# Why do decoders get fried?



## isoc (Jan 23, 2017)

I understand that if a short occurs on a DCC system, and if the DCC system does not automatically shut down power, a decoder can be "fried" and no longer work. I've read this in many threads, and I believe it.

But I don't quite understand it. I thought electricity always likes to take the path of the least resistance, and if the track is shorted, that's the path of least resistance. I can see how, at the point of the short, track can heat, plastic ties melt, etc. I don't understand why a decoder is affected. An explanation, in 150 words or less, would help!

Thanks, Ted


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## DonR (Oct 18, 2012)

There may be a 'static' type electrical interference
that 'could' be the cause of fried decoders. There
also could be a current surge when power is
restored. I really
doubt it though. I have 9 DCC locos on my layout
powered up any time the layout is. There have been
untold short circuits on my tracks. While I try to
clean them up instantly, there have been times
when the short indicator on the controller blinked
on and on. There was no effect on any of my locos.

One thought comes to mind, though. If a loco crosses
from one track to another while the two tracks are
powered by separate boosters a slight phase difference
could do damage.

The only time I've fried a decoder was installing it
on a loco with too high an amp drain.

Maybe one of our more experienced electronics
members can offer more educated information.

Don


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## Brakeman Jake (Mar 8, 2009)

You're right,a short that is limited to the tracks wouldn't toast a decoder.A short aboard the loco definitely will.A poor installation that will have wires rubbing against the frame or the rotating flywheels or two stripped wires that are allowed to touch are good examples.But the most occurring is a motor that is about to fail or shorted thus drawing current beyond the decoder's capacity.

Then you have the earlier MRC sound decoders...they fry...by design.


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## isoc (Jan 23, 2017)

Thanks for the replies, that clarifies things for me. - Ted


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## D&J Railroad (Oct 4, 2013)

To expand on Brakeman Jakes explanation. If you have a decoder running a motor and the motor makes contact with the frame or rail path of power, count on it being fried.


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## crusader27529 (Apr 3, 2016)

Decoders get fried for 2 usual reasons....

First, a short in the motor drive circuit, which overloads the drivers, and destroys them. It is usually caused by a wiring problem during installation, or a wire touching what it shouldn't after installation.

Second, an overvoltage condition in the rails, exceeding the limits of the decoder. This most likely happens when a short occurs, and the booster keeps trying to supply power.....when the short is removed, the booster over-supplies power, and the voltage can spike, which can kill a decoder.

Obviously, this is the short verson of how to kill a decoder, but you get the idea....the decoder is a small computer, and has limitations that can be exceeded without alot of effort. Usually, the damage isn't visable, unfortunately.


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## isoc (Jan 23, 2017)

Thanks for the further clarification! :thumbsup:

I have gone around my modest layout and shorted the track in about a dozen locations (This was *before* I began DCC operations.) and the Powercab always, instantly, cut off the power.

- Ted


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## johnfl68 (Feb 1, 2015)

Part of it has to do with the economy of the decoder. It is possible to make a decoder that does not fail in almost all instances. Unfortunately the cost of that decoder right now would make it so that not many people would want to pay for it.

So it's a balance of making the decoders affordable, and as less susceptible to damage as possible with the parts that can be easily obtained within the cost allotted for the decoder.

The decoders have gotten better over the years, and will continue to do so as better parts and designs come out, and more is learned about how present decoders are being damaged and what cost effective things can be added to designs to prevent the most common causes of damage.


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## andgul (Jan 6, 2015)

Hi there!
I use Lenz system version 3.6 and trafo at 5 Amps/70 watts.
My digital system can be adjust difference power voltage by program LZV100.
I use 16 Volt output to the track.
I can of course also use trafo 3 Amps/45 watts by reduce power feeder to the track and still use 16 Volt output.
Most digital expert recommended 3 Amps and 16 Volt.
Decoder of today are more powerful but still i recommend 16 Volt which are standard in scale H0.
This is more safety for the decoder.


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## alaska railroad (Oct 20, 2015)

Do you think it's possible that a decoder could be fried due to let's say the wheels not turning freely? For instance due to hard grease in the gear box, or binding in the drive system on steam locomotives?


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## crusader27529 (Apr 3, 2016)

alaska railroad said:


> Do you think it's possible that a decoder could be fried due to let's say the wheels not turning freely? For instance due to hard grease in the gear box, or binding in the drive system on steam locomotives?


