Dummy Load for Bench Power Supply - Resistors? - Page 1
Forum section: electronics / beginners

author: DW1961
author_rank: Frequent Contributor
timestamp: November 01, 2021, 07:46:31 pm
content: I'd like to load a bench power supply using some sort of simple dummy load. I was thinking about a couple of 100 watt resistors. I can get two of them for 8 bucks.https://www.amazon.com/Twidec-Wirewound-Resistors-Equipment-RX24-100W1RJ/dp/B07QY6365F/ref=pd_di_sccai_5/137-6696668-9948413?pd_rd_w=aY7wU&pf_rd_p=c9443270-b914-4430-a90b-72e3e7e784e0&pf_rd_r=P69W3XKZ8GKEE2Z97PQ4&pd_rd_r=133a564b-73a6-42d1-918e-03d37798984e&pd_rd_wg=haTE7&pd_rd_i=B07QY6365F&psc=1I'm not sure about the Ohm rating or other things I would need to use a simple resistor for a dummy load, however. Is there a better way that is just as easy and cheap?Is there anyway to run two identical resistors together for x2 power resisted?

author: Benta
author_rank: Super Contributor
timestamp: November 01, 2021, 07:51:11 pm
content: Voltage? Current? 1 ohm won't get you far IMO.

author: tunk
author_rank: Super Contributor
timestamp: November 01, 2021, 08:49:29 pm
content: I'm sure you already know that you also need to attach the resistor(s)to a large heatsink. If you e.g. have a 30V/5A PSU, then you need a150W 6 ohm resistor (R=U/I=30/5=6, W=U*I=30*5=150).

author: Gyro
author_rank: Super Contributor
timestamp: November 01, 2021, 08:58:38 pm
content: I have a selection on self (radiation/convection) cooled tubular rheostats that I use. The chunkiest is 3.3R 10A... 330W @ 33V.These are the sort of things that used to be used in school labs. Maybe try ebay. Note, the single turn rotary ones tend to have lower wiper ratings than end to end of the winding.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 01, 2021, 09:36:40 pm
content: Quote from: Benta on November 01, 2021, 07:51:11 pmI could probably do with 30V 5A but if it isn't too expensive 30V @ 10A would be all I would ever need.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 01, 2021, 09:38:04 pm
content: Quote from: tunk on November 01, 2021, 08:49:29 pmIt would only be running for less than a minute. Still? I should probably just build a cheap one using a small 1 x 4 and then a heat sink. Any ideas?Would a 150W 8 Ohm work?Also what about running smaller wattage resistors in a series? Several smaller ones are cheaper than a single large.

author: Gyro
author_rank: Super Contributor
timestamp: November 01, 2021, 09:44:59 pm
content: If you're load testing a PSU you need to load it for long enough for it to reach a steady state temperature. A few seconds won't tell you whether it can sustain a given load current without frying.

author: Benta
author_rank: Super Contributor
timestamp: November 01, 2021, 09:54:38 pm
content: 30 V at 5 A is 150 W, at 10 A it's 300 W.That heatsink is going to be one heck of a mother. You're more likely looking at forced cooling, either with fans or water.A better idea than resistors could be heating elements in that power range. For example Diesel engine preheaters from a salvage yard. Should be cheap.

author: Jwillis
author_rank: Super Contributor
timestamp: November 01, 2021, 10:02:05 pm
content: Even a cheap 180W electronic load from Banggood would give you a better indication of the potential of you power supplies and batteries. They're not bad . I have a couple of them . One I put a larger MOSFET and Heat sink on and one I put a just larger heat sink on and both will handle 150W easily . For anything above 150W they might be questionable . I have not tried either at that power. Built an >800W for over 150W DUT .

