# A buffer circuit

#### Neilshop

Joined Jan 19, 2020
5
I am not sure if that is what I need but not sure what to call it. I am a model railroader and have a blue tooth controller for one of my small HO trains.
Its output is 0~12 volts DC and 2amps with the capability to reverse polarity.
Is it possible to build a controlled circuit for my large scale train which can draw up to 10 amps at 25 volts DC which would follow the output from the small board? It would need to also reverse polatiry for forward or reverse.
Thank you for any input.
Neil Hart

#### BobTPH

Joined Jun 5, 2013
3,510
If it is following the 0-12 output, how do you get 25V out?

I presume you mean 12V in gives you 25V out at up to 10A.

What you are talking about is a voltage controlled power supply, and a pretty hefty one. A linear supply would need huge heat sinks and a fan. Better would be a switching supply.

An op amp could adjust the voltage range.

Bob

#### crutschow

Joined Mar 14, 2008
27,477
Do you already have a 25V, 10A supply?
If so you could likely use a voltage-controlled PWM circuit to control the voltage to the train with low power loss.

What signal would control the reversal of the voltage polarity?
An H-bridge circuit can perform the reversal to the train.

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#### djsfantasi

Joined Apr 11, 2010
7,772
Is this a DC controller or a DCC Controller? The approach for what you need is different.

#### Neilshop

Joined Jan 19, 2020
5
Ok, it will be a DC controller. Blue Rail make a small "blue tooth" board for HO trains that you control with an app on your cell phone. It runs on 12 Volts DC with a 2 amp output and you can control the speed of the train by the variable voltage out and the direction by the reversing polarity output; again all through the cell phone app. I have a large scale out door train that runs on up to 25 volts DC (by 2 on board 12 volt gell cells in series) . I doubt that I need a total of 10 amps output but I don't want to undersize the board and end up burning it out. The locomotive I plan to run has two motors. I haven't checked the stall current yet but have a DC ammeter on order to enable me to do that. I am wondering if a board can be built that would follow the output from the small board which would be used as the control circuit, then the new board would handle the higher current draw.
Thank you for your interest in my question.
Neil Hart.

#### djsfantasi

Joined Apr 11, 2010
7,772
I looked up Blue Rail. I couldn’t find a DC controller. They were all DCC controllers. Could you supply a link for the board you’re using?

Typically, any remote control of an engine isolates the motor from the track voltage, the track voltage ALWAYS is a maximum DC voltage and that voltage is varied by PWM on board. A normal DCC controller sends commands to the board in the engine, through superimposing the command on the track voltage which are then interpreted by the controller.

Blue Rail has chosen to not send commands through the rail. They open a serial connection with Bluetooth and send commands that way. Similar to DCC but different enough to cause some confusion.

The major problem I see is that you’ll put 25VDC on the rails, and the board’s maximum is 22VDC. Is it possible to run the trains at 22VDC?

Without the link to the actual board, I’m not certain of the current ratings

#### Neilshop

Joined Jan 19, 2020
5
I am sorry for my confusion. You are right, it is a DCC control. For my HO I use a 12 volt battery on board the train. No track power is needed. For the outdoor train I use 2~ 12 volt bateries in series. I will be able to use 12 volts from just one battery to power the Blue Rail board.
Neil.

#### crutschow

Joined Mar 14, 2008
27,477
So why do you want to control the indoor train and outdoor train from the same controller signal?

#### djsfantasi

Joined Apr 11, 2010
7,772
I am sorry for my confusion. You are right, it is a DCC control. For my HO I use a 12 volt battery on board the train. No track power is needed. For the outdoor train I use 2~ 12 volt bateries in series. I will be able to use 12 volts from just one battery to power the Blue Rail board.
Neil.
So why are you using 24V for the outdoor engine? Who manufactured your large scale engine? I’m not aware of any engine that requires more than 24V?

#### Neilshop

Joined Jan 19, 2020
5
I don't need more than 24 volts just up to that amount and in the 8 amp range. The locomotive I want to control is an Aristocraft Mallet with 2 fan cooled motors. The owners manual says to provide 18 to 24 volts and at least 3 amp capacity when using battery power. If I can build a follower circuit to power the train I will purchase another Blue Rail controller so I can control the follower circuit and there-by the train from my cell phone.
Neil.

#### djsfantasi

Joined Apr 11, 2010
7,772
The problem is that you’ll need 24VDC controller while providing 12VDC to the Blue River controller. One way to do this is with a 10A H-Bridge on the output of the DCC Controller.

This link describes how someone approached the problem. I haven’t done anything like this, but I’m familiar with the model railroad environment and DCC.