YES, and no......if the drive was locked, and the stall current exceeded the capacity of the decoder, then YES, it's possible.

But, as long as the wheels aren't completely locked, the current required to drive the mechanism would be higher than normal, but not as high as the stall current......the current could still exceed the capacity of the decoder, but it'd be far less likely.


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## alaska railroad (Oct 20, 2015)

crusader27529 said:


> YES, and no......if the drive was locked, and the stall current exceeded the capacity of the decoder, then YES, it's possible.
> 
> But, as long as the wheels aren't completely locked, the current required to drive the mechanism would be higher than normal, but not as high as the stall current......the current could still exceed the capacity of the decoder, but it'd be far less likely.


Aha, makes perfect sense. Thanks crusader.


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## Panther (Oct 5, 2015)

Doesn't running a train at slow speed significantly increase the current feeding the unit ? It would be like running a computer on a dimmer switch.

Dan


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## crusader27529 (Apr 3, 2016)

Panther said:


> Doesn't running a train at slow speed significantly increase the current feeding the unit ? It would be like running a computer on a dimmer switch.
> 
> Dan


No, because the motor is drivin with PWM (pulse width modulation) and the instantaneous voltage that the motor is driven with is the maximum available......the AVERAGE voltage is less because the motor doesn't get that full voltage all the time.


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## DonR (Oct 18, 2012)

Use your multimeter set to amps in series with
one wire of your track feed. Notice that the slower
your loco goes the current draw is less. This is why
some of us get away with running 4 and 5 locos or more on
a one amp DCC system. They draw a fraction of
an amp each when the speed is a good match for the
real thing.

I see videos of proto switchers at work. They run
at walk speed so the switchmen can throw the points
and get back to the car to uncouple. That's how I run mine
also. 

However, as the loco runs faster or is pulling a
long train you'll see the amps rise.

Don


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## mesenteria (Oct 29, 2015)

crusader27529 said:


> Decoders get fried for 2 usual reasons....
> 
> ..
> 
> ...


The decoder is not so much limited by voltage as it is by amperage. Amperage is 'current'....voltage is NOT 'current'. You could ostensibly put 1000 volts through a decoder as long as individual components on the decoder could handle it, but if you drove 4 amps at the same time, the amperage would be what would heat up the circuitry and melt tiny wires or fry components. If you limited the circuit to 0.5 amps throughput and 100 volts, you would not fry the decoder.

I drove my HO layout on Digitrax's DB 150 set at O Gauge voltage, about 20 volts if I recall. I didn't notice for several weeks. I had no issues with my HO scale decoders because I had no shorts and none of the decoders drew more than their rated 2 amps.


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## mesenteria (Oct 29, 2015)

alaska railroad said:


> Do you think it's possible that a decoder could be fried due to let's say the wheels not turning freely? For instance due to hard grease in the gear box, or binding in the drive system on steam locomotives?


Yes, it's possible. It's the same situation when a person checks the stall current for his motor to ensure the decoder can handle it. If you have an older pancake motor slaved to a modern decoder with an upper amperage limit of 1.5 amps, and the old pancake needs 2 amps or more to keep the locomotive's drive moving at the speed you want, your decoder will heat up in seconds and let its magic smoke out. So, you put your hand over the locomotive and begin to increase your pressure on your hand until the drivers stop spinning under the locomotive. At that point, you quickly note the amperage draw with a proper amp meter and then let go of the locomotive so that the motor is no longer in stall....otherwise, you will let the magic smoke out of the decoder. 

If the motor is being fed more and more voltage to draw more amps in order to shift hard old grease, and it's slaved to a decoder, the decoder will not be able to put through the amperage needed to make the motor break free of the old grease packing. Your decoder will soon fry.

Fortunately, some decoders have thermal sensors that will shut them down if they get too warm. And even so, no decoder should attempt to put through more current/amperage than they are rated to throughput.


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## andgul (Jan 6, 2015)

Too much Amps out to the decoder will make decoder fried.
There is short circuit protect.
There is overload protect.
But if you have too much Amps output to the decoder none of the protect works.
Today decoder works with about 1,2 Amps in scale H0.
Just send 2-3 Amps to the decoder you fried it!
I don´t believe it has to do with the grease on the gear box because you will see and notice wrong with the locomotive and you stop to drive it,right?
I don´t believe either too about the wheels doesn´t Roll correctly.
It will or else make driving else hacking.


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