Rheostat as AC dummy load - Page 1
Forum section: electronics / beginners

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 08:13:31 am
content: What could be correct sizing(Specs) of the Rheostat to take on 2 Amps 230VAC.1. Does the usable wattage and current goes down as you use less than full resistance of rated Rheostat.some info in below thread reply #5https://www.eevblog.com/forum/projects/diy-high-tech-adjustable-acdc-dummy-load/new/#newI prefer Rheostat because it is esily avalable no need of DIY, only issue(Specs) sizing for my requirement.Thanks advance.

author: BravoV
author_rank: Super Contributor
timestamp: October 11, 2020, 08:16:30 am
content: Stop posting duplicate posts ...

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 08:28:47 am
content: Duplicate post deleted can gurus answer my question.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 08:39:57 am
content: Hi Gurus, can someone put light on my question, I need the answers urgently.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 08:59:04 am
content: You question lacks context but if you are literally proposing to wrap a pole in wire and slide a contact up and down it then you need to consider that the current handling capability of the wire will always be the same. So you need to use wire that will always handle the maximum current.So say you have a 1m rheostat and use 230V, say the total resistance is 115 ohms as you propose for 2A and therefore 460W. If you select only 0.5m of your rheostat that will be 57.5 ohms and 4A will flow at 230V or 920W, so your wire must be able to withstand 4A, your W/m will also quadruple as you have twice the power in half the length if you want to get more power or reduce the resistance further you can quickly see where this will go.What is your application?

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 09:23:18 am
content: @Simon, Thank you so much for reply, my applications are testing low watt inverters, voltage stabilizers, Variacs and 1:1 isolation transformers rated at 2 amp current. I want pure resistive load, so selected Rheostat.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 09:34:53 am
content: so get some resistors that will do the power and have the correct resistance. Why do you need a rheostat?

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 09:44:16 am
content: The reason why I selected Rheostat is it can used as a variable load in its range of Resistance, amps and watt. It is available spot on. Getting resisters, heat sinks and cooling fans DIY job I am not comfortable.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 10:10:28 am
content: In this entire context what is the role of Voltage.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 10:13:08 am
content: But if you have a rheostat that need to go from 1kW to 100W you will need to disipate 100 times the power per m of wire at 1kW that you need to at 100W so it is impractical. This is all ohms law which is easy to work out and you need to know it and do the calculations yourself. It is because of the complexity and impossibility of a one size fits all approach of a rheostat that electronic dummy loads exist.

author: TurboTom
author_rank: Super Contributor
timestamp: October 11, 2020, 11:21:10 am
content: What about an electrolytic load? Have a plastic barrel filled with salt water and dip two sheets of stainless steel into it. Depending on the conductivity of the electrolyte and the surface area of metal that's immersed, the current will vary and can be adjusted. Be warned: Since you are probably messing with dangerous voltages, precautions have to be taken that nobody can touch the setup. Moreover, the electrolyte may produce hydrogen / oxygen which is an explosive mixture. So the whole apparatus must be very well vented. The advantage is that you will be using the whole volume of water to dissipate the power so in the power range that you specified, you can probably run it permanently. By adding more electrodes, it's simple to extend the setup to three / multi-phase AC with common ground.The alternative would be using a fairly fast electronic DC load in resistance mode and placing a bridge rectifier in front of it. Testing of inverters may require a filter network.Take care,Thomas

author: BravoV
author_rank: Super Contributor
timestamp: October 11, 2020, 11:59:12 am
content: Quote from: suryaputhra on October 11, 2020, 08:13:31 amQuote from: suryaputhra on October 11, 2020, 09:44:16 amQuote from: suryaputhra on October 11, 2020, 10:10:28 amReading above, its clearly this thread is going no where with the op ignorance.Even the basic electronics like ... Volt = Current x Resistance or Wattage = Current2x Resistance is not in the op's comfort zone.Basically what the op want is, he only want to use rheotat, period. And tell him what rheotat size needed to load the mains line up to 230VAC@2A.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 01:38:23 pm
content: @BravoV, It is Rheostat not rheotatI hope it is not your comfort zone and digging into basics too.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 02:16:12 pm
content: @ TurboTom, Thank you so much, I will seriously look into the setup you suggested. Thanks again.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 03:07:43 pm
content: Do you need infinitely variable steps?

