Guru
Joined: 09/08/2007
Location: AustraliaPosts: 4406
| Posted: 12:46pm 07 May 2017 |
There is a much better way to build an inductance tester than that.
The problem with that circuit, and others like it, is that all the energy has to come from the power source, and is then dumped into a resistive load. Both the power source and the load need to be ENORMOUS to test any serious kind of choke.
A simple practical way to overcome that problem is to recirculate the energy stored in the choke back into a large energy storage capacitor, so the dc power source only has to make up the circuit losses, which can be quite low.
So for example, you can easily test a choke that saturates at fifty amps, requiring an adjustable dc power supply that might need to supply only one or two amps.
Both mosfets turn on and off simultaneously at a constant 50/50% duty cycle, but at an adjustable frequency.
A suitably large low ESR electrolytic, capable of supplying huge peak current supplies the charging energy into the choke, while both mosfets are on.
When both mosfets turn off, the stored energy in the choke returns to the energy storage capacitor through the two shottky diodes.
The energy returned will always be very slightly less due to voltage drops in the diodes and resistive losses through the mosfets and dc resistance of the choke.
The ramp up time will always be very slightly longer than the ramp down time, because of these unavoidable losses, so the current having ramped down, will sit at zero for a short time before the mosfets turn back on again. So its always safe to use a 50/50% duty cycle to drive this, preferably coming from a flip flop.
A high current Hall sensor will be required to monitor the current ramp, as its dc and only goes in one direction, a current transformer will not work.
Start out with your main dc supply at zero volts, and your switching frequency set very high for the shortest on interval. Monitor the current waveform on an oscilloscope.
Gradually increase the applied dc voltage, and lower the switching frequency until you get a nice big current ramp, that fills the oscilloscope screen.
Its then easy to read off the screen the rate of current rise in terms of amps per microsecond at a known applied dc voltage.
The rising ramp will be perfectly linear up to some point, where it will then start to bend upwards, which indicates the onset of saturation. The core might even growl or whine due to magnetostiction at that point.
Don't get carried away with this ! Remember your mosfets and shottky diodes have a current limit above which they will definitely go bang. Work out what the safe limit is for your test rig, and stay below that.
Heat sinking should not be a big issue, as you just wind up the wick, look, and wind it back down again. No need to run it for extended periods at very high power.
If you are using a function generator with push button frequency ranges, do not attempt to switch frequency ranges under significant power. Always reduce the dc voltage to zero, change the frequency range, then wind the volts back up.
Just pushing a frequency range button will likely blow up your mosfets.
Its a very handy tester to have, and very quick and easy to use. But be mindful that its all too easy to tweak things where the peak currents reached are going to be destructive.
The energy storage capacitor is going to have a hard life, so use anything you have on hand to start off with, but plan on eventually getting a really good quality high ripple current rated capacitor. Exploding electrolytics can be dangerous, so take care.
Best capacitors are the Evox RIFA brand, often available on e-bay. These have awesome specifications. They come in a white plastic case and always have screw terminals. Brand new they are horribly expensive in the larger sizes, but if you are patient, something suitable will turn up on e-bay.
Cheers, Tony.