H-Bridge with level shift - MOSFET gate voltage issues?

It's a good point -

I don't have access to a 'scope but would this not apply to most fairly lights with dimming patterns?

My set cannot be using current control as you can odd anywhere from one to 8 sets in parallel after the controller.

I find it doubtful they could be using voltage control given LEDs - or could they?

These are definitely not addressable NeoPixel devices.

It was a good point to raise - to not try to get to clever and bang the PWM through the roof for the sake of flicker control :)

In reality, we wouldn't want to try to get too high with a Pi, but kHz sounds like a good place to be for LEDs.

I've wanted an excuse to get a picoscope or similar. Apart from helping with software debugging, it would be interesting to see what the current (approved) unit puts out.

As long as I stay in the same region, should be fine - there are no black boxes on the strings that could contain much in the way of filtering (or if there are chokes or caps in the LED bases, they would work as well with my controller if the drive pattern is similar)

If you switch at, say, 1 KHz, the radiation efficiency of a string of LEDs as an antenna will be zilch. At the AM broadcast band, where the antanna efficiency works its way up to terrible, you'd be looking at the 1000th harmonic of the drive waveform. That line will contain microscopic energy.

So don't worry about it.

But for extra fun, go software spread-spectrum.

I was thinking about this driving home - the wire is also a balance conductor pair which should reduce radiation too.

We're not talking about a giant single core loop, but a star topology with 2 pair cables. Not twisted pair, but still in close proximity.

:-)

Sadly, AM radio amplitude at your receiver is also microscopic, and a strin g of LEDs can be lot closer to your AM radio (and your neighbour's AM radio s) than the AM transmitter. A long string of LEDs can be physically long en ough to be a appreciable fraction of an AM wavelength so its radiative effi ciency can be above the "zilch" level.

Not good advice.

It will certainly make it harder for the interference detector vans to lock in on your location, and make it less likely that a particular neighbour w ill get irritated by noise interfering with their favourite AM station. Hav ing all of them mad at you might be a different problem.

Filtering out the higher frequency components before the current gets to th e LEDs might be a safer solution, if more expensive.

Ms

ing of LEDs can be lot closer to your AM radio (and your neighbour's AM rad ios) than the AM transmitter. A long string of LEDs can be physically long enough to be a appreciable fraction of an AM wavelength so its radiative ef ficiency can be above the "zilch" level.

ck in on your location, and make it less likely that a particular neighbour will get irritated by noise interfering with their favourite AM station. H aving all of them mad at you might be a different problem.

the LEDs might be a safer solution, if more expensive.

JL is overlooking edge rate of the current pulses. One can expect unattenua ted spectral components in the GHz range. The LED string is a very nonlinea r load which greatly exacerbates the interference problem in this respect, in the way it will snap into high conduction after just a few 10s millivolt output excursion past LED cut-in.

Appropriately sized choke?

Is there a MOS version of this antediluvian part? I could still find several applications of it.

Best regards, Piotr

there are lots of more modern DC-motor and stepper-motor drivers that are similar, all depends on what you need

No, but I am considering that the 1000th harmonic of a square wave has pretty low amplitude. The math is well documented.

At power levels that are undetectable in the thermal/atmospheric noise. Do the math, for Pete's sake.

A forward-biased LED string will "snap into high conduction" in 10s of mV?

That sort of reasoning prevents anything from getting done. But you don't design electronics.

e:

But neglecting the fact that AM radio signal with which that harmonic would interfere also has a pretty low amplitude.

Not that John Larkin seems to have a clue about what mathematics he might b e suggesting that Fred Bloggs ought to do. John Larkin isn't exactly famous for posting his mathematical work, and some of us suspect that he doesn't know any more about what the word actually means than he does about word "d esign".

nterference problem in this respect, in the way it will snap into high cond uction after just a few 10s millivolt output excursion past LED cut-in.

LED's are diodes, and the current through them is an exponential function o f voltage. I've never heard of LEDs acting as "snap-off" diodes.

They are varicaps, so I suppose that it is theoretically possible.

No. That sort of reasoning leads people to test what is actually happening.

The idea of one LED in long string of LED acting as a snap-off diode is arr esting, but it clearly didn't register with John Larkin (who does know abou t snap-off diodes, but only when they are diodes that he wants to use as st ep-recovery diodes).

the 60 Hz harmonics of a standard lamp dimmer certainly can and DO interfere with AM radio.

Basically I need a dual low-side MOSFET driver, but their static load properties are rarely specified.

Best regards, Piotr

O tempora, o mores!

Best regards, Piotr

if you just need low side there are millions of FETs that will work or if you want short circuit and temperature protected the numerous low side switches

Low side *driver*, not a FET. I need two full half-bridge pairs and their static current and transient voltage specs. Vendors don't care much about that, given the usual application.

Best regards, Piotr

Now this is something very close to my needs, thanks!

Best regards, Piotr