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 03:41:04 pm
content: Quote from: Simon on October 11, 2020, 03:07:43 pmIn the field we need sometimes.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 04:14:26 pm
content: So this has to transportable?

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 04:24:17 pm
content: Quote from: Simon on October 11, 2020, 04:14:26 pmYes, based on your inputs we have selected Rheostats and ordered from Amazon India, great source during lock down.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 11, 2020, 04:31:31 pm
content: You need infinite adjustment resolution but no other requirements? I call BS. If you need as close as possible to 12.345678 ohms, I'm 100.000% sure that a solution made using a bank of switchable fixed resistors will get you closer than a sliding rheostat which commonly are plagued by varying contact resistance. And you won't get infinite resolution with the rheostat, either; it's made of wire making discrete turns, and the adjusting the brush position connects to an integer number of turns at once.If you build it from fixed resistors, you'll get:1) Components actually rated for tolerance you need,2) Components actually rated for tempco you need,3) Switches / connectors rated for much lower, and much more stable contact resistances you'll ever get with the rheostat brush,and so on,so while you think you get "infinite" resolution out of the rheostat, in reality you are maybe +/- 5% off after careful adjustment.With say 5 fixed resistor (strings) in switchable binary configuration you'll have 32 different resistances available. Put in series with one fixed resistor (or string) so you never accidentally go too low. I guarantee this will be more accurate than most rheostats you actually end up buying.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 04:32:36 pm
content: Your best thing would be use a selection of resistors in series that you switch up and down to. You could include a rheostat so that you have steps and then vary between those steps.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 04:40:15 pm
content: Quote from: Simon on October 11, 2020, 10:13:08 amOur maximum load is about 200W. It is difficult to find electronic dummy loads in India. We have to DIY. If you suggest any company which sells AC electronic dummy loads please share link we will import it. Thanks in advance.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 04:44:28 pm
content: And the minimum?

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 04:52:59 pm
content: @Siwastaja, @Simon, OK I will go with your suggestions with resisters in series with step up and step down feature. There are resistance boxes available with different configurations does this resistance box fits in our requirement?. Anyway ordered Rheostats are not at great cost, let them be there in the lab instead of cancelling the order( This company sells on Amazon) every company struggling to cope with lock down except online companies like amazon.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 04:54:06 pm
content: Quote from: Simon on October 11, 2020, 04:44:28 pmAbout 60W.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 05:05:51 pm
content: 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 ohmsand 60W/230 = 0.26A, 230V/0.26A = 885 ohmsYou 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.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 05:08:20 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: October 11, 2020, 05:09:47 pm
content: 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.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 05:38:54 pm
content: Quote from: bdunham7 on October 11, 2020, 05:09:47 pm200W 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.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 05:42:23 pm
content: Quote from: Simon on October 11, 2020, 05:05:51 pmI am fortunate enough to have the above design from you, as you mentioned I will figure out the rest.Regards.

author: bdunham7
author_rank: Super Contributor
timestamp: October 11, 2020, 06:08:30 pm
content: Quote from: suryaputhra on October 11, 2020, 05:38:54 pmThen 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=8-23If 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.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 11, 2020, 06:20:33 pm
content: Quote from: bdunham7 on October 11, 2020, 06:08:30 pmThanks for great design and sharing Amazon India link for ceramic resisters I will go ahead with this design.

author: Simon
author_rank: Unknown
timestamp: October 11, 2020, 06:36:37 pm
content: Yes parallel would be easier. just start with your 60W and add more 10W chunks or whatever you require

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2020, 11:31:09 pm
content: Quote from: suryaputhra on October 11, 2020, 08:13:31 amThe wattage decreases as less of the full rheostat element is used.

author: coppercone2
author_rank: Super Contributor
timestamp: October 12, 2020, 01:48:11 am
content: 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.