Can the variable DC output of a modern MRC transistor throttle be run through another transistor thottle circuit? I.E. can the first throttle just limit the maximum voltage to say 9 volts and then the second throttle be used to control the final voltage from 0 to 9 volts to the track?
You "could" .... but, I have to question .... why ?
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
I'm on a tight budget and have a pair of MRC tech 4 200 (the no frills one with just a power and direction switch). Their output is 0v to 15v, but my HO kato locos start moving at about 1v and at 6v they are highballing (making my useable throttle range only about 30%), so I want to drop that max voltage down to something like 8 or 9, which would make my throttle sweep wider with finer control. Thus I want to tie the output of one throttle pack to the circuit of the second throttle (bypassing it's transformer of course).
i can understand building a transistor PWM and using an older throttle to provide the input voltage at the desired level, but several of the MRC throttles are outputing one form or another of pulses and don't output constant DC.
greg - Philadelphia & Reading / Reading
I opened up one of the throttles and there is the circuit fuse disc, four rectifier diodes, a resistor for the power indicator led, and a couple other resistors which I assume is for the pot. I don't see any kind of artificial pulse generator or smoothing capacitor, so I'm guessing it's just using the 60 hz AC line cycle and rectifying it into a track pulse (either full or half pulse, I'm not sure). Can the variable DC output from throttle one be wired into the red power wires of throttle two, or do I need to bypass those diodes and simplify the circuit? Would the ripple from throttle one just get passed through those diodes?
Here is a top and bottom pics of the PCB. The red lines are the diodes, blue are resistors, orange is the LED and yellow is the disc fuse.
gregc i can understand building a transistor PWM and using an older throttle to provide the input voltage at the desired level, but several of the MRC throttles are outputing one form or another of pulses and don't output constant DC.
I also have an old MRC controlmaster X and that uses a pot to control the max voltage to the second pot, so thats where I was getting the idea to use two identical throttles with one throttle to limit the voltage to the other.
It's a full wave rectifier. Those are the simplest possible transistor throttles, just one transistor stage. It appears they use the came PCB for multiple models with additional features.
What you propose may work. An alternative would be to feed something like a 9V AC or DC power supply to the input, rather than feed one power pack to the other. Might actually have to go with a 12V DC wall wart, the two diode drops in the bridge rectifier will already drop it by 1.2V. The transformer that's part of the power pack probably puts out closer to 16V AC. Using the lower voltage power supply to run it, you could still have 2 power packs.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
why 9V?
rrinker It's a full wave rectifier. Those are the simplest possible transistor throttles, just one transistor stage. It appears they use the came PCB for multiple models with additional features. What you propose may work. An alternative would be to feed something like a 9V AC or DC power supply to the input, rather than feed one power pack to the other. Might actually have to go with a 12V DC wall wart, the two diode drops in the bridge rectifier will already drop it by 1.2V. The transformer that's part of the power pack probably puts out closer to 16V AC. Using the lower voltage power supply to run it, you could still have 2 power packs. --Randy
That might be an option as well. I've heard that some wall warts today are just using a rapid switching PWM to drop the voltage rather than a true transformer. I'd rather keep the ripple effect from a true transformer so is there any good way to tell what type of voltage step down a partictular wall wart is?
gregc why 9V?
Because I measured the max throttle output of both MRC throttles and it's around 15.5V. That might have been fine with old open frame motors or the BB athearns but I run all Kato motors and they don't need much voltage. They start crawling around 1.5V and at 6V they are racing down the track. Because of this I can only really use the bottom 1/3 of the throttle range and any slight throttle movements have significant speed changes. If that 6V was at 80% throttle sweep instead of 40% then I'd have more precise speed control.
Right now it's like trying to fine tune in a radio station when the dial knob sweeps the whole radio band in just a 1/4 turn of the knob.
Another question would be is there a way to add a second pot or resistor to the circuit to basically cut the primary pot range in half?
