple.
I have a 1200W 220v clam-shaped pizza oven with two resistors connected in series: the first resistor is under the stone and the second in the upper part of the oven.
After having modified it, the oven reaches the temperature that I wanted. Problem is that the stone gets too hot and burns the pizzas at the bottom.

My solution: put the resistors in parallel and switch off the resistor under the stone when it gets too hot (I have installed a thermocouple to control that).
My problem: the resistors can likely work at 110v but not at 220v. How can I work this out? My weak electric knowledge would suggest me to use a diode to cut the wave, but I guess this is not correct.

Any suggestion would be appreciated.
Maurizio

 

[jackorocko]

My solution: put the resistors in parallel and switch off the resistor under the stone when it gets too hot (I have installed a thermocouple to control that).
My problem: the resistors can likely work at 110v but not at 220v. How can I work this out? My weak electric knowledge would suggest me to use a diode to cut the wave, but I guess this is not correct.

I take it the resistors are failing now that you have increased there power dissipation? Do you happen to know what the power rating is of the resistors? Or at the very least the resistance value of each resistor?

 

[mau]

What I did was to put a couple of 6A diodes in line with the 2 resistors (which are in parallel now).
I have been able to cook a couple of pizzas at 500C, but eventually the temperatue got too high, something melted in the oven and the overload protection took care to break my home circuit.

I don't know if the diode approach is correct and the problem has just been some plastic melting in the oven (I have rewired properly now) or if I should use something like a step down to feed the resistors with 110v.
I don't know the resistors specifications either, except that if connected in series at 220v they consume 1200W and the oven works with a mechanical thermo switch. So I assumed that the resistors are 110v-600W each and did not powered them with 220v now that they are in parallel.
Thanks.

 

[jackorocko]

If we work backwards,

1200W/220V = 5.5A
220V/5.5A = 40 Ohms
40 Ohms/2 = 20 Ohms per resistor

If you parallel 20 Ohm resistors then you have a total resistance of 10Ohms @ 110V which is about 1200W. That would be the same as before, so I don't see why you would want to do that, it wouldn't dissipate any more power then your 220V @ 40 ohms load. But 220V @ 10 Ohms you're dissipating about 4x as much power, no wonder why you smell plastic melting

Are the resistors a piece of metal wire that glow or something, like you find in a toaster? If they where I would get out the multi-meter and put one probe on one end and the second probe close to the other end and work backwards, if you could remove a small piece of that heating element you would reduce it's resistance increasing power dissipation safely.

 

[mau]

Yep, I see that.
Maybe I didn't make my point clear, I just need to have the resistors in parallel in order to be able to switch off the one under the stone (and then do not burn the pizza).

If I cut the 220v wave with diodes, am I giving 110v to the resistors, or should I still consider the voltage 220v? Or using diodes is completely wrong in this contest?

Thanks

 

[jackorocko]

If you add in a diode to make a half-wave rectifier on the incoming 220VAC. The output, pulsating DC component, will be ~100V/10 Ohms = 10A. So you need a diode at least rated for 10A.

What diode are you trying to use? The Vp of one half of a 220VAC signal is 312V, so the reverse breakdown voltage of the diode needs to be rated higher then this I would imagine.

 

[mau]

I'm using two P600m, which are rated 6A.
I didn't find a single diode rated high enough, so bought 2 of these from my local electronic store.
Do you reckon they are not enough?

 

[(*steve*)]

OK, imagine you have 2 resistors (elements) in series that dissipate 1200W at 220V.

This means they have a resistance of around 20 ohms each.

If you place them in parallel, they will dissipate 4800W and you will have a very burnt pizza, possibly with a smoky taste and fireworks effects.

Placing a diode in series with them will reduce the average power dissipation to about 2400W, which is still going to do them no good.

My pizza recommendations are to make your own bases and use thinner toppings. Both of these techniques will require less heat from the top and will possibly get you a better result.

 

[mau]

Hi Steve,
If I'm not an expert in electronic, the same can't be said about pizza.
This is off topic, but I'm doing a Neapolitan pizza, incredibly difficult to raise and cook.
The oven must be at around 500C and the pizza must be cooked in 60-90 seconds.
The result is a super thin and super soft pizza.
The last pizza before the oven breakdown cooked in 60 seconds, and the result has been amazing. I used to cook pizzas in 3 minutes, and the difference is simply unbelivable.
Now all my efforts are in trying to make a relieble oven that can give me again those temperatures.

