Folks,
It has been 40 years since I last attempted to design a simple circuit so I need someone to check this for me.
I tried to insert the circuit diagram into this post, but after trying for nearly an hour to insert a graphic, I gave up. So I am going to have to describe this in words.
I am already wiring my Tortoises to route the frog power based upon the turnout direction. I want to use the other set of Tortoise contacts for something else, so I was wondering how to light the LEDs in the control panel to indicate the direction of throw of the turnout. In this case the turnouts are hidden in a staging yard so I want a positive visual indication on the panel as the which way the turnout is thrown.
I got the idea from reading the Nov. MR article on LED basics. In the article there is mention of using the full wave rectifier from Diodes Inc. (DF005) to generate DC power from the power bus.
I propose to hook up 2 rectifiers "back to back". Imagine rectifier "A" with one of its inputs hooked up to one "side" of the DCC power bus. Let's call that the "Red" pole. Then, take rectifier "B" and hook up one of its inputs to the other pole of DCC power. Let's call that the "Black" pole.
Now, take the other input of both rectifiers "A" and "B" and connect them together. From this common connection, wire this up to the frog power wire. Got it? We have one rectifier connected to "Red" DCC power and frog power and the other rectifier to "Black" power and frog power.
Of course, each rectifier's DC output is used to drive an LED. One LED indicates the turnout one way and the other LED indicates the other direction on the panel.
Here's how it works. Set the turnout to the through direction. Let's suppose that when it is in the through direction then the frog needs to be driven with the "Red" pole of the DCC Power Bus. If so, rectifier "A" will have "Red" and "Red" on its inputs, it will have no input power so its LED will be off. Rectifier "B" will have "Red" and "Black" on its inputs so will generate the 14V DC output that I direct to the LED and it will light (given a 1.4K Ohm load resister to set the current to 10 ma for the LEDs I am using).
When the turnout is thrown the other direction the frog gets "Black" frog power. Then rectifier "A" gets "Red" and "Black" from the common input and will light its LED. Rectifier "B" is quiet with "Black" and "Black" on its inputs so its LED is dark.
This seems like a simple and elegant way to use frog power to light the LEDs on the control panel. The DF005 rectifiers are only about 40 cents so this is a cheap and simple method.
Unless I am missing something about how the rectifiers work. In this proposal the rectifier inputs are "shorted" when the frog power is set to the same power as the one side of the rectifier. But there is no voltage across the terminals and no current so there should be no problem, right?
Thanks for any feedback.
You wouldn't need two rectifiers. One would be fine. Preferable approach would be to NOT use the DCC power to runt he panel LEDs, it's a waste of 'expensive' DCC power. However you do it, you'll have a DC power suppyl with a + and -. Connectthe + to the switch common on the Trotoise, Each pole of said switch contact will go to an LED via a resistor (resistor value depends on LED and power supply voltage). Free terminal of each LED goes to DC -.
Even easier - just wire the LEDs in series with the Tortoise motor contacts. The current is automatically limited by the Tortoise motor and you won't need a lot of extra wiring.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
rrinkerEven easier - just wire the LEDs in series with the Tortoise motor contacts.
Yes, but doesn't this just tell you that you have instructed the Tortoise to move in one direction or the other? I don't think it tells you that the machine actually moved. I think he said that he wanted a positive indication.
It gives as positive an indication as using the contacts - the LED(s) will be dim while the motor is moving (it draws less current when moving) and will jump to full brightness only when the motor stalls at one position or the other.
The only way to get REAL positive feeback is contacts on the turnout itself - after all the linkage could break and the Tortoise move but the points not move. Since we're dealing with a model and not human lives, fully duplicating the fail-safe designs of the prototype is a bit of overkill.
CharlieBedardNow, take the other input of both rectifiers "A" and "B" and connect them together. From this common connection, wire this up to the frog power wire. Got it? We have one rectifier connected to "Red" DCC power and frog power and the other rectifier to "Black" power and frog power.Of course, each rectifier's DC output is used to drive an LED. One LED indicates the turnout one way and the other LED indicates the other direction on the panel.
Hi,
Glad you could post a schematic.
Yes. This is exactly what I had planned to do . Sounds like it will work as I expected.
I neglected to mention in my first post that I am already using one set of contacts to drive the Tortoises themselves using the "yard ladder" idea published in the May 2009 MR. That makes driving the LEDs off the power to the Tortoises themselves a little clumsy as the voltage drops as each Tortoise in the chain is activated. This makes choosing the load resistors variable and time consuming.
I am using the other set of contacts to route frog power which is why I wanted to use the frog power wiring to trigger the LEDs.
The amount of power used is trivial. I alread have a power district planned for the staging yard. The LEDs will draw maybe 150 ma total. Not a big load.
The only drawback I see is that if there is a short in the staging yard my LED indicators go out too. I can live with that.
Thanks to all who replied :-)!
Charlie
Texas ZepherYeah, I believe that will work but it is a whole lot harder than it has to be. Just hook the two LEDs together (same polarity to same polarity) and connect that common wire to the current limiting resistor and then the other side of the resistor to the frog of the turnout. Connect the other side of each LED to each side of the DCC power. The facing polarity LEDs themselves prevent the DCC from shorting. A side benefit is that if there is no power to the frog the LEDs will be out, so they not only indicate which direction the turnout is - but also that the frog is powered.
I think this only works if the track has a DC bias. Since the signal pulses between positive and negative voltages, the DCC bus is essentially an AC-like signal. I think it would cause the LEDs to oscillate.
That is why I think you need to put a rectifier across the bus to generate a clean DC power source.
In quantities of 25 or more, the rectifiers are only 32 cents each. At 64 cents a pair, this seems like a cheap and quick to wire solution. I only need to send 1 wire from each Tortoise to the control panel for LED indication of turnout direction.