7864 12V Transformer

[Lazarus]

Hello All.

Looking at replacing my 9v controllers with the older 12v.

I was told from one of the guys at my lego club that they have a higher amp and as long as you dont run it at max they run the 9v motors very well, Also gives toy more control as you move up the speed steps.

I was doing a bit of googling and found these also had come AC and DC sockets on them what are not track power and for accessory very much like a DC model train controller.

So what i am keen to find out

- Is the a AC/DC socket on these (for accessory's)

- If so what Amps run though these.

End goal is to use these 12v controllers to power my remote switches and my locos.

[Bricktrix]

Hi Lazarus,

The 9v controllers are stepping i.e. 1 step gives 1v, next gives 2v, etc. There is no in-between with these steps, like 1.2v, 1.3v, 1.5v, etc.

The 12v controller is continuous so you do get 1.1v, 1.2v, 1.3v, upwards.

However, during our recent experiments with DCC, we found that the standard 12v dc output of these controllers is actually giving out at max around 16.5v, so be careful with them.

[Lazarus]

The 9v i think have only 4 steps,

i will need to find a sweet spot with the 12v.

But do that have an accessory Port on them. Not just the track power?

I know the track power will go from 0-12v when nob it turned but the AC/DC output should run at constant 12v if it has one at x amount of amps.

Edited July 8, 2013 by Lazarus

[alainneke]

- Is the a AC/DC socket on these (for accessory's)

- If so what Amps run though these.

The 7864 transformer has three outputs:

- sec I: 0-12V DC (as noted by Bricktrix, the maximum voltage is actually 16,5V)

- sec II: 12V DC (also actually 16,5V)

- sec III: 13V AC

The maximum load for I is 8VA

The maximum load for II and III combined is 8VA

If you want more amps, you could take a look at transformer 740:

- sec I: 0-13V DC, 11VA

- sec II: 16V AC, 16VA

- sec III: 3,6V AC, 6VA

Edited July 8, 2013 by alainneke

[Bricktrix]

Yes the output on the side is a constant power source, so you can hook up accessory switches to them as it was designed to do. In fact I power all the points and lights on my display using the 12v switches designed to go with the controller.

[Lazarus]

just to check when you say 8VA you mean 8amps?

Cool sounds like what i am after saves me getting another power supply to for my remote switches.

I like that old school speed leaver they show in the manual for this controller might make that up and find the sweet spot for the 9v motor and make a block so the leaver cant go up to 12v (16v)

With the speed regulator is there anything you can feel when turning it to know you have gone up a notch.

[alainneke]

just to check when you say 8VA you mean 8amps?

No; you should divide this number by the voltage you're operating your trains at: 8VA will give you 0,9 Amps at 9V, or 8 Amps at 1V

I like that old school speed leaver they show in the manual for this controller might make that up and find the sweet spot for the 9v motor and make a block so the leaver cant go up to 12v (16v)

This is what LEGO did to limit the output to 4,5V: by fitting the black knob the other way round, it couldn't be turned all the way and the output voltage was limited

With the speed regulator is there anything you can feel when turning it to know you have gone up a notch.

There is indeed some resistance when turning the knob

[Lazarus]

This output sec II: 12V DC (also actually 16,5V)

would the amps not be at a constant for this are it is DC, understand the above comment for the sec I: 0-12V DC

[alainneke]

This output sec II: 12V DC (also actually 16,5V)

would the amps not be at a constant for this are it is DC, understand the above comment for the sec I: 0-12V DC

That's right!

[Lazarus]

OK so these are my specs for the motors for my turn outs.

Current consumption: 2,800mA

RPM: 20,700rpm

Proper voltage: 2.4~3.0V

I have currently tested on a Hornby select DCC transformer what only puts out around 15v DC and 1 amp.

This has ample power to do what i need but takes to long to charge the caps in side the controller so i suspect it is drains them every time i switch the motor. It did the same for my model train point motors also thats why its my spare as its utter crap.

I run the above motor though a MOM switch so they only get power for a Sec then stop. Do you think the 8 amp 12v controller will kill these due to the increase in amps.