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 01, 2021, 10:47:35 pm
content: You could go with the old Cantenna approach and use a big resistor 6-8 ohms and wire the resistor into a paint can with mineral oil.Not related to the WiFi cantenna.You could also put say 12 100 ohm 5 watt resistors in parallel in the can and end up with 8.3 ohms maybe DC to 50 MHz depending on the resistors.12 5 Watt resistors in parallel give you a 60 Watt load12 10 Wall resistors equals 120Watt load etcBigger resistors equals more watts as do more resistors in parallel.These resistors are cheap and can be configured into any ohm load.You can run these type loads for at least several minutes.Heathkit made one and other manufactures still sell the same thing. These are 50 ohm loads customarily.https://www.orcadxcc.org/content/cantenna_va7jw.pdfHams have been using this Homebrew load for at least 80 years for stuff up to one KW or more. I use one.They solder one end of the resistors to a copper rod or maybe a 12 g wire and connect that to a RF connector. The can itself is the negative. as is the outer RF conductor.You could use any DC connectors you like .Customarily, there is a relief valve on top which is just a little hole covered by a stiff rubber flap.Just don't leave it on for too long, it will heat up. Put it on a pie plate for any possible mineral oil escape. Transformer oil these days is better than mineral oil and is silicone. Good if you can easily procure some. Old transformer oil had PCBs.

author: Gregg
author_rank: Super Contributor
timestamp: November 01, 2021, 11:34:36 pm
content: Automotive filament light bulbs can be series / parallel wired to get just about any load in your range. I personally have a bunch of H4 headlight bulbs with the low beam filament burned out that I use for such purposes.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 02, 2021, 12:19:04 am
content: Quote from: Gyro on November 01, 2021, 09:44:59 pmIt's true. I really do need a longer time. I'll just have to do what it takes.https://www.amazon.com/Resistor-Aluminum-Wirewound-Resistors-Chassis/dp/B08G8JGW1M/ref=sr_1_43?dchild=1&keywords=150W+6+ohm+resistor&qid=1635802995&sr=8-43Those type have their own heat sink. I could just run a 12 computer CPU fan over it in open air, right?

author: DW1961
author_rank: Frequent Contributor
timestamp: November 02, 2021, 12:20:52 am
content: Quote from: Benta on November 01, 2021, 09:54:38 pmWhat about running x2 150 watt in a series?

author: DW1961
author_rank: Frequent Contributor
timestamp: November 02, 2021, 12:27:12 am
content: Quote from: Gregg on November 01, 2021, 11:34:36 pmWill I need a welders glasses at some point?

author: DW1961
author_rank: Frequent Contributor
timestamp: November 02, 2021, 12:36:37 am
content: Quote from: Wallace Gasiewicz on November 01, 2021, 10:47:35 pmWallace, thanks for all of that. Would lower wattage resistor in parallel run cooler than a single large resistor?

author: beanflying
author_rank: Super Contributor
timestamp: November 02, 2021, 01:11:26 am
content: Might be a bit over Engineered but it worksThe Resistors are 50W and I built it as a high power decade as much as for a Dummy Load so 1,2,3 & 4wired with bars so I can series/parallel or parallel two 5pairs for maximum power. Copes easily and stays coolish at 7V and 15A

author: ledtester
author_rank: Super Contributor
timestamp: November 02, 2021, 01:16:13 am
content: You might find this video interesting/helpful:Home Lab Equipment: Making sets of Power Resistors - TheHWcavehttps://youtu.be/Fxp0nxphgYwIncludes a discussion of heatsink requirements.Also mentions that a bank of power resistors is useful for testing AC power supplies.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 02, 2021, 01:35:16 am
content: Got a question about doing this with resistors. After doing some reading, it looks like the resistor load tester will always pull it's maximum wattage from the power supply, or try to. If I tried to limit the current, for example, at the power supply, and the resistors were asking for more power, I'd get an OC on the PSU? In other words, if I built a resistor load tester for 30V/10A 300 watts, and my power supply was set to 150 watts, it would OC (or trip the protections)?

