The 751D and 751K ELECTRONIC TURNOUT SWITCHES

updated February 2023

This space saving circuit is unbeatable for performance and fascia panel design. Read on and see why thousands of these are in use!

A simple circuit, the 751D was developed in answer to numerous requests to from modelers who wanted to switch turnout point position with a momentary pulse and also have the ability to enable constant state display of that turnout's position. The 751D answers that calling for twin coil (Atlas, PECO, etc) type switch machines. As well, the heavy duty MOSFET transistors incorporated in the design allow multiple turnouts to be switched simultaneously making the 751D a "no brainer" choice for crossover switching. 

Later on, the 751K was developed for use with single coil type turnouts (Kato, Rokuhan). The 751K is simply a momentary "H bridge" design that once again utilizes heavy duty MOSFETs in its output stage.

The optional add-on 12" wiring harness simplifies connections

Connect 12VDC power, turnout coils, and LEDs simply. A time saver when it comes to installation. Only $1.20 each!

FAQ's

Q - Can I use the 'AC ACCESSORY' terminals on my model railroad power pack to provide power to the 751 switches?

A - ABSOLUTELY NOT! The 'AC ACCESSORY' connections supply AC voltage. The 751's require DC. If you supply AC to the 751 switches they will cause irreparable damage to your turnout coils. There you go..... you've been warned, discussion over.

Q - What are the power requirements for the 751 series switches?

A - A clean source of 12VDC able to supply current in excess of 2 amps will do the trick for most layouts. Consider that any LEDs that are being used as directional indicators will draw between 8ma to 12ma each. The majority of current draw happens during the first part of the switching pulse and for less than about 1/10 of a second. This is referred to as 'inrush current'. The one exception to the 12V guideline is PECO. These coils like to see between 16VDC to 18VDC, so your DC supply should be capable of delivering this voltage.

Q - Can I use a switched mode power supply (commonly referred to as 'SMPS') to provide power for 751 series switches?

A - Yes and no. If you want to use SMPS type supplies they should be capable of at least 5 amps and should be supplemented by adding several thousand microfarads of bulking storage (at least 20,000uF). SMPS units have a tendency to go nuts if you ask for too much current all at once, which is the case for turnout switching. You may find that when using SMPS with a plurality of turnout coils one switching action may cause LEDs to flicker and coils to buzz in an 'out of control' fashion. If at all possible, stick to a well filtered, better yet regulated, conventional low tech linear power supply. Here's a link to one I recommend at Parts Express in the USA - https://www.parts-express.com/pyramid-ps3kx-138-v-25a-dc-power-supply--120-530

Q - What is the dimensional data for the 751 series?

A - The 751 printed circuit board measures 1.5" X 1". With the toggle switch mounted the behind panel depth requirement is 2.25" minimum to allow for wiring. The toggle switch mounting bushing requires 1/4" hole for attachment to fascia or control panels. 3mm LEDs will need (no surprise here) 3mm holes (that translates to 1/8" for us 'old timers')

 

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