[legoman666]

I dont think you understand how amperage works in simple DC motor circuits. If the max current is listed on the transformer as 2A, that doesn't mean it's going to output 2A. For any given scenario, the current will vary depending on how much resistance there is on the motor and how fast it's spinning. As a lightly loaded DC motor turns, it generates a backwards-flowing electromotive force that resists the current being applied to the motor. The current through the motor drops as the rotational speed increases, and a free-spinning motor has very little current. It is only when a load is applied to the motor that slows the rotor that the current draw through the motor increases. (some quotes from wiki because I couldn't figure out how to word it elegantly)

For example, say we're running a regular 9V train motor at full speed (9V) with nothing attached. No cars, nothing on top of the motor, nothing. The motor will generate back EMF which will drop the voltage across it by... say 5V. Lets make up a number for the internal resistance that this motor has, say 25ohms. (9V - 5V) / 25ohms = .16A.

Now say we build a locomotive on top of the motor. This will increase the resistance the motor encounters and will reduce its speed. By reducing the speed, you also reduce the back EMF. Say the back EMF falls to 2V which will increase the voltage to 7V instead of 4V. Then we have 7V / 25ohms = .28A

Now say you hold the motor still with your hand as you turn the knob on the controller to max. There will be no back EMF and the voltage across the motor will be 9V. 9V / 25ohms = .36A

Stalled motor = max current, motor by itself zooming around track = minimal current. The current being drawn is completely independent of the DC power supply (as long as it can provide enough). You could use a car battery, which is capable of supplying >100A, but would still only supply 12V / 25 ohms = .5A for our hypothetical motor. Remember I made up all of my numbers, but the same thing will happen in real life.

So to answer your question, keep the voltage down between 9V-10V max, and you will keep the current down. 15.6V / 25 ohms = .6A, whereas 9V / 25 ohms = .36A.

Hope this helps, I'm an electrical engineer. This is what I do.

Edited July 9, 2013 by legoman666

[alainneke]

Thanks for the clear explanation, 'legoman666'! The back-EMF value is used on some DCC decoders to measure the speed of the motor and helps the decoder to let the motor/train run at a constant speed. For the record, the free spin/loaded/stalled values of most LEGO 9V motors are listed at http://www.philohome.com/

[paul_delahaye]

It's interesting and I only found out the other day, Lego designed the 12v transformer, so you could invert the black control piece and it then only allowed you to output 4.5v (you could not rotate the power knob any further) this allowed the running of 4.5v motors using the 12v power pack. Quite a clever design, but prone to over powering a motor is the knob popped up a bit and rotated to far or like an earlier poster mentions if the transformer is actually producing more than the stated 12v.

[Lazarus]

Thanks for the Info Guys. Going to order some 12v controllers when i have the cash and do some playing. Sounds like it is going to do what i want it to do what will do wonders for saving space and amount of cables i need around my train set.

[hoeij]

I have some 12V controllers. If you are planning to run more than 1 train on the track, then I would not recommend them.

If you're running 1 train, and then start a second one, the first one will slow way down.

And when you stop the 2nd one in a siding, then the first one will speed way up again.

I think that a typical controller from a model railway behaves better.

I modified one of my 9V controllers to change the output voltage. The 9V controller has a sequence of resistors

*---R0---*---R1---*---R2---*---R3---*---R4---*---R5---*

with a voltage applied to them. This divides the 9 volts as follows: 3V over the first resistor R0, and 1.2 volts over each of R1,..,R5.

The chip that controls the output uses those voltages as base-lines. I placed a variable resistor in parallel with R1.

Turning that variable resistor changes all the base-line voltages (except the first one, that stays 3V). This way I can

adjust the output voltage to almost any value between 3 and 9 volts.

[PedroQ]

On 7/9/2013 at 2:34 AM, legoman666 said:

So to answer your question, keep the voltage down between 9V-10V max, and you will keep the current down. 15.6V / 25 ohms = .6A, whereas 9V / 25 ohms = .36A.

Hope this helps, I'm an electrical engineer. This is what I do.

Maybe I can use your knowledge (I'm a mathematician :)

I'm trying to use Arduinos to form a network of masters/slaves to manage an entire Lego railroad.

Everything is OK when I use a 12v 2.5A led battery --- my circuit includes a MOSFET-P (to avoid reverse polarity issues) and a push-down converter (12v to 5v) to power the Arduino --- but when I tried to substitute the 12v battery by the Lego Transformer the Arduino works only up to the moment where I add a Lego light to the circuit, at that moment it seems that the Transformer does not have enough Amps to run both the Arduino and the light... any ideas/suggestions?

[Brickblock1]

This was 8 years ago!