model railroad power supply

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I have a Outdoor Model Railroad electronic DC throttle that does what I want EXCEPT it only has a 50watt capacity, and my railroad needs a 350 watt supply. I have such a supply, but need a circuit that will duplicate the voltage and polarity of the smaller supply using the input from the larger supply. Help?

Smaller supply= +24 0 -24 vdc, max. 2 amps. chip based design
Larger supply = +24v, max. 25amps massive transformer, regulated output

 

[ronv]

I don't know much about model railroads but it sounds like what you need is an H-bridge to make your 0 - 24 volt supply act like your +24 0 -24 volt supply. Am I on the right track?
Do you know how the little supply controls the voltage? Or the real question is would PWM work?
Tell us a bit more about the set up.

 

[4pyros]

It is impractical to try and combine the two supplys. you need one big one to do what you want. Maybe you can use the controls on the smaller one (Speed pot and directoin switch) to control the big one. Andy

 

[ben7]

It probably would be best to use PWM, because at 24volts the transistors/mosfets in the H-bridge will get HOT when controlled at a steady voltage. Plus, when you control the train with steady voltage the current will go down with the voltage, and the electric train motor will stall, because it gets starved of current.

I would go with the H-bridge on your single rail supply, and the H-bridge being controlled by Pulse Width Modulation, or PWM.

H-bridge, replace the switches with mosfets.
**broken link removed**

555 timer PWM circuit.
**broken link removed**
555 output goes to the h-bridge mosfets through a resistor.

 

[[email protected]]

ronv - you are right. the small supply is in a canned electroniics module that I want to use, and I can't get a circuit diagram

4pyros - same reply as ronv

ben7 - what would such a circuit look like?

all - I need the voltage output of the canned (small) module to control the larger supply. this would be a part of an integrated automation set-up.

 

[[email protected]]

I just had a clarifing thought, input = 24-0-24 at 50w, need output to be 24-0-24 at around 350w

 

[4pyros]

Dan; Can we get more info on the supplys you have, make modle number. I am asuming you have no controls on the trains at all just a pick up and a motor right?

this would be a part of an integrated automation set-up.

What is your finale goal? You should work toards
your finale goal. Are you going to computer control? That would make a big difference on the power supply control needed. Andy

 

[KeepItSimpleStupid]

Take a look at this **broken link removed** for a possible starting point,

I don't think your doing -24- 0 - 24 at 50 W as a control signal, just -24-0-24 that's probably PWM.

So, why not determine the polarity to get a DIR signal.
Set a point around 0 V to get not enable/brake.
Pass either the scaled down signal or the inverted scaled down signal to PWM.

It's just an idea to go with. I'm not sure how to do it.

Electronic circuit pseudocode:

PWM = PWM scaled to 0 to 5 or 0 to -5 depending on polarity.
PWM = Absolute value of PWM; Get a 0 to 5 volt logic signal

If freq <> 0 then enable motors ; Not Enable = TRUE

If coming from a running condition to stop condition, then enable brake; (OPTIONAL)

 

[ronv]

KISS has the question that needs an answer. It is possible (not likely) that your little supply is analog. In other words to go forward it could put plus 24 volts across the motor and in the other direction put minus 24 volts across the motor with ground always on the same track. Or (more likely) it is PWM with an H-bridge in which case it would just reverse which track that has the +24 volts on it. Either way it can be done. How handy are you?
Is there a reason why you want to keep the little one? Does it do some extra "stuff"?

 

[KeepItSimpleStupid]

Very good points ronv. Dynamic braking is an example of "extra stuff"

 

[4pyros]

Outdoor Model Railroads mostly have only two rails but without Knowing just what he has we will not be able to help. Andy

 

[[email protected]]

here is a block diagram of the power supply that I wish to use to control my large power supply. this module is a part of an automation package.
In Out
PC Communications

Overload Protection Microprocessor

Power Supply Conditioning Digitally Controlled Voltage Adjust Direction Control
+ Input - Input +- Output +- Output

The instructions say that the +- Input should not exceed 24vdc, 2a
I presume that the input to the digitally controlled voltage adjust is PWM, but without a circuit diagram from the manufacturer, I can't be sure. I assume that the output could control a latching relay to handle direction from my supply. I just need a module to allow a larger amperage input to be used. a circuit diagram would greatly simplify matters, to be sure.

 

[4pyros]

What is the name of the automation package you are trying to use? What is the modle number of the power supply you are using now? Andy

 

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The SmartCab module from "CTI-Electronics.com"

 

[[email protected]]

My outdoor railroad uses 0-24vdc power.

 

[KeepItSimpleStupid]

Smart Cab supplies a fully regulated DC output, controllable in 100 discrete digital steps. It continually monitors its output, maintaining output voltage to within 0.1% regardless of variations in load. For smooth, even starts and steady low-speed performance, Smart Cab employs pulsed operation at the lowest throttle settings, switching automatically to a continuous voltage output once the train reaches cruising speed.

Smart Cab employs a novel user-selectable output voltage range so that all of its 100 speed settings are available for use by your locomotives, regardless of gauge. None are wasted on speeds that run your trains faster than you want them to run. Other built-in features include variable momentum settings, direction control, braking, short-circuit and over-temperature protection.

If I read this right, it's PWM combined with voltage regulation at 100 steps. Messy. Add dynamic braking which is more messy. Not sure how thy do momentum. Just vary the speed?

A high power op-amp anyonw with two 24V supplies?

I doubt you want togo this https://www.electro-tech-online.com/custompdfs/2010/11/PA03U_J.pdf route?

 

[[email protected]]

I would be happy with just voltage regulation. My big power supply also has other built-in features, so voltage only would be just fine.

 

[[email protected]]

Again, just voltage regulation is fine. Direction and other stuff I can do.

 

[[email protected]]

My other thought is to just buy the module now instead of later and see what IC's they are using, and find substitutes of the desired current rating.

 

[[email protected]]

to KeepitSimpleStupid---
Actually, this PA03 doesn't look bad. However, I guess that I'd have to call tech support at the manufacturer to understand their engineering talk on their data sheet. i.e., just what is the control input, and several other of the mounting and usage instructions...