Rheostat as AC dummy load

[Simon]

Actually the above calculation is not right, I'll leave it to you to figure it out. maybe it's (885-264)/140 = the resistor value per W or maybe i am still talking out of my arse but you get the idea.

[bdunham7]

 Could you clarify your power requirements?  In one place you say 200W max, then another you mention 2 amps, which is closer to 500W.

Rheostats are going to be harder to control than you think.  You may get pretty close to continuous control, but the noise and drift will make it difficult to be precise, if precision is what you need.   I was going to suggest either nichrome wire and ceramic insulators to make a 'variable toaster', or alternatively just using a bunch of small light bulbs with an equal number of switches.  If you are willing to use properly specified resistors, then all the better.  To figure out what will work, you need to know the minimum and maximum load you want to provide and the minimum step.  Surely something like 1 watt steps would suffice?  Perhaps 5 watt?  The larger the step you are willing to accept, the simpler your design will be.

[suryaputhra]

Could you clarify your power requirements?  In one place you say 200W max, then another you mention 2 amps, which is closer to 500W.

Rheostats are going to be harder to control than you think.  You may get pretty close to continuous control, but the noise and drift will make it difficult to be precise, if precision is what you need.   I was going to suggest either nichrome wire and ceramic insulators to make a 'variable toaster', or alternatively just using a bunch of small light bulbs with an equal number of switches.  If you are willing to use properly specified resistors, then all the better.  To figure out what will work, you need to know the minimum and maximum load you want to provide and the minimum step.  Surely something like 1 watt steps would suffice?  Perhaps 5 watt?  The larger the step you are willing to accept, the simpler your design will be.

200W maximum, just thought of having 2 amp for an added advantage in future. Anyway discorded Rheostat idea and moving towards Resistance in series as you experts suggested. 10W step up is suffice for us.

[suryaputhra]

You can get such boxes, they are called decade boxes but the power each resistor can handle is small. You will also have many values that are much higher than you need, they are also rather expensive at least in Europe.

So 200W/230V = 0.87A, 230V/0.87A = 264 ohms

and 60W/230 = 0.26A, 230V/0.26A = 885 ohms

You need a 264 ohm 200W resistor, how many steps from 60W to 200W? 10W steps? and 1W?

so 264/200 = 1.32 ohms for every 1W reduction, so add 13 13.2 ohm resistors in series and 10x 1.32 ohm resistors.

So you have 13x13.2, then the 264 then the 10x 1.32, you move your contacts at each end, at one end you have 10W increments, at the other you have 1W increments.

I'll leave you to figure out the power ratings of the resistors.

I am fortunate enough to have the above design from you, as you mentioned I will figure out the rest.
Regards.

[bdunham7]

200W maximum, just thought of having 2 amp for an added advantage in future. Anyway discorded Rheostat idea and moving towards Resistance in series as you experts suggested. 10W step up is suffice for us.

Then I think the simplest and easiest to understand design is to use parallel elements, combined into groups so you can select the power you want by adding.

Here are some resistors that would work--you just need 40 of them.

https://www.amazon.in/ELECTROPRIME-Ceramic-Cement-Power-Resistors/dp/B07L45L95W/ref=sr_1_23?dchild=1&keywords=power+resistor&qid=1602436955&sr=8-23

If you put 20 of them in parallel, you would have 105 watts at 230 volts as they are about 5.3 watts each.  If you want more accuracy, you can measure each one (they are 5% tolerance) and add an additional 470R or 560R resistor in series with each to give you as close to 10,580 ohms as you can reasonably get--this number gives you 5.0 watts at 230VACrms.  For a group of 20, you can just use 19 instead and it will be pretty close.  And, of course, you can measure the group if you have an accurate DMM.

So you need 5 switches and you put the resistors in parallel in groups of 20, 10, 4, 4 and 2, with a switch for each one.  This gives you 100W, 50W, 20W, 20W and 10W.  Just turn on the switches that add up to your desired load and you can have any number from 10 to 200 in steps of 10.  If you want to expand, you can add either a group of 38 more of these for an additional 200W, or for a smaller step, just one more and one more switch for 5 watt steps.  So, 80 of these and 7 switches would give you from 5W to 400W in 5W steps.

[suryaputhra]

200W maximum, just thought of having 2 amp for an added advantage in future. Anyway discorded Rheostat idea and moving towards Resistance in series as you experts suggested. 10W step up is suffice for us.

Then I think the simplest and easiest to understand design is to use parallel elements, combined into groups so you can select the power you want by adding.

Here are some resistors that would work--you just need 40 of them.

https://www.amazon.in/ELECTROPRIME-Ceramic-Cement-Power-Resistors/dp/B07L45L95W/ref=sr_1_23?dchild=1&keywords=power+resistor&qid=1602436955&sr=8-23

If you put 20 of them in parallel, you would have 105 watts at 230 volts as they are about 5.3 watts each.  If you want more accuracy, you can measure each one (they are 5% tolerance) and add an additional 470R or 560R resistor in series with each to give you as close to 10,580 ohms as you can reasonably get--this number gives you 5.0 watts at 230VACrms. 

So you need 5 switches and you put the resistors in parallel in groups of 20, 10, 4, 4 and 2, with a switch for each one.  This gives you 100W, 50W, 20W, 20W and 10W.  Just turn on the switches that add up to your desired load and you can have any number from 10 to 200 in steps of 10.  If you want to expand, you can add either a group of 38 more of these for an additional 200W, or for a smaller step, just one more and one more switch for 5 watt steps.  So, 80 of these and 7 switches would give you from 5W to 400W in 5W steps.

Thanks for great design and sharing Amazon India link for ceramic resisters I will go ahead with this design.

[Simon]

Yes parallel would be easier. just start with your 60W and add more 10W chunks or whatever you require

[David Hess]

1. Does the usable wattage and current goes down as you use less than full resistance of rated Rheostat.

The wattage decreases as less of the full rheostat element is used.

[coppercone2]

this question relates to kiln wall load, the amount of heat per square inch allowed to be generated in a kiln in relation to the surface area of the kiln.

What happens when you exceed the load is obviously aging of wire.

It would be nice if a rheostat manufacturer gave you the figure so this can be used to generate a SOA curve for a rheostat. A watts per angular degree figure maybe.

If you have a bare rheostat you could take the cover off, see what the temperature steady state is reached along the structure at maximum power, then try to do some figuring. The idea being that, no matter at what angle you set the rheostat, the skin temperature at the portion that is being heated does not go past the temperature reached steady state under full load at full resistance..

Then you can draw a little graph on the rheostat and call it a day and take off 10% to be sure its not going to break immediately.