Voltage drop on return line  - Page 1
Forum section: electronics / beginners

author: joniengr081
author_rank: Regular Contributor
timestamp: July 16, 2024, 10:36:18 am
content: Consider the power supply located at 4 meter away from the PCB. The power cables consist of the main power cable and the return current cable.If the PCB needs 2 A at 3.3 V. Do we consider only the voltage drop through 4 meter cable ?The voltage to be set at the power supply would be 3.3 V + voltage drop through 4 meter cable. Do we also need to consider the voltage drop through the return current cable ?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 16, 2024, 10:40:14 am
content: Of course. Other things to consider:* Is there ground-referenced signalling to other devices? If yes, is it acceptable that ground potentials are so much different?* If you set the power supply to higher voltage to compensate for the drop, can you guarantee your load current is always on, and constant? If not, the device is seeing overvoltage during lower consumption (e.g., before starting operations). Can it handle this overvoltage?For very low voltages such as 3.3V, local regulation is usually the solution. Make the board accept 5V (e.g., accept range of 4 to 6V), which is also easily available from USB power supplies, and use 3.3V regulators on-board.

author: joniengr081
author_rank: Regular Contributor
timestamp: July 16, 2024, 10:48:39 am
content: With 4 meter cable between the PCB and the power supply. Would there be a voltage drop on the cables. The answer is yes. Now the question is how much in numbers.V_drop = Current (2 A) * Cable Resistance (4 meter or 8 meter)

author: Siwastaja
author_rank: Super Contributor
timestamp: July 16, 2024, 11:02:25 am
content: U = R * I, where R is the resistance of the 8 meters of wire.And, for example, if the total voltage drop is 0.4V, then the device would see 2.9V over it, with the negative input 0.2V higher than the power supply -, and positive input 0.2V lower than the power supply +.Note that in reality cable resistance is not a finely controlled parameter - even if you buy wire rated to exact same cross-sectional area, resistance could easily vary by tens of %, so you get the exact answer by taking a measurement.Also remember that contacts (connectors, screw terminals etc) also have resistance, something that might produce a significant voltage drop, too.

author: joniengr081
author_rank: Regular Contributor
timestamp: July 16, 2024, 11:16:12 am
content: I am getting it now.Let's suppose there is 0.4 V total voltage drop for 2 A current on a 2 meter cable of given given resistance. The 0.2 V will drop on +ve supply cable and the remaining 0.2 V will drop on the return current cable.In other words on the PCB the positive supply connector will be 0.2 V lower then the power supply +ve and the return current connector will be 0.2 V higher then the power supply -ve.What setting do I need at the power supply to get 3.3 V on the PCB.3.3 V+ 0.4 V= 3.7 V ?This way the positive supply connector on the PCB will be 3.5 V and the return current on the PCB will be 0.2 V. The voltage difference on the PCB will be 3.3 V. But the absolute zero voltage will be on the power supply connector, not on the PCB.

author: Ian.M
author_rank: Super Contributor
timestamp: July 16, 2024, 11:26:45 am
content: Does the power supply support remote sensing, and is its output floating with respect to ground?If so you simply run a pair of thin sense wires next to your + and - power cables, connect them to the power cables at the far end (your PCB), and to the PSU's sense terminals and the PSU will take care of adjusting its actual output voltage to maintain a regulated 3.3V (or whatever you set) at the far PCB. See the PSU's manual for details, including any jumpers or links that may need to be removed to use remote sensing. Don't forget to put them back afterwards so you don't get a nasty surprise next time you use the PSU *without* remote sensing.If the PSU is *NOT* floating with respect to ground, and your PCB connects to anything else grounded or you try to test it with an ordinary scope probe, you can run into the ground potential difference issue Siwastaja mentioned. This can vary from mildly compromising noise immunity, through stopping stuff working, up to causing actual damage to your PCB or grounded stuff connected to it.

author: joniengr081
author_rank: Regular Contributor
timestamp: July 16, 2024, 11:36:54 am
content: I am not sure if our power supply have remote sensing option.

author: Ian.M
author_rank: Super Contributor
timestamp: July 16, 2024, 11:38:36 am
content: So examine it looking for sense terminals (usually clearly marked) or RTFM and see if it mentions remote sensing anywhere.

author: selcuk
author_rank: Frequent Contributor
timestamp: July 16, 2024, 11:44:58 am
content: Is 2A load a resistive heater or a decent circuit? 4 meter cable is not a pure resistor. It is resistor + inductor + capacitor + RF antenna. You may get interesting effects on your circuit.

author: tunk
author_rank: Super Contributor
timestamp: July 16, 2024, 11:57:45 am
content: What's your device?If you know the square area of the cable, then it's simpleto calculate it's resistance using the resistivity of copper.

