Large Scale Central

Should My MRC 6200's Power be this Dirty?

Electronics Power and Sound
#1

I was working with the Sierra Supercap disconnect and got everything working perfectly, on battery. When I tried it with the MRC 6200, it just hummed. Eventually, it “caught” and worked OK.

So I was checking the d/c tap and on the a/c scale on my multimeter, I get a bit over 9 volts a/c (1/2 the a/c tap voltage) on the d/c tap when all the way up. The a/c on the d/c tap climbs with the d/c voltage. OK, probably a bad rectifier.

So I pulled out the other MRC6200 and got the same results. Both units seem to work fine and certainly power the trains. They have always made lousy test units for my IC projects due to spurious ???

If there anyone with an MRC6200 that can confirm this a/c on the d/c tap?

Thanks

#2

Todd, mine powers a pair of 12 volt (DC) relays, and I never had an issue with it. But all that stuff is now locked up for the winter. Maybe you just need to invest in some BFCs

#3

They power relays, motors, etc., just fine. It is the delicate electronic stuff (e.g., 555 chip circuits) that always seems to get “confused” and need a ton of filtering. These are only used on my test bench. If stuff works with these, it works with anything.

The railroad itself uses three 12.5 amp, 24-volt Meanwells through three Train Engineers.

#4

I decided to make one MRC into a “lab standard” unit.

I enlarged an existing vent hole and put a 4pdt on the dc output that selects between the usual output and a rectified/filtered output (3,300 mfd @ 50 volts with a “bleed down” resistor). The cap actually develops ~30 volts.

As an added bonus, when I use the lab standard mode, I don’t have to worry about the position of/accidently moving the “Direction” switch on the front panel and ruining my test or burning out my project.

#5

I gave mine away, it is a cheap unit, and actually, unfiltered, rectified AC gives a bit of “pulse power” effect that actually can help locos to start more easily.

(remember the original “pulse power” that was basically half wave ac, just the positive half of the AC signal?)

They are notorious for popping output transistors too (I am pretty sure they were transistors).

See Dave Bodnar’s excellent writeup:

http://www.trainelectronics.com/MRC_6200/

Greg

#6

OK, I found a way to clean up the MRC without generating heat or wasting power.

I used a 4pdt to select between the normal output and the normal output that has been put through a bridge rectifier. This takes 2 poles. The third pole closes the connection between the bridge and the 3,300 mfd, 50 volt cap. The cap has two 4,200 ohm, 1/2 watt resistors in parallel (2,100 ohm 1 watt) tied across the cap so it will bleed down when not under a load. If we didn’t disconnect the cap/resistor from the bridge, it would put a small, continual, unnecessary load on the system when used in the original manner.

Because the the cap will develop 30 volts on the output, (and who wants to accidently fry their delicate electronics?), I felt it necessary to limit the output voltage when used with the cap. Experiements with various “draw down” resistors showed they consume too many watts, and gets too hot. The resistor needs to be about 12 ohms or less and will consume most of the power and/or the cap has to be much smaller limiting its filtering ability. Furthermore, as the resistor becomes effective in reducing the voltage, it looses its effectivness in filtering the a/c component.

There had to be another way.

It occured to me that all I needed to do was to use the fourth pole on the switch to introduce a series resistor between the the speed pot and the power FETs. This then serves as a “speed control” when selecting the filtered d/c. And when I put a load on the system, I’m not getting the a/c component. I used a value of 39,000 ohms and the max voltage is now a safe 20 volts at full throttle. And, the “current indicator light” is not nearly as bright at full throttle as when the filtered supply is not selected, so I’m not unduly straining the system.