uCurrent design with relays - Page 1
Forum section: electronics / projects

author: alexscep
author_rank: Contributor
timestamp: April 03, 2015, 07:29:18 pm
content: Im having some trouble converting the uCurrent schematic from Dave Jones to be used with relays instead of manually operated switches. The problem is the following, the output of the opamp MAX4239, is not zero when no current is flowing through the selected sense-resistor. I'm having trouble finding the cause for this. When i am pushing a current through the sense-resistor i get a reading that is 10mV off. The strange thing is, when i am drawing a current, the value i am reading from my adc is also incorrect but this time with a different value. Does anybody have any idea what could be happening here?I also included the relevant part of my schematic to further clarify the situation.

author: adam1213
author_rank: Regular Contributor
timestamp: April 04, 2015, 05:18:41 am
content: Your schematic appears to use most of the resistor values from the original schematic. However in the original schematic there is always a 10k resistor between the input terminals (r2). On uA and mA range your circuit does not include this.

author: mij59
author_rank: Frequent Contributor
timestamp: April 04, 2015, 06:30:46 am
content: With a single power supply it's not possible to get exactly zero volts output, check the datasheet, look for max output swing of the opamp.You need to add a negative power supply rail.

author: dom0
author_rank: Super Contributor
timestamp: April 04, 2015, 11:39:25 am
content: He has separate AGND and VGND...For the record: Probably thermal offset due to the relays or the pinout of the relay used in assembly doesn't match the one of the layout and you're actually measuring the contact resistance, too.

author: alexscep
author_rank: Contributor
timestamp: April 05, 2015, 06:12:20 pm
content: Dear dom0,could you further explain thermal offset of relay-contacts. And maybe a way how i could solve this? But will the relay-contacts also have a thermal offset when i don't draw any current through them, because the opamp is not loading the previous stage?

author: Seekonk
author_rank: Super Contributor
timestamp: April 05, 2015, 08:26:21 pm
content: I'd skip the sense side relay and just put all the resistors in series and sense off the top one.

author: alexscep
author_rank: Contributor
timestamp: April 06, 2015, 10:07:50 am
content: Dear Seekonk,wouldn't this introduce input inbalance of the MAX4239 and therefore would introduce different offsetvoltages due to the biascurrents of the inputs?

author: alexscep
author_rank: Contributor
timestamp: April 07, 2015, 02:26:45 pm
content: Dear Seekonk,Do you mean something in this trend?I got this out of an old school book of mine.

author: dom0
author_rank: Super Contributor
timestamp: April 07, 2015, 02:35:20 pm
content: Yep

author: alexscep
author_rank: Contributor
timestamp: April 08, 2015, 06:50:22 am
content: The error was indeed introduced by the thermal offset of the relays. After removing them and connecting a direct wire between the sense-resistors and opamp, the error was gone. Now I will be using a similar approach to the schematic in my previous post. I will keep you updated about the result.Alex

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2015, 04:59:42 pm
content: The relays at the OP-Input could be replaced with CMOS switches or similar.Another Option would be having the 3 shunts in series (thus needs slightly different values, like 9.99 Ohms) - this is how old meters usually solved switching the current range, without the need for an extra switch.

author: pelud
author_rank: Regular Contributor
timestamp: April 08, 2015, 06:20:20 pm
content: High end multi-meters use latching relays so that they don't need to leave the coil power up all the time, and thus don't generate any heat causing a thermal gradient. For example, the HP 34401A uses a variant of the Panasonic/NAiS DS2E-SL2-DC5V relay as seen in this post here:https://www.eevblog.com/forum/reviews/hp34401-teardown/.

author: alexscep
author_rank: Contributor
timestamp: April 08, 2015, 07:08:48 pm
content: Pelud,thanks for mentioning these relays, they offer superb contact resistance, down to 50 mohm , which is quite low. I was initially working with 100 mohm relays.These new ones bring the burden voltage down with a factor two.Great tip thanks

author: alexscep
author_rank: Contributor
timestamp: April 08, 2015, 07:11:48 pm
content: Kleinstein,I was also considering analog switches, but i really want as few control signals as possible. An analog switch would introduce two extra signals.

author: alexscep
author_rank: Contributor
timestamp: April 08, 2015, 07:56:30 pm
content: I just quickly draw the main idea for the setup. I am using two latching relays for current-switching. Does anybody have any remarks about it? Something that could go wrong or could it be done simpler?

author: pelud
author_rank: Regular Contributor
timestamp: April 08, 2015, 09:32:57 pm
content: I can't claim to be an expert in electronics by any means, but I been thinking a lot about a similar project using an LTC2440 to measure the current. You can bypass the contacts on the lower section of REL2 and go straight to the top of the 0.01 ohm resistor from REL1. This will further reduce the contact resistance on the lowest range (where it matters). I think this scheme would also work with regular relays as the thermal offset will only be in series with the load, not the measurement points on the resistors.

author: alexscep
author_rank: Contributor
timestamp: April 13, 2015, 11:47:06 am
content: Hi again,does anybody have any experience with these latching relays? I am using a panasonic DS1E-S-DC3V relay. These use a single coil for set and reset conditions. The direction of the current-pulse through the coil determines the set or reset condition.

author: alexscep
author_rank: Contributor
timestamp: May 12, 2015, 03:12:15 pm
content: Hey everybody,i am still struggling with the realisation of this uCurrent with relays. I am currently using the approach previously given by pelud (see drawn schematic) , but for some reason, negative currents don't want to work. Does anybody have a clue what is going on here. Positive currents are no problem. And with not working, i mean the amplifier outputs a voltage wich has an extra offset to it, but i can't determine where this is coming from. Its particulary strange that this does not occur while measuring positive currents.

author: Kalvin
author_rank: Super Contributor
timestamp: May 12, 2015, 05:10:07 pm
content: Maybe you need negative power supply so that the op amp's output can swing to the negative side of the ground.

author: alexscep
author_rank: Contributor
timestamp: May 12, 2015, 05:12:23 pm
content: But the opamp is situated in a virtual ground setup, such as employed by Dave in his uCurrent design, so this shouldn't be the issue.What else could it be? Something with the relay's?

author: Kalvin
author_rank: Super Contributor
timestamp: May 12, 2015, 05:21:02 pm
content: Ok. I tried to interpret the schematics shown in the first post, so I couldn't quite figure out that AGND and VGND are at different potential - although they are different nets. What is the voltage of AGND in U$3 pin 4 referenced to VGND? And what is the voltage of U$3 pin 7 referenced to VGND?

author: alexscep
author_rank: Contributor
timestamp: May 12, 2015, 05:24:10 pm
content: The overall supply is 3V3 and the virtual ground is introduced half-way. Thus you have an +/-1.65 symmetrical supply for the opamp.

author: Kalvin
author_rank: Super Contributor
timestamp: May 12, 2015, 06:00:40 pm
content: Does it make any difference if you remove R1 and measure directly from the op amp's output?Btw, MAX4239's input's VCM range is VGND-0.1 ... VCC-1.4, thus you are working quite close to the upper limit here. It may not be significant here, but something to be aware of.