Adjustable "dummy" load - Page 1
Forum section: electronics / beginners

author: Laurynas
author_rank: Contributor
timestamp: December 08, 2013, 09:09:29 pm
content: I'm trying to design an adjustable load circuit (attached, 12V powered, 10A max target, would use an active heatsink for mosfet)I wish to use it together with an ATX PSU to have a cheap lab power supply alternative.Since I'm not from electronics background, first of all I'd like to ask if this type of load can be used at all to limit current flowing via a circuit if it's ground would be connected to the mosfet and it would be powered off the same PSU as this load.Other questions would be:I came up with R3, R4, R5, RV1 contraption to provide calibratable 1:1000 multiplication. It doesn't look elegant at all. What's the proper way to provide such multiplier to an op-amp?Is there something else that's wrong/silly/broken in this schematic?

author: Laurynas
author_rank: Contributor
timestamp: December 10, 2013, 06:20:14 pm
content: anyone?

author: prenato
author_rank: Regular Contributor
timestamp: December 10, 2013, 07:18:59 pm
content: Hi Laurynas,I have a circuit in my website that you could use as another reference:http://paulorenato.com/joomla/index.php?option=com_content&view=article&id=91:constant-current-load&catid=4:projects&Itemid=4An LM324 can sense voltages down to ground (see "Common mode input range" in the datasheet). Not sure if your opamp can do that but you should confirm in the datasheet. I wonder why you are using such a small sense resistor (0.01 Ohm)? The PCB trace resistance itself could be that much and makes you susceptible to errors. Plus I would advise against using such a high gain in the OPAMP as it will multiply the voltage offset errors in the amplifier (that's another reason not to use such a small sense resistor). You would be better off with a Gain of 1 using a follower (like in my circuit) or use a very modest gain.Paulo

author: M0BSW
author_rank: Frequent Contributor
timestamp: December 10, 2013, 08:29:19 pm
content: Quote from: prenato on December 10, 2013, 07:18:59 pmHi Paulo, I've just created a shortcut to your site, as this is something I'd like to build next Year.Paul

author: prenato
author_rank: Regular Contributor
timestamp: December 10, 2013, 08:43:15 pm
content: QuoteGreat. Let us know how it turns-out!

author: Laurynas
author_rank: Contributor
timestamp: December 10, 2013, 09:14:12 pm
content: Quote from: prenato on December 10, 2013, 07:18:59 pmNice circuit, I'll use it as a starting pointI have not used op-amps before so i don't know the practical noise limits to amplification factor. I did use 0.01 Ohm current sense resistor with no amplification using a comparator(MCP6542) before, so i took that as a reference.Another reason is that i'm an SMD fan, and i wanted 1206 resistor to handle 10A without heat issues while having no effective voltage drop for circuit being tested. But thanks for the warning - I'll try to avoid such high multiplication factors.MCP602 datasheet says that it has VSS-0.3 to VDD -1.2V input range so it should work well here.Thank you for your help!Laurynas

author: prenato
author_rank: Regular Contributor
timestamp: December 10, 2013, 09:35:12 pm
content: Quote from: Laurynas on December 10, 2013, 09:14:12 pmThanks. I looked briefly at the MCP602 Datasheet and I see "Input Offset Voltage" it 0.7mV typical and +/-2mV max. This is an error you get right at the input, even without any gain. Now imagine you are drawing 1A through your 0.01 resistor. That's a 1*0.010 = 10 mV signal you will try to measure. This means your error is up to +/-20% (+/-2mV/10mV) which isn't very good. The situation gets even worst if you are drawing only say, 100 mA (offset error of 2 mV is actually *larger* than your measurement voltage of 1 mV). This is why I would suggest considering either increasing your sense resistor, or use an OPAMP with much lower offset voltage.Hope this helps,Paulo

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: December 10, 2013, 09:49:40 pm
content: Here is one I have been working on and sorry about the screwed schematic I am still learning Black Board..

author: Laurynas
author_rank: Contributor
timestamp: December 10, 2013, 10:38:59 pm
content: Quote from: prenato on December 10, 2013, 09:35:12 pmAll these troubles from wanting to get a very wide range of control from 1mA to 10A. I looked into devices like MAX9938, but they don't help by "orders of magnitude" as seems to be needed here.I think I'll look into providing alternative current paths depending on set current range with different sense resistors. This should help me jump over these offsets while using low power resistors.

author: prenato
author_rank: Regular Contributor
timestamp: December 10, 2013, 10:54:46 pm
content: Quote from: Laurynas on December 10, 2013, 10:38:59 pmAnother thing that can help is if you use a Voltmeter with a different range (say 0-0.1V instead of 0-10V)The panel meter I used had jumper settings for that If I recall correctly...

author: fcb
author_rank: Super Contributor
timestamp: December 11, 2013, 01:04:09 am
content: MCP602 opamp has absolute max. rating of 7.0V

author: Laurynas
author_rank: Contributor
timestamp: December 11, 2013, 07:00:48 am
content: Quote from: fcb on December 11, 2013, 01:04:09 amthanks,missed that, another reason not to use high amplification