Battery powered constant current source - Page 1
Forum section: electronics / projects

author: jflick
author_rank: Newbie
timestamp: February 19, 2023, 03:01:52 am
content: I would like some advice for an idea I have for field service related device.I work in heavy industry with large motors, eddy current brakes, and high power low ohm resistor banks.I would like to use a 18V lithium battery pack normally used for Dewalt or Milwaukee power tools to power a constant current source to measure low ohms/fractional ohms using a DMM to measure the voltage drop across the motor winding or resistor and calculating the resistance based on this voltage.I cobbled together a constant current source using a LM317 adjustable voltage regulator as the constant current source and set it to 500mA. It worked ok but gets a bit hot even with a heat sink. I used a 4 AA battery pack as the voltage supply.With the LM317 with 4AA batteries I am limited to about 2 ohms or less or the current will begin to drop. I used alligator clips to connect the current source to the motor under test which seemed to work fine.I would like opinions on this type of setup. Is there any advantage to increasing or decreasing the current through the current source? I and all our technicians use Fluke meters, so decent but certainly not lab grade. I can buy battery adapters and build the circuit piggy backed onto the adapter.Thanks for any recommendations.

author: moffy
author_rank: Super Contributor
timestamp: February 19, 2023, 03:42:09 am
content: The best and possibly simplest battery powered milliohm meter I have seen is:https://www.analog.com/en/design-notes/batterypowered-circuit-measures-milli-ohms-and-micro-ohms.htmland you only need a 9V battery.P.S. Thanks pqass, link changed to one that works, same application note AN106/Maxim.

author: pqass
author_rank: Super Contributor
timestamp: February 19, 2023, 04:00:15 am
content: Quote from: moffy on February 19, 2023, 03:42:09 amThe link does not work; you have a trailing comma.

author: jflick
author_rank: Newbie
timestamp: February 19, 2023, 04:28:28 am
content: Thanks for that link. The Maxim devices are obsolete but I should be able to find available replacements. It does look like a very simple circuit, and more than accurate for my needs.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2023, 06:38:43 am
content: A 18 V supply is a bit on the high side for a current source. So the LM317 or any other linear driver gets quite hot. This can especially be an issue when the LM317 is also used to set the current, as it is not that temperature stable. So ideally the power driver and voltage reference should be separete.A motor / transformer can be highly inductive and not evenry constant current regulator likes this. Some DMM may even start to oscillate under such conditions. One may want some RC snubber in parallel.500 mA and a large inductor can create dangerous inductive kick back. To protect the current source some extra diodes can help. One may have to treat the DUT as life for the test.

author: iMo
author_rank: Super Contributor
timestamp: February 19, 2023, 07:51:02 am
content: With your 18V you would need a switching regulator to get say 1A test current.With LM317 set to 1.2V and 1A test current the power loss at the 317 would be 16.8W of heat (plus 1.2W at the 1.2ohm resistor)..4AA batteries (6V) and the 317 create 6W heat with 1A test current.Also mind with the above schematics (Analog) you would need 1.5V battery (except the 9V one) capable to provide the test current.

author: coppercone2
author_rank: Super Contributor
timestamp: February 19, 2023, 09:02:43 am
content: I built something similar (750mA current source that is biased by 16x? AA cells). My solution was to put a big CPU copper heat sink on it.BTW those temps on the chip will drop like 30 degrees with thermal paste, I tried before and after w/ arctic silver. I don't see the problem with a big heat sink and maybe larger cells... if you are working on big devices big tools make senseI think the drill battery would work fine, just increase the size of the heat sink and make sure the thermal bonding is right. And put a fuse on it. I used Arctic MX4 pasteThe benefit of more current is that you have more voltage to measure, and the wetting current of stuff. The device manufacturer should/often gives the test current they use for their resistance measurements. There is one measurement for dry and one measurement for wet contacts. For just windings, it should have little effect unless you have noise problems, you want small current so there is less heating going on.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 19, 2023, 09:20:59 am
content: I like the simplicity of an LM317 with a single resistor to program the current, but it is inefficient, especially when using a high input voltage.A very simple way to limit the heat (and extend battery life) is to just add a switch. Then you first wire up your measurement circuit (with clamps, so you don't have to hold probes), and press the switch for a second or two.Or you connect your DMM with clips, and just hold the wires of your current generator to the circuit.You could use a LED power IC. There are plenty of LED drivers which produce a (settable) constant current, and use an SMPS based topology. I do not know what accuracy and stability they have, nor do I know what you need.

author: Gyro
author_rank: Super Contributor
timestamp: February 19, 2023, 10:23:07 am
content: Quote from: jflick on February 19, 2023, 04:28:28 amHere's one I built about 20 years ago, based on that Maxim App note. I used an LM385 reference, OP295 and BUZ10 mosfet, also probably obsolete but easy to substitute more recent parts. It works extremely well and is still on original batteries.https://www.eevblog.com/forum/projects/design-review-ac-milliohm-meter/msg4589653/#msg4589653

author: jflick
author_rank: Newbie
timestamp: February 20, 2023, 02:05:55 am
content: Thanks to everyone for your input and ideas.