author: ledtester
author_rank: Super Contributor
timestamp: November 02, 2021, 01:51:49 am
content: Those smaller "100W" resistors definitely need a good heatsink... here's a pic comparing them to a 8-ohm 120W power resistor made by Dale in the 70s:

author: wizard69
author_rank: Super Contributor
timestamp: November 02, 2021, 01:54:14 am
content: That is one pretty load.Quote from: beanflying on November 02, 2021, 01:11:26 am

author: beanflying
author_rank: Super Contributor
timestamp: November 02, 2021, 01:56:45 am
content: Quote from: DW1961 on November 02, 2021, 01:35:16 amYou need to think of each single resistor being capable of dissipating X watts depending on how you wire them together. ** Also assumes they are properly heatsinked.Below is some figures I did a while ago on my home made decade boxes (the top two are not relevant and not shown). The two lower figures are for the test setup shown above at 2.5the limit is the 4resistor in one of the legs being 3.4A for maximum 50W (I^2*R=50W) dissipation. So using this and basic Resistor theory the box will run at twice this current or about 7A throught the two 5legs. If you then apply Ohms Law you get a maximum voltage of 7x2.5 or around 17VThe other configuration which is the current and voltage measurements you see above is all four resistors paralleled so around 0.5with the limiting resistor being the 1in this case so maximum current (I^2*R=50W) is circa 7A. In this setup the current/power is shared across the stack but the others will be handling less of it.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 02, 2021, 02:55:28 am
content: As a rough rule of thumb, those aluminium cased power resistors are basically halved in power rating when not attached to a heat sink.

author: David Hess
author_rank: Super Contributor
timestamp: November 02, 2021, 03:45:35 am
content: Cement power resistors have the lowest price per watt so I find the best deal on 5, 10, or 20 watt resistors in a value that I can series and parallel to get the resistance and power rating that I need.

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 02, 2021, 04:30:39 am
content: Incandescent lamps are cheap.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 02, 2021, 11:21:22 am
content: A bunch of lower (not TOO low) resistors help a little in that the coolant surrounds more of the resistor. the idea is more surface area with a bunch of smaller resistors.Probably unnecessary if you can find bigger resistors cheap.Look on ebay and see the price point, I think it will favor the 5 or 10 watt cement resistors, probably 100 ohms.Someone had a model of one of these using resistors soldered to can lids in two levels also. there were many resistors in each level, so he made the load with parallel and series. Then put it into a paint can with mineral oil.You can also use a bigger can like a metal 5 gallon can but it becomes heavy.These things are made for 50 Ohms but you can build one for whatever Ohm you want.The can lids kept the resistors away from each other to maximize the coolant contact. Lots of people actually made and used and liked this thing.Someone brought up incandescent bulbs:Electric incandescent light bulbs work but resistance changes with different voltages when they heat up. This idea was also used by Hams a lot because it was cheap.This works but it is not very stable resistance. Also would require a bunch of incandescents in parallel. But it does light up!!!!lI think there is also a rush of current as the bulb light up.AC incandescent bulbs work just fine with DC. Also heat sinking is less of a problem since they are made to run hot.Remember Resistance is not Futile. Resistance is voltage over current . Refer to OHM. HA!!!

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2021, 11:22:30 am
content: Quote from: BrokenYugo on November 02, 2021, 04:30:39 amAnd unfortunately very non-linear. The cold resistance can be 5...6 times lower than at temperature.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 02, 2021, 05:06:13 pm
content: Quote from: TERRA Operative on November 02, 2021, 02:55:28 amI was researching and some say de-rate them by x4 also, not the usual 50%. So a 100 w resistor would be max sustained 25 watts.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 02, 2021, 05:11:25 pm
content: I've been looking at Nichrome wire, also. I could wind that around ceramic posts. Then just connect where I need the wattage to be. However, what you have then is basically exposed heater coils. I'd have to put them in a box with a fan. Then, creating my own space heater. lol

author: Stray Electron
author_rank: Super Contributor
timestamp: November 02, 2021, 06:25:29 pm
content: Quote from: DW1961 on November 01, 2021, 07:46:31 pmIf you want to keep the original resistance than you need to use four equal value resisters. Two in series and then that in parallel to two others in series. Each string will have double the resistance but 2R paralleled to 2R would yield 1R. The advantage sn that the heat dissipation is quadrupled.

