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Voltage reduction for lighting


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On 01/08/2020 at 16:57, mikeg said:

An update, I have managed to find the end of the connections between the batteries and the copper strips and have unsoldered it and have been able to measure the amps for the lights.

A coach with 10 LEDs measured 0.2 amps, another with 14 LEDs was 0.33 amps. I have 3 others that have 8, 12 & 13 but have not been able to measure the amps as the batteries are flat and I thought that as they are all wired up the same the amps will be between the 0.2 & 0.33, so can a solution be given.

 The track voltage is 12.9v DCC which I have checked and the batteries are 2 AA in series and measure 2.7 to 2.9 volts they are approximately 20 years old but have not been used for long periods, I have used them 5 to 7 times in the 6 years I have had them.

 

 I hope that you now have all the information you require, the lock down period has been very useful for sorting out my models and layout.

 

regards mike 

if you assume the rectified DCC will be about 16v on load (with a smoothing capacitor) then you need to drop 16 - 2.8 =13.2v, with a current of 200mA for 10 LEDs (that's very high) you would need about about 68Ω (13.2 v/ 0.22A) but it would need to be a 3W resistor and it would get very hot.

The best bet would be a Buck convertor (Hobbyhorses's suggestion) as they are a switched mode device with typically 95% conversion efficiency. (so little heat to dissipate)

 

I've used buck convertors for powering an Arduino projects from a rectified16V AC supply, when using LM317s they got too hot to touch.

Edited by melmerby
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5 hours ago, mikeg said:

Thanks Richard, I have put in a bridge rectifier across track supply and it’s output voltage is 12.5 so I can workout what I need from you answer. Having now checked all the coaches I have been wondering if it would be possible to connect them all together from the one set of pickups, I think I can get small 2 pin plug & socket connectors for between the coaches and as I can now get to the copper strips in each coach I could easily join them all together. 

Can you see any problem with this approach and can I just add all the amps together and use one resister to the supply in the pickup coach?

 

Hope I am not being a pest but electrics is not something that I fully understand so thanks for your help and time.

 

Regards mike 

No that would not work. All resistors dissipate heat the higher the current the more heat. The more heat needed to be dissipated physically the larger the surface area of the resistor needs to be. It is a fine balance not allowing a resistor to get too hot since that begins to alter the value of the resistor. This is the reason why it is recommended to provide each LED in a circuit with its own resistor thus each individual resistor has minimal heat to dissipate.

 

Going of your figures for your coach lighting of 12.5V supply and 0.33A current draw you would require a resistor value of approx 38 Ohms, the nearest preferred value would probably be 39 Ohms. However, the total power that the resistor would need to cope with is just over 4W, you would have to go up to a 5W resistor. Resistors of that wattage are available round about 25mm long and in the region of 6mm round so doable. The more current you draw by linking all your coach lights together the smaller the ohmic value of the resistor will become but bigger physically the resistor will become  to manage the higher the wattage. As an example I recently had to install a power resistor in a circuit it’s Ohmic value was a mere 25 Ohms but it required a 50W resistor. This beast was about 60 mm long encased in an Aluminium heatsink approx. 16mm diam.
 

Your requirement for a single resistor solution was always going to be less than ideal and I think you are going to have to experiment with the proposed resistor and see how much heat it generates before installing it into your coach.

 

Richard

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19 hours ago, melmerby said:

if you assume the rectified DCC will be about 16v on load (with a smoothing capacitor)

 

The OP said it's 12.9V DCC. The rectified voltage will be slightly less than this due to the losses in the diodes. It doesn't matter whether you measure on load or not.

 

If you are rectifying the DCC in a lot of coaches then you should really use fast recovery diodes or bridges to avoid distorting the DCC waveform.

 

 

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13 hours ago, Tricky Dicky said:

Going of your figures for your coach lighting of 12.5V supply and 0.33A current draw you would require a resistor value of approx 38 Ohms,

 

Looks like you used ohms law R=V/i and calculated 12.5/.33. You need to subtract the voltage across the LEDs?

 

Still a high power dissipation though :)

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1 hour ago, Crosland said:

 

The OP said it's 12.9V DCC. The rectified voltage will be slightly less than this due to the losses in the diodes. It doesn't matter whether you measure on load or not.

 

If you are rectifying the DCC in a lot of coaches then you should really use fast recovery diodes or bridges to avoid distorting the DCC waveform.

