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Low Voltage Inverter Circuit.

Griznoid

New Member
Hi. I need a 12VDC to 16VAC approx 3Amp inverter circuit for a model railroading application: Specifically, to supply a 16vac motor using a 12vdc battery. Anything like that doable or available? Please advise.
 

gophert

Well-Known Member
Most Helpful Member
Hi. I need a 12VDC to 16VAC approx 3Amp inverter circuit for a model railroading application: Specifically, to supply a 16vac motor using a 12vdc battery. Anything like that doable or available? Please advise.
Walmart, Amazon, Advance auto or Staples all have something similar to this inverter. Get something 150w or more and then plug your standard 16v model railroad transformer into it.

 

Griznoid

New Member
Thanx, guys, for your prompt replies. But since each battery supply is in each engine or its tender, cramming all this in, especially in HO models, would prove very difficult. Sorry for not going into detail initially, but the newest thing in model railroading is eliminating track -fed power to locomotives, and instead using radio control to power to the loco motor from on-board lith-pro batteries. Most model motors are DC, so it's not a problem, but I have a number of O gauge older engines that have AC motors in them, and I am trying to figure out if it is practical to re-power these units with batteries. Hope my problem makes more sense now. BTW, if you google Tam Valley Dead Rail, you can get a good idea of what I am up against. Thanx again for your replies.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
O gauge "AC" motors will run perfectly well on DC, especially with a bit of rewiring.

They are in fact a "universal" motor - AC or DC - with an armature and wound field coils.
The commonest setup on such as Lionel ones is that the field coil is centre-tapped and the centre linked to one of the motor brushes.

Some makes have a permanent magnet kit available to replace the field coil, converting them to normal DC-only motors. Examples: http://www.esu.eu/en/products/accessories/permanent-magnets/

Without doing that, there are three ways to wire those, at least:
Note that you cannot reverse a universal motor simply by reversing the overall supply polarity - the motor connections must be changed; that's how they can run on AC as well.


1: Use the free brush wire as one connection and one or the other end of the field coil for the other connection, depending on which direction you want, using a small relay - or a couple of rectifier diodes, opposite polarities to each of the field coil ends so one or the other is fed depending on the supply polarity.

That's simplest, but least power efficient.


2: Disconnect the link between the motor brush and field coil.
Use the two ends of the coil in series with the two motor brushes. Reverse the field connections to reverse the motor rotation, using a relay.

That give more torque for a lower supply current, but lower maximum speed.


3: Disconnect the link between the motor brush and field coil.

Then use a separate small step-down switching regulator to power the field directly via the end terminals, all the time the motor power is enabled. It probably needs about a quarter to half an amp at a volt or so. You are in effect making the motor work as if it had a permanent magnet field, as long as power is on.

You can then treat the two brush wires as a conventional DC motor and use any normal motor controller with it, reversing by reversing the polarity to the motor.
That's the only option without replacing the motor or field coil that will allow "back emf" speed monitoring and control.


Another option, depending on the motor and mechanics, is just to replace the universal motor with a "can" motor, a more modern conventional permanent magnet type. They are generally rather more efficient than the older ones so better suited to battery operation.

Again, conversion kits are available for some types but I don't know exactly which or were to look for those.


Note that there is a forum on RMWEB about RC conversions.
You can now apparently get some kind of RC receiver that outputs DCC format data, so you can use a standard loco DCC decoder to give full motor/sound/lighting etc. control.

There is also quite a bit of stuff on "thngiverse" relating to "OS Railway", which is an O gauge RC train system.
Another approach may be to substitute power bogies from suitable designs on their in to your loco bodies?
 

Griznoid

New Member
Thanx, Mr Jenkins, for taking the time to answer my query in such great detail. You have given me a lot to consider, so now I will get to work reading and experimenting! Please be assured that I will place a "What I did and Why" comment on this Forum when once I move forward successfully.
Thanx again for your interest and help. Jim P
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Just had a chance to have a look at the "tam valley dead rail" - that's one of the types of systems I'd seen in passing, using a radio link but still DCC data format and DCC decoder.

To clarify the "method one" I mentioned above, this is the two-diode minimal mod circuit:

And this is the type of thing I was thinking of for method three, a little switching regulator that can supply a low voltage at reasonable current and very high conversion efficiency; eg. the load from a 12V input would be about a tenth the field current at eg. 1.2V, so for 250mA field (if it even needs that much voltage) it's only using 25mA extra from the battery, to give full compatibility with a conventional DCC decoder across the armature connections, including back emf speed regulation.

 

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