SimonBa

3 hours ago, Suzie said:

I don't think this is a problem that you need to worry about! It will only happen while the button is pressed and those currents are pretty small and hard to measure.

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 :-)

5 hours ago, Sol said:

You could always refer to  Model Railroad Electronics  by Peter J Thorne for a few circuits

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

3 hours ago, SimonBa said:

http://tinyurl.com/y8zxlfp9

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.

8 hours ago, Suzie said:

You won't need heatsinks.

 

Use 24V latching relays, they are readily available. It keeps things simple - one less PSU to have to worry about. Nowadays I usually use a single 12V PSU with an adjustable DC-DC step up to give me 24V, and I can wind up the voltage higher if I need it (sometimes need 35V with dodgy wiring and sticky points).

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

1 hour ago, MAURICE040947 said:

So in order to simplify the number of motors available we have decided to drop the PL-10W and PL-10WE from the range permanently. "

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).

2 hours ago, Suzie said:

PL10 standard should be 4 Ohms giving 6A current at 24V. The TIP120, 121, 122 are 5A continuous with a good heatsink, but should be OK I guess for a short 6A pulse - I have used them to drive a matrix directly with about 30A on a good day with a short pulse and no heatsink.

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.

6 hours ago, John ks said:

If you provide local CDU feed & return busses then the local return & feed busses only need to carry the firing current to the points on that bus &

Then the Main return & feed busses only need to carry the charging current to the CDU's

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.

5 hours ago, kevinlms said:

A Darlington Pair, would continue to conduct if switched on.

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