author: Ian.M
author_rank: Super Contributor
timestamp: July 16, 2024, 12:00:24 pm
content: Quote from: selcuk on July 16, 2024, 11:44:58 amYes. Even with remote sensing, if your PCB is anything with logic ICs or a MCU, you may need additional decoupling capacitance at the PCB end if it hasn't been designed with adequate on-board bulk decoupling. Keep the power leads loosely twisted together to minimise induced EMI pickup.Quote from: tunk on July 16, 2024, 11:57:45 amIf you don't know the Cross Section Area (CSA) of stranded wire, you can measure the diameter of one strand with a micrometer, calculate the CSA of the strand (=D2𝝅/4), and multiply by the strand count. Alternatively, due to how the strands pack together, its approximately 90% of the area calculated from the overall core diameter.Unfortunately nowadays there is so much dodgy wire that contains low grade copper or even that isn't copper but has a thin copper coating to fool the unwary, so unless you bought your wire from a reputable distributer that can trace each individual lot back to the manufacturer, you may well find that using the standard resistivity for electrical grade copper seriously underestimates the voltage drop! If the calculated and measured voltage drops don't match up, your DMM is not out of calibration and you've accounted for thermal EMFs, you've got fake or crappy copper wire, not fit for purpose!

author: joniengr081
author_rank: Regular Contributor
timestamp: July 16, 2024, 12:24:26 pm
content: The PCB will have analog and digital ICs and MCU. The remote sensing at the power supply will set the power supply output higher then 3.3 V, right ? But the ground on the PCB can not be at 0 V. How can we make it at 0 V potential.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 16, 2024, 01:53:31 pm
content: Quote from: joniengr081 on July 16, 2024, 12:24:26 pmSo it will probably not consume continuously constant current. With current fluctuations the voltage drop at cables will also fluctuate.Quote from: joniengr081 on July 16, 2024, 12:24:26 pmYes. It will set supply output to ensure 3V3 at the place where sensing wires are connected. With changing current the output voltage will be adequately changed to ensure 3V3 where it is needed.Quote from: joniengr081 on July 16, 2024, 12:24:26 pmWhere we have 0V it depends on our decision. There are no something like absolute 0V. Even local Ground potential changes (sometimes high and rapidly - consider what happens during thunder strike).So you need only assume that you have 0V at your PCB and you have some negative voltage at power supply output.

author: TimFox
author_rank: Super Contributor
timestamp: July 16, 2024, 04:11:23 pm
content: Remote sensing can be very useful for high currents.However, watch out for the details of the four-terminal connection: many power supplies have a diode between each sense terminal and its corresponding output terminal, to prevent the supply generating too much voltage when the sense connection is open.With such a circuit, you must avoid excessive voltage drop along each high-current wire that results in its diode going forward-bias.Normally, the current in each sense lead is small (maybe 1 mA), so its wire can be much smaller than the high-current output wire.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 16, 2024, 04:52:58 pm
content: Quote from: joniengr081 on July 16, 2024, 12:24:26 pmMixed signal (digital + analog) PCB drawing 2A at 3.3V, supplied with 4 meters of cable -forget aboutdirectly supplying it with 3.3V. Redesign the board with a voltage regulator, and supply with 5V (or 12V or whatever). Higher supply voltage to the board reduces current, reducing voltage drop, but also allows you to have more variation in the voltage.Most 3.3V rated ICs are 3.6V recommended maximum. If you try to compensate for voltage drops by outputting higher voltage, you easily exceed that. Plus parasitic inductance of the supply cable can be a problem.Local regulation makes life easier.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 17, 2024, 09:59:27 am
content: It is more logical to send a higher voltage through the cable, and then use a local voltage regulator on the other side.I have a homebrew automation network at home, which sends 24V power and RS485 data over CAT-5 cables, and each node has it's own SMPS. CAT-5 is limited to about 1A, with linear regulators (each node 10mA to 100mA) the maximum would be around 20 nodes on a cable, but with the SMPS, a higher input voltage results in less current though the cable for the same power, and I guess I could add 100 nodes before I have to add some auxiliary power supply in another location.Voltage drop over the GND wire is also a concern, although not a problem when done correctly. With CAT-5 and 1A, the voltage drop on a long cable can be as much as 2V per conductor (they are not trans atlantic). That would mean that data received on a node at the far end of the cable, can be 2V below it's local GND level if the data is sent from a node near to the power supply. GND levels change along the cable, but data does not, because current is low. That was one of the reasons for choosing RS485 drivers. These are officially specified to work with data levels between -7V and +12V. (which is 7V above the "normal" 5V power rail)

author: joniengr081
author_rank: Regular Contributor
timestamp: July 18, 2024, 11:47:18 am
content: Thanks for comments. I think I will either use standard power supply with four wire, two for remote sensing. The other solution is to have linear regulators on the PCB and power up the PCB with 5 V or 7 V. The regulators on the PCB will generate 3.3 V.Regardless of the application onw which I am working.How do we design the remote sense at the custom power supply. Let's we do not have a standard commercial power supply. But using a DC-DC converter. How can we add the feature of remote sensing at the DC-DC converter such that the voltage at the PCB remains at 3.3 V with long wires and the ground at 0 V for different currents through the long cables ?

author: PGPG
author_rank: Frequent Contributor
timestamp: July 18, 2024, 04:17:48 pm
content: Quote from: joniengr081 on July 18, 2024, 11:47:18 amThat way you will solve the problem of having local voltage being 3V3 but you will not make cable currents lower so you will have the same voltage drop in GND wire.Quote from: joniengr081 on July 18, 2024, 11:47:18 amYou have to have the separate power outputs from converter and differential input of feedback voltage. I can't tell more as I have never designed/used such supply.