|BD8 Power Supply|
|The original BD8’s have been designed so that they need no external power supply. But since there is some electronics on the BD8, some sort of supply is required and the BD8 takes its supply from both the LocoNet and the rail power. In reality, most of the BD8 power supply is coming from the LocoNet sync lines, as the BD8 has to be (at least partially) powered, when rail power is down.
However, this power supply scheme is not really suited to a good operation of the Digitrax system, since adding BD8’s collapses the LocoNet sync signal to an unacceptable level. Although there is no defined limit as to the number of BD8’s that can be connected to the LocoNet, a reasonable number is considered to be around 4-5 BD8’s. Adding more BD8’s may collapse the LocoNet sync signal to such a low level that the throttles won’t work anymore.
I have made some trials to power my BD8’s externally, using the existing power circuitry, but with no success.
Since I own a good number of these devices and didn’t want to loose my investment, I decided to look at a better way to power the BD8’s, that would be independent of the LocoNet and the rail power. I finally came up with the modification described below which allows to power the BD8’s from an external AC source and also add some interesting features to the BD8 operation.
This modification requires a rather extensive modification of the BD8 and also making a small PCB for the new supply. So, this modification is not for the faint-hearted.
The diagram at left (click on it for an enlarged view) shows the original BD8 power supply scheme. As can be seen, most of the power is coming from the LocoNet sync signals. Also, when rail power is off, the LM339N modules (the detectors) are powered with a low level voltage not allowing the detectors to operate properly. This is the reason why when rail power is shut off (either by switching power off, or due to a short circuit on the layout), all track sections controlled by BD8’s appear as occupied. When rail power is on, then the LM339N modules are properly supplied (about 7.5 volts) and actual track occupancy is reported.
Also, the design is based on some amount of current flowing between the BD8 ground and the LocoNet ground, thus creating a ground loop and a significant voltage drop (about 3 volts) through R14.
The new power supply scheme is shown at right (click for an enlarged view). It is a basic power supply scheme working from an external 12VAC power supply and comprised of a diode bridge, two voltage regulators and the required capacitors, for decoupling. You may notice that the LM339N detectors modules are now supplied by a 12V voltage (instead of 7.5V previously). This is not a problem as these modules can be supplied with voltages up to 36V without any performance change, and allows to use a more common 12V voltage regulator.
As this new scheme eliminates the ground current between the BD8 ground and the LocoNet ground, some levels are now shifted in the LocoNet interface and require some minor modification of the LocoNet interface, for proper operation (not shown on the schematics).
The new power supply scheme offer the following advantages over the original one :
|Bill of Materials
Parts for the new PCB :
Replacement parts for the BD8 :
Note : the references for the replacement parts are the references shown on the BD8 board.
All these parts should be obtained for around 5-6 $ per BD8 to be modified.
The implementation consists in removing a number of components from the BD8, replacing some other components, building the new power supply PCB and connecting it to the BD8.
We’ll look at this modification step-by-step :
Step 1 : Removing components
The figure below shows all the components to be removed / replaced. The best way to remove these components is to use solder wick, as described in the box below :
|After removing all the components described above, the BD8 card should look like what’s shown below :|
|Step 2 : Adding new components
After removing all components required from the BD8, it’s now time to add a few components (resistors) that will replace some of the original components. These components are shown by red arrows in the above picture :
That’s all there is to do on the BD8 card, for the time being. The next step is to build the small PCB which will be later installed on top of the BD8.
|Step 3 : The new PCB
The figure at right shows the small PCB (bottom view/solder side) that should be made to accomodate the new components for the Power Supply. This PCB is 1.1 x 1.9 inches. It is a single sided PCB that can be easily made at home.
Caution : These images are not to scale. If you are interested by this PCB, I can provide a 1:1 scale image file, upon request.
The figure at left shows the assembled PCB (top view/component side). Refer to the above parts list for components to be installed on this PCB and make sure that all electrolytic capacitor have their positive side as indicated on the drawing. The two voltage regulators have their 3 leads soldered to the PCB and their upper tab attached to the PCB using brass srews/washers/nuts. The four “heavy” holes indicates the places from where this PCB will be connected to the BD8, in corresponding holes, just below.
In order to prepare connecting this PCB to the BD8, do the following :
The new PCB, shown assembled at right, is now ready to be installed on the BD8.
|Step 4 : Installing the new PCB on the BD8
Insert the four wires protruding from the bottom of the small PCB in the corresponding holes of the BD8, indicated by blue circles in the figure at left.The two wires from capacitors C2 and C3 (negative leads) goes in the holes marked as “G”, while the +12V and +5V wires goes to the corresponding holes. Attach the small PCB to the BD8 using a screw/washer/nut and a 3/8 in. spacer between the two boards at the mounting hole location and solder the four wires at the bottom of the BD8. The figure at right shows a side view of the final assembly. You can see the spacer used for mounting the PCB, as well as the wires going from the top PCB to the bottom PCB.
The resulting BD8, with its new power supply, is shown in the picture below, along with its connections to the Power supply.
You are now ready to use your BD8 by connecting the 12VAC supply and the Booster ground, as indicated and enjoy…
Note that the Rail B connection is not used anymore and does not need to be connected.
|For sure, this modification will void the BD8 warranty. But who still has a BD8 under warranty, nowadays ?|