Constant Current Charger for NCE throttles.

 

 

See also – Procab & Cab04 “On Board” Chargers.

 

Mark Schutzer provides details for a 50 mA “On Board” Charger that charges the batteries whenever the throttle is plugged into the Cab Bus. Select “NCE Articles” in the L/H column at Mark’s web site at:

 

http://markschutzer.com/

 

Modifying a 12 volt “Wall Wart” for an “external” Charger.

 

All types of batteries including NiCd and NiMH rechargeables, are suitable for NCE Radio throttles due to a special on board, step up “Sepic” regulator from Linear Technologies, that supplies the required voltage to the “board” as long as the battery pack can supply the required current.

 

As can be seen, the Procab with 4 AAA = 4.8 volts and the Cab04 with 2 AAA = 2.4 volts, provide different voltages to the Radio board, but both work. For more operating capacity, I have fitted an extra 2 AAA batteries for a total of 4 batteries to a Cab04ER, so the owner could operate two 8 hour operating session on a week end, without having to charge the batteries. He did not have an “on board” charging socket.

 

NiMH batteries have:

1.      more capacity than NiCds

2.     have a lower discharge current rate than NiCds but are suitable for low current applications like this charger.

3.     do not suffer from the memory effect.

4.     are more “environmentally friendly”.

 

Throttle current draws:

·       The Procab with the screen ON = 125 mAs

·       The Procab with the screen OFF = 65mas.

·       The Cab04 = 65mAs.

 

Charging batteries.

Charging NiMH and NiCd batteries, requires a constant current for a prescribed time. Typical charge rates are:

 

 

where C  is the single battery’s Amp Hour capacity.

 

Always check the particular brand of battery’s specification to be sure that they can be charged at the fast rate.

 

With the medium and fast rates, there has to be some for of “disconnecting the batteries” when the batteries are fully charged, other wise they may be damaged that may cause electrolyte leakage and if they are still in the throttles, may damage the throttle.

 

For ease of charging and simple charger design, I have put together a slow charger, that can be left connected to the batteries, in extreme cases, indefinitely, but generally overnight. My own “charging regime” has me just charging my throttles when they are flat or when I remember. Prior to an operating session, I just charge all my throttles. These chargers are simple, so few can be made.

 

I have made a little box that can be powered from the DCC track in case I need an emergency charge to complete a “radio” session. This emergency charger uses a bridge rectifier, a 555 timer, a few other components and a normally open push button switch and a relay that I leave in my “traveling kit”. This provides .5 Amp to allow me to finish an operating session.

 

Mark Schutzer on the Yahoo NCE group has designed a charger built inside the Procab that uses Cab Bus power that will require soldering surface mount components that only modelers with soldering surface mounted components should attempt, not for everyone. Select the Procab.doc at the Files section at NCE Yahoo groups. You have to be a member of the NCE Yahoo group, easy to join, 5 minutes.

 

To build the constant current charger, use a 9 volt DC 1/2 amp power pack, available from Jaycar Electronics part number MP-3007 and attach components to a tag strip attached to the plastic case. You may have to add a heat sink for the LM317T voltage regulator.

 

For more information about charge rates and how to charge NiMH batteries, see Powerstream or Panasonic.

 

According to some NiMH manufacturers, the NiMH batteries should not be trickle charged indefinitely.

 

This is my simple constant charger circuit with only a few components and assembled in a short time. Use the values I have included.

 

 

This is not my design, I have just copied the circuit from some specification “typical” notes that I have seen for 3 Terminal Adjustable Voltage Regulators that the LM317 is.

 

Parts List with Jaycar part numbers

 

 

Values of Current limit Resistor.

 

50mAs = 25.0 ohms          55mAs = 22.73 ohms        60mAs = 20.83 ohms          65mAs = 19.23 ohms         70mAs = 17.86 ohms

75mAs = 16.67 ohms         80mAs = 15.63 ohms         85mAs = 14.71 ohms          90mAs = 13.89 ohms         95mAs = 13.15 ohms

100mAs = 12.5 ohms

 

Use the closest ½ watt resistor. It is not critical to get the .1C value right, close enough is good enough. For 65 mAs, 60 to 70 mAs is ok.

 

Resistors See Colour Code Chart for reading resistor values. For more information on resistors and basic electronics

 

I have added the LED resistor and LED to give an indication of charging current. Illuminated equals charging, extinguished means no connection to batteries.

 

Use 12 ohm resistor to give the 2 .2 volts for the LED. This resistor and LED can be left out for a more simple charger.

 

The two chargers I have made are shown below.

 

 

Left: The cheaper and easier to make unit with the regulator, resistors and LED mounted on a tag strip with a heat sink for the regulator, all secured to a 240/9 volt DC power pack.

Both units have a fuse on the output.

 

Nothing pretty, but practical, easy and cheap $20, to make.

 

 

 

 

 

 

 

 

 

 

 

 

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The more “professional” looking unit. All components bought from Jaycar Electronics including the case that has a 3 pin 240 volt AC plug molded into the base.

 

This unit charges at a constant current of 70 mAs.

 

It does not automatically shut down – charges as long as it is connected.

 

As made my 700 – 900 mAh NiMH rechargeable batteries will be charged in 16 hours (overnight). No damage if left indefinitely.

 

The 22 ohm 1 watt resistor has been “adjusted” by soldering a 100 ohm ½ watt resistor in parallel to give 70 mAs charge rate.

 

To increase charge rate add another parallel resistor to this location.

 

Remove 250 mAs fuse an insert an ammeter to check charging current with unit plugged into a throttle.

 

When I added this I had to add a parallel resistor to the LED “indicator” resistor to reduce the current through the LED from 25 mAs to 17 mAs.

 

The LED resistor (under the LED) is adjusted to give about 20 mAs through LED by placing a resistor in parallel.

 

LED illuminated indicates charging current is flowing. OFF indicates no power to the unit or no connection to the batteries confirming connection to the batteries.

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Procab and Cab04 Charging socket.

 

Fit a 2.5 mm power socket available from Electronics suppliers next to “transmitting LED” in case of throttle. The smallest size socket you can get and drill appropriate hole in case.

 

Connecting Instructions:  Remove original red “positive” wire from “1” and connect to socket “2”, the terminal of the socket that is going to be powered when the plug is inserted into socket. The other terminal “3” is disconnected when plug inserted, but is connected to “2” when plug is removed.

 

Connect wire (can be any colour) from “3” to board “positive” connection at “1”.

 

Remove some insulation from the “black” wire at “5”  where both the original red and black wires run around the mounting screw tube, and solder a wire (any colour) at “5” and connect to “6”. This will be the “Negative” of the charging socket.

 

 

The Constant Current Charger charging circuit for this charger can be easily made by using a LM317, 3 terminal adjustable voltage regulator and can adjust the “constant current” by just changing the value of the resistor.

 

To get the smallest socket to fit in between the circuit boards I used mono (two contacts), phono sub miniature connectors instead of power connectors, due only passing 50 – 100 mAs current. A 2.5 mm mono socket, Jaycar no PS-0105 and the corresponding plug, part no PP-0100.

 

The socket has a switch inside it, that when the plug is inserted it will disconnect the radio board from the batteries when charging, if wired up as per the instructions above.