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Has anyone tried to use a thermal circuit breaker or 1 Amp fuse
between an engine pick-up wire and the decoder to protect the decoder
from short circuits?

Rotortips

don't think they will fit in scales lower than O

zekda99
--- rotortips <Rotortips@...> wrote:


---------------------------------
Has anyone tried to use a thermal circuit breaker or 1
Amp fuse
between an engine pick-up wire and the decoder to
protect the decoder
from short circuits?

Rotortips









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Fuses react much too slowly to offer protection to the dainty components inside a decoder.
DonV

--- rotortips <Rotortips@s...> wrote:
Has anyone tried to use a thermal circuit breaker or 1
Amp fuse
between an engine pick-up wire and the decoder to
protect the decoder
from short circuits?

No
First!!!
A fuse does not protect the item being powered. It protects the power
supply.

If your decoder shorts out, burns up or melts and draws the maximum
amout of current your supply can provide, the circuit breaker ( or
electronic equivilant inside the booster) will disconnect power from
the rails to prevent damage TO THE BOOSTER. It can not prevent damage
to the decoder, because the decoder is already damaged. Adding a fuse
to the engine simply duplicates the fuse that is already in the booster.

Second!!!

There is nothing that can "protect" a loco from a short circuit!

Nor is there any need to protect a loco from a short circuit. If you
have a shots circuit on your layout (IE laying a crowbar across the
rails) then the voltage on the track is "shorted out". That is to say
that there is no voltage (or almost no voltag) across the rails. A
short can no more do damage to the loco than turning off the power or
lifting the loco from the track. In fact, better power supplies include
a "crowbar" circuit. This is a circuit that monitors the output of the
power supply, and, if the supply malfunctions and starts to put out TO
MUCH VOLTAGE, the circuit shorts out the supply output in order to blow
the supplies fuse.

B0B

Exactly, Bob! While I'm really, really new (as in weeks) to DCC and
not an electrical guru at all, I have read and heard so much about
the crying fear of decoders being blown because there is a short. I
agree that if there's a derailment and the wheels short (like your
crowbar example)that's a power supply concern and not a decoder
problem.

Ignoring the "obvious" wiring errors, the only way I can see a
decoder
being blown because of a short is if:
1)the decoder output is connected directly to the rails, which is
possible if the motor is not, or rather no longer, isolated
2) or if the wires, or some component on the PCB, ground outs on the
metal chassis,
3) or the wires somehow become cross connected because their solder
connection breaks and the wire moves or their insulation is worn away
4) the decoder self destructs internally because of heat or excess
current draw or component aging

Seems to me most of these are preventable in the first place by
studying the loco, the decoder and its needs, existing wiring if any
and the careful placement, wire routing, connecting and soldering of
the decoder and its connecting wires, including the speaker and its
leads.

QUESTION: Is there some other way, that I'm not seeing and should be
much worried about, to short out the decoder and that is what
generates all this talk and fears?

Thanks in advance. Ken

--- In WiringForDCC@..., "bobgrosh" <bobgrosh@h...>
wrote:

--- rotortips <Rotortips@s...> wrote:
Has anyone tried to use a thermal circuit breaker or 1
Amp fuse
between an engine pick-up wire and the decoder to
protect the decoder
from short circuits?

No
First!!!
A fuse does not protect the item being powered. It protects the

power

supply.

Hi Ken!

You asked:

QUESTION: Is there some other way, that I'm not seeing and should

be

much worried about, to short out the decoder and that is what
generates all this talk and fears?

If the decoder is properly wired, not under rated for the load, and
does not take a dip in the "very realistic lake", chances are that
only a motor failure could take out a decoder.

My perspective is from garden scale where 3, 5 and 8 amp decoders are
the norm. (so are real lakes) So, this may not apply to you if you
are in one of the small scales.

What we have learned from garden railroads may be important in small
scales too.


Burning out decoders:
1 - Stall current:


Most people measure the stall current wrong. You have to monitor the
input voltage and the current with the motor locked.