author: Gyro
author_rank: Super Contributor
timestamp: November 02, 2021, 07:08:56 pm
content: Quote from: DW1961 on November 02, 2021, 05:11:25 pmYep, that's what it is!Remember that Nichrome isn't solderable (aside from the temperature of the joint). You need to crimp the connections - or you can get high temperature ceramic screw terminal blocks, eg.https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1311&_nkw=ceramic+terminal+block&_sacat=0

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 02, 2021, 09:34:02 pm
content: Need lots of nichrome wire and be careful, I almost started my bench on fire as a kidYou would be making a toasterYou can also wind the wire around a ceramic tube and put it in the Cantenna. Ater measueing the ohms to your satisfaction.

author: pqass
author_rank: Super Contributor
timestamp: November 03, 2021, 12:48:58 am
content: I used to futz around with power resistors but then watched mikeselectricstuff whip-up a crude e-load so I made my own. It was really really easy and have used it many times to test power supplies and batteries.For less than $20 you could easily create a 360W load with a pot, LM324, 4xMOSFETs, 4x4x1R 1W resistors, and a handful of passives. I've used an old 6cm2CPU heatsink but if you're hard-up, just attach the MOSFETs to one end of a long Al plate and submerge the other end in a bucket of water (keeping just the MOSFETs above the water level).See attached schematic. Make four of these for the full 360W. Just tie the MOSFET drains together as your positive post, ground is your negative post. The pot wiper is attached to all +in pins of the opamps. You could use fewer modules but I wouldn't push more than 100W per MOSFET module.http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgoqoQFMBaMMAKAEMQUUAWEbvKtzh8BIYuBRRk8YTAzdCPbGEKEEebGnlRo8xZRlww2bWEMsATn2H9BxQiKpUzkS9drde3exU9SXrgDu7r68XLwIfkGcPKExXj6uAGZ8PrYUhIKimiDYOiRo3GAI4cTGLMEImakOVR68rgDmGVSZeC0gig1QbnVxfW1SqAgsAMYh6UL1Q7DwEAjQxCaQxHglhEJ4xFw6Rixg2ht8KuB+CGCSEAFOs0gASnQAzgCWjwAubAB2o3QszUfnSQAlDtJwsZYdbCEdp9BS8CBgu6Q6GQtBDay5JwzEZIo7YKGdMJ4UFSIQSdEwEbBMB+OHxPhHVwAB1OXiONIiF3AQxYLK6cX5gwRPWC-MiEWq4pFnVigwG+GlHIoXIBXOZMrZvDFfmF0X5dNhjN5NUcJoQGAcuoqJrl1UG0T6Uu8tSibjxBKORQczgMcGtzs6CoD5u9LAAHp11HwSiAMAgkNx43xOLwAJJ3ABiAAUUJRw51qtwLWJVnwMOJ4e07gATR4ACgA9p8AJQAHQAjjB8AgADStx4ANUajw7aAHfcHw7rbwAFm329gB-miA5uLFsIm+Jhk7EADIAWU03HzahhCHEAnEVUklfAd37Yf73H7z0+-aZbAsbAANt+6N+WAbagjnIIRSAxQhoG6K5MkAkB2hA6xwLMTooO5CAUGgEYgA

author: Electro Fan
author_rank: Super Contributor
timestamp: November 03, 2021, 03:31:28 am
content: Quote from: wizard69 on November 02, 2021, 01:54:14 am+1

author: wizard69
author_rank: Super Contributor
timestamp: November 03, 2021, 03:59:31 am
content: Well if you are going that way , almost any material with a resistance would work. Steel banding used for heavy shipping for example could be coiled up to create a load. In either case you have two problems. One is that you are creating an inductor which might be important. The other thing is that you have a very nice fire starter.Somebody mentioned above "the resistor in the paint can" approach. That is a very good idea if you need to test for an extended time. You can also air cool. I've seen huge motor drives in industry with massive wire loads mounted above the control panels that are fan cooled. You can build the same thing if you keep an eye on safety.Quote from: DW1961 on November 02, 2021, 05:11:25 pm

author: DW1961
author_rank: Frequent Contributor
timestamp: November 03, 2021, 04:27:03 am
content: Quote from: pqass on November 03, 2021, 12:48:58 amWould that only test 12 or 30V? If I could find a kit I would buy it and just assemble it myself.

