Block Detector Description

Refer to the Block Detector Schematic

DCC power to the track is an AC signal of 10 to 16 volts. DCC transmits binary information on this AC "carrier" by varying the width of the positive and negative peaks of the wave. See DCC Basics for more information. This block detector design has limited drive current capacity and should be connected to high impedance input devices such as logic gates or op amps. This design is based a circuit by Rob Paisley

Each block of track to be detected must be electrically isolated by appropriate gaps in the rails of the track. Two 1N5400 diodes are connected in series with one leg of the DCC power to a block of track. Whenever a locomotive or car draws power from the track within the block, an AC voltage will be present across these diodes. The 1N5400 diodes have a 3 amp current rating and should be adequate for HO and N scale multiple locomotives trains. Higher current diodes should be used for larger scales. More on current and short circuit protection below.

The secondary 8 ohm winding of the audio transformer is connected across the 1N5400 diodes. The voltage across the diodes is stepped up to a useable level by the primary winding of the transformer. The actual voltage present in the primary transformer winding depends on the total current being drawn within the block.

The remaining components of the circuit are connected to form a voltage doubling power supply. The 1N4148 diodes and transformer center tap provide half-wave rectification of the transformer primary voltage. Each capacitor is charged to the primary voltage on alternate half cycles. The voltage across both capacitors is double the primary transformer voltage.

The capacitor values and load resistor effect the response time of the block detector. The larger the capacitors, the further into the block the train will be before its presence is reported. The capacitors also minimize signal transients as the train enters or leaves the block as well as transients causes by dirty track and wheels. The resistor serves to discharge the capacitors once the block is empty. The value determines the length of the discharge period. This value should be large enough to minimize signal transients.

The 1000 ohm resistor and 5.1 volt zener diode form a voltage divider and are included for protection of the inputs to the logic circuits connected to the detector. These components ensure that the detector output voltage does not rise above the zener diode voltage rating. Without these components, a large current draw or short circuit within the block will cause the output voltage of the detector to rise well above normal logic levels possibly damaging the attached input circuits.

The following table summarizes the detector DC output voltage under various loads. The voltages were measured across the two capacitors and vary depending on train speed, length, track grade, lighting and the number of locomotives/cars in the block.

Load Description Measured Voltage Stopped Measured Voltage Moving
10,000 ohm resistor across the track 1.3  
Car with two 10,000 ohm resistor wheels (5,000 effective) 2.75  
Single HO scale P2000 GP30 locomotive 3.20 5.65
Two HO scale Bachmann Spectrum series F7 locomotives 4.75 6.10
Single HO scale Athearn Genesis USRA 4-6-2 Light 3.75 5.90
All four locomotives in same block 6.00  

The input voltage necessary to reliably register as a "high" condition varies based on the IC family being used. At a supply voltage VCC of 5 volts, the "74LS" family requires a minimum 2 volt level. The "74HC" family, at the same 5 volt VCC, requires a minimum 3.25 volt level. Select the resistor wheel value depending on the input level requirement and the number of wheels on the car.

Short Circuits and Total Block Current: Most DCC power boosters are capable of delivering more than the 3 amp current rating of the 1N5400 diode. Overload circuits in the DCC power booster will normally shut down track power in a short circuit condition and protect the diodes. Short term spikes above the maximum diode current limit can probably be tolerated. Long term operation at or above maximum current levels will cause diode heating and eventual failure. If a large number of locomotives under heavy load conditions are expected to be simultaneously operated in a block, measurement of the total current draw under worst case conditions should be made. Diodes with appropriate current ratings can then be substituted, or additional diodes wired in parallel, to increase current capacity.

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Copyright © 2006 Don Buczynski
San Diego, California