Kato DC Transformer ripple cleanup for DCC loco.

[SkipGear]

Just a quick tech tip. May have already been mentioned but I couldn't find it.....

Had a customer who is starting out, come in the other day. He has a couple loco's, including a Bachmann 4-6-0 w/DCC and had been running around on Atlas snap track and an ancient transformer. He recently picked up a Kato M1 set with a loop of track and their transformer. The Bachmann 4-6-0 did not like the Kato transformer. The loco would only run in reverse. This version is a much more updated version of the transformer than I could find reference too anywhere. The original only had a half dozen components on the board. This one is more like 40-50 SMD pieces. I assume they have now added a true pulse power to their transformers.

Anyhow, this new pulse power version messes with DCC installed loco's. I tried a few of my loco's on his transformer and they all seemed to have some weird qwirks.

The fix is not very difficult once I traced the board. I wanted to put a filter capacitor in the circuit before the forward and reverse switch so I could use an electrolytic with a higher capacitance range. The forward reverse switch provides a simple place to install the capacitor. I chose a 220uf 16V honestly because that was the first one I came across with a voltage higher than the output of the transformer in my spare parts stash. I tested the board for polarity and installed the capacitor + to + and - to -, tested the loco and it works perfectly now. A larger capacitor may help smooth the ripple more but this was enough to make it work correctly with the Bachmann loco.

[peteski]

Good idea for running DCC locos on DC, although I would have thought that more than 220uF would have been needed to smooth out the ripple. But if it works, it's all good.

I'm also thinking that when running straight DC locos, that original ripple likely gives better slow speed performance. It would be easy to wire a switch in series with the cap, to be able to take it out of the circuit when running straight DC locos. Unless of course this throttle is dedicated to testing DCC-equipped locos on DC (or it is only used for testing, not slow speed switching ops).

[Point353]

If a manufacturer is going to claim that their DCC-equipped loco is truly dual-mode, and therefore also operable from a "DC" power pack, then shouldn't that capacitor be factory-installed in the loco?

[Maletrain]

If a manufacturer is going to claim that their DCC-equipped loco is truly dual-mode, and therefore also operable from a "DC" power pack, then shouldn't that capacitor be factory-installed in the loco?

If the capacitor was in the loco, wired across the rail pick-ups, wouldn't that make trouble for the DCC mode of operation in a "dual mode" loco?

Putting it in the DC power pack, maybe along with a cut-out switch the user can operate, seems to make the loco really be dual mode and the transformer "DCC compatible" without harming low speed DC capability of that transformer for DC locomotives.

[SkipGear]

If a manufacturer is going to claim that their DCC-equipped loco is truly dual-mode, and therefore also operable from a "DC" power pack, then shouldn't that capacitor be factory-installed in the loco?

Having the capacitor in the loco would filter the DCC signal to the decoder. That is why sound decoders have trouble on low power programming tracks. They have keep alive capacitors that somewhat effect them the same way.

It didn't really need much filtering as the loco ran fine in reverse, something in the pulse was confusing the decoder and would not let it go forward. I don't think the capacitor is enough to effect it in DC operation. I personally prefer pure DC over pulse power anyhow. Pulse power was aimed at the older motors which took a more voltage to get them started. Modern skewed armature, low current motors really don't need pulse power, at least in N scale.

[Point353]

If the capacitor was in the loco, wired across the rail pick-ups, wouldn't that make trouble for the DCC mode of operation in a "dual mode" loco?

Having the capacitor in the loco would filter the DCC signal to the decoder.

How does a dual-mode decoder determine if it's operating on DC versus DCC?
It has to be able to detect the difference and then respond appropriately to the type of input voltage/signals.
If it can detect that difference, then it can also switch in the capacitor when it detects the DC mode of operation.

[SkipGear]

A DCC decoder is looking for a discernable digital signal. If it doesn't see that signal, it reverts to DC operation. In DC operation, it still powers the decoder and the decoder runs the loco, it just uses input voltage as a reference for speed, not a digital signal. It still has to be able to see a signal to determine what to do and a capacitor on the power side of the decoder would filter that out, making it only work in analog.

The pulses from the transformer mimic the signal that the decoder is looking for, in this case, too closely. The decoder thinks it is a DCC signal but is not able to understand it so it does nothing. Some decoders deal with pulse power better than others, some pulse power transformers are more prone to cause problems than others. This was just a bad combination.

It's not really the decoders fault, dual mode decoders are designed around using pure DC per the NMRA. A DCC decoder can be programmed to come to a stop when it sees pure DC. Some users do this so if something fails on the layout and DC power is introduced to the rails, all the DCC loco's don't take off running on analog.