author: exe
author_rank: Unknown
timestamp: February 20, 2023, 07:48:24 am
content: Quote from: moffy on February 19, 2023, 03:42:09 amWhy they add a gate resistor? I'd understand if they used a compensation capacitor, but they don't. Imho this makes circuit less stable due to RC (where C is transistor's gate) phase shifting.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: February 20, 2023, 09:38:14 am
content: in the schematic R5 (0.9Ω) is given as three resistors in parallel: 1Ω // 10Ω // 1000Ω. this is incorrect, it should be 1Ω // 10Ω // 100Ω. the difference is minor (0.9% vs 0.1% error), but a glaring maths failure.an even better choice would be 1Ω // 18Ω // 18Ω, which isexactly0.9Ω.cheers,rob :-)

author: Gyro
author_rank: Super Contributor
timestamp: February 20, 2023, 10:13:08 am
content: Quote from: exe on February 20, 2023, 07:48:24 amI think to isolate the gate capacitance of the mofsfet gate. Looking at the MAX480 datasheet (for the first time), although it is a slow (12V/ms) opamp, its output stability is specified as up to 650pF and the IRF540 gate capacitance is 1.7nF.Using the faster OP295 in my unit, I added a stability cap from the opamp output to the inverting input, 1nF irrc.Quote from: robert.rozee on February 20, 2023, 09:38:14 amHa, good catch, I'm surprised that wasn't spotted. In mine I used a salvaged set of 100mR, 900mR, 9R and 90R (added 1mA range) shunt resistors.

author: exe
author_rank: Unknown
timestamp: February 20, 2023, 11:41:50 am
content: Quote from: Gyro on February 20, 2023, 10:13:08 amThat's my point. Unless I got this completely wrong, bigger resistor causes more harm in this case. To "isolate", one needs to put a capacitor that would provide a "shorter" path to negative terminal. Otherwise that bigger resistor alone slows down feedback loop, causing instability. See attached figure (look at Cf). Source:https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.html.

author: Psi
author_rank: Super Contributor
timestamp: February 20, 2023, 11:59:39 am
content: If you want something quick an dirty you could probably repurpose a meanwell DCDC CC LED driver.Most of them have analog control for the current, but check the part numbers to be sure.So you could have a few switches or a pot for various load currents.You'd need a dummy load to ensure the output voltage was always above 2V as that is the min output, but that shouldnt be a problem.You wont produce much heat at 2V.Add test resistor in series and measure with DMM, done.Something like this will take 6-36V input and step down with 1.2A current regulation to 2-30V, with adj analog current control.And costs less than $20https://www.digikey.co.nz/en/products/detail/mean-well-usa-inc/LDD-1200LW/7704729

author: Gyro
author_rank: Super Contributor
timestamp: February 20, 2023, 01:15:30 pm
content: Quote from: exe on February 20, 2023, 11:41:50 amI never built the circuit with the app note Maxim parts, so can't really comment on the stability but I remember the same basic circuit (sans D cell) appeared in another app note for a wider range floating current source used in their labs. Yes, you could adopt a more complex stabilisation network but I suspect that the glacially slow MAX480 micropower opamp was sufficient to maintain loop stability, even with the relatively slow RC network conprising the series resistor and mosfet gate (I haven't done the sums).In my case, the heavily slugging of the OP295 with the feedback capacitor was sufficient to maintain stability. Even with relatively inductive loads, I didn't seen any oscillation when scoping. In this sort of application, slow is good.

author: trobbins
author_rank: Frequent Contributor
timestamp: March 28, 2023, 08:21:48 am
content: I'm using that Maxim style circuit and getting quite good accuracy and repeatability, so using it as a transfer current reference for meter calibration. I've used a 1 ohm HS50 Arcol resistor as the single sensing resistance, and switched the reference voltage (from an LM336) presented to the opamp to achieve either 0.1 or 1A. For practicality and portability I use a small USB 2 amp plugpack as a generic 5V grunt supply for both the reference current and the control power for the LM336-2.5 and an LT1013 opamp, and I use a logic fet (MTP3055VL). As a transfer current device, the external current meter or shunt or whatever 'load' typically has a low burden voltage, and repeatable performance and tolerance is pretty good up to 1V burden (ie. up to 1 ohm external load). With that setup the FET is bypassed with a 7.5 ohm 10W shunt resistor to reduce the current passing through the FET (and hence heat-sinking) and achieve a repeatable accuracy of better than 0.01%.This is a poor-mans current reference, so I tested a few cheap sense resistors for tempco for starters. Two 10W ceramic block resistors worked well in one unit, and the Arcol works well. Although seemingly similar RS 50W resistors showed gross tempco variation and in particular related to temp change at the end terminals - very quirky.

author: drvtech
author_rank: Regular Contributor
timestamp: March 28, 2023, 09:46:17 am
content: Quote from: trobbins on March 28, 2023, 08:21:48 amCould be due to thermocouple effects between your probes and the terminations. It's important to use two identical (in terms of alloy and plating) probes when measuring low voltages in the presence of temperature fluctuations. You also need to be sure the two ends of the resistor are being heated together.

author: trobbins
author_rank: Frequent Contributor
timestamp: March 28, 2023, 11:00:39 am
content: I used soldered copper wire kelvin connections to the three sample 50W resistors to give the same test conditions. 2 samples were RS branded, and 1 sample was OEM Arcol.

author: trobbins
author_rank: Frequent Contributor
timestamp: March 29, 2023, 12:33:59 am
content: I used a USB 2 lead to connect to a 5Vdc power supply, so it works well with a USB battery pack as well as the ubiquitous small switchmode plug-packs or USB hubs etc.