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 Power inductor tester 
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1048
Location: Lacey, WA USA
Very cool. Tests power inductors. When the curve breaks, that is the point at which the core saturates. You never want the core to go into saturation, as the inductance will drop suddenly and current go up quickly. Inductance can be calculated by the slope of the curve, before and after saturation.

My kind of simple circuit, two 555 timers driving some power transistors.

http://elm-chan.org/works/lchk/report.html

Schematic:
Image

A: 21.5μH
B: Rated current = 1.7A
C: Saturated at 3A (ferrite core saturats sharply)
D: 3.2μH (after core-saturation, it works as air-core inductor, so the peak current must be kept lower than core-saturation point)
Image

Kainka Labs did a video about an updated version, they will be doing a Kickstarter and it will be open source.
https://youtu.be/3Y_WcuIenSM

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Steve Greenfield AE7HD


Tue Jul 18, 2017 12:00 pm
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1048
Location: Lacey, WA USA
In my junkbox, I have some 0.02 ohm 1% resistors, so I can parallel a couple. 555 timers galore! A few high uF capacitors. And a selection of power MOSFETs, resistors, and transistors. I'll try and build this in time for the Capacitor/Inductor class I'm giving next Thursday the 27th.

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Steve Greenfield AE7HD


Thu Jul 20, 2017 2:04 pm
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1048
Location: Lacey, WA USA
I'm putting together a slightly modified version of the elm-chan circuit. I'm using some scavenged parts including two low ESR 4,700uF Al electro and four 22uF really low ESR ceramic capacitors. My current sense resistor is two paralleled 0.02 ohm 1% that were current sense resistors in a Lithium Ion pack, and the MOSFET is pulled from a dead car booster amp.

I'll make the first 555 timer have selectable slower clock speeds so I don't need such a high current power supply for high current inductors. I have some 10 turn front panel potentiometers for VR1, but I think they are 10k so I will need to increase C4 and C5 by a factor of 10. I think I'll use a multi-pole switch and maybe add a few extra ranges on their, as I'd like to use this on some very high inductance coils such as AC transformers.

The bold lines around the circuit from the 0-10V variable supply indicate the high current pathway. C1 and C2 must supply high ripple currents, leaving the variable power supply to provide much lower peak currents to keep them charged up. That high current pathway should be as short as possible and with wide PCB traces or heavy gauge wire. 16 or 14 gauge should be good for up to 10A over a short distance.

I have some 5 way binding posts rated at 30A for the inductor connection. The only thing that will be tricky, from my junkbox, is D1 across the coil under test. Whatever the peak current is when the MOSFET is turned off, will flow through D1. So it must be a fast turn-on diode that can withstand peak currents of over 10A. The long term current rating can be lower.

As it happens, although the 1N4007 is meant for powerline rectification, it has a very fast turn-on time. Where it fails for things like switching power supplies is that the off-time is too long. But for this, we only care about turn-on time and peak current. Looking at the datasheet, 1A continuous, 30A non-repetitive single peak 8.3ms half sine wave.

http://www.mouser.com/ds/2/149/1N4007-888322.pdf

Looking at page 3 figure 3, with 12A 8.3ms half-sine pulses, it can withstand 40 in a row at 60Hz. In this case, pulses will be -much- shorter than 8.3ms, so this should be good for those 10A pulses with room to spare.

Note that the 9V supply for the 555 timers cannot be the same as the variable supply. The 9V is probably simplest to get from a 9V battery for complete isolation to avoid ground loops from the high current section. It should be able to drive a MOSFET gate, even non-logic MOSFETs. It could be 9 to 15V. A 555 timer can put out reasonably high current. But because current starts out at zero with an inductor, rise time isn't too bad of an issue. And turn-off time is more a matter of power dissipation because it doesn't affect any measured parameters.

Hmm.... I forgot, I ordered some logic level MOSFETs that are rated for 30A and at 5V Gate drive, 0.047 ohms Drain to Source.

https://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf

I do so love this stuff...

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Steve Greenfield AE7HD


Sat Jul 29, 2017 6:07 pm
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