author: pqass
author_rank: Super Contributor
timestamp: November 03, 2021, 05:46:37 am
content: Quote from: DW1961 on November 03, 2021, 04:27:03 amActually, it would depend on the MOSFET. The IRFP250 datasheet says it is rated to 200V or 30A max. so any voltage or current below these limits. However, practical circuits should give a good buffer and stay below 175V.But what really limits the circuit is the heat transfer capability of the package. The TO-247 package is better at transferring heat from the die to the environment then say, a TO-220. That's why I suggested to keep each MOSFET at 100W max. So you can do 175V@571mA or 30V@3.3A or 10V@10A. In the circuit shown, I chose to limit the current to 3A. If you increase this design limit then that changes the wattage of the 0.25R shunt; eg. 25V@4A keeps the MOSFET to the 100W limit but now the shunt must be 4.7W+margin (probably should double that). You can try this yourself in the simulation by lowering the 150K resistor to 100K and the voltage source from +30V to +25V.As I suggested, if you parallel 4 of these circuits then you multiply the 3A individual circuit limit by 4 or 12A. And if you stay at 30V or below, you won't overshoot the 100W limit per MOSFET or a combined 400W. If you plan on inputting more than 30V then you'll have to turn-down the pot. Say, about half-way (1.6A per circuit) for up to 60V, etc.This circuit can be assembled on a small perfboard; 1 IC+passives with short leads to 4 mounted MOSFETs, shunts, and a pot. I think it's a good beginner project.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 03, 2021, 06:51:20 am
content: Quote from: Stray Electron on November 02, 2021, 06:25:29 pmI think that's what I'm going to do, but can I run more than 4 and maintain 6ohms? I was thinking 8 100w resistors. Max 300w watts, divided by 8 resistors should keep them pretty cool?

author: theHWcave
author_rank: Regular Contributor
timestamp: November 03, 2021, 09:27:32 am
content: Have a look at this videohttps://youtu.be/Fxp0nxphgYwwhere I build a power resistor setup for this purpose (and many other uses). These really have become invaluable over time. The idea is to use 10, 20, 40, and 80 Ohm (and 1, 2,4,8) because by parallel and serial connections that gives you an incredible wide selection of possible uses and power ratings. I calculated a table of resistances and power ratings of all possible combinations which you find on my Github pagehttps://github.com/TheHWcave/BCD-Power-Resistors

author: beanflying
author_rank: Super Contributor
timestamp: November 03, 2021, 12:11:49 pm
content: Quote from: theHWcave on November 03, 2021, 09:27:32 amNice idea. The reason I went 1,2,3 & 4 rather than BCD was I could get two matching pairs of 5No right or wrong just that was my reasoning at the time.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 04, 2021, 12:00:02 am
content: Here is a solution a 6.8 Watt 200 Watt resistor, Put it on a big heatsink with a fan or put it on a smaller heatsink in a can of oil.US Stock 6.8 ohm 6R8 200W Watt Aluminum Housed Metal Case Wirewound ResistorsJust copy the above and past it into e bay

author: DW1961
author_rank: Frequent Contributor
timestamp: November 04, 2021, 07:06:10 am
content: Got something on the way. I'll post after I wire it up.Thanks for all of the help and ideas from everyone.

