I'm constructing a petrol-electric loco for 5" gauge. B&S engine
turning a car alternator and also a 12V DC motor (ex power shower) as
a generator to supply the alternator field current through a high
power rated rheostat. The alternator output runs a pair of 24V, 12A
DC motors to drive the wheels.
Reversing the connection between the motors and alternator will
obviously reverse the loco. I have quite a number of double pole,
double throw switches rated at 10A 250V AC which can perform this
connection reversing. Its pretty essential that the current is at
zero before this reversing occurs and this is easy enough - just turn
the speed control down to zero.
The question is - how many of the AC rated switches should I be using
in parallel to safely carry the DC load assuming the load is zero when
switching occurs? How are switch ratings assigned? Is it based on
switching full load current and issues of arcing or on current
capacity to avoid overheating?
cheers for your thoughts,
If there's no current passing through the switch when the contacts open,
you'll likely get away with using one switch per traction motor. You
might even get away with using one switch for both motors, but that's
pushing your luck. Of course, the switch manufacturer won't officially
approve of this, but they might not approve of you using the switch for
DC at all (DC creates more persistent arcs). But it'll probably work
fine until you flip the switch with the motor running.
Arcing is what destroys switch contacts. There will be little heating of
the contacts when they are closed, probably less than a watt of power loss.
If the switches use a common type of mounting, like the 15/32" round
hole, use one switch and see what happens. If it doesn't work you can
always buy a more suitable switch to fit the same hole.
It's essential, yes!!!
In practice I think you'll have to consider the cases when the motor is
switched off when running at full whack, when the switch is switched on
when the voltage is at full, and even the case when it is switched on in
reverse when going full speed forward.
You can't really rely on it never, ever, happening.
If you don't design for the ignorant they'll always get you - and you
thinking they are stupid, which they probably aren't anyway, won't
change anything. Besides, you may fall and accidentally operate the
A switch rated at 10A AC will probably not be able to reliably switch
more than 2A DC.
When a switch opens an arc is formed, and this can quickly destroy the
switch, maybe leaving it permanently on with the contacts welded
together, or maybe permanently off, melting, catching fire, or even
In 50 Hz AC the voltage is zero 100 times a second, and this quenches
the arc quite quickly. In DC however, this doesn't happen, so switch
ratings are much lower for DC than for AC.
When a switch closes, it bounces open for a short time - and again an
arc is formed. This isn't usually as bad as an opening arc, but it can
still weld the switch closed, again leaving it permanently on.
If a permanently on situation is dangerous - and it sounds it - then
just for that reason, never mind idiots or accidental operation, you'll
pretty much have to use a much beefier switch.
For reliability people don't usually use electrical or electronic
components at their full ratings. Manufacturers tend to like to give the
biggest numbers they can, and that's maybe under specific conditions etc
- it is usual practice to derate to no more than 60%, and even less for
semiconductors. This doesn't mean they won't run at full rating, but
reliability tends to suck if you do.
Your switch will probably not be able to carry much more than 10A (AC or
DC) continuously before the tiny copper bits inside start to melt. The
manufacturer's rating is really about switching off, and to a lesser
extent about switching on, but they don't usually make the tiny copper
bits any thicker than they have to.
Unfortunately, switches don't really parallel very well - more current
tends to go through one switch than the other. Bipolar transistors are
notoriously bad for this, but it happens in any kind of switch, even
MOSFETs (though MOSFETS tend to cancel this out a bit). The higher
current in one switch will most likely kill reliability.
Also, if one switch welds on, any paralleled switches won't make any
difference - unless of course someone switches one of them to the other
direction, when you'll get a huge short circuit!
It would be be best, by *far*, to use a single switch for the 12 A each
motor takes. Even a single 10 A unsuitably-rated switch would be better
than two paralleled switches in your case - one switch switched the
wrong way, and BANG!. Wires melting, fires everywhere. molten and
vaporised copper spraying everywhere ... DO *NOT* PARALLEL SWITCHES HERE!
It may be better to use just one switch for the whole 24A the two motors
take, but I don't insist on that. :) Are you ever going to want only one
motor to operate?
Lastly, the loads will be inductive. A lower voltage usually means a
better switch-off capability, but an inductive load will cause the
voltage to rise when it's switched off, which will probably more than
overcome that advantage.
