7mm Heljan Class 33 DCC lighting

I wonder if somebody can help me. I'm converting a Heljan O-gauge Class 33 to DCC using a Zimo MX699KV decoder. It's one of the early Class 33s with red/black/yellow wiring (see photo below).

I've already wired up the motors and speaker and now need to sort out the lighting. I've dispensed with the Heljan DC board (which looks like it has resistors on it) but I'm unsure as to whether I need to include resistors between the Zimo decoder and LEDs. I've seen a post on this forum for a similar Class 33/MX699 conversion which appears not to use resistors (although that was for a later 33 than mine). I've also seen a DCC conversion guide on another web site which clearly shows resistors being used.

Can anybody advise - I'm obviously keen not to blow the LEDs!
Ian

I'm not up to date with Zimo decoders but in short - yes you do need a resistor when using LEDs for lighting.

There is a small sentence on page 34, blink (pardon the pun) and you miss it, of the user guide.

LED’s, on the other hand, always require a series resistor; if however, the resistor is designed to operate at 5 V, the PWM dimming is also sufficient at a track voltage of 25V (in this case the set-ting would be CV #60 = 50, so a reduction by one fifth, 1/5th).

I have used a few Zimo decoders now, including a MX699 and I have never used a resistor, I haven't had any issues with LEDs blowing.

Richard

richard carr said:

I have used a few Zimo decoders now, including a MX699 and I have never used a resistor, I haven't had any issues with LEDs blowing.

Richard

Click to expand...

Hi Richard,

I tend to agree with you if we were talking about the latest Heljan models, but with the older ones, as is the case here, I think that resistors will be required as they were only installed on the PCB, the later ones have a 'secondary' resistor adjacent to the LED and therefore replacements are not required. John Gymer (YouChoos) is still advocating installing the resistors in the current models despite them not being required.

regards

Mike

Mike

I have an original 33, it hasn't got any resistors fitted by me and the lights all work fine.
It is an ESU XL sound decoder in it and you can turn down the brightness which I will have done.

I have a feeling that even the early 33 had resistors built into the LEDs but it is a long time since I did that one.

Ian

Have you got a picture of the LEDs ?


Richard

Thanks Richard, Mike. I removed one of the small PCBs that includes the LEDs. Photo below. Do you think any of the components inside the white markings might be resistors?
Ian

Ian

Those are resistors, I think the original 33 might have been the first loco with these style lights, you don't need any additional resistors.

Do you agree Mike ?

Richard

Hi Richard,

yes. I would think so.

regards

Mike

Sorted. I wired up the lighting PCB directly to the decoder, took a deep breath and clicked F0. Lights worked! Thanks for all your help.
Class 26 next.
Ian