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Registered Member #4078
Joined: Tue Aug 30 2011, 12:53PM
Location: Sydney, Australia
Posts: 19
What's the consensus on the best way to measure the output current capability of a flyback+driver?
I presume a regular DMM won't appreciate having the output fed into it..? Though at a short circuit the output voltage will drop very low, so maybe its ok?
Or an HV probe + HV resistor array + ohms law perhaps?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
The only way i can think of doing this, (and im trying) is the use of a 5kV 2.2M resistor (for example) then put that in series with the Hv lead to your normal load (in my case a DeSeversky Lifter) then differentially measure the voltage drop, and use ohms law, and integration of the area enclosed will yied power. using hall stuff and clamp-ons, seem to be noise prone.
With differential-resistive current measurement you can still get credible redaings in when the enviroment is harsh.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Sample accurately the current's waveform at a low voltage. then amplify it 30dB. Dump that into a water bath load and measure the temperature rise. then extract the RMS value.
Registered Member #3904
Joined: Sat May 21 2011, 10:32AM
Location:
Posts: 28
I've had success with the cheap DC analog current panel meters you can find on eBay. For example, do a search for "DC 0-100mA Analog Current Panel Meter" and you'll find loads.
Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
You don't need to measure the output side. Measure the current drawn by the total circuit. All of this flows through the flyback primary and switching device.
Hence, find the turns ratio of your flyback transformer. Multiply the current by the turns ratio and then the current in the secondary will be some fraction of the current in the primary.
Say the total current flowing in the driver circuit is 2Amps, and you find the turns ratio is 1: 200. Simply turn this into a fraction to find I secondary...
2 * 1/200 = 0.01 A= 10mA. In practice there will be losses (heat) in the dirver, and in the transformer windings, - so not completely accurate, but it will give you a very good idea.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
As long as a current meter is well isolated from ground then there is not a big problem, because the voltage drop across a current meter's leads will be minimal at the small currents common in a flyback output. The meter itself will be floating at the output voltage of the flyback, but it doesn't arc to anything unless you bring the opposite side of the flyback, or ground, near it. Still there are some risks, so it's best to pick up something cheap and simple as Jamie suggests ;)
The current in the arc will have a very strange shape, and a high frequency, so unless you have a big filter capacitor to smooth it out to DC then I have to guess that the current meter will not be completely accurate though it's probably in the ballpark of the average current.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
astralhighway wrote ...
You don't need to measure the output side. Measure the current drawn by the total circuit. All of this flows through the flyback primary and switching device.
Hence, find the turns ratio of your flyback transformer. Multiply the current by the turns ratio and then the current in the secondary will be some fraction of the current in the primary.
Say the total current flowing in the driver circuit is 2Amps, and you find the turns ratio is 1: 200. Simply turn this into a fraction to find I secondary...
2 * 1/200 = 0.01 A= 10mA. In practice there will be losses (heat) in the dirver, and in the transformer windings, - so not completely accurate, but it will give you a very good idea.
Im going to measure the input and output of a flyback in the next few weeks. And ill bet these types of calculations are way off. flybacks can lose energy as heat in multiple ways. also the fact that they store energy unlike everyother type transformer means you could really be off.
Alot of people claim to be putting in 500 W - 1 kW int a ZVS, but i wonder how much of that really comes out of the secondary, what if its as low as 30-50% ?
The calorimeter idea might be usuable though id use oil with the whole driver+flyback in a thermos, then do the math.
Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Patrick, you say:
'...also the fact that they store energy unlike everyother type transformer means you could really be off.' Show me any transformer that doesn't store energy. It's not possible. Transformers all store energy, they're inductive. All inductors store energy.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
astralhighway wrote ...
Patrick, you say:
'...also the fact that they store energy unlike everyother type transformer means you could really be off.' Show me any transformer that doesn't store energy. It's not possible. Transformers all store energy, they're inductive. All inductors store energy.
Energy storage is minimized and unwanted in every other kind of transformer, but the flyback is dependent on the air gap, and magnitizing inductance.
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Measuring a flyback's current
