(Almost) zero miliamps on the power supply display with no load connected - Page 1
Forum section: electronics / beginners

author: bodzio_stawski
author_rank: Regular Contributor
timestamp: February 01, 2024, 11:39:04 am
content: Hi!I have recently been wondering about the methodology of measuring current in power supplies, which shows the current drawn from them with an accuracy of 1 mA. (I have one similar to the one below).https://www.amazon.com/Adjustable-Switching-Regulated-Interface-Adjustments/dp/B09XTWKVLD/ref=sr_1_21?crid=UBUETUWMHPLS&keywords=bench+power+supply+5A&qid=1706787155&sprefix=bench+power+supply+5a%2Caps%2C205&sr=8-21I noticed that when I do not connect any load to this power supply, after pressing OUTPUT it should simply display 0.000A, but it displays once 0.000A and once 0.005A. When I connect a resistor to the power supply outputs, 5mA is always added to the current measured by the power supply.So, for example, when I connect a 1kohm resistor and set it to 5V, I see 0.010A instead of 0.005A, etc.Is this behavior of the power supply normal or some hidden fault?

author: inse
author_rank: Super Contributor
timestamp: February 01, 2024, 12:07:59 pm
content: Don’t confuse accuracy with resolution

author: bodzio_stawski
author_rank: Regular Contributor
timestamp: February 01, 2024, 12:13:40 pm
content: Perhaps this was the impression, but the very fact of the existence of non-zero current despite the lack of load connection worried me.

author: inse
author_rank: Super Contributor
timestamp: February 01, 2024, 12:17:10 pm
content: Perhaps there is some kind of discharge resistor across the output clamps

author: Buriedcode
author_rank: Super Contributor
timestamp: February 01, 2024, 12:19:06 pm
content: Quote from: bodzio_stawski on February 01, 2024, 12:13:40 pmYou mean non-zerodisplayedcurrent. You are assuming the power supply (and the display) is measuring the output current perfectly - it isn't. There will be errors, including offsets, which in this case appears to be around 2.5mA - and if the resolution of the measurement is 5mA, then it will toggle between the two nearest values - 0.000 and 0.005.If it always adds 5mA to the diplsay, then the offset iwth +5mA, or + 0.005A. Some instruments allow calibration against a known reference, so you can zero these offsets out.

author: bodzio_stawski
author_rank: Regular Contributor
timestamp: February 01, 2024, 12:39:23 pm
content: The port can show 0.005, 0.006, 0.007 depending on the load, so it's not about resolution. To make matters worse, what is displayed depends on the voltage set. At 5V the current still fluctuates between 0 and 0.005A, while at e.g. 20V it is permanently 0.005A. However, when you wait a few minutes or more, the value fluctuates between 0.005 and 0.000, pointing often to 0.000, and finally stops at 0.000. PolskiBut when you then turn the power off and on again, those few milliamps will show up on the display again without any load connected.Unfortunately, it is not possible to calibrate the power supply (at least without intervening inside the device).

author: Terry Bites
author_rank: Super Contributor
timestamp: February 01, 2024, 01:48:03 pm
content: 1% accuracy is hard to achieve with the best off the shelf parts. You'll see boasty specs for precision current sense amplifiers. That's all well and good but the typical external shunt resistor will add a nasty temcpco of 0.39%/'C. If you go from +25 to +50 you'll expeience 10% drift, ouch.There are devices with integrated shunts with a much better tempco they tend to be aimed at high current measurement. eg The INA250 looks like good choice for lower currents. One day I'll read the datasheet.

author: bodzio_stawski
author_rank: Regular Contributor
timestamp: February 01, 2024, 02:14:41 pm
content: Quote from: Terry Bites on February 01, 2024, 01:48:03 pmWell, it's true, I don't expect super accuracy from a somewhat unknown device. However, I am curious how the power supply's own ammeter can measure any self-current when there is no load. I wonder what is a source of that effect on display. I admit that I approached this matter somewhat in the way that the "lack of an electrical quantity" cannot generate any value error. Do other brands of laboratory power supplies have similar currents?

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 01, 2024, 02:31:04 pm
content: The specs said that current display accuracy is +/- 0.5% +5 digit. The fact that it has the +5 digit if it displays 5 cound more or less it's in specs. You shouldn't expect too much accuracy.

author: Manul
author_rank: Super Contributor
timestamp: February 01, 2024, 04:21:23 pm
content: Quote from: bodzio_stawski on February 01, 2024, 02:14:41 pmCurrent is usually measured by using a shunt, aka current sense resistor. When current flows, there is a voltage drop. That voltage is what is measured. Lab power supply must be able to measure ralatively big range of current, from milliamps to amps. The sense resistor must be low value, to not heat up too much at high currents. Lets say it is 50 milliohms. But then low currents are difficult to measure. At 1mA you get voltage drop of only 50 microvolts! You ask, how can anything at all come out of nothing at zero current? We are dealing here with very low voltages, we need amplifier. Any amplifier will have offset and gain errors. Plus there could be other issues, like ground potential shifts in the circuit board. Current sense resistor is usually measured by using differential amplifier and ground shifts should not affect measurement - theoretically. But the measured voltage likely does not go as differential signal all the way to ADC, so ground shifts might still affect it. And yes, there are ADC errors too. There is also noise. Long story short, it is not easy to achieve microvolt precision. When there is no signal present, what is still present are errors and noise, there is no way to fully avoid that.

author: MarkT
author_rank: Frequent Contributor
timestamp: February 01, 2024, 08:11:50 pm
content: Suspect they just fake any reading below 0.005 to 0 to hide the offset error (which would be more annoying in most situations).

author: bdunham7
author_rank: Super Contributor
timestamp: February 01, 2024, 08:24:03 pm
content: Quote from: bodzio_stawski on February 01, 2024, 02:14:41 pmThere are scaling errors, linearity errors and nulling (zero) errors. They all have their various causes, but there's nothing unusual in having some noise or fluctuations at the zero point any more than there would be having the same noise or fluctuations at some other point. Its a universal issue with almost all types of meters. It's not the "electrical quantity" or lack thereof, it's the meter itself that generates the error.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 01, 2024, 08:30:57 pm
content: From your description, the current reading would appear to have 5mA resolution, so what you see is, unfortunately, normal. Yes, say the current measurement is done with a 10-bit resolution, if say the current range is up to 5A, there that's your about 5mA resolution. Nothing you can do about it, and not a sign of defect as long as it was documented in the power supply's specs. But don't expect neither good resolution nor good accuracy with these power supplies. The reading is just meant to give you a rough idea. If you need better, use a multimeter.