switchmode gyrator

I'm sure that there are other reasons why an inductor will be used besides the lack of time/energy/resources to 'design them out'.

Energy storage is just one means to an end.

RL

Sure. We want to design some dummy loads that will simulate relays, solenoids, stepper motors, torque motors, with programmable R and L. Seemed to me that using caps to make fake inductors would be a good way to do that.

It's at least an interesting idea to play with. Maybe we can switchmode simulate R+L all at once. We would have to store energy and dissipate power to do that.

Relays and motors do not behave like simple inductors. For example, while a relay armature is moving, the back EMF is high enough to make the current _drop_ briefly. Modelling that requires more than a simple gyrator.

Jeroen Belleman

PM DC motors look capacitive over a broad frequency range.

Cheers

Phil Hobbs

My customer is building giant rackmount boxes full of heavy inductors as part of his dummy loads. We want to replace them.

Given a generalized switching impedance simulator, I guess one could model a DC motor.

I am considering a powered impedance simulator, not the theoretical gyrator. Just sort of a gyrator.

If space, weight and stray magnetic fields aren't a problem, big inductors can sometimes turn out to be remarkably cheap and efficient compared with more complex solutions.

Loads - the simpler the better unless there's a return on the investment.

One nice rhing about inductors is that they tend not to break.

Unless it's meant to fly or be hand-carried, weight doesn't sell. Used to be a factor in shipping costs.

RL

Investing in heavy duty castors for the racks would get more bang for the buck.

RL

Quote your customer and see if they're as enthused.

Remind him that he has 'free' loads just crying out for burn-in.

RL

We have learned that our customers care a great deal about rack space and cables. And they used to have techs that built things like load boxes, and when those old guys retired they didn't replace them.

We may make some smaller R+L dummy loads by switching real resistors and tapped inductors. That is brain straining, especially specifying the tapped inductors.

I was musing on designing a switchmode programmable synthetic R+L box with essentially infinite programmable resolution, and using caps for the energy storage instead of real inductors.

I suppose there can be a generalized impedance simulator.

It's hard to get a forklift into some of the test cells.

Then be carefull, because you're dealing with the wankers who are still hanging around with their fingers . . . .

They have billions of dollars to spend, so we're polite to them.

And we don't want customers who are smart enough to design their own electronics.

Many inductive loads can produce kilovolts when switched off suddenly. If you build a switched-mode equivalent, unless it contains a large real output inductor, your class-D output stage might need supply rails of kilovolts to emulate the real inductor accurately. That sounds expensive.

If the load you are emulating contains a TVS or other clamp as part of the load, then it would be easier to emulate, but you wouldn't be able to emulate the fault condition where the clamp fails open.

An electronic load would of course have specified voltage, current, and power limits.

But some of the simulated inductance - a few mH maybe - could be real inductors, and they would fly back a bunch. We do need to accept a customer PWM drive and sort of behave right.

I don't think that kilovolt flybacks are common in real areospace systems. That could damage wire insulation and interfere with other stuff.

Yes, the customer's drivers have flyback diodes or equivlent, or just a continuous low-Z PWM drive. We can't simulate every corner case, lightning strikes or major system faults.

It's just an interesting problem, simulating a pretty high power inductor without a hundred pounds of iron. What I'd never thought much about is how terrible inductors are at storing energy. Inductor:capacitor:supercap:battery are roughly 1000:1 steps in energy density.

On the plus side, you can (so to speak) charge and discharge them a good

Cheers

Phil Hobbs

I haven't sold many of these, but it was interesting to design.

Better to make it a floating inductor instead of a grounded one like gyrators are.

Bob Pease shows how:

That's differential but not floating relative to the opamp power supplies.

Every channel will of course need its own dc/dc converter, so it really looks like an isolated resistor+inductor to the customer.

We do that a lot, floating circuits. The Coilcraft planar transformers are great for powering isolated stuff.