 

 

True.

Senior moment, converting from RMS, not a virtual square wave.:D

12.9v seems rather low anyway and working on 16v would give a greater voltage margin to allow for.

I still think resistors are not the way to go as there will be a lot of heat to dissipate in a confined space.

I include in that linear voltage regulators which will need to drop & dissipate about the same voltage & power.

IMHO Buck convertors are the way to go.

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Keith I have had to drop the output voltage of the Z21 as half the points on my layout did not work when set at 16 volts but set at 13 the accessory bus measurement is 12 volts and all the points now work and the loco’s still operate OKAY.

There is plenty of room for large components in 4 out of the 5 coaches there is a space approximately 60x40x40 the odd one is the size of two AA batteries stood on end (a toilet compartment).

 I am starting to think that my best option is a medium size chip with a small resistance to allow programming then run two wires through the coaches to work the rest. Fixing pickups to 1 coach was not easy but to do 4 more life is to short!

 

Regards mike 

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24 minutes ago, mikeg said:

Keith I have had to drop the output voltage of the Z21 as half the points on my layout did not work when set at 16 volts but set at 13 the accessory bus measurement is 12 volts and all the points now work and the loco’s still operate OKAY.

There is plenty of room for large components in 4 out of the 5 coaches there is a space approximately 60x40x40 the odd one is the size of two AA batteries stood on end (a toilet compartment).

 I am starting to think that my best option is a medium size chip with a small resistance to allow programming then run two wires through the coaches to work the rest. Fixing pickups to 1 coach was not easy but to do 4 more life is to short!

 

Regards mike 

Hi Mike.

You really do have some different components to most others:)

I havent heard of points that wont work above 12v

As regards space when using resistors, they will get hot and unless adequately ventilated the area they are in will heat up.

That is why my personal choice would be Buck convertors (one per coach). They are cheap, can be set to the required output voltage and will run much cooler than any resistor.

The ones I bought have a selection of choosable fixed outputs and also a trimpot to allow setting to other voltages.

 

Ten for just over £5 inc postage:

https://www.aliexpress.com/item/4000259403990.html?algo_pvid=2d222d49-b318-42f2-b75d-907c13d8c866&algo_expid=2d222d49-b318-42f2-b75d-907c13d8c866-23&btsid=0b0a187b15965594409483166ee9e7&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

 

I would use these to make a bridge rectifier:

https://uk.farnell.com/on-semiconductor/uf4003/rectifier-fast-1a-200v-do-214al/dp/2454192?st=UF4003

With lots of rectifiers in circuit you could have problems with the DCC signal due to slow recovery unless you use fast ones.

 

Standard rectifiers, such as 1N4000 series are really too slow for the 8kHz DCC signal to operate properly. (the odd one or two !n4000 based bridges aren't a great problem but when you have several it becomes more critical.

What happens is that when the polarity reverses the rectifier is still conducting for a few microseconds which effective is a short circuit across the DCC signal and can really mess things up.

Ultrafasts switch much much quicker and are therefore less of a problem.

Edited by melmerby
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Thanks Keith, is there a good uk supplier that I could use to purchase the Buck converters, I don’t use online shopping I prefer to deal direct and pay cash (it’s an age thing). I have decided that I am not going to convert any of the other coaches and will run a supply from the one I have already done, I have the miniature 2 pin plugs & sockets that can go under the couplings as they will be out of sight, that’s one advantage of O gauge.

Thanks again for your help and time.

 

regards 

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1 hour ago, mikeg said:

Thanks Keith, is there a good uk supplier that I could use to purchase the Buck converters, I don’t use online shopping I prefer to deal direct and pay cash (it’s an age thing).

 

regards 

Hi Mike

I don't think you will find such components on the High St these days as there aren't many (any?) "hobby" component suppliers left.

The best I could suggest might be CPC/Farnell but it is online only, or Radiospares also online but they do have some trade counters scattered around the country.

RS do a click and collect service although I haven't used it so don't know what the terms are for that service. You might just be able to turn up and pay, but I don't know.

 

Re the "age thing"

I do 99% of my purchases online and pay by card and I'm 74!:)

 

EDIT

Neither Farnell or RS seem to have the typical modules shown earlier on here, so you may need to buy online.

E-bay sell them

Edited by melmerby
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