Will a 2.2KOhm 10W resistor work as a dummy load for DC supply? - Page 1
Forum section: electronics / beginners

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 17, 2017, 04:16:37 am
content: I have a 2.2k 10Watt power resistor and I'm testing a power supply (The 1.25 to 12V one)Will this work as a load?What about a 10Ohm 10 watt?

author: drussell
author_rank: Super Contributor
timestamp: February 17, 2017, 04:53:20 am
content: https://en.wikipedia.org/wiki/Ohm's_law

author: tautech
author_rank: Super Contributor
timestamp: February 17, 2017, 05:00:49 am
content: A few auto bulbs are good. 6V ones from smaller kickstart bikes, 12V from stop and tail indicators or instrumentation bulbs. eg. a 12V 21W bulb draws ~1.75 A and you can push them to 15V for short periods for a bit more draw.

author: Brumby
author_rank: Unknown
timestamp: February 17, 2017, 05:30:26 am
content: 2.2k resistor on 1.25v will pull just under 0.6mA and dissipate around 0.7mW2.2k resistor on 12v will pull around 5.5mA and dissipate around 65mWNeither of these are much of a test on a power supply - and a 10W resistor isn't going to get noticeably warm.10ohm resistor on 1.25v will pull 125mA and dissipate around 156mWAgain, this isn't much of a test.10ohm resistor on 12v will pull 1.2A and dissipate 14.4WThis IS a better test for the power supply - so long as it is rated to deliver at least 1.2A. However, it's not much fun for the resistor, since this exceeds its power rating. If you wanted to do a test at this current for 5 seconds, I wouldn't be too worried - but leaving it for 5 minutes wouldnotbe on my list of 'good ideas'.As suggested above, use Ohm's Law to work out appropriate resistor values and calculate power dissipation to make sure your load can handle it. (Suggestion - work through the figures from what I have posted above and see if you get the same answers.)Automotive bulbs are not a bad choice either - but just remember to work out the current from the voltage and power figures. Measuring the filament resistance when it is cold won't give you the right figure - because the filament resistance increases as it heats up.AND - as with any testing procedure - make sure you are performing your tests within the specifications of the device under test (DUT) .... unless you are testing tofailure.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 17, 2017, 07:14:49 am
content: Yes I did use ohms law came up with the same values in the third post. My question should have been: Do dummy loads need to be running your power supply at their limit to get consistent results? Such as the 10 ohm draws about 1 amp at 12 volts. Transformer is rated at ~15watts (15VACx1100ma).

author: amirm
author_rank: Regular Contributor
timestamp: February 17, 2017, 07:25:42 am
content: Consistent for what?With Power Supplies you want to test them to their rated limit. If that is 3 amps, then you want to see if it can actually put out that much current. You work backward from that to determine the dummy load resistance you need.

author: Brumby
author_rank: Unknown
timestamp: February 17, 2017, 07:34:29 am
content: This:Quote from: amirm on February 17, 2017, 07:25:42 am

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 17, 2017, 07:48:00 am
content: Quote from: Brumby on February 17, 2017, 07:34:29 amThat:Quote from: amirm on February 17, 2017, 07:25:42 am

author: Rick Law
author_rank: Super Contributor
timestamp: February 17, 2017, 03:50:47 pm
content: Quote from: tautech on February 17, 2017, 05:00:49 amAnother vote for car-bulbs. Besides being fully capable of dissipating the heat, you have visual confirmation that it is still going and visual reminder that it is HOT.The only draw back is that their resistance vary. Well, even a fixed resistor will have varying resistance depending on temperature, but bulbs varies a lot more.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 17, 2017, 04:00:55 pm
content: Light bulbs have a big inrush current that settles as the element heats up. Some supplies see that as a problem, even if the supply is working as intended.Sent from my horrible mobile....

author: FrankBuss
author_rank: Unknown
timestamp: February 17, 2017, 04:34:19 pm
content: I can recommend these resistors, if you need more power:http://cgi.ebay.de/201054222850The seller has other values as well. Cheaper than a professional electronics load, if you don't need to test many different ranges. But more expensive than building your own electronics load, which is easy:

author: innkeeper
author_rank: Unknown
timestamp: February 17, 2017, 06:55:15 pm
content: So no a resistance its not a constant current load....since voltage is part of the I = E/Ras the voltage increases, to keep a constant current the resistance needs to also increase.this is a good use of a constant current load... but there not cheap...so then comes the question... how to load test a supply without one of those fancy smancy constant current loads....you use different resistor values for different voltage levels that meet your test current requirements.what are those requirements... the rule of thumb I use is to test within 20% of maximum. (prevents over stressing your transformer, regulator etc)Here is what I would do personally.assuming max rating of the 15vac transformer is 1100maI would load it to 80% as my max working current (20% safety margin)so 1100ma * .8 = 880ma max12v / 880ma = 13.64 ohms.power = 880ma * 12v = 10.56w * 1.2 (saftey margin) = 12.7w13.64 ohms is a odd value, so id do a little rounding id use nothing lower then 14 ohms with a min power disputation of 13watt 12 v.@12v 14 ohms 13watts10 ohms @ 10 watts would draw too much current and would be under rated for power dissipation at 12v.at the low end 1.25v..1.25v / 880ma = 1.42 ohms1.25 * 880ma = 1.1watts * 1.2 = 1.32 wattsso at 1.25v1.42 ohms at a min of 1.32 watts. (id likely round up on the resistance to a value I could easily make up or buy and use a 2watt resistor)----------------------if all you have is a 10 ohm resistor at 10w... and you want to use that to load your supply, then don't exceed lets say 8w (that 20% margin again)so the max voltage id run to load the supply would be...8w / 880ma = 9.09vso lets call it 9Vbut your current draw will be much lower then maximum at 1.25v .... 125ma--------if you want an off the shelf resistor you can test though the whole range...get a 15 ohm 15w resistor (3 series 5ohm 5w resistors will do)but again the maximum current will be drawn at the highest voltage setting.It very much depends what your goals are in testing the supply in choosing how your going to load it.as others have recommended, light bulbs can also be a good cheap load but, be carful there not drawing to much current.I hope this helps

