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Minimising EM noise from PWM switched fairly lights?

Started by Tim Watts October 24, 2019
Hi,

This year's project is to drive 3 strings of 30V DC LED outdoor fairy 
lights from Raspberry Pi's[1]


I've been on here before last year and took some greet advice with 
respect to H-Bridges and found some, along with DC-DC converters that 
will enable me to interface a 30V DC supply to the Pi and the Pi back to 
the lights.


The only thing I am mindful of is not to inadvertantly blat out a ton of 
EM noise.

So I was wondering if anyone (without guarantees of course) might be 
able to offer any rule of thumb advice please?


1) I'll keep the PWM frequency as low as possible without causing 
visible flickering to passing motor cars (which is sometime I can test 
by driving by, road speed limit is 30mph so not super fast).

2) I'm thinking to pop a suitable choke inline with each string of 
lights after the H-Bridge to block the higher harmonics. This is the bit 
I'm not sure of. What would be a suitable number of Henrys as a function 
of drive current and base frequency of the PWM?

3) Are there any inexpensive ways of doing a quick and dirty 
verification of the amount of noise being emitted? I don't have a 
'scope, though I could get a picoscope type unit if sticking an antenna 
of some sort on it would provide useful tests???


Many thanks,

Tim
On 24/10/2019 09:57, Tim Watts wrote:
> 3) Are there any inexpensive ways of doing a quick and dirty > verification of the amount of noise being emitted? I don't have a > 'scope, though I could get a picoscope type unit if sticking an antenna > of some sort on it would provide useful tests??? >
Cheapest, simplest, quickest: get an old-school battery powered pocket AM/FM radio. Find a quiet spot on the AM broadcast band and start sniffing your unit under test for RFI. piglet
piglet wrote:

---------------
> > > Cheapest, simplest, quickest: get an old-school battery powered pocket > AM/FM radio. Find a quiet spot on the AM broadcast band and start > sniffing your unit under test for RFI. >
** Ah, the $10 portable spectrum analyser. Analogue TVs with rabbit-ears are also good at the same task. Built a simple, 18V linear reg once using a BD139 while watching CH9 on TV in the same room. When energised, it wiped out the pix - a 10nF ceramic soon came to the rescue. Another time, I was getting regular, slow impulse noise spikes breaking into my scope and guitar amps. Tracked it down with a AM portable to a local Pharmacist, 65 metres away, with it a dodgy neon sign in the window. Who woulda thunk ? ..... Phil
On 24/10/2019 09:57, Tim Watts wrote:
> Hi, > > This year's project is to drive 3 strings of 30V DC LED outdoor fairy > lights from Raspberry Pi's[1] > > > I've been on here before last year and took some greet advice with > respect to H-Bridges and found some, along with DC-DC converters that > will enable me to interface a 30V DC supply to the Pi and the Pi back to > the lights. > > > The only thing I am mindful of is not to inadvertantly blat out a ton of > EM noise. > > So I was wondering if anyone (without guarantees of course) might be > able to offer any rule of thumb advice please? > > > 1) I'll keep the PWM frequency as low as possible without causing > visible flickering to passing motor cars (which is sometime I can test > by driving by, road speed limit is 30mph so not super fast). > > 2) I'm thinking to pop a suitable choke inline with each string of > lights after the H-Bridge to block the higher harmonics. This is the bit > I'm not sure of. What would be a suitable number of Henrys as a function > of drive current and base frequency of the PWM? > > 3) Are there any inexpensive ways of doing a quick and dirty > verification of the amount of noise being emitted? I don't have a > 'scope, though I could get a picoscope type unit if sticking an antenna > of some sort on it would provide useful tests??? > > > Many thanks, > > Tim
Why bother for a private project ? -- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
On 24/10/2019 10:16, piglet wrote:
> On 24/10/2019 09:57, Tim Watts wrote: >> 3) Are there any inexpensive ways of doing a quick and dirty >> verification of the amount of noise being emitted? I don't have a >> 'scope, though I could get a picoscope type unit if sticking an >> antenna of some sort on it would provide useful tests??? >> > > Cheapest, simplest, quickest: get an old-school battery powered pocket > AM/FM radio. Find a quiet spot on the AM broadcast band and start > sniffing your unit under test for RFI. > > piglet >
Thanks for that :) I could baseline the current controller on the same lights with it first to get some idea of what level is acceptable.
On 24/10/2019 10:28, TTman wrote:
> On 24/10/2019 09:57, Tim Watts wrote:
> Why bother for a private project ? >
I'd like to not piss the neighbours off when their TVs or radios stop working :) Or worse, interefere with some comms system that matters. I'm in England, my plot is 100x75 feet so we're all fairly close to each other.
On 24/10/2019 09:57, Tim Watts wrote:
> Hi,
> The only thing I am mindful of is not to inadvertantly blat out a ton of > EM noise. > > So I was wondering if anyone (without guarantees of course) might be > able to offer any rule of thumb advice please? > > 1) I'll keep the PWM frequency as low as possible without causing > visible flickering to passing motor cars (which is sometime I can test > by driving by, road speed limit is 30mph so not super fast). > > 2) I'm thinking to pop a suitable choke inline with each string of > lights after the H-Bridge to block the higher harmonics. This is the bit > I'm not sure of. What would be a suitable number of Henrys as a function > of drive current and base frequency of the PWM?
You want a small capacitor across the load to provide a local path for higher frequencies to attenuate them further. Try this simple URL: https://www.electronics-notes.com/articles/radio/rf-filters/constant-k-simple-low-pass-lc-rf-filter-design-calculations.php If you lowpass it to below 30kHz you probably won't annoy any hams. (or jam local 60kHz radio clocks)
> 3) Are there any inexpensive ways of doing a quick and dirty > verification of the amount of noise being emitted? I don't have a > 'scope, though I could get a picoscope type unit if sticking an antenna > of some sort on it would provide useful tests???
AM radio tuned to an otherwise quiet spot. You would have to be doing something really silly to end up broadcasting MHz harmonics though. -- Regards, Martin Brown
On Thursday, October 24, 2019 at 7:57:56 PM UTC+11, Tim Watts wrote:

<snip>
 
> 1) I'll keep the PWM frequency as low as possible without causing > visible flickering to passing motor cars (which is sometime I can test > by driving by, road speed limit is 30mph so not super fast).
It's the edges that generate the high frequency content, so minimising the PWM frequency isn't all that helpful.
> 2) I'm thinking to pop a suitable choke inline with each string of > lights after the H-Bridge to block the higher harmonics. This is the bit > I'm not sure of. What would be a suitable number of Henrys as a function > of drive current and base frequency of the PWM?
The aim is to stop the high-frequency content getting out into the string of lights (which acts like an aerial broad-casting and high-frequenes you may push aronud the loop), so put the choke as close as possible to the H-bridge, and immediately follow it with a capacitor to the return connection of the H-bridge. This is an L-section filter. As second one right up against it will attentuate any high frequency content even further. Any wound inductor has interwinding capacitance, and a self-resonant frequency. It doesn't do much to attentuate frequencies higher than the self-resonant frequency. Ferrite beads and chips aren't wound, but don't have much inductance.
> 3) Are there any inexpensive ways of doing a quick and dirty > verification of the amount of noise being emitted? I don't have a > 'scope, though I could get a picoscope type unit if sticking an antenna > of some sort on it would provide useful tests???
Transistor radio - as everybody else has pointed out. -- Bill Sloman, Sydney
On 24/10/2019 11:49, Martin Brown wrote:

> You want a small capacitor across the load to provide a local path for > higher frequencies to attenuate them further. Try this simple URL: > > https://www.electronics-notes.com/articles/radio/rf-filters/constant-k-simple-low-pass-lc-rf-filter-design-calculations.php >
Hi Martin, That's great - thanks! I'll play with some numbers and see what looks sane. Sounds like a job for a bit of sold fashioned tagboard. I'll have 4 modules (filter, DC-DC, H-Bridge and Pi) to mount on an acrylic plate to sit inside of an airtight food/fridge box which is ideal for the job. Gland the cables in and out and it'll sit on the grass right next to the lights it's driving. The plan is to use a Pi Zero/W and access it over WiFi - 3 sets in total, one for each fairy string. It's more for my son who wants to dabble with some coding and asked if we could make a controller for our existing Xmas lights.
> > If you lowpass it to below 30kHz you probably won't annoy any hams. > (or jam local 60kHz radio clocks)
Thank you - good design limit. I have 60kHz radio clocks, so hopefully any badness will be apparant to me fairy quickly.
> >> 3) Are there any inexpensive ways of doing a quick and dirty >> verification of the amount of noise being emitted? I don't have a >> 'scope, though I could get a picoscope type unit if sticking an >> antenna of some sort on it would provide useful tests??? > > AM radio tuned to an otherwise quiet spot. You would have to be doing > something really silly to end up broadcasting MHz harmonics though. >
I'd hope it would be hard :) Actual drive frequency depends on whether the Pi's 2 hardware PWMs are phase locked which which case I will use 2 to drive a single H-Bridge. If not, I'll have to do it in software and it will be really slow. I have an el-cheapo logic analyser USB widget so I'll test that soon. If this works, next will be to make a 4th box but drive some neopixels in fairy string format which will add a dash of colour to the whole thing. Many many thanks for your kind suggestions :)
On Thu, 24 Oct 2019 09:57:49 +0100, Tim Watts <tw@example.com> wrote:

>Hi, > >This year's project is to drive 3 strings of 30V DC LED outdoor fairy >lights from Raspberry Pi's[1] > > >I've been on here before last year and took some greet advice with >respect to H-Bridges and found some, along with DC-DC converters that >will enable me to interface a 30V DC supply to the Pi and the Pi back to >the lights. > > >The only thing I am mindful of is not to inadvertantly blat out a ton of >EM noise. > >So I was wondering if anyone (without guarantees of course) might be >able to offer any rule of thumb advice please? > > >1) I'll keep the PWM frequency as low as possible without causing >visible flickering to passing motor cars (which is sometime I can test >by driving by, road speed limit is 30mph so not super fast). > >2) I'm thinking to pop a suitable choke inline with each string of >lights after the H-Bridge to block the higher harmonics. This is the bit >I'm not sure of. What would be a suitable number of Henrys as a function >of drive current and base frequency of the PWM? > >3) Are there any inexpensive ways of doing a quick and dirty >verification of the amount of noise being emitted? I don't have a >'scope, though I could get a picoscope type unit if sticking an antenna >of some sort on it would provide useful tests??? > > >Many thanks, > >Tim
At a low PWM rate, say 100 Hz, anything radiated in the AM band will be ballpark the 10,000th harmonic of the switch frequency. The energy will be nil. Slow down the switching edges a bit if you like. Add mosfet gate resistors to soften the edges. -- John Larkin Highland Technology, Inc lunatic fringe electronics