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Decoder Advice - live framed chassis


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I need some decoder advice for a particularly risky install...

 

I have built a PDK 47xx which unfortunately came with live Markits wheels on one side.  Now I realise at this point I should have saved myself a lot of pain and just bought a set of insulated wheels, but I didnt so I am where I am...)

 

The decoder needs to be small enough to fit between the frames (likely the tender frames) in an insulated box), with 4 wires to the loco to connect with the motor, pickup and frame.   There is no room on the loco to fit the decoder given the boiler is filled with lead and motor (although I suppose it could go under the cab maybe.  

 

So I am looking for a recommendation for a small decoder which is either very resilient to short circuits in the power feeds (or has a suitable idiot proof warranty that I can get it replaced if I kill it)  

 

In addition I would be interested to hear anything else I should be doing to try and mitigate the risk (other than changing the wheels!) 

 

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The motor terminals must be electrically separate from the pickups in order to ensure both the decoder feeds to the motor are separate from the feeds to the pickups. Check this with a meter. Thus you may have to separate the motor from the frame with insulation. 

I can't advise on the best decoder as I suspect most will go up in smoke if the motor terminals are shorted to the pickups. 

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8 minutes ago, grriff said:

I can't advise on the best decoder as I suspect most will go up in smoke if the motor terminals are shorted to the pickups. 

 

Some DCC decoder brands, such as Lenz and Zimo, do have short circuit and overload protection. 

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As Grriff implies, the issue is whether the motor terminals can short, either to each other, or to either rail (via the body or pickups).  Such shorts will destroy many decoders, though some (including those RFS mentions) have limited protection built in.   

Provided you're certain of the insulation of the motor terminals and motor leads, then the decoder should be safe. 

 

Which then leaves track and pickup short circuits, such as a wheel touching the body.   Any decoder should be able to cope with track or pickup shorts.  Though some seem more robust than others.   If you have such shorts regularly, you may damage the pickups. 

I'd put in a circa £20 decoder, from Zimo, Lenz or ESU.   

 

 

One risk from live-frame kit built models is the "live body".  Which can then connect electrically to other bodies via couplings or buffers.  There have been cases of short circuits running down the entire train, doing damage to each vehicle by annealing metal parts in each vehicle.     So, if possible, try to insulate the buffers and couplings on the model.  

 

-  Nigel

 

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Is it not a bit cheeky to ask about warranty’s when you believe up front before purchasing that your proposed installation could kill it???

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24 minutes ago, Nigelcliffe said:

As Grriff implies, the issue is whether the motor terminals can short, either to each other, or to either rail (via the body or pickups).  Such shorts will destroy many decoders, though some (including those RFS mentions) have limited protection built in.   

Provided you're certain of the insulation of the motor terminals and motor leads, then the decoder should be safe. 

 

Which then leaves track and pickup short circuits, such as a wheel touching the body.   Any decoder should be able to cope with track or pickup shorts.  Though some seem more robust than others.   If you have such shorts regularly, you may damage the pickups. 

I'd put in a circa £20 decoder, from Zimo, Lenz or ESU.   

 

One risk from live-frame kit built models is the "live body".  Which can then connect electrically to other bodies via couplings or buffers.  There have been cases of short circuits running down the entire train, doing damage to each vehicle by annealing metal parts in each vehicle.     So, if possible, try to insulate the buffers and couplings on the model.  

-  Nigel


Thanks to Gryff and yourself for that explanation, I hadnt thought about the risk of transfer via the body.  Fortunately in this case the loco has a resin boiler, and while I harbour a particular dislike of resin parts in loco kits, I think in this case it has done me a favour.   My worry was more towards shorts coming from the track, and the risk to the decoder itself.  I think in that case insulate the motor pickups (just in case), shrink wrap the soldered wire connections, and fully insulate the decoder (my current logic is to place it in a plasticard box).

I cant tell if my usual Zimo MX617 includes short circuit protection, but if not I will go for a Lenz Silver.  (The latter's year one warranty covering installation failure makes it all the more tempting.)   

 

 

50 minutes ago, Jonboy said:

Is it not a bit cheeky to ask about warranty’s when you believe up front before purchasing that your proposed installation could kill it???

 

Not really, the point of a warranty is to mitigate your risk (with the acceptance you will be paying a higher price in order to obtain it).  The installation still shouldn't fail, but there is a much higher chance of failure than with a standard installation so it makes sense to take it into consideration.   

That said, the Zimo chip (without warranty) is £20, a Lenz Silver mini would be £34,  so the delta between the cost of a replacement decoder and the protection of the warranty is negligible so might as well stick with the Zimo (Assuming it has the short protection which I believe it does).   

 

Either way it will be good to finally get the model running at last. 

 

 

 

  

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3 minutes ago, The Fatadder said:


Thanks to Gryff and yourself for that explanation, I hadnt thought about the risk of transfer via the body.  Fortunately in this case the loco has a resin boiler, and while I harbour a particular dislike of resin parts in loco kits, I think in this case it has done me a favour.   My worry was more towards shorts coming from the track, and the risk to the decoder itself.  I think in that case insulate the motor pickups (just in case), shrink wrap the soldered wire connections, and fully insulate the decoder (my current logic is to place it in a plasticard box).