For example if the supply only puts out 5 amps at 18 volts and you
measure 4 amps but don't look at the voltage, (which may have dropped
to 9 volts) you could wrongly believe that the stall current is only
4 amps, when in fact it may draw 10 or 11 amps at the full 18 volts.
All to often I've seen people measure the maximum output current of
their power packs when they thought they were measuring stall current.
Most decoders can not handle more than their rated stall current for
more than a second.

DO NOT ASSUME THAT IF YOU NEVER STALL YOUR LOCO OR THE WHEELS WILL
SLIP FIRST, THAT WILL SAVE YOU.

When you first give a motor it's first kick from a decoder, the motor
is stooped. IE! IT IS STALLED!!! A motor will always draw it's stall
current for some period of time before it starts moving.

The point is, Always make sure the stall current (or peak current)
rating is grater than the stall current of the motor. If the loco
manufacture does not tell you the stall current of the motor, measure
it while monitoring and maintaining full voltage.

It seems likely to me (because I have seen this happen more than
once,) an under sized decoder straining to get cars moving will fail
when there is a derailment somewhere in the train . The
operator/owner blames the failure on a short because of the
derailment. He defends his conclusion because he saw lights on the
layout flicker. In reality, the decoder was blown because the
derailment stalled his loco and the lights flickered when vital parts
of the decoder melted.

Burning out decoders:
2 - Failed motors:

Poor quality motors can fail. They can short out. In this case a fuse
between the motor and the decoder (not between the decoder and track)
might protect the decoder. However, if a failed motor shorts out it's
windings, it will draw much more current than the stall current. If
that happens for more than a few milliseconds the H-bridge will be
fried. Most fuses will not open the circuit fast enough to provide
any meaningful protection for the motor output of the decoder.

In garden railroads, experienced railroaders opt for decoders with
built in over current, short circuit and thermal overload protection.
( Such decoders will simply flash all the lights if mis-wired and
placed on a track.) All that fancy circuitry probably does not fit in
HO or N. ( But then you guys don't put tachometer monitored brush-
less DC cooling fans on your decoders either.) :>)

In Garden scale popular locos like the SD45 have four motors, each
drawing more current than your whole train. If a motor fails it is
usually replaced. Even if is decided not to repair the loco, we
usually remove the decoder for use in some other loco, We have almost
no custom decoders designed for a single loco, one or two sizes fit
all. If a small scale loco burns out the motor and it can't be
repaired, then the decoder possibly will not fit in some other loco
it was designed for. Because of this, and considering the cost of a
decoder, it may not we smart to put try to put expensive protection
circuitry in small scale locos.

If you want to protect the decoders from motor failures, then there
is only one way I know of to do this. Limit the available current to
the decoder to the maximum amount of current they can safely supply.
That is what power managers like the PM42 are designed for, and why
some people use power districts with multiple smaller boosters rather
than one giant booster.

For example on my "G" scal layout, I could use 3 amp decoders in some
locos, but I don't. I use 5 amp decoders, that can dandle 10 amps
peak. My boosters are all 8 amps each, so if a motor shorted out, the
decoder only has to handle 8 amps, even tho it is designed to handle
10. As a result, the booster for that train will always cut off
before the decoder is damaged.

Don't know if this helps answer your question, but it's the best I
can come up with.

B0B

Wow! You packed a lot of info in that answer. So, the tests I did
before, where I set the O/P to 14Volts DC for a running engine and
then stalled the motor and got a .91A current draw @about a 10 volt
reading really isn't necessarally a .91 stall current? OOPS. Now I
have to figure out how to keep the voltage up @14VDC.

Never heard that before about the stall current being present @
startup, but it makes sense.

I appreciate the explanations, Bob. They help.
Ken

--- In WiringForDCC@..., "bobgrosh" <bobgrosh@h...>
wrote:

Burning out decoders:
1 - Stall current:

B0B