Inductive clamp energy

In message , The Phantom wrote

That was posted over 8 hours ago. Nothing in abse yet.

I was able to grab it about 7pm EDT last night, 9 hrs ago. But I haven't read or even more than glanced at it yet (it looked complicated), so you're at no disadvantage, so far!

Well, it does take a little while to digest it all.

Briefly, what they're showing is something that, to borrow a phrase John Popelish used nearly 3 years ago, "...we knew but didn't realize." Any of us could figure it out, if only we realized we should.

As shown in this app note:

the suppressor (clamp) can be connected across the switch *or* across the relay coil (inductor).

If the clamp is across the inductor, then a simple zener can't be used, because the zener will conduct in the forward direction when the switch is on; another diode must be placed in series with the zener. A bidirectional suppressor (two zeners in one package, or a MOV) solves that problem, but may be more expensive or hard to get.

If the clamp is across the switch, then a simple zener can be used, but as shown in the paper I posted on ABSE, the dissipation in the clamp is not just due to the energy stored in the inductance.

The thing to realize is that when the clamp is across the switch and the switch turns off, the current in the inductor is decaying and the power source, inductor and clamp are all carrying the same current. The direction of current in the DC power source is such as to represent energy flow *out* of the source. Since we believe in conservation of energy, we see that the energy that must be absorbed by the dissipative elements (mainly the resistance of the wire of the inductor and the clamp) will be

The authors of the paper I posted over on ABSE show that if the clamp voltage (when the clamp is across the switch) is only slightly higher than the power source voltage, the time for the inductor current to decay is increased, and the energy the clamp must absorb can be substantially more than L*I^2/2.

The situation is different if the clamp is across the inductor, and this factor should be taken into account when deciding where to place the clamp.

The increase in energy to be absorbed when the clamp, or snubber, is across the switch is a phenomenon that also should be taken into account in switching power supplies.

The Phantom wrote: (snip)

The possible up side of having the snubber across the switch (that might pay for the higher power dissipation during inductive discharge, in some cases) is that the supply current is continuous during the process, so the supply voltage is not bumped up as much as when the snubber is across the coil and the supply current switches of as fast as the switch current drops.

Generally, I would rather use bypass capacitance to smooth the bump than dump extra energy into the clamp.

The Phantom wrote: (snip)

(snip)

When I read this, I assumed you were remembering something someone else had said, but had my name attached to the memory. I was wrong. I found it with Google and enjoyed reading the thread all over again. It is a shame my brain is full and I have to lose a memory to make room for a new one.

I don't get out much...What is ABSE?

If your newsgroup browser supports it, you can use this link to get directly to the indicated article: news: snipped-for-privacy@4ax.com

Otherwise, look for alt.binaries.schematics.electronic and search for the subject looking like: Energy in clamps--from SED - Inductive Energy Calculations...

Jon

He's a googlie - they don't get binaries.

What Jon needs to do is contact his ISP, and get the IP/URL of his newsserver, and find a real newsreader (butthook distress sucks), and then he can post/read anywhere he wants to.

Cheers! Rich