A selection of Questions

[Crosland]

"Don't do it, Ethel!"

 

If the windings are slightly dissimilar, then one of the two will win over the other and the transformer will end up as an exhibit that will displease the local plastic surgeon and one's fire inurance company. It's far cheaper to buy an adequately-rated transformer for the job.

Alarmist nonsense.

 

Paralleling the outputs of a transformer like this is fine, and is done all the time.

 

Just make sure you connect them in phase.

 

Andrew Crosland

[noiseboy72]

Please explain why it isn't good to do it with the independent supplies advocated in the original post.

 

As per my post, if you use 2 transormers and disconnect the input to just one of them, mains voltage will be present at the disconnected input!

 

Always use 1 transformer and parrallell the output windings for more current.

[Ray H]

When I wrote the OP I was thinking of parelling the two (isolated) secondary windings (of the same nominal voltage) off the same transformer, each winding being rated at around 1 amp. My thought was that this would provide double the current at the same base voltage.

 

Fortunately, since then I have located a larger transformer, again with two isolated secondary windings, both delivering 12v at around 4 amps. My intention now is to connect those two outputs in series to give around 24v at 4 amp which I think should be enough to ensure the SEEP motors fire each time.

 

I've been advised against using a CDU with the motors wired through their inbuilt switch as (so I understand it) the CDU effectively delivers DC and the resultant spark across the switch terminals will soon wear the switch out something that shouldn't happen with an AC supply.

 

Many thanks for the comments and observations and apologies if I've unintentionally caused any grief.

[melmerby]

As per my post, if you use 2 transormers and disconnect the input to just one of them, mains voltage will be present at the disconnected input!

 

Always use 1 transformer and parrallell the output windings for more current.

 

Wire them up permanently and it wont happen. If I was using 2 transformers that is what I would do, I would never have separate mains connexions. If necessary cut off any mains plugs and wire them through a suitable (mains rated) connector.

 

Keith

[noiseboy72]

When I wrote the OP I was thinking of parelling the two (isolated) secondary windings (of the same nominal voltage) off the same transformer, each winding being rated at around 1 amp. My thought was that this would provide double the current at the same base voltage.

 

Fortunately, since then I have located a larger transformer, again with two isolated secondary windings, both delivering 12v at around 4 amps. My intention now is to connect those two outputs in series to give around 24v at 4 amp which I think should be enough to ensure the SEEP motors fire each time.

 

I've been advised against using a CDU with the motors wired through their inbuilt switch as (so I understand it) the CDU effectively delivers DC and the resultant spark across the switch terminals will soon wear the switch out something that shouldn't happen with an AC supply.

 

Many thanks for the comments and observations and apologies if I've unintentionally caused any grief.

 

 

Either connection method is fine on a single transformer. Despite the postings of others, I would not reccomend coupling 2 transformers unless you are a competant electronics engineer. It is all too easy to delve into a faulty device and forget about any little short cuts you might have made...

 

To reduce the spark on a DC powered point motor, a diode & capacitor across the switch will prolong the switch life. The spark is probably back EMF generated by the coil in the point motor, so the supression circuit will cure this issue. The higher AC voltage may well shorten the life of the switch due to arcing, and this will be harder to correct.

[Ray H]

Thanks for the further comments.

 

I thought that I'd read somewhere on here recently that (within reason) the higher the voltage to operate the PMs the better.

 

Another reason for wiring the PMs through their in built switch (and any associated common crossing [frog] polarity through separate contacts on the external switch that operates the PM) is that it minimises the number of external connections to the PM. The PM only needs three wires when wired this way. A spare PM can be kept ready wired up - there will be over two dozen motors on the layout - and changed over quite quickly. With the more conventional wiring of a PM, the wire count is doubled, extending the changeover time and increasing the risk of a dry joint on one of the PM's connections..

[Donw]

If you use 24v AC you will be switching at various currents. If you use a CDU charged from 12v it should charge to around 20v but will discharge when it fires so the current will be lower when the switch breaks. Just use one with a moderate sized capacitor. The circuits are quite easy.

Don

[melmerby]

If you use 24v AC you will be switching at various currents. If you use a CDU charged from 12v it should charge to around 20v but will discharge when it fires so the current will be lower when the switch breaks. Just use one with a moderate sized capacitor. The circuits are quite easy.

Don

 

If you charge from 12v DC you get 12v DC, if you use a bridge rectified 12v AC you should get around 17v (12v x 1.414).

 

With a CDU the current pulse is what gets the motor moving, the bigger the capacitor the longer it will take for the current to discharge to a given figure and the more chance of a sticky point motor moving completely.

 

The current from a CDU will be much diminished by the time the switch opens and the inertia of the solenoid completes the movement.

 

Keith

[BR60103]

You learn something every day. I had never heard of piggy-backed switches until now.

Were they an H&M accessory or did someone else supply them?

 

Keith

 

I have a few of these; they are H&M. The part is the top plate of the H&M motor and IIRC came with the extra spacers and longer bolts required.

It effectively made the point motor a DPDT switch. I feel the drag from the extra contacts probably ups the current demand.

[Ray H]

Does anyone know the best way to guard against the inadvertent transposed connection of a Gaugemaster HH walkabout controller?

 

It seems a little strange that they are supplied devoid of connector and any obvious safeguards with just four loose wires at the end of the lead. I appreciate that we're talking low voltage but I'd feel safer if it were possible to guard against the unwanted especially as I presume connecting the DC output from the controller across the AC windings of a transformer could short out the latter and do one or both serious harm. Would a 2 amp fuse - the controllers are rated at 1¼ amp - connected to one of the transformer's AC winding's suffice?

 

 

[Donw]

If you charge from 12v DC you get 12v DC, if you use a bridge rectified 12v AC you should get around 17v (12v x 1.414).

 

With a CDU the current pulse is what gets the motor moving, the bigger the capacitor the longer it will take for the current to discharge to a given figure and the more chance of a sticky point motor moving completely.

 

The current from a CDU will be much diminished by the time the switch opens and the inertia of the solenoid completes the movement.

 

Keith

 

Your right of course I was thinking of the 16vac used for 12vdc. I said 20v because many of these give slightly more than the quoted volts on no load. With a CDU it charges up to the maximum. I measured over 20v on one I tested for a friend.

 

Don