SimonBa

I probably shouldn't trust the circuit simulator implicitly, but it shows rapidly the varying minute voltages and currents when the button isn't pressed. I'll try not to worry about it :-)

I've just nabbed a copy off a well known auction site.

Hmm - now I'm confused. With an independent 12Vdc supply for the activation of the transistors, suddenly there's some stray tiny voltages and currents ringing around the darlington pair.

Thanks Suzie - just for fun, I decided to simulate the circuit using lushprojects online tool, including a CDU for good measure (I took John's advice and repositioned the point motor in the circuit so the transistor switches the ground of the coil rather than the feed). The graphs are interesting, and reassuring. CDU in the design is based on RK Education's circuit. If anyone fancies having a play, it's here: http://tinyurl.com/y8zxlfp9

Curses! Their higher Ω meant I would be able to drive them with c.2A rather than c.6A (if they are 11Ω rather than 4Ω) which had lots of useful consequences. Better go and buy a job lot before they disappear (I'll need about 70 in total).

Thanks - I was hoping that since the action is momentary and the duty cycle is light I could get away without heatsinks. The control panel PSU will also be driving LEDs route indicators via 12V (or 5V) latching relays taking their coil power from the output side of the diode matrix, so I was considering a separate supply from the CDU charging PSU. Check my math? Assuming 4Ω for the motor coil, that gives 6A via the TIP, so 6mA minimum maximum to the base when hfe >=1000 and if supplied from a 12V supply that means the base resistor ought to be around 1k7 - 2k2 (with the 1.4V drop across the TIP from the CDU factored in). I was planning to use 1k5 1/4W resistors in this case. Figures dependent on motor coil resistance of course, hence the original question.

That's an interesting and useful mod - thanks. Not quite sure what effect moving the transistors has, but I'll give it some thought. Haven't planned to go the Arduino route.

Would it? I plan to fire the points from a diode matrix with routes initiated by push button momentary contact switches, and had assumed (perhaps incorrectly) that once the push button was released the base current would drop to zero and the TIP12x would turn off. ( I do have a multimeter, just no PL-10s or PL-10Ws to hand - the layout is in the planning stages)

Hi all, Would someone do me a favour and measure the coil resistance on the PL-10 and a PL-10W Peco point motors please? I find a number of values quoted around the 'net (and am surprised Peco don't provide specs). Reason for asking is that I'm planning a layout with a complex diode matrix that will need to shift up to 10 points at a time, via a number of CDUs distributed around the place. Rather than attempt to push several dozen amps through subminiature push buttons on the mimic panel, I'm going to use a pair of transistors (actually a pair of darlington pairs, TIP121s) mounted locally to the point motors that will switch in the relevant coil to the local CDU bus when the mimic's route selection button is pressed. The current to the transistors only needs to be a few mA each which will reduce the gauge needed for the wire from the panel to the point motor and allow the use of smaller panel buttons. This approach will also reduce the length of cable run from CDU to each individual motor. The CDU common return buses will be >ahem< somewhat hefty - probably 30A mains wire :-) Without knowing the resistance of the motor coils it's difficult to calculate the values for various things. I'm also trying to decide whether to use 12Vdc or 5Vdc for the mimic panel PSU and obviously that choice also impacts on resistor values etc. Anyone able to provide the data I need, it'd be much appreciated. I've attached the circuit for the small stripboard carrying the transistors and flyback diodes that will go near to each point motor. Cheers Simon