Designing a product for possible manufacturing? - Page 1
Forum section: electronics / projects

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: July 05, 2016, 07:01:51 pm
content: Hello again everyone. I have cobbled together an arduino based battery tester.It works but was not an easily buildable device. It is fairly simple:1602 LCD4 N channel mosfets that can handle 1A or moreArduino MicroButtonBuzzer4 4 ohm resistors used as a load and current sense5 10k resistors4 0.56?F capacitorsSome battery holdersAnd 4 LEDs.The arduino does current sensing by measuring the voltage drop on the 4 ohm resistors. Using my bench supply's mAh counter as a reference, I was able to get repeatable results within 0.1% of my bench supply.However, the resistors are 100w aluminum chassis resistors mounted to a heatsink with forced airflow. So they barely get warm. I was concerned about acuracy if they got hot due to the resistance changing with temp. I thought I could use a .1ohm current sense resistor or something, but the arduino's ADC has a far too small resolution to get meaningful readings. So I measured the resistors and used ohms law.So keeping the resistors at a fairly stable temp, this thing works extremely well.If I were to design a PCB for this thing, what enclosure would be good and cheap? What about cooling? Aluminum chassis resistors aren't cheap.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: July 05, 2016, 08:25:37 pm
content: Forgot to add that these would be handmade by me.

author: trophosphere
author_rank: Frequent Contributor
timestamp: July 05, 2016, 08:49:27 pm
content: A pre-made enclosure would be the least hassle. Themanufacturersin this thread all have extensive catalogs. Metal would provide the option of conducting the heat of your sense resistors. Placing a fan blowing or sucking air across the resistor would be an alternative. I'm being vague because other pieces of critical information is missing such as expected power the resistor is supposed to dissipate is missing.As for your sense resistors, how much of your ADC's range are you using? You can, as an alternative, go ahead and use a 0.1 ohm resistor and feed it through an amplifier to maximize the usage of your ADC's range. If you are worried about temperature variation causing unacceptable error in your readings, you can implement software compensation. Thermally bond a temperature sensor to your resistor and have your software use a lookup table to provide offset adjustment is a suggestion.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: July 05, 2016, 09:00:23 pm
content: Quote from: trophosphere on July 05, 2016, 08:49:27 pmI am only dissipating 4W per resistor. So a total of a measley 16w.I am directly feeding the battery voltage and resistor voltages into the arduino, so in this config, I am using most of the ADC range, with 800mv to spare.I have been thinking about using LM317s as a constant current load, and then I could simply count the seconds the battery is discharging. However, it would be nice to add a current sense resistor in case the load current changes for some reason, or I want to add an adjustment pot for current.

author: trophosphere
author_rank: Frequent Contributor
timestamp: July 05, 2016, 09:17:07 pm
content: It may be cheaper then to reduce your sense resistor to 0.1 ohms and just scale it up to your ADC's range with an amplifier as then the amount of power you will be dissipating will be very small. Likely can get away with free air (convection) in a plastic enclosure with holes. This, of course, goes out the window if you want to include load resistors in your design.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: July 05, 2016, 09:59:47 pm
content: Quote from: trophosphere on July 05, 2016, 09:17:07 pmYeah its going to include load resistors. Do you have any recomendations as to what amps I should use?I could use 0.1ohm sense resistors and then some ceramic power resistors with a fan. They would probably not stay exactly 4 ohms, but that won't matter with a seperate sense resistor.

author: trophosphere
author_rank: Frequent Contributor
timestamp: July 05, 2016, 11:21:28 pm
content: Quote from: iamdarkyoshi on July 05, 2016, 09:59:47 pmMy usual jellybean op-amps for basic instrumentation includeLMV722andOPA2335.