 

[(*steve*)]

So it used to work and now it doesn't?

What has changed? You need to fix that.

If the problem is that the bottom is too hot, then you may be able to either use a thicker stone, or allow a little cool air into the bottom of the unit. Alternatively, you could allow some ventilation between the bottom and the top of the unit (I once had a pizza oven that did this in order to boost the amount of heat above the pizza.

My favourite pizzas are currently:

1) Roast lamb chunks with roast pumpkin, sweet potato and a minty sauce.

2) Lamb chunks with tzatziki, feta, and red onion.

3) Chicken tikka with a spicy yoghurt base, onion, mint, coriander, and a squeeze of lime (before serving).

Tell me more about your Neapolitan pizza. (I'm sure the moderator won't mind)

 

[timothy48342]

If you add in a diode to make a half-wave rectifier on the incoming 220VAC. The output, pulsating DC component, will be ~100V/10 Ohms = 10A. So you need a diode at least rated for 10A.

What diode are you trying to use? The Vp of one half of a 220VAC signal is 312V, so the reverse breakdown voltage of the diode needs to be rated higher then this I would imagine.

I don't know how to calculate the RMS of pulsed 110 DC, but assuming that's true a 10A diode is needed when both elements are in parallel (10ohm), would a 5A diode work for 1 element by itself? (20ohm) Then use a seperate diode for each heating element (resistor).

By the way(1), the datasheet says, "Average Rectified Output Current: 6A" I'm not sure what "average rectified" means.
By the way(2), the ds says, the "M" version of the P600 has 1000V reverse breakdown voltage, so no problem there.
By the way(3), the ds says, max opperating temp is only 150C, so these can't be mounted too close to the inside of a 500F oven and certainly not anywhere near the heating element of that oven.

My first (and farorite) job in my life was working in a pizza shop. I will never ever get sick of pizza. Pizza, gooood.

--tim

 

[(*steve*)]

Just for fun Timothy48342, calculate the power dissipated by those elements in that configuration.

A simple estimate is that the rectified mains will result in half the dissipation (due to the same voltage for half the time) as the full 220VAC.

Note that that are each rated at 600W.

Here is the calculation for you... (1/2) * (220^2)/20

 

[timothy48342]

Now that wasn't very fun at all.

So, since there is 220 peak posative voltage during the part of the cycle they are on, 220 is used to calculate the wattage. (times 50% for half the cycle)
So about 1200 watts, through a 600W element. (No fun, no fun.)

So what is needed then? A 220 to 110 step down transformer and no diodes, and then parallel with a switch?

Or try to find another one like that original pizza oven that broke down.

-t

 

[(*steve*)]

Thinking about it overnight, I would measure the resistance of the two elements and see if there is any difference between them.

We have been assuming they are the same, and that may not be true.

 

[War_Spigot]

If it's possible, making the second resistor's resistance higher(if that's feasible) would result in a lower voltage across it as well as a lower current. Unless I'm thinking wrong...

 

[mau]

Hi Steve,

I got excellent results switching off the lower resistor at the right time, so I'm hoping to go that way again.
About the neapolitan pizza, it is extremely simple in the ingredients, yet difficult to cook. There are only two types of Neapolitan pizza. The traditional type is called Marinara, and the topping is only tomato, oil, garlic and oregano. The "evolution", which is the one that 99% of people order, is called Margherita. The topping is tomato, mozzarella (coming from a very restricted area in Italy, nothing else) and basil. Google it and have a look at some images.
Cheers

 

[mau]

Hi Tim,
I mounted the diodes in a location that I thought safe, but I could have been wrong and maybe the temperature was over 150C.
I'll try again in a different location. That could have been the problem, I noticed the operating temperature yesterday as well.
BTW, mine is a 500C oven, not a 500F. There's real heat there!

 

[cjdelphi]

I suggest you eat pizza less and when you do buy it from a professional - but seriously i'm too scared to go near mains voltages with a resistor lol