I cant tell if my usual Zimo MX617 includes short circuit protection, but if not I will go for a Lenz Silver.  (The latter's year one warranty covering installation failure makes it all the more tempting.)   

  

 

Which motor is being used, and what's its maximum current on DC running ?   I'm a little concerned that a MX617 is a bit small, and later you discuss a Lenz "mini" which is also small, yet this is a fairly substantial loco (you talk about lead-filled boiler, etc.).   I had in mind next size up, so "Lenz Standard" or Zimo MX623.    With a very efficient motor, the smaller decoders will be OK. 

 

Paper box or paper packet will do, though a plastic box is fine.  There is heat dissipation to consider, so I'd avoid tight wrapping, but try to use something which is vented in some way.  

 

 

 

Track short circuits (ie. coming off rails, causing left-wheel or body to touch right rail) should not affect any decoder.   

 

 

- Nigel

 

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I’m sure ive used the small Zimo in a couple of RTR applications, but you’ve got me doubting myself now and it may well be I’ve used another small decoder

 

the loco has a Mashima (I think it’s a 1420) with high level gear box.  Even with a boiler full of lead it only weighs 255 grams thanks to that resin firebox/boiler.

 

My big issue is space, because the boiler needs weight, the firebox is full to the brim with motor, and the tender is a solid block of resin for the lower half.  The only place I can find for the decoder is between the tender frames.  Which only gives me 10-11 mm of width to play with 

Edited by The Fatadder
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Just wrap the MX617 in a layer of ordinary masking tape. I do this now as a matter of course with all types of decoder I have used recently and haven't any problems to date, overheating etc. Even such as TTS where heating is supposed to be a recurring problem. Can't say it won't happen because I guess it might hinge on how often/how hard the loco/decoder is worked. But it does ensure full electrical insulation from any/all body/chassis contact. If you wrap width ways you can leave a gap front and back for the wires and through draught ventilation. 

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1 hour ago, The Fatadder said:

I’m sure ive used the small Zimbabwean in a couple of RTR applications, but you’ve got me doubting myself now and it may well be I’ve used another small decoder

 

the loco has a Mashima (I think it’s a 1420) with high level gear box.  Even with a boiler full of lead it only weighs 255 grams thanks to that resin firebox/boiler.

 

 

Ok. with that motor/gearbox, I'd be reasonably certain its OK with a small decoder.   If you have a DC ammeter and can check it on DC under load, that would be a good confirmation, but I'd be pretty confident of it being acceptable.  

 

- Nigel

 

 

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2 hours ago, Izzy said:

Just wrap the MX617 in a layer of ordinary masking tape. I do this now as a matter of course with all types of decoder I have used recently and haven't any problems to date, overheating etc. 

IME masking tape loses it stickyness over time and will come loose. What sort of space is available between the chassis and the decoder  - could pieces of plastic card be glued on  the adjacent metalwork.

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21 hours ago, Nigelcliffe said:

 

Ok. with that motor/gearbox, I'd be reasonably certain its OK with a small decoder.   If you have a DC ammeter and can check it on DC under load, that would be a good confirmation, but I'd be pretty confident of it being acceptable.  

 

- Nigel

 

 

Current across the motor or across the track?

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59 minutes ago, The Fatadder said:

Current across the motor or across the track?

 

Generally one measures current in series....   

Put ammeter in series with track feed on DC  (ie. one wire from controller goes to one ammeter lead, the other ammeter lead goes to the track, thus ammeter is in series)   and test what the loco draws when running over a range of speeds, ideally with a load (train) on it, or pushing against a stationary object.  

Make sure for initial tests that ammeter has adequate range (1A minimum, preferably quite a bit more) otherwise ammeter will either blow its internal fuse or be damaged.  Once you have an initial reading on a high current setting, you can optionally reduce the setting if confident the motor doesn't exceed the new value selected on the meter.   

Take care to not cause a short circuit on the track with the meter connected in series, otherwise its likely to blow a fuse or damage the meter.   

Note that quite a lot of cheaper multi-meters require the red lead to be moved to the secondary connection for higher current readings.  

 

   

 

 

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I am sure the MX617 will be able to cope without problem. My testing notes indicate a Mashima 1833 draws around 160-190Ma when running light at up to 12v, and at 12v stall 750Ma. A smaller 1630 draws slightly less running light and 600Ma at 12v stall. They both start on around 60Ma. Both these figures are well within the MX617 specs of 0.8A continuous and 1.5A max so I believe there will be no issue with the smaller 1420.

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On 16/01/2021 at 05:31, The Fatadder said:

I think in that case insulate the motor pickups (just in case), shrink wrap the soldered wire connections, and fully insulate the decoder (my current logic is to place it in a plasticard box).

Rich, you definitely need to make sure you've isolated the motor connections both from the chassis and the pickups (not "just in case"!). Once you've done that just install the decoder in the usual way, with the red wire to the pickups, the black wire to the loco chassis and the orange and grey wires to the motor. I've done this several times on old Tri-ang chassis with X04 motors by just slipping a piece of insulation over both motor spring arms instead of just one.

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