## Really basic Ohms law question on an AC Circuit.

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I am planning to use an older toaster oven to make a DIY reflow oven. Specifically the oven is a Hamilton Beach Model 31507R which is 120v @ 60 Hz and is 1300W. First things first I never trust a product label. I pulled the cover off to gain access to the heating elements, to confirm the resistance values of the four heating elements. They are wired in series so Rt= R1 + R2 .....  for a total of 44.4 Ohms (11.1 each). Now using my known Voltage of 120V and the stated Wattage on the label I can say that:

R=V^2/W - plug it in and you get 120^2/1300 = 11.076 Ohms<-- That is about what I measured on one element BUT there are 4 wired in series.

I=P/V - that yields 1300/120 = 10.8333... Amps <-- I hope this is incorrect and that little oven does not pull almost 11 Amps.

If I use numbers I measured then I get the following result:

W=V^2/R - with my values is 120^2/44.4 = 324.324... Watts <-- This is with the known 120V and the measured total resistance of 44.4 Ohms.

I=P/V - gives a much different value of 324.324/44.4 = 2.702 Amps more what I would have guessed for a tiny toaster oven.

I must be missing something elemental here series resistors should be totaled before Ohms law is applied correct? That statement assumes you are looking at the entire circuit not a single node. Wiki states "The total resistance of resistors in series is equal to the sum of their individual resistances." I know there is a problem with my math. If anyone would be willing to check / correct me I would really appreciate it. I know I can simply test the amperage of the device with my meter however I have a 10 amp fuse, so if Hamilton Beach is telling the truth about the Wattage I will smoke my fuse. One last thing that just occurred to me, is that this is AC and I should have used Z for impedance not R for Ohms but this is a purely resistive AC circuit and the equations are well, equal...... for that condition. Really if anyone out there is willing to flog me for my error I would be grateful I have spent half my day trying to decide how many magic pixies are in that wire.

I do have pictures of the oven wiring but they are freaking huge 1.2+Megs and I didnt want to kill anyone's mobile data. I need the calculated amperage to connect the correct tool for an actual amperage measurement. I can borrow a clamp meter if I have to. Once I have the numbers in place I can decide on going with a TRIAC or being shoved into using an SSR.

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Last Edited: Sun. Apr 15, 2018 - 10:31 PM

Interesting question.

We'll see what others say.

Note, however, that measuring a resistive heating element's resistance when the device is off, and at room temperature, is its cold resistance.

This is likely your worst case, and gives a high in-rush current when one first turns it on.

Very quickly, however, as the heating element warms up its steady state resistance is much higher.

So, perhaps 10A is the initial inrush current, and 2 A is the steady state current?

Know that your controller, obviously, has to handle the in-rush current, plus a safety factor.

JC

That's actually really good information as it would affect the overall operating temperature of the electrical components, also calibration and power consumption. I might try and figure a way to graph that cold to operating temp just for fun. I really don't care so much about switching over to SMD but the Oven project just seems really interesting as temperature affects all things electrical. Being able to accurately measure and control the temperature of a small enclosed space seems to be a good thing to burn brain cells on. God my grandkids are right I am a geek.

I recall a project like that in Circuit Cellar Mag about 10 years ago, should be a good read before you start.

Jim

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Yeah I am not sure if I read that exact one but I have read about 15 or so different build articles. I will not disclose the hours I've wasted on youtube watching videos. I have done quite a bit of research but no one mentioned this issue with strange wattage labeling (or just crappy math skills jury's still out). I just remember how power tools and vacuum cleaners were sold in the eighties, like listing the max inrush current of a motor to look better than the competition. I am a bit cynical looking at consumer goods labeling.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

LOL Now there is absolutely no fun in that! I feel sure I can purchase an entire DIY PCB oven from Ebay. I could also fork over the 3-5 grand for a lab unit. I am not building the PCB toaster because I need the fastest or cheapest solution, I actually enjoy learning how to do things. Its the journey not the destination, that matters to me. If you want to learn something you have to actually do something. I know I can order the power supply for the microcontroller circuit that will manage the heating elements cheaper and easier than building it myself. That does not mean I have any intention of doing so.

Ok, then get a known resistance, put it in series and measure the voltage across it. Note - you are dealing with mains voltage, so ensure you don’t become part of the circuit.

Regarding resistance, did you measure resistance of one heating element, or resistance of whole circuit? Are you sure that all the heating elements are connected in series? Maybe they are actually connected as two parallel branches with two elements connected in series in every branch? The resistance of such circuit would be the same as resistance of one element.

And regarding resistance change with temperature, it should increase with temperature increase, so I think it is not possible that 44ohms at low temperature changes to 11ohms when the heating elements are warm.

This reply has been marked as the solution.

Kartman you are a genius thank you! MarekJ71 you had part of it as well. If your initial circuit analysis is faulty the math will never work <--- note to self. So two of the elements are connected in series. So are the other two. The issue with the math was that those pairs of elements are connected in parallel. What made me look at the wiring again was Kartmans comment, so I went back and checked again and sure enough 60V across two points! That poor dim light bulb over my head went off when I saw a voltage divider. I really appreciate your help with this. I don't know anyone where I live that has any interest in electronics, industrial controls or anything close so it is refreshing to get to talk about something I enjoy. I am just very inexperienced with the subject.

