Sub 10 ohm accurate power resistors or accurate electronic load. - Page 1
Forum section: electronics / general

author: beanflying
author_rank: Super Contributor
timestamp: November 21, 2017, 09:42:32 am
content: Starting to look at building a decent load (3-500W) with some monitoring for LiPo batteries. Not so concerned about constant current or load but accurate measurement and stable resistors over a range of operation temps.I have a small stash of Swiss Kanthal A1 1 sq.mm which runs at 1.86 ohms per meter. The only figures on the data sheet shows it's resistance at sub 1% to 400 C. Is there a better alternative at a sensible price (what is evanohm worth?) ?https://www.kanthal.com/en/products/material-datasheets/wire/resistance-heating-wire-and-resistance-wire/kanthal-a-1/The current stash of 1-4 ohm 50W resistors I have are around 1% accuracy but their temp co are fairly average. Also a .05% 80A 50mV shunt and a handful of Vishay sense resistors.So not wanting to design the load here but it is a question of what are my alternatives when it comes to high power low temp co and accurate resistors? Or do I forget about passives as an option and go down the FET path?In the past I have used the Kanthal in 6-12 wires between brass bus bars drilled and clamped as shunts and resistors. But looking around here Copper Rules

author: mikerj
author_rank: Super Contributor
timestamp: November 21, 2017, 12:06:09 pm
content: If you aren't worried about having a constant current load, then is it really important that your resistive load have an ultra low tempco? Provided you accurately measure the current through the load and the voltage across it rather than assuming a constant resistance, then you can take any changes in resistance into account.

author: beanflying
author_rank: Super Contributor
timestamp: November 21, 2017, 12:37:21 pm
content: I was hoping not to have to add a shunt or current sense if the resistors were more temperature stable/accurate. Looking at 3 or 6S LiPo packs of 2000-5000 mAh So 10-20V under load at 25-50A btw. Gets values of 0.2-1 ohm range.

author: tszaboo
author_rank: Super Contributor
timestamp: November 21, 2017, 01:08:33 pm
content: You have to use two resistors in series. One is a shunt, measuring the current, the other one is the load. The two has different jobs to do, and different requirements.You want something like few microohm resistor, 4 wire, say: 1mOhm @ 50A is 2.5W, and only gives you 50mV. Tempco of 50ppm is typical in this resistance range. Better ones are available, $$$.And place this in series with this the load. This could be a wire placed in a bucket of water, to a highly configurable block of FETs. It doesnt really matter.

author: The Soulman
author_rank: Super Contributor
timestamp: November 22, 2017, 12:26:28 am
content: Using highly stable resistors makes sense for this application, as then only the voltage over time has to be recorded and others can becalculated with that and the *known* resistance.However that may be difficult/expensive to do with decent "good enough" accuracy.I've built a dc dummy load and used a couple of lm317's in parallel in cc-mode combined to draw 1A, a useful number for diy sized battery's.That is good from 5 Volts up to about 15 Volts (15 Watts dissipated) above that I can connect regular 12 Ohm (50 Watt) power resistors in series for higher voltage's.Accuracy from those lm317's is pretty good with time and temperature, deviation on mine is below 0,1 %These are on a good sized heat-sink, keeping temps low helps.

author: beanflying
author_rank: Super Contributor
timestamp: November 22, 2017, 09:49:50 am
content: Actually got around to testing the Arcol HS50 F 's I had today. 4 wire on the Agilent. Well within the 1% spec0.99461.99353.00554.0010Tested at about 28 ambient which is a bit irrelevant if they are going into a power setup.Leaning toward building an Electronic load at this stage I guess but still something about playing with basic passive components

author: bopcph
author_rank: Contributor
timestamp: November 22, 2017, 07:22:28 pm
content: Some 16-17 years ago I was working for a company that produced some pretty capable PSU's - 500V/20A !We tested these PSU's with resistor loads for 30 min. at each test - CC and CV at both low and high AC supply (180 vac and 260 vac).So I guess you can imagine the temperature in the test-lab in the summer :-)Putting the PSU in CC or CV mode by loading with resistors was a tedious task (thick cabels and screew terminals) so we decided to produce an active load.One heatsink tunnel and 20 big MosFETs - very efficient and easy to manage :-)If you decide to do that kind of load start small and with low band wide in your feedback loop - MOSFET's are very fast, also when they "pop" one at a time , as a machinegun - all 20Slow turn on (more loading) and fast turn off (less loading).

author: beanflying
author_rank: Super Contributor
timestamp: November 22, 2017, 10:33:02 pm
content: Not sure I still have it but I had a 100A Aveox brushless speed controller fail in flight 200km'hr smoke trail coming out of a plane and 8 popped fets out of 12. These things are getting slammed hard on and off and the fets act as a propeller brake too.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: November 24, 2017, 12:39:37 am
content: Quote from: The Soulman on November 22, 2017, 12:26:28 amI did something like this once for a factory test jig, for telecom power supplies. The jig provided a programmable constant-current load for the UUT, 5V to 12V, 20A. Ended up using a number of 3-terminal regulators each sinking a fixed current, plus a power FET in a moderately slow control loop, all in parallel.When loading the UUT, the jig would switch on the 3-terminal regulators (one at a time or all at once, depending on the need) to sink nearly all of the required current. The jig would then enable the FET circuit. The FET circuit would "clean up" any error, between the programmed setpoint current and the current sensed through a 4-wire resistor made of bare manganin wire. So it would compensate for thermal drift in load resistors, in regulator characteristics, etc.When removing the load, the jig would do the reverse sequence: variable FET load off, then the fixed loads off. Worked pretty well. The regulators and load resistors were cheap, and I got a metre of manganin wire as a sample. The jig had lots of heatsinking, and a careful placement of fans in a push-pull arrangement for forced air cooling.