Have you built a resistive load bank?  Ideas? - Page 1
Forum section: electronics / projects

author: forrestc
author_rank: Unknown
timestamp: January 24, 2019, 01:16:21 am
content: I've come to the conclusion that I need to add a resistive load to the arsenal here. I have a couple of electronic loads but there are times that I just need a good old fashioned resistive load.I know I've seen pictures/schematics of loads that others have built on here, but I don't seem to come up with any of them. This is a good time to post them if you want to show off your work. I'd love to see how everyone dealt with thermals.One thing I'm trying to get my head around is how best to arrange the resistors with switches so that I can switch them in and out to be able to have various options as far as the resistance of the load while maintaining power dissipation capacity. I.E. some sort of switchable parallel/series arrangement.I'm looking at needing a resistance down to around 2 ohms and as high as around 50 ohms. My maximum power will probably be around 10 ohms when I'm loading a 48V source up to around 5A. This would be ~240W. I briefly thought about using some 100 ohm >25 watt resistors and having switches to parallel how many ever I needed, but then I realized that I'd need 50 of them with switches to get down to to 2 ohms. I guess I could switch them to 50 ohm >50watt resistors. Then I'd only need 25 of them. Still a pain, but if that's what's needed...So I'm asking for pictures/schematics/url links/etc to loads that everyone has built. And any pointers as far as arrangement/thermals.

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 24, 2019, 09:29:09 am
content: Quote from: forrestc on January 24, 2019, 01:16:21 amHere is mine

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 24, 2019, 12:42:37 pm
content: These boards from Ali / Ebay / China cost about EUR 2, completely build:Schematics is extremely simple.All 4 resistors are placed parallel, with each a switch in series.As you can see, the resistors are 20Ohm, 10 Ohm, 4.7Ohm and 2.2Ohm.I also have a drawer with random power resistors scavenged from old equipment and use crocodile clamps on them every now and then to heat them up.I dislike the aluminimum resistors which are ment to be mounted on a piece of aluminimum to cool them.These are very sensitive to overheating and get damaged easily.I like the square white cement resistors. as in the link from not1xor1https://www.aliexpress.com/wholesale?SearchText=cement+resistorThese are cheap reliable and almost indestructable.You can also significantly increase their power handling capabilities by bolting them to a piece of aluminimum.You can also clamp these between 2 aluminum profiles. Drill holes through the profiles between the resistors and make a nice strong sandwitch with some cooling paste. Add more profiles for better cooling.If you buy bags of the same value, (for example 10 pieces of 1 Ohm) and put them in series with 11 banana jacks, then you can use them as 1, 2, 3, ... 10 Ohm. With extra Banana cords you can then also increase the power handling by shorting the ends of the string and use the ends and the middle as a 5 Ohm resistor. (But I Like Zero999's idea better of using 1, 2, 4, 8 in series).If you want to build a big box of power resistors, then decide in advance if you want to include a fan and manual switch or a thermostat.Reserving a bit of space to add a fan later if needed is probably the least you should do.It can also be usefull to build a single resistor out of a combination of 2 series and 2 parralel. Resistance stays the same, but power handling is quadrupled. This is handy if you find a cheap bag of 20 resisors with the same value.If you build one of these with 1 Ohm resistros and another with 10 Ohm resistors, then you're already covered over 3 decades of resistance.Make some basic calculations of the power requirements of your resistors. Once you get over a few Kohm it becomes unlikely you'll need high power resistors, while a 1 Ohm 50W resistor easily gets overheated.Mike'sElectricStuf simply uses enamaled copper wire which he dumps in a bucket of water as a load restance. Very good Idea for when you need a 1kW resistor for occasional use. for more frequent use you can use heating elements for warming glasses of liquid. Or heating elements from washing machines or other equipment such as frying pans.If you want to callibrate power resistors you can use regular 5% tolerance resistors and take 1 value in E12 higher and then put another in parallel.For example, you need a 1 Ohm resistor, so take 1.2 Ohm. and put a resistor of around 6Ohm (6.parralell to it to bring it down to 1Ohm again.The idea is to always keep the resulting resistance just above 1Ohm. The extra resistors carry a low current and do not need to handle much power. Low restances are of course easily measured with a high current source and 2 DMM's.

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2019, 01:13:11 pm
content: Use resistors going up in powers of two, then any arbitrary resistance value can be generated.If the resistors are connected in series, then they can be bypassed to generate different resistance values.If the resistors are in parallel, then they need to be open circuited.Series is easier to calculate, suppose you have 1, 2, 4, and 8. That can give any value between 1 and 15R, just by adding the resistances together.Parallel is more tricky. Rather than resistances, think of conductances i.e. 1/R, so 1, 2, 4 and 8 Ohm resistors, would give 1S, 0.5S, 0.25S and 0.125S, resulting in a conductance range of 0.125S to 1.875S or resistance range 0.533Ohm to 8Ohm. Any resistance value would need to be converted to conductance, before deciding which resistors to switch into the circuit.

author: Howardlong
author_rank: Super Contributor
timestamp: January 24, 2019, 01:33:41 pm
content: Bucket of water and resistance wire, a la Mikeselectricstuff?

author: bdivi
author_rank: Regular Contributor
timestamp: January 25, 2019, 12:23:49 pm
content: I used a couple of 50W resistors on a 3mm aluminium plate inside an ATX power supply case. I even kept the fan for forced air circulation when I push too much power onto the load.I configured the following values - 1, 1, 2, 2, 4, 8, 16, 24 Ohm. By connecting the resistors in parallel and/or series I can get anything between 0.3 and 58 Ohm. Each resistor is independently wired to a pair of banana sockets so I can manage the combination by external wire bridges.The picture is from the start of the process. I will take some more shots in the next days.

author: bdivi
author_rank: Regular Contributor
timestamp: January 26, 2019, 04:22:25 pm
content: Some more pics