Also MarekJ71

"And regarding resistance change with temperature, it should increase with temperature increase, so I think it is not possible that 44ohms at low temperature changes to 11ohms when the heating elements are warm."

The 11 Ohm resistance was a single element removed from the circuit and checked. The 44 ohm number was the total resistance (Which was wrong due to Series vs Parallel). As to the resistance change in the heating element. I did a SMALL bit of data collection and some research my readings amounted to the following;

70 deg F = 11.1 Ohms

150 deg F = 10.8 Ohms

250 deg F = 10.4 Ohms

350 deg F = 9.7 Ohms

Which was really counter intuitive so I dug around and found the Physics Forum with this thread which covered the topic well. The math got over my head in a hurry but it is explained well enough to get it.

Thank you again for leading me in the right direction!

Just use your multimeter to measure the current it draws (assuming you have a high enough range)...short the meter leads together at first (to bypass the meter), so the meter is bypassed until the heaters warm up.

Use an oven with a blower (or add one) to keep the heating stirred up & much more even.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Hi Mark,

Most toasters, if not all, will use a specific heating element made from nichrome wire whose claim to fame is its increasing resistance with increasing temperature. Have a look at this reference https://en.wikipedia.org/wiki/Ni...

Nichrome wire is not just used for toast!

Best of luck being a geek... my granddaughters love exploring the things that surround my desk. I currently have them interested in magnets of all shapes, sizes and strengths. They are 8 and 6.

Ross McKenzie ValuSoft Melbourne Australia

Most multimeters are only good for 10A. I’d be wary of using my Fluke considering the fuse is \$15. Using a 0.1ohm 5W resistor would be safer or as suggested, a plug in energy meter.

Kartman wrote:
Using a 0.1ohm 5W resistor would be safer
At the expected 11A that will be 12W into your resistor.

0.1 Ohm is however a convenient value as a shunt for experiments. So use 4 of them, in the same way as your heating elements are connected now.

Or put 10 1 Ohm 2W resistors parrallel, or some other combination.

If you plan to do more experiments with this then I recommend to build such a shunt resistor in a sturdy box with some (isolated ?) banana plugs and also make a box with a dual throw switch with built in neon light for the 120Vac. Then you can connect an experiment without any voltage present, and throw the switch from a safe distance.

There are dedicated switches for this. BigClive on Youtube has a nice commercial version of such a switch with connectors that can only be accessed when the box is opened and no voltage is present.

I have found these "WAGO" connectors to be very convenient for building temporary experiments:

https://www.aliexpress.com/wholesale?SearchText=wago

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Mon. Apr 16, 2018 - 02:19 AM

A 5W resistor should last long enough to do a measurement!!!! You can never trust the info you get on the internet.

Kartman wrote:
A 5W resistor should last long enough to do a measurement!!!
True, but with this overload the resistor could be releasing some smoke, which might induce a "panic" reaction. That is not a good mix with 120Vac. Then there is the difference between the "cold" current and the "hot" current of the heating elements and being able to leave it on for a few minutes seems to be an advantage here.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Somehow my attempt at humour got lost.....

I would suggest using a resistor with less resistance. At 10 milliohm and 10 A of current, you get 1 W and 0.1 V on the resistor. Relatively low power and voltage high enough to measure with any reasonable voltmeter.

Paulvdh wrote:

Kartman wrote:
Using a 0.1ohm 5W resistor would be safer
At the expected 11A that will be 12W into your resistor.

0.1 Ohm is however a convenient value as a shunt for experiments. So use 4 of them, in the same way as your heating elements are connected now.

Or put 10 1 Ohm 2W resistors parrallel, or some other combination.

If you plan to do more experiments with this then I recommend to build such a shunt resistor in a sturdy box with some (isolated ?) banana plugs and also make a box with a dual throw switch with built in neon light for the 120Vac. Then you can connect an experiment without any voltage present, and throw the switch from a safe distance.

There are dedicated switches for this. BigClive on Youtube has a nice commercial version of such a switch with connectors that can only be accessed when the box is opened and no voltage is present.

I have found these "WAGO" connectors to be very convenient for building temporary experiments:

https://www.aliexpress.com/wholesale?SearchText=wago

GAAAHHH! Had to show me something else cool to buy didnt you Kartman.... Thanks!

Not sure why you want to know the current, but ...

If the label says 1300 watts, it probably approaches that at some point -- assuming that the info comes from the rating label which is is required by whatever agencies approved it for use. I assume that UL was one of them. That rating is there to prevent you from overloading a circuit and causing a circuit breaker to fail -- or in the worst case, a fire.

To verify this, you could probably modify a suitably rated extension cord with and add a decent  inline fuseholder. Then you could get some tubular fuses of various sizes and see where it starts to blow them. Make sure that you they're rated for 250 VAC -- that's pretty standard.

Then you can plug it in, turn it on and see if it pops a fuse. No issues with having a probe slip and getting a nasty shock. Adding a properly rated fuseholder to an extension cors should not be a big deal -- you should be able to break the hot line and use twist type connectors to splice in the fuseholder. Don't do it if you're not comfortable with mains type connections.

Be safe,

hj

Using blowing fuses would probably only give a rather crude accuracy (one step above better than nothing).  Cheapo fuses have a pretty wide latitude.

As mentioned, the heater rating would probably be more reliable.

You can never trust the info you get on the internet.

Trust me, I never do.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sun. Apr 22, 2018 - 08:00 AM