Digitrax Booster Current Capability

greenspun.com : LUSENET : Wiring for DCC : One Thread

I have an "8 amp Chief" that appears to fall quite short of 8 amps. It trips off at just under 7 amps RMS input at 18 VAC from a 10 amp Loy's Toys transformer. To me, this implies that the output current at the trip point is under 6 amps. The problem is that typical heavy large scale trains can draw well more than half of the booster capability so just adding more power districts won't do much good as two such trains can easily be expected to migrate into any one power district.

If the booster REALLY did 8 amps, I would be able to get by most of the time. Can anybody tell me if an 8 amp Chief really makes a full 8 amps all the time? I know about the output voltage trim. Is there an adjustment for trip current?

- gws

-- George Schreyer (gwschreyer@usa.net), September 29, 1999



It is more then likely a thermal problem then that of the actual current draw. Yes I know they are related, but try putting a fan on the DCS100. Next is what do you have the track voltage set to, HO or large scale, reason I ask is you also need to take into account the voltage drop across the booster, that is from the AC input to the actual digital voltage, the bi-polar DC on the output. Keeping in mind that with an AC input voltage, the actual voltage on the boosters DC input is as in your case 18vac time, 1.4 = 25.2 - 1.4 = 23.8 volts DC at the boosters DC input stage. Or more properly stated on the boosters filter caps. Take this voltage and subtract the actual digital track voltage to find the drop across the booster. Now take this voltage drop, and multiply by the amps being used and you have an idea of how many watts of heat need to be handled by the DCS100 heat sinks. As ex: 23.8 - 12.6 typical HO setting = 11.2 volts dropped times 5 amps = 56 watts. Now just think of the heat of a typical 60 watt lightbulb. At 8 amps it becomes 89.6 watts. Thus the reason for the need to have air moving across the heatsink, otherwise thermal shut down befor max amperage output.Remember Always Have Fun and Enjoy!, Don Crano Akron, Oh NMRA #096211 mailto:donc@sssnet.com Visit Model Railroading with DCC at: http://pages.sssnet.com/donc/

-- Don Crano (donc@sssnet.com), September 29, 1999.

Don thanks, however it is set to large scale AND the output voltage has been boosted to 23 volts rms to minimize the booster dissipation AND it already has a fan blowing right on the heat sink. Even at extended operation right below the trip point, it hardly gets warm.

I have estimated the output trip current at somewhere between 5.5 and 6 amps.

-- George Schreyer (gwschreyer@usa.net), September 29, 1999.

Booster trip current is relative easy to measure. 1) Use a small full wave bridge rectifier (4 diodes) and a resistor load on the DC side (say 4.7K ohms) and a dc voltmeter to measure/verify the DCC booster track voltage output. (Add 1.2 volt dc to the reading to make up for 2 diode drops.) 2) Temporarily clip an adjustable power resistor across the track to draw power. Start with 4 ohms, (~4 amps for 16 V at G scale). Keep lowering the ohms value by 0.5 ohms till the DCC booster trips out. 3) Calculate trip current as I=V/R.

This calculated trip value will/should be quite repeatable. If it is not near the rated 8 amps, contact Digitrax for advice. One or more of your locos may be drawing high peak currents, causing it to momentarily exceed the trip limit. This may not show up with an average or rms reading ammeter. Raising the pwm modulation frequency of the decoder may help, as would putting a resistor in series with the motor leads.

-- Don Vollrath (dvollrath@magnetek.com), October 04, 1999.

I ended up doing it exactly this way. I found that the booster really does 8 amps at trip, but it sags to 16 volts (2 ohm load). I assume that this is the way that they really work. They are rated at 8 amps, but not usuable at 8 amps. The 8 amp Chief makes a good 5 amp system.

I'll have to find another way to attach a better booster to the Chief.

- gws

-- George Schreyer (gwschreyer@usa.net), October 04, 1999.

When you measured the voltage sag, did you happen to measure the input voltage at the time?

Remember Always Have Fun and Enjoy!, Don Crano Akron, Oh NMRA #096211 mailto:donc@sssnet.com Visit Model Railroading with DCC at: http://pages.sssnet.com/donc/

-- Don Crano (donc@sssnet.com), October 04, 1999.

I did measure the input voltage, it dropped 0.66 volts (Loy's Toy's 10 amp transformer) so that this wasn't the major cause. I've elected to investigate getting a real 200+ VA booster to use following the Chief.

- gws

-- George Schreyer (gwschreyer@usa.net), October 19, 1999.

Moderation questions? read the FAQ