I believe on this forum you have to save your images to some third party host and link to it. No thanks. I’ll scan the Tansey article and email it to you.
Charlie
having now seen both articles, it seems the Smith article from Nov 2000 is the simpler approach. it’s application to a yard ladder, where just one turnout needs to be diverging while all others would be set normal is very straight forward.
it wires all the motors for normal position using one supply (+) thru a ~2k resistor and the other motor terminal to ground (common). a single pole rotary switch can then be used to connect the motor terminal with the resistor to the opposite supply (-) causing the motor to the diverging position.
in the above case only one motor needs to be set to diverging. but is describes how using diodes connected between the motor and rotary switch, more than one motor can be diverging.
it is somewhat confusing how it describes using a center tap transformer and half-wave rectification to generate +18V, -9V and ground. i see no reason why wall warts, ~18V and ~9V can’t be used and avoid the need to build the half wave rectifiers. (note that only one direction would be “slow-motion”)
the Tansey articles uses a single power supply voltage. it wires the negative supply to both motor terminals thru a 560 ohm resistor. connecting the postive supply to either terminal overrides the negative voltage causing the motor to switch.
the articles describes using diodes as well as an SPDT contact on the motor to route the positive voltage.
the obvious advantage of the Tansey approach is a single supply but requires twice as many resistors.
Now I’ve had a chance to look at both articles thanks to Greg forwarding Robert’s email. They both seem to be viable and they both have advantages/disadvantages. The Smith approach may be a bit simpler, especially if wall worts are used as Greg suggests, but it does require two. Also, 18V wall worts may be harder to find. A higher voltage unit could be substituted but the 2.2K resistors would have to be changed. Interestingly the Tortoise actually sees 9-10 volts while the motor is spinning due to back emf. When it hits the stop it sees a bit over 4 volts which is enough to hold it in position. The speed is slower than with 12V but not too noticeable. The circuit does overcome the sequential turnout problem that I see as the primary disadvantage of the Tansey approach. However, the Smith approach does require two power supplies of rather non standard values (-9V and +18V). The Tansey approach uses a single 12V supply which is very useful for many other applications on the layout. The extra resistors and/or diodes with either approach are about a toss up and nothing like the full diode matrix would require. Both approaches are clever solutions and either will work. Your choice. Over to you, Robert.
not sure why you bring this up.
don’t know what the bemf voltage of the tortoise while turning. bemf is proportional to RPM. bemf is zero when a motor isi not turning. so more mechanical resistance causes the motor to turn slower which reduces the bemf, increasing the current thru the motor and increasing the force.
there’s more force holding it in place while stalled than while turning.
Greg,
Only to point out that the motor sees a respectable 9-10V while running. This could be a concern when the Tortoise is returning to its “normal” position, being supplied via the 2.2K resistor. Usually the Tortoise would be supplied from a relatively stiff power source. The 9-10V readings are a bit of a guess due to the sampling and display delays of my DVM.
the Tortoise instructions only specify a max voltage of ~12V, but they draw about the same amount of current as an LED (max 20ma). so a relatively lightweight supply, what you might use for driving LEDs on a panel is usually sufficient
max ~8ma flows thru a 2.2k resistor driven by 18V, or ~13 ma when that terminal is driven directly by -9V.
i’m using a pair of LM324 op-amps to drive pairs of Tortise machines in an interlock controller
“Clever” it is. Though current exchanges between the two of you about bemf is complex. I’m still thinking the Tansey Variation A (with resistors only instead of Variation B with resistors and diodes) is still going to be my selection. The advantage of using one power source readily attainable in my electronic discard box (a 12 volt 500 mA wall wart should do the trick and cost nothing to me now) outweighs the “downside” of watching Tortoise machines run sequentially instead of simultaneously. For the four track yard the track furthest from the main that means waiting 20 seconds instead of five. I can live with that in a staging yard.
Speaking of resistors, Tansey identifies 560 ohms, 1 watt, with three identification bands of green, blue, and brown. My search found these identified by the vendor as 560 ohms 1 watt, but the identification bands don’t match the article’s description:
https://vetco.net/products/560-ohm-1-watt-resistor/nte-1w156
Has the resistor identification system undergone a change since 2009 or are there different kinds of resistors? If I need to use a different resistor, your suggestion please. And just to make sure, resistors do not have an “orientation” in the circuit. There is no “in” and “out” side; it just has to be in the circuit between other components to reduce the current. Right?
Thanks for the education gentlemen. Keep it up and someday I might have enough intelligence to be insulted from time to time.
Robert
it’s a generic picture of a resistor in the link. the same for all resistors
right, resistors are not polarized
A few points. First, Greg has delt with the resistor codes and polarity. The correct code for 560 ohms is green, blue,brown.
One half watt resistors are plenty adequate. I never did understand Tansey’s 1 watt recommendation. The actual dissipation at 12V is about 1/4 watt (12x12/560).
A tortoise takes 2-3 seconds to fully move. Therefore the worst case for your four motors in line would be 12 seconds, but only if all four had to change from their previous positions. That is highly unlikely. In your case The usual operation will be two motors moving; the base main turnout and one other.
I assume you plan to wire the opposite ends of each individual spur in parallel. BTW, if you do you could use two 270 ohm 1 watt resistors per Tortoise pair instead of four 560 ohm 1/2 watt resistors. 270 ohms is the closest standard value to two 560 Rs in parallel.
Each dendrite in Tanseys configuration will draw about 31 mA. at 12V so your 500 mA wall wort should be good for 16 Tortoises. I think you’re OK.
Charlie
I’m late to this discussion, but am interested as I am trying to use Tansey’s Option A myself and wanted to find someone who had actually implemented it. The issue I am having is that if I choose position 1 on the rotary switch, for example, then the first resistor that receives power gets very hot - too hot to touch even. And the same thing happens as I move to other positions on the rotary, so it’s not just one resistor. Since I can’t believe that this is how this setup is supposed to function, I’m wondering if any of you that actually use Tansey’s config could provide me with any guidance as to what I might be doing wrong. Thank you.
Yes, that first resistor has to dissipate ~0.257 watts (12 x 12 / 560), so I would expect it to get pretty hot. 0.257 watts may not sound like much, but consider that the typical through hole resistor that we often use in model railroading application is often only rated at 1/4 or 1/2 watt.
Thanks for the response. I’m no electronics expert, but I am using the 560 ohms 1 watt resistor that Tansey recommends. Should a 1 watt resistor get that hot dissipating .25 watts?
Probably.
It finally occurred to me to get out my multimeter and actually measure the voltage of my power supply. I was just using an old wall wart that stated the output as 12v DC, 400mA. Turns out it was putting out 17 volts. So using your formula: 17 x 17 / 560 is over .5 watts. Once I tried using a PS that actually measure 12v output, I got very little heat on the resistor. Thanks again for your responses.