I don't know offhand where you'll find a suitable switch for 24 A
inductive loads @ 12V DC, but I'd start by looking at car/lorry parts.
Hope I've already answered the rest of your questions, as below. It
seems I've written quite a lot about one switch - and could go on a lot
-- Peter Fairbrother
Most toggle switches are inherently fast-break by their design, but there's
nothing stopping you using a pair of DPDT relays to do the job, you can get 30A
rated ones with Lucar terminals.
Make sure you have a fuse in the circuit, as if the relays are not mechanically
linked, there could be a small problem if one fails to operate for any
Peter & Rita Forbes
Hi Peter, Peter and Chris
thanks for all your thoughts, especially the possibility of someone
(even me) selecting reverse when the loco is at full speed. Thinking
about this is a bit scary - the diode pack in the alternator would
conduct if the motor switch(es) were reversed, producing some (maybe
lots of) braking. Not a pleasant thought.
How about ensuring that the switch(es) cannot be changed over unless
the mechanical brake that I'm also fitting is engaged and the speed
control is set to zero? Some form of rugged mechanical interlock
should make things somewhat safer.
If I can ensure zero load at switch over by such interlocking, do you
still object to parallelling switches to obtain higher capacity? My
plan was to couple as many as required in such a way that a single
control lever moves them all so there's no possibility of me
forgetting to move one.
I cannot think of any need to have separate control over the two
motors, so one switch or group could serve both for simplicity. I'm
not keen on relays as they need their own control supply.
thanks again for your insight and thoughts,
You can get 4-way power relays, we had one of the Matsushita J series knocking
around, I'll see if that is still in the workshop 'spares' bin.
Albright do DC motor reversing contactors, and some of their product can be got
from fork truck dealers.
You could rig up a simple speed detector on the wheels or axles to prevent
change of direction until the loco was at rest.
Peter & Rita Forbes
Got some 1/4"x1/8" brass bar and a few bits of piano wire to make contact
clamping springs, a bit of Delrin and a bit of Tufnol sheet???
Make a lever change over switch with a little delrin handle and a Tufnol base.
If you're worried about limited contact life due to arcing, fit spring loaded
It ain't high tech, this is railway engineering :-)
Toby pretended :
I would rate an AC switch as able to carry, not switch, the same DC
current plus a bit. So 10amp AC rated switch should easily be able to
to carry 12amps DC continuously.
However, were you to make or break the DC load with the switch it is
liable to arc and destroy the switch, so you do need some form of
interlock to prevent this. Perhaps a cover over the switch and as you
lift the cover it breaks the supply.
Heavy duty switches similar to auto starting solenoids are commonly used to
control 12VDC vehicle winches in both directions. They tend to be a bit
Electric lift trucks also use similar switches.
Don Young (USA)
I'm against it, as no matter how good your control lever is they will
not all switch at the same time, which would cause a momentary
short-circuit. Bad news if the alternator can supply any significant
If they are interlocked so they can't be operated when any current is
flowing then you don't really need to parallel them anyway - but try and
get some switches rated to least 15A for each engine, and preferably
higher. One switch for each engine doesn't have as much potential for
disaster as paralleled switches.
Plus, as I said, switches don't parallel well anyway. So basically,
forget all about paralleling switches, it's a bad idea.
I must admit I'm a bit surprised you aren't using a battery, I thought
that was the point of fuel/electric hybrids, to boost acceleration when
starting so you need a smaller fuel engine, and maybe to recover energy
However you already *have* a suitable control supply - the output from
the small genny before it feeds the rheostat and alternator feed coils.
Relays have several other advantages beyond availability, current
capacity and cheapness. First, the switch on the "dashboard" doesn't
have to carry the full current, and can be almost anything you like.
Second, the main current-carrying wiring does not have to be rerouted
via the dashboard, as the relays can be inserted almost anywhere, and
thus the main wiring can be shorter.
Those are the main reasons why cars and lorries use relays. If you have
trouble finding suitable relays, one relay for each motor is also okay
and might be useful for diagnostics, a low power running mode, etc.
If you want a neutral as well as forward and reverse positions you could
go for two normally open relays, one for forward and one for reverse,
rather than a single changeover relay, with a low-current direction
switch on the dashboard with a center-off position.
You could even use a keyswitch on the dashboard, deterring unauthorised
operation at fairs etc. while leaving the motor running.