author: Electro Fan
author_rank: Super Contributor
timestamp: November 05, 2021, 04:40:22 am
content: Quote from: Wallace Gasiewicz on November 04, 2021, 12:00:02 amLooks like they might also have similar 200 watt 10 ohm resistors for about the same price.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 05, 2021, 09:47:49 am
content: Quote from: pqass on November 03, 2021, 05:46:37 amEven 100 W from a TO247 part needs a good heat sink.Beside the total power and voltage there can also be lower SOA limit. So not every combination of voltage and current to give the 100 W may be reliable. Especially high power at a relatively high voltage can be a problem and lead to failure (some samples are OK, some may fail). The SOA specs on MOSFETs are a tricky topic - often there is no curve in the DS and even if there is may not be valid.The resistor version has definitely some advantage in reliablity and may get a way with a smaller heat sink. However it is less flexible. With the resistors the chances are smaller to get fakes from Ebay or Banggood. So lower cost sources may be acceptable.As a crude test an old toaster may make a cheap high power resistors, usually better than incandesant lamps.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 05, 2021, 05:18:26 pm
content: Quote from: theHWcave on November 03, 2021, 09:27:32 amHA, found that a couple nights ago even before you posted. I watched the entire video. Very good. That's sort of what I am doing, except I'm going to use 4 100 watt 6 ohm resistors in various series or parallel to get what I need. I already spent $29.00, so it's enough for now. Heatsinks are expensive. I think I may have gone too small on the heatsink, too, but it is what it is for now. (Any biggest on the sink and the prices doubled or more)Heat sink:Resistors:https://www.amazon.com/gp/product/B07KJZYW7J/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1Heat Sink:https://www.amazon.com/gp/product/B08T8837ZH/ref=ppx_yo_dt_b_asin_title_o00_s01?ie=UTF8&psc=1The heat sink is only 5.9 x 2.75 x 0.38 but has 28 fins on it.I'll also attach a decent 12 V CPU cooler to it if I need it.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 05, 2021, 05:21:35 pm
content: Quote from: Kleinstein on November 05, 2021, 09:47:49 amA friend of mine mentioned using an old toaster, or even a cheapo 10 dollar new one. However, wouldn't that have a fixed ohm/power rating? Those even small toasters can use a lot of power.

author: Jwillis
author_rank: Super Contributor
timestamp: November 05, 2021, 05:28:52 pm
content: Quote from: Kleinstein on November 05, 2021, 09:47:49 amFor a reliable electronic load I would only suggest Linear MOSFETS .Theirs really only a couple manufacturers that produce reliable Linear MOSFETs . Infineon and IXYS . And they're not usually cheap .Nonlinear MOSFETs are tricky, as you say, because theirs no defined SOA for the DC region and can differ significantly from one chip to the next , even on same makes and models .Power resistors from Ebay or Banggood I wouldn't go as far as calling them fakes but they can be of low quality . I've had a couple aluminum shelled power resistors exploded out the ends when pushed close to their maximum rated power. Pretty scary events . Could be just isolated cases but I don't push them far anymore.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 05, 2021, 08:20:14 pm
content: Quote from: Jwillis on November 05, 2021, 05:28:52 pmI've read reviews of that happening, but many reviews don't report any problems. I won't be trying my luck with them. Well, maybe I will. lol

author: pqass
author_rank: Super Contributor
timestamp: November 06, 2021, 01:59:45 am
content: Quote from: Jwillis on November 05, 2021, 05:28:52 pmOkay, if 100W per module is too ambitious then just use 8 parallel modules at 50W per. Surely they can do that much. The cost per module (MOSFET, 0.25R shunt, opamp, and passives) is less than $4 (Digikey prices). Power resistors aren't cheap either. The e-load solution is simple, repeatable, and you gain dialing-in any power you may need without thinking; no discrete resistor combinations. What's not to love?Well, there is Kerry Wong's 400W/100A beast but each  MOSFET is $32!http://www.kerrywong.com/2017/01/15/a-400w-1kw-peak-100a-electronic-load-using-linear-mosfets/I'd rather go with lot more but less stressed devices. It's good enough for HPAK. They use 8 modules of IRFP140's. I'm sure they were preselected by grey-bearded virgins. Although, I don't know what's up with the parallel IRF540! See attachment and/or page 26 here:https://dev.xdevs.com/attachments/download/148/Agilent_N3304_component_shematic_5989-6691EN.pdf

