Am i doing this LED work correct?

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Ironduke
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Re: Am i doing this LED work correct?

Postby Ironduke » Mon Oct 22, 2018 1:14 am

Ok so, from what I can gather the DB107 rectifier has a reverse recovery time of 500ns which is referred to as "standard recovery time". I'm not sure how to calculate the max frequency vs the Trr, especially since we're talking about a square wave here instead of a sine wave.
But I would note that I haven't found many DIY decoder circuits that use Schottky diodes in the rectifier stage. Has anyone run this through an oscilloscope?
Regards
Rob

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Re: Am i doing this LED work correct?

Postby End2end » Mon Oct 22, 2018 1:20 am

My electronic components shopping list for my coach lighting. (Excluding the vero board that I soldered everything too)

Capacitor 470uF @ 35v - https://www.ebay.co.uk/itm/Electrolytic ... 6SjZAMxcnw
Bridge rectifier 1.5A Single Phase @ 50V (W00M5 in the listing) - https://www.ebay.co.uk/itm/1-5A-Single- ... TNA99k0yUQ
1K resistor (1/4 W)
Hope it helps.
Thanks
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Re: Am i doing this LED work correct?

Postby collectors » Mon Oct 22, 2018 8:02 am

TimberSurf wrote:I may be scaremongering here, as you may NOT have a problem, but technically, you should use a fast recovery diode network (bridge rectifier) or commonly known as Schottky1 diode.
Normal diodes are only designed for (50Hz) 'mains' frequency, but DCC is in the region of 20,000hz (20KHZ) and normal diodes cannot react fast enough, causing an unwanted spike that distorts the waveform, most notably on Hornby systems (non NMRA compliant)!

I would recommend something like a MB6 (example) / pdf

1 amended


Well done :D , i do like someone with the best product & also saves me money. At £1.49 for 50, i might buy 2 lots out of sheer extravagance. lol. I would love to know how they do these prices inc P&P. I can only assume its a world wide reciprocal postal agreement.
Thanks again.

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Re: Am i doing this LED work correct?

Postby TimberSurf » Mon Oct 22, 2018 9:16 am

I can't find a brilliant video I once found on the subject, so without getting really technical, problems can be had with ringing on DCC.
This is in the form of spikes at the leading edge of the square wave. In essence, these induced spikes can cause 70v peak to peak to be seen. There are fixes, like emi filters at the end of long runs, snubbers, ferrite cores and even TVS diodes can be fitted in the loco. All of these do not eliminate the cause!
Certain conditions (unavoidable) can be caused simply by the size and location of wiring/track, allowing the system to act as a radio frequency pickup aerial. But a lot can be attributed directly to know generators like diodes with inadequacy recovery times.

The bottom line is that most of the time on a small layout, little problems occur. The point is, that if you increase the the size or number of instances of potential problems, the risk is increased accordingly. i.e. run 3 coaches with lights with normal bridge rectifier = no problem, run 16 coaches with lights with normal bridge rectifier = decoders mysteriously misbehaving!

It's a question of good practice. I see so many threads on forums about decoders suddenly with no reason (that have performed faultlessly previously for a long time) throwing their dummy out, it makes me wonder if the guy next door happened to get his DIY arc welder out that day?
There is never a detailed list of the home grown electronics hanging off the DCC system or how many auxiliary devices are hanging on it! All of which can have an effect. As there are few true DCC experts (i.e. not available for home visits) and the technical vagaries of DCC almost make it a black art, the only recourse is to be cautious and use best practice to eliminate issues at the beginning.

I am sorry to have to wear a black hat, but a little bit of good knowledge can go a long way, towards getting onto the right track (pun intended)
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Re: Am i doing this LED work correct?

Postby collectors » Mon Oct 22, 2018 9:37 am

TimberSurf wrote:I can't find a brilliant video I once found on the subject, so without getting really technical, problems can be had with ringing on DCC.
This is in the form of spikes at the leading edge of the square wave. In essence, these induced spikes can cause 70v peak to peak to be seen. There are fixes, like emi filters at the end of long runs, snubbers, ferrite cores and even TVS diodes can be fitted in the loco. All of these do not eliminate the cause!
Certain conditions (unavoidable) can be caused simply by the size and location of wiring/track, allowing the system to act as a radio frequency pickup aerial. But a lot can be attributed directly to know generators like diodes with inadequacy recovery times.

The bottom line is that most of the time on a small layout, little problems occur. The point is, that if you increase the the size or number of instances of potential problems, the risk is increased accordingly. i.e. run 3 coaches with lights with normal bridge rectifier = no problem, run 16 coaches with lights with normal bridge rectifier = decoders mysteriously misbehaving!

It's a question of good practice. I see so many threads on forums about decoders suddenly with no reason (that have performed faultlessly previously for a long time) throwing their dummy out, it makes me wonder if the guy next door happened to get his DIY arc welder out that day?
There is never a detailed list of the home grown electronics hanging off the DCC system or how many auxiliary devices are hanging on it! All of which can have an effect. As there are few true DCC experts (i.e. not available for home visits) and the technical vagaries of DCC almost make it a black art, the only recourse is to be cautious and use best practice to eliminate issues at the beginning.

I am sorry to have to wear a black hat, but a little bit of good knowledge can go a long way, towards getting onto the right track (pun intended)


Thanks for the info. Fortunately i will only have one passenger train with carriage lights. But it is on 60met of track. The only thing i might have problems with is there is a 240volt mains cable running in parallel with some the dcc cables, i am not sure if i will get problems with any induction/interference? The dcc pair have been twisted.

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Re: Am i doing this LED work correct?

Postby TimberSurf » Mon Oct 22, 2018 10:11 am

60 meters?
If your track is that long, you should definitely look at snubbers at each end! :(

6 meters. hmm not so bad
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Re: Am i doing this LED work correct?

Postby collectors » Mon Oct 22, 2018 11:00 am

TimberSurf wrote:60 meters?
If your track is that long, you should definitely look at snubbers at each end! :(
6 meters. hmm not so bad


Sounds long, but that is a twin track in a sort of figure of eight on 3 area's & 3 snubbers fitted, that i got the idea from your site. Thanks

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Re: Am i doing this LED work correct?

Postby End2end » Mon Oct 22, 2018 11:07 am

Erm....what's a snubber? :?
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Re: Am i doing this LED work correct?

Postby TimberSurf » Mon Oct 22, 2018 12:18 pm

End2end wrote:Erm....what's a snubber? :?
Thanks
End2end

An RC snubber (Resistance Capacative) is an electric circuit intended to suppress voltage spikes.
Usually consisting of a resistor and a capacitor in series across (in parallel with) the supply.
As a 'tuned' circuit, components are selected to suppress frequencies other than the one your trying to get through.
Sometimes referred to as an EMI filter.
Commonly used on AC relay coils and motors to suppress BEMF and inverter hamonics (hundreds of £'s), but also used on coms networks to suppress induced and radiated EMI.
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Re: Am i doing this LED work correct?

Postby End2end » Mon Oct 22, 2018 12:26 pm

Oh a filter! Now i get it. :)
I use filters all the time in music prodution to remove unwanted frequencies.
Thanks
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