How to use the uCurrent Gold with a voltage/current divider - Page 1
Forum section: electronics / beginners

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: November 30, 2017, 02:20:45 pm
content: Hello,I'm using a uCurrent Gold to measure the current flowing through an electrochemical cell (a voltage applied across two gold or palladium electrodes immersed in aqueous electrolyte solution). I have an issue with being limited to select from just three shunt resistors each three decades apart.Let's say the maximum for a range is 1000mA but my current is 1.2mA. Tthe problem is that the output from the uCurrent is very small and noisy. I have to select the next more sensitive range. e.g., switch from the 1mV/1mA range to 1mV/uA range which allows a maximum current of 1mA. Now I am exceeding the range. I know that for some circumstances I can put a resistor in parallel across the current input terminals.So...I have tried what I thought was the obvious, namely make a simplevoltagedivider circuit to send, say, 10% of the current through the uCurrent but I cannot because the negative input terminal is connected to the output's negative terminal which, in turn, connects to ground on my analog/digital converter I'm using to record the current (it has single-ended inputs only). I'm at a loss (not being an electronics wizard) as to how to divide the current. My electrolyte solution has an impedance between approx. 100 to 10kohm. Any resistors used to divide the voltage must therefore be significantly less. 10ohm would be okay for a 10kohm solution but a 100ohm solution would need something less than 1ohm (I have a 0.15ohm shunt resistor to hand).Sorry for the wordiness. Any guidance greatly appreciatedJohn.

author: BNElecEng
author_rank: Regular Contributor
timestamp: November 30, 2017, 03:46:55 pm
content: I think you already mentioned a good solution. The 1mv per uA range uses a 10 ohm resistor. If you parallel another 10 ohm resistor then you get half the sensitivity, so you'll only get 0.5mV per uA and so double the measurement possible before hitting the overrange limit. If your current is 1.2mA then with this modification you should get 600mV as an output voltage.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: November 30, 2017, 04:53:49 pm
content: Thanks for your quick reply.Such is Sod's law that I typically have this issues on the 1mV/1mA range which uses a 0.01ohm shunt. It just so happens I have one that I can put in parallel which will help in some cases. I could buy another couple but then I'm approaching the cost of the uCurrent itself unless it' possible to get them with less precision.Anyway, I'll see how I get along. Thanks again.John.

author: BNElecEng
author_rank: Regular Contributor
timestamp: November 30, 2017, 05:50:53 pm
content: In the back of my mind I remember a modification to a ucurrent to increase the supply voltage which then increases the achievable range.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: November 30, 2017, 07:22:31 pm
content: Yes, I read that you can just apply a higher voltage. I've been reluctant to try that and have stuck to using a button cell for fear of destroying the uCurrent. However, I did test a buck down converter with a 9V battery to deliver the 3V. I'll be brave and try the ~5V Dave describes.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 19, 2018, 03:34:32 pm
content: Hi,I thought I'd give an update of where I have ended up with this. I added four precision resistors (0.1, 1.2, 100 and 1K) in parallel across the input terminals with each independently switchable via a DIP switch. I removed the (very nice) binding posts and soldered my connections directly to the board. I also use a 5V DC supply to get the extra range. The 5V comes from a buck down-converter connected to a 9V battery. I tried using a DC source that I use to power other components in my system but the fact that the output of the uCurrent is connected to ground upset everything. Nevertheless, the battery lasts a long time. I have found that most of my samples of interest for my measurements result in currents in the many 100's of mA and sometimes up to 2A at ~5V.This little board has made such a difference to the capabilities of my instrument.John.