author: amirm
author_rank: Regular Contributor
timestamp: February 17, 2017, 07:08:40 pm
content: Quote from: FrankBuss on February 17, 2017, 04:34:19 pmGood suggestion. I have a bunch of these I use for amplifier testing. They handle a lot of power given their large size.

author: james_s
author_rank: Super Contributor
timestamp: February 17, 2017, 07:36:01 pm
content: Play around with thishttp://www.falstad.com/circuit/You can wire a virtual resistor to a virtual voltage source and experiment with the values and *see* the current flow. If you're a visual thinker it's going to be more clear than plugging numbers into the equations for Ohms law.

author: FrankBuss
author_rank: Unknown
timestamp: February 17, 2017, 07:52:47 pm
content: Quote from: james_s on February 17, 2017, 07:36:01 pmOr you can use the 10 ohm resistor with 12 V and *smell* the current flow

author: rx8pilot
author_rank: Super Contributor
timestamp: February 17, 2017, 08:11:49 pm
content: Quote from: FrankBuss on February 17, 2017, 07:52:47 pmThere is someone out there with a 1% calibrated current nose. They can tell you the current within 1% just before the fire. ;-)

author: Brumby
author_rank: Unknown
timestamp: February 18, 2017, 12:10:31 am
content: For those of us with uncalibrated noses, we can use a FLIR camera. For those with restricted equipment budgets, a thermocouple probe can give an indication - and for those even more cash strapped, just count the layers of skin removed when you grab it after 1 minute.

author: Gregg
author_rank: Super Contributor
timestamp: February 19, 2017, 03:09:32 am
content: If you are serious about testing your power supply, I have some 500 watt 2.0 ohm open wire resistors that I can send you. Similar size to these but with thicker resistance wire:http://www.ebay.com/itm/Cutler-Hammer-G3AP1500-Porcelain-Base-Wirewound-Resistor-15-Ohm-420-W-Lot-of-2-/351949659932A good source of cheap fixed 12V, 5V, 3V power supplies are old computer power supplies. They are perfect for the tinkerer that may let some smoke out while on a budget.PM me if you want the resistors.

author: Brumby
author_rank: Unknown
timestamp: February 19, 2017, 03:25:55 am
content: With 4 of those, it would be just the sort of thing I could use to test a 13.8V 25A power supply I have.

author: IanB
author_rank: Super Contributor
timestamp: February 19, 2017, 03:43:21 am
content: Quote from: innkeeper on February 17, 2017, 06:55:15 pmThe trouble with a constant current load is it contains a feedback control loop to vary the resistance to maintain the set current.If you connect one of these to a regulated power supply, then that power supply also contains a feedback control loop to maintain the set voltage. If you are unlucky the two control loops will fight each other and the whole system will go unstable.IHMO, it's really not a bad idea to test power supplies with constant resistance loads to avoid such troubles.

author: Gregg
author_rank: Super Contributor
timestamp: February 19, 2017, 04:46:02 am
content: I forgot to add to my previous post that open nichrome resistors that I mentioned can be tapped almost anywhere the coils are exposed to vary the resistance. Most any electric heater that uses nichrome elements that are accessible can be used for load banks with a little ingenuity.

author: Brumby
author_rank: Unknown
timestamp: February 19, 2017, 05:38:47 am
content: I've got a little Nichrome - some..... where......... ............

author: boffin
author_rank: Unknown
timestamp: February 19, 2017, 07:02:42 am
content: Quote from: raspberrypi on February 17, 2017, 04:16:37 amHeadlight bulb from a car, 12v, 30-50w. Find an old sealed beam type as they're fairly robust.

author: james_s
author_rank: Super Contributor
timestamp: February 19, 2017, 08:07:18 am
content: Headlight bulbs are 55/60W mostly for lowbeam/highbeam. They do make good test loads, although for a LM317 based supply they're going to be too big a load. The regulator is only good for 1.5A.

author: tautech
author_rank: Super Contributor
timestamp: February 19, 2017, 08:23:48 am
content: Quote from: james_s on February 19, 2017, 08:07:18 amYet perfect for a LM338 based PSU.Gotta be careful with halogen units though, they'll burn/melt most things they come in contact with.