Hello AllI'm in the process of pre-wiring 2 BDL 168's which will be fed from a single PM42. In the PM42 manual there is a note that says "Digitrax does not recommend sharing a power supply between one or more PM42's and any other device on the layout."I have a 12VDC 1.5 amp which is plenty of power for the 3 boards. Is there a reason that it's not recommended or would it be ok to do?Thanks
Springfield PA
I guess it's because a couple of sentences before that the manual says "One power supply can power more than one PM42 as long as it provides 125ma per PM42 and is not used to power any other device. Do not share a power supply between your PM42 and any command station or booster on the layout"
Following that is the original post in Bold. It just sound like an engineer on drugs who can't make up his mind. Typically a power source can be used for multiple devices. The only exception is when a device somehow feeds back through the power. In this day and age it's pretty rare.
I have 2 of the PM-42 that I have had powered by one of the Digitrax 5amp power supplies for about three and a half years with no problems of any kind. I also noticed it had more then enough output do do this. Other than I have not got them properly programed. Though they do work ok. That's an other story though.
Any way my other Digitrax components, UP5, UR91, DB-150, do each have there own transformers. Mostly because it was easier wiring that way.
Ken G Price My N-Scale Layout
Digitrax Super Empire Builder Radio System. South Valley Texas Railroad. SVTRR
N-Scale out west. 1996-1998 or so! UP, SP, Missouri Pacific, C&NW.
The issue is that some things have a full bridge rectifier ont heir inputs and some have just a single diode for half wave rectification. Powering a mix of devices from the same power suppyl withotu carefully checking the connections on the board can result in some very messed up grounds. All like devices can be powered by a common supply - but mixed devices should generally have their own. That's written in there because just using a dedicated power supply is easier and will ALWAYS work. Combining devices to a single power supply can introduce issues if you are not careful and/or don't understand the way the power input circuitry is designed. Hence the recommendations in the manual. There is an analysis of what works with what on the JMRI site, as well as some diagrams on teh circuitry on the Digitrax Yahoo Group.
My PM42 has a dedicated supply, I will be running a bus around the layotu to power my UP5's, tapping off with the proper coax power connectors. The whole daisy chain a wire fromt eh solder holes on the board method shown int he manual makes the loconet cable the ground - every circuit needs to be complete, and 1 wire is not a complete circuit. By pluggin in to the power jack on the side with a power source, the loconet cable is reserved for loconet.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
What Randy and David B. said
After all, if you're talking about PM42's and BDL168's, it obviously means you're investing in your DCC system. Why cause yourself headaches (and possible equipment damage) by being cheap with power supplies instead of listening to the manufacturer's recommendations?
Besides, good power supplies really aren't all that expensive. For example, I'm using the 12v model of one of these to power my DS64's. It may cost a couple dollars more than the handful of wall warts it would take to replace it, but for only a couple dollars more I'm getting a high-quality power supply with a clean output and a 3-year warranty.
Edit: Fixed spelling!
I highly recommend following Digitrax Suggestion on having separate power supplies for each. We installed 5 PM 42's at our club layout powered by two Power supplies (two on one, three on another). We were using a digitrax PS 515 5 amp power brick which is AC.
We fried two of the PM 42's (they were the only ones on that power supply). The bridge rectifiers literally lit on fire. What had happened was that they got wired out of phase and it caused them to smoke because of a ground loop issue. I replaced the bridge rectifiers and placed them on separate power supplies with a 1/4 amp circuit breaker to prevent further damage.
The other three PM 42's were fine, however, we did convert them to all having separate power supplies just to be on the safe side.
Just thought I would add my since we did have an issue with this.
Colorado Front Range Railroad: http://www.coloradofrontrangerr.com/
Thanks for the replies.
Since I'm using a DC supply to start I'll go ahead and use 1 for the 3 boards since rectifier configurations won't matter. (Voltage is already rectified)
I can see where poorly engineered boards could have a problem with some half wave rectified and some full wave. The half wave units would only draw power from half of the AC source which would totally throw the full wave boards out of balance, but probably only if drawing near the AC sources max power. If the club supplies AC on the future locations I'll be sure to stay away from sharing though.
I had thought about running an AC bus and putting bridge rectifiers at every drop to get DC< but you still have to be VERY careful to wire them all in phase. Probably not worth the risk.
Good Point. I wouldn't risk it with AC after reading about different rectifiers in the boards. The forums have saved us several bucks so far, including the BDL4 issue, transponders and now this.
Thanks again.
PM42's have half wave rectifiers. BDL168's have full wave rectifiers. Don't run both off of the same supply. You can run multiple PM42's with the same supply. (my club has 7 on one DC supply). Run multiple BDL168's off of another supply. Just make sure the supply is capable of powering the sum of all units connected to it. If you use an AC power supply, phase must be the same on all. Easier to use a DC supply.
Martin Myers
I believe that's the other way around - or at least the PM42 is full wave. The very first component in the lower left (with the edge connector facing you) is a bridge rectifier. BDL168 is full wave as well - power input is int he middle of the card edge connector and rigth between the second and third groups of detection bridges is a bridge rectifier, just below the voltage regulator.
I ran a single PM42 with a single BDL for a half hour off of the same DC supply. The rectifiers were cool after the half hour so all looks good. I'll add the second BDL when it gets in but I don't expect a problem.
If the PM42 only has a half wave rectifier I would expect it to only work with one polarity when using a DC supply. This would confirm what randy noted that it must be at least a full wave.
I finally found the page Bob Jacobsen posted on the JMRI site with his power analysis:
http://jmri.sourceforge.net/help/en/html/hardware/loconet/DigitraxPower/
The old DS54 is half wave, The SE8C might be - I don't have one or have a good enough photo of one to look. My observation on the PM42 is made by looking at a physical board - it clearly has a full wave rectifier component with the AC pins connected to the pins on the edge card where you attach the external power supply.
Thanks for the link
You are so right, Randy. My PM42 does have a full wave rectifier. It is the old DS54 that has half wave.