Discussion on booster heat sinking due booster cutting out after 5 minutes.
This refers to a Digitrax DCS100, but can apply to all boosters when operating near or just below their rated capacity. One fix recommended by Don Crano is to reduce the A.C. voltage input that will reduce the power dissipated by the booster. Read on.........
Bill Roger 23 Dec 05 on the Digitrax Yahoo group.
When I run trains they run fine, for about 5 minutes, but then the DCS100 will shut down like it's a short. The trains were running fine up to this point. At the time I was running 5 F7 units all powered 2 with sound, and two powered P2K's. And I have to reset the DCS100 back to the N scale voltage setting before I can get it to attempt to go back online, but of course I get the beeps and it shuts down again. The Ramp meter never exceeded 3.5 amps. So what causes the thing to shut down? This happens all the time now. Anyone got any suggestions as to what I should check?
Don Crano replies:
Reading over this thread, it does appear your DCS is having problems with thermal shut down. I read that you are getting a reading of approx. 3.5 Amps when it happens after approx. 5 minutes, assuming this is 3.5 Amps average and not a peak reading.
One question that does not appear to have been asked, what are you using for a power supply for this DCS100, is it AC or DC and what is the voltage of this supply, actual not rated?
This is important when we are trying to find how much heat the booster heat sinks are having to dissipate.
Here is why:
Digitrax boosters will handle AC input from 12Vrms to 20Vrms. Or DC from 12.6Vdc to 28Vdc.
But what you want, that is the real question. How much heat the booster has to dissipate is related to the voltage drop across it, time the current being drawn. And this voltage drop may not be what one really thinks it is.
Keep in mind, Power is the energy dissipated in an electrical or electronic circuit or component. Or electrical energy to do 'work'. In this case Power defined as Wattage, P=ExI or simply Wattage equals Voltage [E] times Current [I], which equals heat over time.
As example using a 18Vrms transformer, and assume your HO track voltage is typically 15V. Keep in mind boosters will work with either AC or DC input power, and you might note the difference in max voltage input, 20Vrms or 28Vdc, here is why and what the actual input voltage is.
18Vrms, times 1.414 = 25.45V peak minus the diode voltage drop of the internal diode bridge of the booster would be 25.45 - 1.4V = 24.05Vdc on the internal filter caps of the booster. OK now we assume the track voltage is 14V so 24.05V - 14V = 10.05 volts dropped across the booster. This would be 10.05V times 5 Amps = 50.25 Watts of heat on the boosters heat sink. If you were to use the N scale track voltage of 12V, it would be 24.05 - 12 = 12.05V times 5 Amps = 60.25 Watts.
Where if you were using say 14Vrms input this would be 14 * 1.414 = 19.80 -
1.4 = 18.40 - 14 = 4.4 * 5 = 22 Watts of heat on the heat sink. If N scale then 18.40 - 12 = 6.4V time 5 Amps = 32 Watts.
So remember with AC you need to calc the peak voltage, then subtract the diode voltage drop to get the true voltage on the boosters internal filter caps. With DC input, you only need to subtract the internal diode voltage drop from the DC input. And you really want to keep the voltage on the internal filter caps just 2 to 4 volts above track voltage to keep the heat down. But of course how much current the booster has going through it also effect the total heat as well. It is the voltage across the booster times the current through it that will set how much Wattage will have to be dissipated as heat. Also keep in mind an heat sink works best with airflow across it. So the more heat it needs to dissipate, the more air that may be required for it to do it. So placing a fan blowing on the heat sink works wonders here as well.
So it is not to hard to see that the input voltage should be selected for the track voltage to be used, other wise there is just more heat that the booster's heat sink has to dissipate. A good rule to use to keep heat down to the minimum is 12.6Vrms ac to 14Vrms ac for N scale Track voltage, 14Vrms ac to 16Vrms ac for HO track voltage, and 18Vrms ac to 20Vrms ac for large scale track voltage. Again for a filtered DC supply simply add the 1.4V diode voltage drop and add a couple volts over selected track voltage to provide booster regulation stability, something like 14.0Vdc for N scale, 18.0Vdc for HO, and 23.0Vdc for large scale track voltages. Note Digitrax nominal track voltages are: 12.4V for N scale, 15V for HO, and 20V for large scale. The actual scale is not what determines the track voltage, but what the Scale Switch is set to does. Or simply it is not all that uncommon to run an HO layout on N scale track voltage for better slow speed performance, etc.
Jim Albanowski replied.