[Point353]

A DCC decoder is looking for a discernable digital signal. If it doesn't see that signal, it reverts to DC operation. In DC operation, it still powers the decoder and the decoder runs the loco, it just uses input voltage as a reference for speed, not a digital signal. It still has to be able to see a signal to determine what to do and a capacitor on the power side of the decoder would filter that out, making it only work in analog.

The pulses from the transformer mimic the signal that the decoder is looking for, in this case, too closely. The decoder thinks it is a DCC signal but is not able to understand it so it does nothing. Some decoders deal with pulse power better than others, some pulse power transformers are more prone to cause problems than others. This was just a bad combination.

It's not really the decoders fault, dual mode decoders are designed around using pure DC per the NMRA. A DCC decoder can be programmed to come to a stop when it sees pure DC. Some users do this so if something fails on the layout and DC power is introduced to the rails, all the DCC loco's don't take off running on analog.

Is the frequency of the pulses/ripple generated by that new Kato power pack in the same range as DCC control signals?

[SkipGear]

I have to assume it's close. I don't have an oscilloscope to test it.

[peteski]

Is the frequency of the pulses/ripple generated by that new Kato power pack in the same range as DCC control signals?

Probably not.  Ripple in Kato DC power pack is likely standard 120Hz ripple which occurs when a full-wave (bridge) rectifier converts 60Hz AC voltage to pulsating DC.  DCC signal packets (voltage at the track) are pulses which have uneven and shifting pulse locations (related to the digital information contained in the packets).

I believe the problem is with the decoder firmware. Most likely it is the firmware (programming inside the decoder's microprocessor).  If the routines which detect and interpret info in the DCC packets are well written, they will ignore the constant 120Hz ripple. But the firmware routines, usually in less expensive decoders) is so basic that it will "think" the 120Hz DC ripple is DCC signal, and try to interpret it as DCC data packets, causing the loco to misbehave.

Just like with other computer software, some programs are better written than others.

The choice of hardware in  the decoder could also be partially responsible for this. The microcontroller (computer) on the decoder has limited internal storage space (ROM) for the the program that runs on it. Less expensive microcontroller chips have less internal memory than more advanced microcontroller. When storage space is limited, the programming is simplified,  DCC packet detection is less robust.

[greenwizard88]

Tony, were you able to test the Bachmann on DCC too? My memory of it is fuzzy, but I seem to recall my K4 giving me trouble, where it would only run in reverse on my Digitrax system. I can't recall if it was a decoder issue or mechanical, etc., but you might want to try it on DCC too and make sure that it doesn't need to get fixed by Bachmann before their warranty expires.

Also, nice fix. I've been holding off on getting one of those new Kato power packs because I didn't want to deal with PWM, especially where Paragon 3 decoders are concerned 

[peteski]

Also, nice fix. I've been holding off on getting one of those new Kato power packs because I didn't want to deal with PWM, especially where Paragon 3 decoders are concerned 

Kato power pack (at least the old one) was not a PWM throttle. It was a simple transistorized throttle using unfiltered rectified voltage. 
I don't see adding a filter cap on the output of a PWM throttle without also adding a series-connected inductor. Or at least I never seen such design.

Tony, is there a way you could post a closeup photo of the component side of the new Kato throttle?

[SkipGear]

Kato power pack (at least the old one) was not a PWM throttle. It was a simple transistorized throttle using unfiltered rectified voltage. 
I don't see adding a filter cap on the output of a PWM throttle without also adding a series-connected inductor. Or at least I never seen such design.

Tony, is there a way you could post a closeup photo of the component side of the new Kato throttle?

I've already given it back to the customer. It is not a simple circuit any more. It's all SMD components, no obvious rectifier diodes.

I was considering an inductor but figured I would try the simple route first. K.I.S.S is always my rule. I'm sure there are more elaborate solutions but there was no need for them here.

[peteski]

I've already given it back to the customer. It is not a simple circuit any more. It's all SMD components, no obvious rectifier diodes.

I was considering an inductor but figured I would try the simple route first. K.I.S.S is always my rule. I'm sure there are more elaborate solutions but there was no need for them here.

Interesting. so, it could now be a PWM throttle.  Is the 120V AC fed directly to the throttle, or is there a wall-wart type of supply?  If it is a wall-wart, then the rectifier and filter caps could be in the wall-wart.

[Point353]

Interesting. so, it could now be a PWM throttle.  Is the 120V AC fed directly to the throttle, or is there a wall-wart type of supply?  If it is a wall-wart, then the rectifier and filter caps could be in the wall-wart.