Reducing 12v to 6v for light on 16mm loco

[Gilbert]

I'm building a small 16mm/SM32  WW1 US Baldwin loco that I'd like to fit with small light - the nearest light to what I need is the 6V RO 1374 here:

 

http://www.model-dockyard.com/acatalog/robbe-Searchlights.html

 

I assume the light output on the Zimo MX 645 I am using is 12v and I'd be glad of any advice on my best approach - I will probably have front and rear lights but only one will be on at any time.

 

Thanks in advance for any help

 

Chris

Edited September 21, 2017 by Gilbert

[brigo]

I'm building a small 16mm/SM32  WW1 US Baldwin loco that I'd like to fit with small light - the nearest light to what I need is the 6V RO 1374 here:

 

http://www.model-dockyard.com/acatalog/robbe-Searchlights.html

 

I assume the light output on the Zimo MX 645 I am using is 12v and I'd be glad of any advice on my best approach - I will probably have front and rear lights but only one will be on at any time.

 

Thanks in advance for any help

 

Chris

Hi Chris

 

Would suggest putting a zener diode in series with the lamp, such as this from Maplin

 

https://www.maplin.co.uk/p/bzx85c-6v2-210ma-zener-diode-qf48c

 

The band on the end of the diode goes to the more positive terminal, ie, either the common positive with the other end to the lamp, or to the lamp and the other end to the function output.

 

Brian

[Pete the Elaner]

They are 100mA bulbs.

Would the decoder cope that that current draw? I thought they were intended to drive LEDs which take a lot less current?

[Gilbert]

They are 100mA bulbs.

Would the decoder cope that that current draw? I thought they were intended to drive LEDs which take a lot less current?

Thanks -  I'll check that out - as you may have worked out I am a bit of a novice in this area.

[brigo]

The Zimo 645 has a maximum total function output of 0.8 Amps, so should cope with a 100mA load. Depends what else is being used.  With a bulb it would be best to utilize the soft start function CV#125 = 52 to prevent the inrush current tripping the short circuit protection.

 

Brian

[Pete the Elaner]

Thanks -  I'll check that out - as you may have worked out I am a bit of a novice in this area.

It was helpful that you explained exactly what you wanted to achieve.

[Gilbert]

The manual says this -

Maximum total function output, continuous *)……….  MX645 ......... 0.8 A

Maximum continuous current for LED outputs ....……MX640, MX642, MX644………..…... 10 mA ea

 

 

 the MX645 does not get a specific comment about light output ? Will I get away with it?

 

The Zimo 645 has a maximum total function output of 0.8 Amps, so should cope with a 100mA load. Depends what else is being used.  With a bulb it would be best to utilize the soft start function CV#125 = 52 to prevent the inrush current tripping the short circuit protection.

 

Brian

 

Thanks

Our posts crossed

[Steven B]

Watch out for how you wire the lamp. Using the blue wire results in roughly twice the Voltage being applied compared to using one of the track feeds (as per 6 pin decoders).

 

 

 

Steven B.

[Crosland]

Watch out for how you wire the lamp. Using the blue wire results in roughly twice the Voltage being applied compared to using one of the track feeds (as per 6 pin decoders).

 

 

 

Steven B.

 

Strictly true, but being pedantic:

 

Using a track feed with a 6-pin decoder results in the bulb only being lit 50% of the time, the average voltage will be half the track voltage but the peak voltage applied to the bulb will still be the track voltage.

 

Using the blue wire powers the bulb all the time and the average voltage will be the same as the peak, i.e. the track voltage.

[Nigelcliffe]

The manual says this -

Maximum total function output, continuous *)……….  MX645 ......... 0.8 A

Maximum continuous current for LED outputs ....……MX640, MX642, MX644………..…... 10 mA ea

 

 

 the MX645 does not get a specific comment about light output ? Will I get away with it?

 

 

Thanks

Our posts crossed

 

Yes. 

 

The LED outputs are for the low volt, low current "logic" outputs of the decoder.  

The main (normal lighting) decoder function outputs are well within the load you are proposing. 

 

The main issue is protecting the bulbs described from over-volts, not protecting the decoder.  The output volts of the decoder will be related to the track voltage, which in turn depends on the DCC system in use.  Don't assume it will be 12v at the decoder function output, it could be quite a bit more. 

 

 

 

- Nigel

[RAF96]

 

 

The main issue is protecting the bulbs described from over-volts, not protecting the decoder.  The output volts of the decoder will be related to the track voltage, which in turn depends on the DCC system in use.  Don't assume it will be 12v at the decoder function output, it could be quite a bit more. 

 

 

 

- Nigel

 

 

I cannot ever see a decoder blowing a bulb, as Ohms Law dictates the end game and most decoders are limited on function output to mA so x available volts will never amount to the requirements for blowing a bulb - an LED yes. The decoder self regulates from track voltage (within NMRA limits) to function or motor output voltage and design limits on current..

Edited September 23, 2017 by RAFHAAA96

[Crosland]

I cannot ever see a decoder blowing a bulb, as Ohms Law dictates the end game and most decoders are limited on function output to mA so x available volts will never amount to the requirements for blowing a bulb - an LED yes. The decoder self regulates from track voltage (within NMRA limits) to function or motor output voltage and design limits on current..

 

The decoder does not regulate anything, apart from a low voltage supply to the processor. The track voltage is whatever is supplied by the booster and is simply rectified by the decoder. Motor outputs may have current and/or thermal limiting. Function outputs are almost invariably simple common emitter transistor switches, which switch one end of the load (e.g. bulb) to 0V, with no current limiting. The other end of the load connects to the blue wire which is at the track voltage.

 

As Nigel implies, some systems can be putting out 18 V or more, all well within the NMRA specs. The voltage at a function output will be a couple of volts less, allowing for the rectifier and the transistors Vce(sat).

 

I wouldn't trust using a 12V GoW bulb if there's a chance of a high track voltage. Even if the bulb dosn't blow or the decoder output doesn't fail, the bulb will run a lot hotter and risk damage to the model. I believe (but could be mistaken) some European manufacturers started fitting higher voltage bulbs with the advent of DCC.