Isn't the heatsink a bit too small to dissipate 60 watts? I didn't go look up my charts the area required but from my
tinkering with audio I would be looking at something 2 to 3 times the area and mass of the heatsink Digitrax uses. I see where a lot of folks have added fans I would think that should be a "recommended practice" for anyone running any reasonable number of engines... 2 to 3 amps worth.
Don Crano replied:
I would agree, it would appear to be a little on the short side for dissipation of around 60 Watts. But then again the sizing of heatsinks is almost an art form in it's self. The actual size is only part of the formula, always a larger physical size will mean more dissipation, but at added cost and may or may not be the best for the application at hand. The actual selection of a properly sized heatsink is based on multiple factors such as thermal resistance and degrees C / Watt, keeping in mind that the whole process of removing heat from a semiconductors active area, the die or junction, involves many separate thermal transfers, both internal and external.
We start with the actual semiconductor it's self, if the thermal resistance of the active devices can be reduced, besides increasing reliability, we also reduce the cost/size of the required heatsink. Next we have the semiconductor case to heatsink thermal resistance, proper mounting, compounding, and if required insulation. If these two thermal resistances can be reduced enough, the required heatsink can be quite a bit smaller than would otherwise be required for the same semiconductor operating temps. The last but not least is the actual mounting of the heat sink to case, both position, that is vertical fins, and thermal resistance to case. Here we have vertical fins to aid in the conduction and/or convection of heat, radiation also enters here, that is a matte black anodized aluminum. Again all to reduce thermal resistance. For conduction and convection to be effective, we need to take into account ambient temperature, and assure a constant air flow, thus the vertical fin mounting, if a constant air flow can not be assured, here a fan works well, ensuring that there is always plenty of cooler air at the surface of the heatsink. It also generates turbulence to aid convection as well.
Now back to our boosters. I doubt if the heat sink was ever designed to
handle a continuous 60 Watts. Remember the heat on the heatsinks are based
on the Voltage across the booster, that is the difference between the input Voltage and the output Voltage, times the current through the booster. Thus the only time there would be say 60 Watts of heat on the heatsinks would be when the input Voltage is higher then it should be. Or as noted in prior message, if we have a 10.05 volt dropped across the booster, as would be with an 18Vrms transformer and a 14V track Voltage, for ref. 18Vrms, times
1.414 = 25.45V peak minus the diode voltage drop of the internal diode bridge of the booster would be 25.45 - 1.4V = 24.05Vdc on the internal filter caps of the booster. This would be 10.05V times 5 Amps = 50.25 Watts of heat on the boosters heat sink. With Nscale track Voltage of 12V, this is now a drop across the booster of 12.05, it would be 24.05 - 12 = 12.05V times 5 Amps = 60.25 Watts.
By simply dropping the input Voltage by using say a 14Vrms transformer, we now reduce the Wattage to 22 for HO track voltage [14v] or 32 Watts for Nscale track voltage 12V. Note all rating are based on the full 5 Amps available from the booster/power supply.
A good rule to use to keep heat down to the minimum is 12.6Vrms ac to 14Vrms ac for Nscale Track voltage, 14Vrms ac to 16Vrms ac for HO track voltage, and 18Vrms ac to 20Vrms ac for large scale track voltage. Again for a filtered DC supply simply add the 1.4V diode voltage drop and add a couple volts over selected track voltage to provide booster regulation stability, something like 14.0Vdc for Nscale, 18.0Vdc for HO, and 23.0Vdc for large scale track voltages. Note Digitrax nominal track voltages are: 12.4V for Nscale, 15V for HO, and 20V for large scale. The actual scale is not what determines the track voltage, but what the Scale Switch is set to does. Or simply it is not all that uncommon to run an HO layout on Nscale track voltage for better slow speed performance, etc.
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Jim replied again:
Exactly, it isn't easy... here is a site I use for audio projects;
I guess what I was fishing for was since we seem to see this voltage/amperage/heat issue a lot here it simple just to say if "you're running 3 amps continuous or more from your 5 amp booster, stick a fan on it"? If your powering your booster with higher than the minimum recommended input voltage expect lower than the maximum current output.
We answer a lot of very basic questions and try to help and I can't help notice that the same issues come up on a fairly constant basis.
I'm sure it works like this... I bought it, I have questions, I found the group I'll ask... No harm there, but.
Since there are so many postings here it's quite understandable that folks who are new or have never seen this problem before will ask rather than spend a large of time reasearching, they could really use a FAQ file that would include this and other topics. There are enough folks out there with real expertise that could vet a proccedure to be included in the FAQ file and might answer a lot of the basic questions quickly.
Joy this year has been helping folks with simple problems that were huge to them, fixing the trains that won't run before Christmas... it doesn't get any better than that.
Merry Christmas (to all),