Above: Brian Millar's Water Flow Diagram.
FREDs (Flashing Red End Devices) are battery powered lamps fixed to the rear of a train as a safety device. Regulations for their use vary from state to state and the lights may be turned off in the daytime. The circuit below provides a mens of recharging the FRED battery while models are running on powered rails.
Using Voltage Regulators: It is important to note that Voltage regulators come in many different ratings (Volts IN and Volts OUT as well as amperage) and care should be taken to ensure you have chosen the correct regulator for the job.
On the wiring diagram you'll notice the resistor marked 'R', you can simply use a resistor in this spot and set a pre determined voltage by way of the formula given (required voltage output x 96 -120 =? ohms.
An example of how this formula works is thus:- lets say you require a 3 volt output 3x96=288-120=168 ohms so the resistor you require is 168 ohms or as close as you can get to that number.
You can again use the formula and instead of a resistor use a potentiometer, either a trim pot or a larger potentiometer, so you can then adjust the voltage output, remember the rating on the pot must cover the required resistance, if you use a pot who's rating is too much then you won't get much adjustment between off and full on.
Also remember voltage regulators are polarity sensitive so make sure the voltage IN is as per diagram. If you are using this circuit where the voltage can be reversed eg: in a loco Forward/Reverse then you will need to put in a bridge rectifier and attach this circuit to the positive output leg of the bridge rectifier, keeping in mind that this circuit has a 2.5 volt loss and a bridge rectifier also has a 2.5 volt loss, which will mean you will overall have a 5 volt loss, so for example if you had to begin with a 12 volt input via a bridge rectifier the most output from this circuit would be 7 volts.
The easiest way to understand how this circuit works is to imagine the voltage IN put and voltage output as being a water pipe with a tee piece in the middle and a tap fitted to the third or bottom outlet of the tee piece, if you were to open the tap, then the water instead of going out the normal outlet would also run out the tap depending how far you opened the tap which means the amount of water coming out of the tap reduces the amount of water coming out of the normal outlet, so if you open the tap fully then no water would be coming out of the normal outlet. (Hope I haven't lost you all).
[At this point, see the animated Water Flow Diagram at the top of this page.]
Now the circuit is a adjustable circuit which you can adjust at random or preset, however there is another even more simple circuit using a different type of voltage regulator and you don't need the capacitors etc.
These type of regulators are normally identified by having a number 78?? for example 7805 this code means positive (78) 05 means 5 volts, another example 7808 = positive / 8 volts or 7812 = positive 12 volts the second 2 digits mean what voltage the output is, the best thing with using these type of regulators is you don't need the circuit in the diagram, you simply hook them up the same as the regulator in the diagram except without the other caps and resistors etc. however if say you have a 7805 but wanted to have say a 6 volt output by adding a 1 amp diode on the base leg (middle leg) this will increase the output by .6 volt or if you add 2 diodes then you would get 1.2 volt increase, you can also limit the output by adding a diode on the output leg will decrease the output by .6 volt.
Note: By putting diodes on the output leg will also create a type pulse voltage, which makes this a handy little set-up for charging some types of batteries.
Types of Regulators: There are 2 main types of voltage regulators.
The regulator I have depicted in the diagram is the flat pack the other one in the photo is the T03 case type which are more expensive but more robust as well.
You may also have noticed in the photo a small plug these plugs are great for hooking up flat pack regulators as well as any other circuitry you may have and saves on soldering. If you use a T03 type then you will need to check which is the base from memory it is the case itself, they only have 2 legs on them and the case serves as another leg. If you see a voltage regulator with 79 on the code that means it is meant for negative circuit (I have never used them and you'd need to find out from someone who knows what circuit is required for them.
Using Power Transistors: If you need to build a variable speed controller which requires the need to handle a greater amperage than 3 amps. output, then you can achieve this by using a power transistor and drive it with the adjustable voltage regulator circuit.
Power Transistors have different ratings so you will need to talk to someone who can work out which one you'll need. In my controller I use a MJ15003 power transistor and drive it with a LM317 (both are in T03 cases) my output is 15volts/15amps.
last updated: 19/07/08.