How to measure the effect of long wire on Powersupply - Page 1
Forum section: electronics / general

author: Wize93
author_rank: Newbie
timestamp: January 11, 2023, 02:27:33 am
content: Hi, I'm working on a project considering driving a LED load 25m from the power source. I'm concerned about the inductive kickback that may be induced on the power supply. Whats the proper way to test this scenario? Based on 18 AWG wire 50m length, the inductance is about 107uH.It is currently regulated, and its 24V 1khz PWM signal varies the duty cycle to regulate the current provided to the LED. It this module:https://www.tridonic.com/com/en/download/data_sheets/Driver_LCA_35W_24V_one4all_SC_PRE_en.pdfI measured the falling edge at the PSU when the wiring to load was 30cm which was 5.6us.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 11, 2023, 07:29:25 am
content: Just add a freewheeling diode (physically right next to the switching MOSFET) so that nothing is "induced on power supply" even with 10000000000000000H of inductance.If you expect the inductance to be large enough for the thing to operate in CCM (i.e., enough energy is stored so that there is not enough time for current to reach zero during off-time), then freewheeling diode has to be schottky type (or if not schottky, at least super fast reverse recovery / soft reverse recovery).With the diode, energy stored in the inductance continues flowing during off-time, powering the LEDs as usual.To reduce emissions, you probably want to slow the switching edge down a bit; simplest way to achieve this is to add a series gate resistor.

author: Wize93
author_rank: Newbie
timestamp: January 11, 2023, 02:24:53 pm
content: Emissions are not a concern since the wire is going to be underground. Adding a freewheeling diode is the standard practice I understand, but adding external circuits is not an option. I just want to test the wire at the source to see any inductive spike. Is there a particular way to probe it other than using a scope probe to observe waveform transitions?

author: berke
author_rank: Frequent Contributor
timestamp: January 11, 2023, 03:47:00 pm
content: Dave does it with a high-voltage probe in episode 1409 "The DANGERS of Inductor Back EMF"

author: Wize93
author_rank: Newbie
timestamp: January 11, 2023, 04:19:29 pm
content: Thanks for the link I'll look into it, I think I'm fine since I'm dealing with inductance as a transmission line and not coil inductance I should be fine in terms of voltage.

author: Faringdon
author_rank: Super Contributor
timestamp: January 11, 2023, 09:38:32 pm
content: yes youre right, though, AYK, the inductance will dominate in that 25m cable...so the freewheel diode likely to be needed.There is no other way to see the exact height of the voltage spike than a scope probe on the circuit.Even then it may avalache the PWM dimming FET and so will look smaller than it otherwise would.

author: Wize93
author_rank: Newbie
timestamp: January 12, 2023, 07:50:48 pm
content: The implementation was canned as flickering at the LED load when drastic changes were made to change the current at the load through the PWM voltage signal

author: floobydust
author_rank: Super Contributor
timestamp: January 12, 2023, 08:57:39 pm
content: I didn't expect the power supply CC to remain stable with large cable inductance.

author: Wize93
author_rank: Newbie
timestamp: January 12, 2023, 09:53:05 pm
content: Quote from: floobydust on January 12, 2023, 08:57:39 pm100% I knew before implementing this was a bad idea from what I learned from school about transmission lines and patristic inductance. It seems to remain stable once you set a set point but there is a huge fluctuation of the set point caused by flickering during change.