With a center normally off position on the dashboard switch you can be
pretty sure one relay has opened before the other closes. You could also
disable the operating supply from one relay when the other relay is on,
and do some slightly finicky stuff so that the relays simply wouldn't
switch on if the engine was already travelling, but it's too early in
the morning to describe it..
-- Peter Fairbrother
Contact arcing is no problem because you have a low voltage
known polarity DC source driving a motor load.
Arc generation can be eliminated by a reverse polarity
connected diode across each pair of switch contacts. Forward
conduction of the diodes short cicuits the inductive overswing
and 24v is not enough initiate and arc.
It's natural to think of four separate diodes but a neater
solution is possible with a standard full wave bridge rectifier -
the internal connections happen to be correct.
Your existing connections - the DC supply in to the two
changeover wipers and the fixed contacts to the motor. Connect
rectifier AC terminals to motor, rectifier + and - terminals to
supply DC+ and supply DC-.
No continuous current flows through the
rectifier so the rectifier can be pretty small - 5A is ample.
Hi all again,
Thanks once again for your thoughts and insight.
How about a pair of these
They are simple change-over relays rated at 40A 28V DC with 12V
coils. I could use the normally closed contacts for forward running
and the normally open contacts for occasional reversing. Movement of
a switch to energise the reversing coils could be interlocked with the
brakes and speed control.
Hadn't read that bit carefully - I presume it's an ordinary 12V nominal
output car alternator?
If so, why are you using 24V motors?
Running them on 12V, you'll probably only get about 24W  delivered
power from each?
-- Peter Fairbrother
 at 4A and 50% efficiency - depends on type, gearing etc, but that's
Also I wrote:
I worded my thought entirely wrongly, and ended up not saying what I
meant. Sorry, I'm a bit 'lergy just now, and not thinking too clearly.
The problem is not (just) the back emf from the motor, it's that the
alternator voltage will spike if the load is removed, and as there is no
voltage regulator and the field is manually controlled that could come
to quite a high voltage, especially during a disconnection under high
Reverse-connected diodes can't protect against this sort of spiking, as
the spike voltage is in the forward direction.
Also spiking might well blow the rectifier diodes in the alternator,
they are/were* very sensitive to overvoltage.
* (it's a while since I did this sort of stuff, and things may have
changed - but in those days anything over 60V would kill them)
It's just possible that even a residual field might cause a damagingly
high voltage if the alternator speed is high and there is nothing to
soak up the voltage, but eg a 10W 470ohm resistor or 48V bulb should
protect against this while taking minimal current under normal operating
conditions, assuming the field current is zero when switching occurs and
we are only talking about residual fields.
BTW, 24V most certainly can arc! Arc welders often work at less than
BTW2, the Maplin relays look okay, but a bit expensive - try a scrappy,
or ebay if you want new. It might be an idea to use a pair for each
motor, as they are designed for 40A at 12V, and 24A at 24V may be a bit
I couldn't find any 40A DPCO ones either!
-- Peter Fairbrother
It must be a very long time. I've yet to see automobile diodes
rated for less than 200v PIV and most test out at more than 400v
Both stick and MIG welders initiate an arc by virtue of the built
in self inductance and the absence of any reverse diode to absorb
the inductive overswing. In MIG welders the inductance is a
separately added component. In stick welders it is the
deliberately high leakage inductance of the power transformer.
Without this inductance a low voltage arc is self extinguishing.
Yep, mid 70's. New cars had alternators by then, but many old ones still
I've yet to see automobile diodes
That's good to know, in the old days the diodes were the weakest link. I
blew several up!
Sure, but that's for arcs which are deliberately stabilised over wide
Not necessarily. It depends on the arc length (and other things).
Besides which, an arc doesn't have to last long to weld a switch shut.
A mate of mine regularly stick welds using two 12V car batteries, and
sometimes only uses one. I can't do it myself, I tend to stick the
electrode to the work, but he does - and there is no inductor involved.
Get a 6V motorcycle battery, and touch a wire to both terminals - you'll
see sparks fly. That's caused by an arc, and it's very similar to the
action of a switch.
-- Peter Fairbrother
Replying to myself:
I just read up a bit on modern alternators, and it seems you can get 24
V out of an alternator at reasonable amps by using your method - so
please ignore and forget about the above post.
-- Peter Fairbrother