Is it possible to use a keep alive device of some sort with a DC locomotive? I have some Hobbytown of Boston DC locomotives that dont have very good electrical because even though some are newer than this, the electrical system pick up was designed in the 60s, and doesnt work that well. Is it possilbe to use a keep alive device for them when using DC? And or would it work on a dcc track when the locomotive is dc? Dont have the effort or patience at this time to wire them up for dcc.
Im asking because im trying to find out how many cars 2 of my oldest Hobbytowns can pull because they have the old Pittman DC91 motors in them, but the train cars have a tendency to tip over if the loco spontainuously loosed power due to old electrical pickup (because so far, iv gotten to 135 properly weighted cars. The cars dont tip if the train keeps moving Smoothley).
The problem with keep-alives on DC locomotives is that it turns them into battery locomotives ā they wonāt respond accurately to the setting of the throttle voltage, and they will continue to run when the voltage is decreased or shut off.
What you want is a comparatively small capacitor-resistor-diode combination, which will charge following the track voltage and maintain that voltage for a small time before discharging ā perhaps an eighth to a quarter of a second. That will be enough oomph to get you over a gap or patch of dirty rail without running away.
you can try adding pairs of polarized electrolytic capacitors wired in series but oppositie polarity (i.e. negative to negative) making the pair non-polar, able to charge to + or - voltage. No need for diode/resistor in DCC keep alive
They charge as voltage increases and provide current when voltage is disrupted.
Just to be sure you know, he is talking about a straight-DC keep-alive, not DCC. All it is meant to do is be able to keep the motor voltage up across interruptions (and in doing so, source sufficient current to keep voltage from sagging) without having a ābattery-powered toyā run-on when the DC voltage is intentionally decreased or shut off in āemergenciesā.
what do you think iām missing?
the capacitor is charged to track voltage and provides some current when track power drops because of a dirt spot. But the capacitor will discharge quicklly and follow the throttle/track voltage
thereās no use for a diode/resistor used with DCC keep alives because thereās no decoder, the track voltage is the motor votlage
The resistor is to prevent too-rapid discharge if the track voltage spikes to zero suddenly, which could cause excessive arcing at the motor brushes or between wheels and track.
The diode is to ensure that the capacitor discharge goes through the motor.
wonāt a resistor in series with the cap defeat the purpose, allowing too little current to keep the motor running
if thereās dirty track, where else can the current go but thru the motor.
and what if the voltage is negative to run the motor in reverse?
What ever I would use, it needs to be very small. Not much room in the older models with the dc 91 motor because on top of a HUGE motor in there, there are 2 lead weights making this little f7 weigh literally 2 lbs in total.
capacitors sufficient in capacitance to keeps things running arenāt small. If thereās no room, in the locomotive, is there room in the tender?
Many manufacturers make very small form-factor supercapacitors ā the problem is that theyāre inherently low-voltage devices and extremely sensitive to overvoltage, so they would need to be ganged with very fast clamping overvoltage protection not just against spikes but too much track voltage.
I would tend to agree that routing all the pickups through a cap mounted in the tender, or in a B-unit or attached car, would be a preferred approachā¦
Connecting wires between the units for track power will make for much more reliable power pickup.
I have also added wipers on the insulated wheels, but this gets somewhat involved and will only work on units with metal insulated wheels, not the older ones that had plastic wheels.
Mark Vinski
I have an awful lot of Hobbytowns .They run very well but they have one problem with the pickup , the original flywheel drives with DC 70 motors had one lead that was connected through the frame resulting path relied in the frame having contact with the rear truck so the only contact depended on the frame resting on truck. You need to provide a wired path from the brush directly to the rear truck. Use light weight wire flexible wire, I often tin the the end of the stranded flexible wire that will contact the rear truck. Then loosen the gearbox cover and slip this in between the cover and gear box tighten the cover so that it will hold the wire, it is important that this is light gauge wire 28 or 30 gauge . I use old set cords for business telephones The other lead is connected to the front truck gear tower.. This is a good connection because the instructions tell you to use one of the gear tower screws to hold that wire This truck is insulated frome the frame so there is no short. doing this will cure your pickup problem. I also do this on the yard switcher kit.
Ron High
wouldnāt a wired path from the loco to the tender frame be sufficient? Isnāt the problem a poor contact between the drawbar and pin on the loco?
Surely with Hobbytown drives heās talking about diesels.
I did talk about Hobbytowns. Hobbytowns have 4 wheel pickup insulated on 2 wheels and the opposite wheel are uninsulated on that same truck . As far as I know most newer diesel models have all wheel pickup with all wheels insulated and wipers mounted to contact the metal wheels wired to the motor of course DCC is more complex. The Hobbytowns are diecast metal frames with solid brass flywheels which are larger than todays models. They are quite heavy ..Some Hobbytown engines have diecast metal bodies the E7 AB, FA2 AB ,PA1 and RS 3. A lot of the drives are just that ,you can adapt almost any body shell to the drives. I donāt do DCC so there is a lot of room for aditional weight which will help with pickup.
I am sorry if I was not clear in my original post.
Ron High
Be careful, most newer DC power supplies have some type of PWM output. Can a capacitor handle that?
The primary purpose of PWM is to approximate a lower continuous voltage by varying the timing and/or duration of āonā time.
In most model applications, you want the pulse train āsmoothedā as much as possible, so the motor sees continuous controlled DC voltage. This can be done by a capacitor (normal ārippleā filters, which would do the same for typical PWM as they do for rectified AC, use both capacitors and inductors, but we arenāt concerned with constant current in this particular application).
The capacitor should not be damaged by the effects of āpulsesā at a high enough frequency.
This is the hobbytown in question regaurding space contraint.
The others that I have dont have a space problem like the pair I have 1 of shown above.
Yes I am
Actually, no they donāt.
Sheldon