GASnBRASSAnother question would be is there a way to add a second pot or resistor to the circuit to basically cut the primary pot range in half?
the voltage reduced using resistors depends not only on the resistance, but the current drawn as well.
a 7809, or any 3 pin 9V regulator chip can be used to reduce the output voltage from a higher voltage source to 9V regardless of the current being drawn. you can add one to the input of the transistor throttle circuit you are considering using. you can also use an unused ~12V DC wall wart supply
GASnBRASSBecause of this I can only really use the bottom 1/3 of the throttle range and any slight throttle movements have significant speed changes. If that 6V was at 80% throttle sweep instead of 40% then I'd have more precise speed control
what you could do with your transistor throttle circuit is put two pots in series, one with a knob connected to gnd used to control the speed and a second that is fixed connected to Vcc which limits the range of the first.
if your building a PWM type throttle, the effect might be to limit the duty cycle range from 0 to 70%
gregc GASnBRASS Because of this I can only really use the bottom 1/3 of the throttle range and any slight throttle movements have significant speed changes. If that 6V was at 80% throttle sweep instead of 40% then I'd have more precise speed control what you could do with your transistor throttle circuit is put two pots in series, one with a knob connected to gnd used to control the speed and a second that is fixed connected to Vcc which limits the range of the first. if your building a PWM type throttle, the effect might be to limit the duty cycle range from 0 to 70%
GASnBRASS Because of this I can only really use the bottom 1/3 of the throttle range and any slight throttle movements have significant speed changes. If that 6V was at 80% throttle sweep instead of 40% then I'd have more precise speed control
The pot appears to be a linear 10K ohm (according to my meter). So my useable range is from 10K to about 5K ohms on the pot. Any less resistance to the base pin and the motor runs too fast. I think the only way to properly use the entire throttle sweep is to get a linear 5K ohm pot and a 5K ohm resistor between the wiper output and the transistor base.
Also I see a resistor bridging between the unrectified AC power to the base pin (blue line closest to the transformer wires). Are they adding a fixed voltage AC half ripple on top of the final output voltage? Any harm if I clip that resistor out?
GASnBRASSThe pot appears to be a linear 10K ohm (according to my meter). So my useable range is from 10K to about 5K ohms on the pot. Any less resistance to the base pin and the motor runs too fast. I think the only way to properly use the entire throttle sweep is to get a linear 5K ohm pot and a 5K ohm resistor between the wiper output and the transistor base.
or put a 10k resistor i between the pot and Vcc
GASnBRASSAlso I see a resistor bridging between the unrectified AC power to the base pin (blue line closest to the transformer wires). Are they adding a fixed voltage AC half ripple on top of the final output voltage? Any harm if I clip that resistor out?
they may be using that to trigger a pulse
from an older thread http://cs.trains.com/mrr/f/744/p/269380/3053917.aspx
gregcgregc i happened to see a product review for the MRC Tech II models 1400/1500/2400/2500 in RMC, i believe march april 1980. It showed scope traces. Those traces showed rectified AC. A switch on some of the models controlled whether it was half or full wave rectified. the may 1980 RMC completed the review of the 1500/2500. It had described the 1500/2500 as having momentum/brake and the 2400/2500 as higher powered versions of 1400/1500. but the 2500 uses PWM unlike the other 3. The review showed scope traces for the 1500/2500 at 4 throttle settings. PWM pulses are ~16 msec apart, which is the period of 60 Hz. I could see some 60Hz ripple riding on top of the wider PWM pulses. the article discusses motor heating as a disadvantage of PWM, which it probably is at 60 Hz. I believe modern day decoders use processors that can generate PWM pulse at 200 kHz.
i happened to see a product review for the MRC Tech II models 1400/1500/2400/2500 in RMC, i believe march april 1980. It showed scope traces. Those traces showed rectified AC. A switch on some of the models controlled whether it was half or full wave rectified.