author: DW1961
author_rank: Frequent Contributor
timestamp: November 09, 2021, 09:44:54 am
content: It's not pretty, but it works. Heat sink is waaaaaaaaaaaaaaay to small. I ran it fanless at 50 watts for about 10 minutes. I think it hit around 80C. Vendor said it was good to 130C. I'll put a fan on it tomorrow and test it with quite a bit more power. It's going to work just fine, though. I need to find a bigger sink and a housing with banana female so I can have it in a box and swap resistance easily. I'd like to have it all self contained and use a 120mm CPU fan that moves some air through the box (open on each end.). The hard part is finding parts to do that.

author: Gyro
author_rank: Super Contributor
timestamp: November 09, 2021, 10:10:31 am
content: Hopefully you've used a smear of heatsink compound underneath the resistors. With a milled / extruded surfaces, the actual metal to metal contact area is minimal, so the resistors will end up running significantly hotter than the heatsink.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 09, 2021, 07:50:08 pm
content: Quote from: Gyro on November 09, 2021, 10:10:31 amI was going too, but if you look at the screw holes on the resistors, the design is bad. Both holes are on the same side. What I did do is tape down some emery cloth and sanded the bottom of the resistors until they were as flat as they are going to get. Then I did the same thing to the heat sink. It's transferring heat. Between the resistors, the heatsink was the same temp as the resistors. That heatsink is just way too small. It would probably help a lot if even just the fins were 1" instead of .38". That would nearly triple the area. Next time I'll insist on getting staggered screw holes, or even all corners, but hard to find on Amazon.25 minutes for the first test, then I just let it run until the temp stabilized, around 10 minutes each after the initial 25 minute warm up time,. each test. Ambient temp around 23C:050W stable @ 054C: Low RPM 120mm computer case fan.100W stable @ 073C: Higher RPM 120mm computer case fan.150W stable @ 096C: Higher RPM 120mm computer case fan.I accidentally threw away my Nidec Servo Gentle Typhoon 1850RPM CPU radiator fans. Those things moved 58CFM with high pressure. So I can't do a test with those wind tunnels. In any event, it looks like adding a fan, in some future configuration/open ended box, will cool that set up just fine. A better heat sink would allow higher wattage, though. Each resistor is rated at 100w.As suggested above, I ran one set of 100W 6ohms resistors in parallel and another 100W 6ohms resistors in parallel and then ran those two parallel groups in a series to get 6 ohms. Since all of the resistors are 6 ohms, I'm assuming power is divided by each resistor?If so, at 150 watts, each resistor is working at roughly 37.5 watts each? That's still far below their rated 100 watt rating, which means they could use much better cooling. I should be able to get 200+ watts out of them in this parallel configuration (derated 50%). And for short periods even 300 watts (75w each resistor).

author: tkamiya
author_rank: Super Contributor
timestamp: November 16, 2021, 07:10:26 am
content: Don't forget simple incandescent light bulbs! A caveat is, as it heats up its ohmic value will increase (draws less current) if you just want to load it up, it's a cheap way to go. Buy buy a piece of wood, a bunch of ceramic sockets, and some wires, you can make all kinds of dummy loads. Replacement is cheap, too.I actually have 40 watts, 60 watts, 100 watts, 150 watts, 500 watts, 1500 watts, and 2000 watts (all rated at 120 volts) bulbs. They come in handy when I have to test extreme situations. 2000 watts bulb is about the size of a toddler's head!

author: theHWcave
author_rank: Regular Contributor
timestamp: November 17, 2021, 07:52:25 pm
content: Totally agree with @tkamiya. Don't forget light bulbs (incandescent of course). Another use case is wiring them in series with the mains input of a device as current limit in case there is a short circuit in the power supply. If the bulb(s) light up brightly, you have a problem.