high power constant current dummy load sense resistor - Page 1
Forum section: electronics / projects

author: electroguy
author_rank: Regular Contributor
timestamp: June 05, 2012, 02:26:51 am
content: Hi,sorry about starting a new thread, but I didn't want to hijack the other high current load thread by allen (which is pretty good).Thinking about building a 1A to 20A dummy load.I have a question about the current sense resistor. If i want to use a 0.001R to reduce the heat dissipated in it, can this be done without introducing too much error? (such as:http://www.jaycar.com.au/productView.asp?ID=QP5412) or even an SMD 0.001R 1WAt 20A this would only be a 20mV voltage, and a 0.4W resistor compared to a 4W if it was 0.01R or 40W if it was 0.1RThat's all fine at say 20A, but when down at 1A (or less?), the voltage drop is only 1mV. Assuming i use low offset op amps which are readily available and accurate, what are the issue with sensing 1mV? My main concern is the pot used to set the input voltage, i can set its range from say 0 to 50mV, but will there be too much error/noise? is there a better way to do this?The circuit i have is nearly identical to post 46 of the following thread:https://www.eevblog.com/forum/projects-designs-and-technical-stuff/project-high-power-constant-current-programmable-load/msg115374/#msg115374i was thinking, can a 10x gain (or 100x gain?) op amp (or instrument amp/diff amp) be used where R5 is, to make the range 10mV at 1A load to 0.2V at 20A load as opposed to 1mv to 20mV? Or will this add new problems/stability errors?thanks.

author: amspire
author_rank: Super Contributor
timestamp: June 05, 2012, 05:34:15 am
content: The using a 1milliohm resistor is fine.Look at the LTSpice files and circuits I posted:https://www.eevblog.com/forum/projects-designs-and-technical-stuff/project-high-power-constant-current-programmable-load/msg116944/#msg116944https://www.eevblog.com/forum/projects-designs-and-technical-stuff/project-high-power-constant-current-programmable-load/msg117494/#msg117494The opamps I suggested are $1.66 each and have a 2uV maximum offset (usually much less).You do not want any gain - a x10 gain means that 1mV in gives 10mV across the output resistor. You are actually talking about adding in a loss of ten times, and I think you are better just having 0-20mV input and unity gain. If you want 0-100mV in resulting in 0-10mV on the sense resistor, just add a divider by 10 resistive divider on the input.Richard.

author: electroguy
author_rank: Regular Contributor
timestamp: June 05, 2012, 06:40:44 am
content: excellent, thanks for the response. I was just worried about noise or ripple on the input pot exceeding the 20mV... And i was thinking of one day adding a DAC instead of a pot, but as you said, i could use a 3.3V output from the DAC, and then use a resistive divider on it's output.So realistically, using the op amps you mentioned, what is the lowest current i could measure with a 0.001R shunt resistor? Could i go down to 0.1A? that would mean 100uV... at what stage will noise (or power supply ripple?) become an issue?

author: amspire
author_rank: Super Contributor
timestamp: June 05, 2012, 07:10:07 am
content: Quote from: electroguy on June 05, 2012, 06:40:44 amThese opamps must have a supply from a voltage regulator, and so power supply noise is not an issue.As far as DC regulation goes, the opamp will easily regulate down to the uV level, and these opamps have no 1/f shot noise that normal opamps will have at low frequencies. The will be some AC noise that will be partly due to the opamp, and partly to layout dependent noise picked up from the rest of the circuit. The opamp noise will be at around the 10mA current level with the 1mOhm resistor.Richard.

author: electroguy
author_rank: Regular Contributor
timestamp: June 05, 2012, 08:28:25 am
content: cheers thanks for that.

author: electroguy
author_rank: Regular Contributor
timestamp: June 06, 2012, 04:03:39 am
content: I want to measure the current so i can display it, so i will need to amplify it by 100 (or more) as my a-d can't read that low, any suggestions? op amp or instrument/diff amp? advantages/disadvantages?Also, if i want to add over temp, over voltage/over current/over power shutdown, is it best to pull the FET gate to GND or the + pos op amp terminal to GND. I guess both will give the same required outcome, but i guess pulling FET gate to GND might add an extra level of safety if the OP AMP went bananas? thoughts/opinions?

author: digsys
author_rank: Unknown
timestamp: June 06, 2012, 09:15:36 am
content: QuoteINA198 .. my favourite Current amp - x100 fixed, up to 80V sense

author: hlavac
author_rank: Frequent Contributor
timestamp: June 07, 2012, 12:43:56 am
content: With 20A the current path will have to have some serious copper cross section area or it will burn.On a 1oz copper PCB, think of a 0.5 inches minimum trace width for temperature rise of 20C above ambient.With tiny/SMD resistor, even when the voltage drop in the resistor itself will be small, you will have problems with its leads... would need to make massive blobs of solder or something.Also, if the resistor is rated 1W and you send 1W thru it it will get hot as hell and will not last long.You would be better off paralleling 10 0.1R 1W resistors spread across the wide trace... look how Dave did it in his current sink or power supply.

author: electroguy
author_rank: Regular Contributor
timestamp: June 07, 2012, 02:46:25 am
content: hlavac,you are correct, i was going to parallel, but i was trying to keep the example simple, as i was mainly concerned with reading the small voltages at 1mv and below.but thank you for reminding me, yes there will definitely be thick copper, top and bottom, and multiple resistors. realistically, it will probably only be used up to 10A, and so total power dissipation is less than 1/10th of a watt in the resistors.