the may 1980 RMC completed the review of the 1500/2500. It had described the 1500/2500 as having momentum/brake and the 2400/2500 as higher powered versions of 1400/1500.
but the 2500 uses PWM unlike the other 3. The review showed scope traces for the 1500/2500 at 4 throttle settings. PWM pulses are ~16 msec apart, which is the period of 60 Hz. I could see some 60Hz ripple riding on top of the wider PWM pulses.
the article discusses motor heating as a disadvantage of PWM, which it probably is at 60 Hz. I believe modern day decoders use processors that can generate PWM pulse at 200 kHz.
gregc or put a 10k resistor i between the pot and Vcc
I'll try the resistor first. I'm not good with schematics terms so does that mean add the resistor inline on the pot's + input? Thank you so much for your help!
GASnBRASSgregc or put a 10k resistor i between the pot and Vcc I'll try the resistor first. I'm not good with schematics terms so does that mean add the resistor inline on the pot's + input?
I'll try the resistor first. I'm not good with schematics terms so does that mean add the resistor inline on the pot's + input?
the middle case below which using a 10k trim pot which provides some adjustment. A 10k pot and 10k trim pot set to 10k reduces the range to 50%. A trim setting of 4.2k reduces range to 70%
Because the OP mentioned that with 9V tot he track his locos are already moving at near-warp speeds. N scale - such fun! LOL J/K. It's MUCH better these days, the stuff I had back in the day would easily exceed 200smph at full throttle on a standard "12V" DC power pack.
Most of my HO Kato motors exibit a growl on the MRC that isn't present on a filtered DC throttle. However I found the filtered DC is poor at starting a loco from a stop because of stiction. I clipped that resistor in the MRC that was putting an AC spike into the base of the transistor and reassembled the throttle. It still has decent starting control because of the DC ripple but eliminating the AC spike cut out alot of the growl, so that was a pleasant result. Now to just order a few parts and try to widen my throttle sweep. Thanks!
I checked out the suggestion of swapping the transformer and I found a few wall wart 120 VAC to 12 VAC transformers by Hammond for not much money ($16-24 each). One is rated 1.6amps/20VA and the other is 3.3amps/40VA. After voltage drop through the diode rectifier and the transistor I'd probably have 9 or 10V at max throttle instead of 15.5V. Any reason to use one over the other for a single throttle/single locomotive, or could both throttles be run on the larger transformer?
do you really want to use an wall wart with AC output?
isn't the peak voltage from AC Vac/0.7?
gregc do you really want to use an wall wart with AC output? isn't the peak voltage from AC Vac/0.7?
Doesn't a wall wart transformer wired to the PCB do the same thing as the original internal transformer, just at a reduced voltage? Here is the specs for the wall wart transformers I was looking at:
https://www.mouser.com/ProductDetail/546-BPD2E
https://www.mouser.com/ProductDetail/Hammond-Manufacturing/BPE2E?qs=sGAEpiMZZMvwUzoUXIIvyQPvPmwnNFGyRJuauysO3wk%3D
The fixed accesories AC output on my throttle from the internal transformer is 18.5V AC. The peak DC output is 15.5V after circuit losses. By switching it to 12V AC input the accessories output would be 12V AC and the peak DC output should be somewhere around 9V, correct? I never need more than about 6V to the track, so this in theory should utilize more range on the throttle sweep.
so this is to replace the transformer in the MRC throttle, not supply your external throttle board?
if 15.5V is 83% of 18.5V, then replacing the transformer with a 10.8V transformer should result in a ~9V DC output.
Only if there is a filter capacitor, which MRC doesn't use (probably on purpose, to get pulsating DC which is better at starting DC motors.). The capacitor allows the rectified DC to pump to peak, instead of RMS. Without a cap, RMS is about all you get. Less 2 diode drops in the rectifier.
rrinkerThe capacitor allows the rectified DC to pump to peak, instead of RMS.
certainly if no current is being drawn, otherwise there is more output than input power