potentiometer as variable load ?  - Page 1
Forum section: electronics / beginners

author: Flump
author_rank: Frequent Contributor
timestamp: July 19, 2014, 07:54:39 pm
content: I would like a variable load to test 14v dc power supplies with and came across this on ebayWirewound Ceramic Potentiometer Adjustable Rheostat Resistor 200W 2K Ohmhttp://www.ebay.co.uk/itm/Wirewound-Ceramic-Potentiometer-Adjustable-Rheostat-Resistor-200W-2K-Ohm-/121276855637?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item1c3ca9f555and wondered if that could be used ?Ideally I would like a load that can go from say 2A up to 40-50Abut that pot should be good for 0-14A if I can sort some cooling out,was thinking about fan cooling it from underneath.

author: rob77
author_rank: Super Contributor
timestamp: July 19, 2014, 08:36:48 pm
content: Ohm's law !at 14V DC you need 14Ohm to get only 1Amp... with a 2KOhm pot you'll have really hard times to set it to 14Ohm.if you want to load the supply with 14Amps , then you will have to turn it down to 1 Ohm... that's impossible with a 2K pot.you should rather build a electronic dummy load.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 19, 2014, 09:27:10 pm
content: As Rob77 stated, that pot won't do what you want. To put 14 amps through a pot, a 1 ohm pot would have to be rated for 200 watts. They do exist. I have a 1 ohm 300W pot and I see even larger ones on sale on ebay. Try searching for < "1 ohm" rheostat > then repeat for 2, 3, etc. ohms.You could also build up your load out of multiple fixed resistors. This might turn out to be cheaper than a pot. I used to work for a telephone company. To test our large batteries and powerplants we had load boxes that were like switched resistance substitution boxes except instead of clip leads they had really big booster cables and large cooling fans! These things had ratings of many kilowatts. They had a big advantage over solid-state load boxes in that they were totally bulletproof. It was almost impossible to hurt them.By the way, I don't recommend using incandescent light bulbs as loads due to their very high starting surge.Ed

author: Flump
author_rank: Frequent Contributor
timestamp: July 19, 2014, 10:44:25 pm
content: ok thanks guys I will scrub that idea then

author: Monkeh
author_rank: Super Contributor
timestamp: July 19, 2014, 10:52:45 pm
content: Much better off building a constant-current load. Dave did a video on them, many on the forum have built their own (myself included). Relatively easy to scale.

author: retrolefty
author_rank: Super Contributor
timestamp: July 19, 2014, 11:18:56 pm
content: I agree that rheostats can be problematic as a variable load. I suspect that say a 200 watt unit is rated when using it's maximum resistance and you can very well reach a current limit of the wire if dialed down to just a few percent of range, where the localized heating can melt the wire.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 19, 2014, 11:45:48 pm
content: Quote from: retrolefty on July 19, 2014, 11:18:56 pmNo, they're not problematic. You're correct that they're rated at their maximum resistance. Just remember that the rheostat has a maximum current and stay within that. For example, Ohm's law tells me that my 1 ohm 300 watt rheostat has a current limit of about 17 amps. As long as I monitor the current and stay below that, it will never overheat. Since you're likely loading a power supply, you would typically be monitoring the current anyway so there's nothing extra required.Ed

author: WarSim
author_rank: Frequent Contributor
timestamp: July 20, 2014, 03:27:06 am
content: The comment about problematic was likely about granularity of control. A rheostat has about 300deg of adjustment a manual dynamic load normally has 3000deg of adjustment.Sent from my iPad using Tapatalk

author: robrenz
author_rank: Super Contributor
timestamp: July 20, 2014, 03:43:53 am
content: I made a set of power resistorsshown hereto load some of my supplies

author: Flump
author_rank: Frequent Contributor
timestamp: July 20, 2014, 08:43:05 am
content: Thanks for the replies everyone.How about some of these in parallel ?I like your idea robrenz and might copy thatbut try and source a big enough heatsinkto screw them down on.http://www.ebay.co.uk/itm/141351942097?_trksid=p2055119.m1438.l2649&var=440488678421&ssPageName=STRK%3AMEBIDX%3AITif I got 6 x 2R then I could have the following values6 = 0.333R , 42A , 588W5 = 0.400R , 35A , 490W4 = 0.500R , 28A , 392W3 = 0.666R , 21A , 295W2 = 01.00R , 14A , 196W1 = 02.00R , 07A , 98WWould this workout ?

author: Fank1
author_rank: Regular Contributor
timestamp: July 20, 2014, 01:22:26 pm
content: The pot is only 200W at 2K.Max current is 100ma

author: qno
author_rank: Frequent Contributor
timestamp: July 20, 2014, 09:25:52 pm
content: You need some big switches to switch 40A.And big wire diameter, plugs etc.It is going to be quite expensive.

author: G7PSK
author_rank: Super Contributor
timestamp: July 21, 2014, 07:33:02 am
content: You could use one or more elements from an old fashioned electric fire, I have done that in the past. Most are either 1 or 1.5 KW per bar, all I did was attach a wire to a jubilee clip and slide it along until I had the load required and it dose not matter if they get hot as that is what they are designed to do.http://www.ebay.co.uk/itm/VINTAGE-RETRO-ELECTRIC-FIRE-heating-elements-x-2-/151355337465?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item233d7b2af9

author: grumpydoc
author_rank: Super Contributor
timestamp: July 21, 2014, 07:53:32 am
content: QuoteYes it would - I have a dummy load for heavier current stuff made out of 16x 1R 50W resistors arranged in a string with various tap-offs so I can create series/parallel combinations as needed.Butthose 100W wirewound resistors don't magically dissipate 100W, there's a datasheetherefor a similar one which says derate by 50% for no heatsink but that feels a bit optimistic - you need to work hard to keep them cool if running close to the maximum power. I have mine bolted to a fairly meaty heatsink and have 3x 120mm fans running at them but 600W is as high as I can go without it melting.You can see the dummy load in this photo, you can't see the fans as they are attached to the back - at 600W the temp rise is 40-60oC above ambient.

author: Flump
author_rank: Frequent Contributor
timestamp: July 21, 2014, 05:17:23 pm
content: those arcol resistors are nice,the cheap ebay ones i linked to are pretty crap qualityas i had some before and tried to solder a wire to one of the legsbut then it pulled right out the side of the resistor.i see people use mosfets for loads so ill see if i can find a schematicthat isn't to complicated and try make a small one up to see how it goes.BIG heatsink is on my shopping list.