Troller dual power pack

[Rich_S]

The transistor in simplest terms, acts as a switch with the pot controlling how much on or off the transistor is (again - simple description). This isolates the voltage from the load giving the same control, no matter what type or number of motors) being controlled.

Don

Don, It has been 30 years since I've done any electronics work, so I have a few questions. Are they using the transistor as a amplifier? I understand that a transistor is an electronic switch, by applying a voltage to the base of the transistor it will pass a voltage from the collector to the emitter, remove the voltage from the base and the transistor stops conducting.  The POT will also control the current flow, but are they using the transistor to amplify the current output to the track?  I don't see anything in the circuit to help regulate the load. It's been many years since I've worked on solid state circuits, but I would think you'd need some time of feedback from the track attached to the base of the transistor to control the load?

I know Cap's were used on the output of the bridge, they were used as filters to minimize any buzzing or surging you'd see from the bridge.     

[peteski]

My question, why are they using the POT to control the transistor, what advantage does this have over simply controlling the current and voltage with the potentiometer? Is the transistor being used as a filter?

What you describing was one of the original early DC throttle designs.  It used a a rheostat (which is another name for a high-power variable resistor aka. pot or potentiometer) connected in series with the load (the locomotive) to control the voltage (and thus current) delivered to the locomotive.   That worked but didn't give very fine range of control (especially when old throttles were used for newer models which had efficient low-current motors.  In those cases, due to the fairly low maximum resistance of the rheostat, the loco would run rather fast, even at very slow throttle settings.

Those were eventually replaced by transistorized throttles.  The transistor acts as a variable resistor which has a much wider range of resistance than rheostats had.  It acts as a buffer. allowing a much smaller and lower current to control the output. This allows it to be controlled by much smaller pot and gives much finer control over the output voltage and current. Plus it also allows for easily adding other functions such as momentum to the throttle.

If you want the technical details, the transistor here is used in a common-collector amplifier (not as a switch).  For even more technical details see https://en.wikipedia.org/wiki/Common_collector . No need to read the entire mambo-jumbo - the Applications chapter gives a good overvie of this circuit.

[Rich_S]

What you describing was one of the original early DC throttle designs.  It used a a rheostat (which is another name for a high-power variable resistor aka. pot or potentiometer) connected in series with the load (the locomotive) to control the voltage (and thus current) delivered to the locomotive.   That worked but didn't give very fine range of control (especially when old throttles were used for newer models which had efficient low-current motors.  In those cases, due to the fairly low maximum resistance of the rheostat, the loco would run rather fast, even at very slow throttle settings.

Those were eventually replaced by transistorized throttles.  The transistor acts as a variable resistor which has a much wider range of resistance than rheostats had.  It acts as a buffer. allowing a much smaller and lower current to control the output. This allows it to be controlled by much smaller pot and gives much finer control over the output voltage and current. Plus it also allows for easily adding other functions such as momentum to the throttle.

If you want the technical details, the transistor here is used in a common-collector amplifier (not as a switch).  For even more technical details see https://en.wikipedia.org/wiki/Common_collector . No need to read the entire mambo-jumbo - the Applications chapter gives a good overvie of this circuit.

Thank you Pete, you've answered my questions.

[sd75i]

   I have finally taken cover off!  I should figure out something in the next 10 months!

[sd75i]

 Oh boy!  Help?

[mmagliaro]

Try to get in closer with another photo, if you can.

Meanwhile, the first two things I'd check would be the output from that power transformer,
and those two glass-bulb thermal breakers I see in there. 

Info in the picture below.  Test the transformer with the power on (throttle does not have to be turned up... doesn't matter).  Measure AC VOLTAGE as shown.  I'd expect to see anything from 12 to 24 volts AC.

THEN TURN THE POWER OFF AND UNPLUG IT!!!    before you test the two glass bulbs with an ohmmeter.
You should see zero ohms across those glass bulb things.  If you see an open circuit (infinite ohms), that's a problem.

[Rich_S]

THEN TURN THE POWER OFF AND UNPLUG IT!!!    before you test the two glass bulbs with an ohmmeter.
You should see zero ohms across those glass bulb things.  If you see an open circuit (infinite ohms), that's a problem.

You need to isolate one side of the thermal circuit breaker to check if it's open. Leaving it attached to the circuit, you'll read the circuit through the transformer, even if the circuit breaker is open it will appear to be OK. Same applies to the other thermal circuit breaker.

[mmagliaro]

I don't think you HAVE to isolate those glass bulbs.  There will be some winding resistance in that transformer.
If the ohmmeter doesn't read dead zero across that bulb, something's wrong with it, and I doubt it will read zero
even with the transformer in there.

But I do take your meaning.  It would probably be more certain.
sd75i... Can you unsolder one lead of each glass bulb before you test across it with an ohmmeter?
Be quick.  A lot of soldering heat can damage those things.

[tehachapifan]

This thing sure brings back some memories! Had one with my HO layout when I was about 15. I recall the momentum and braking features as working very well with my Athearn locos.

[sd75i]

  Sorry i was working for last 4 days.  Ok item 1 was measured in AC and highest reading was .1.  Item2 and 3 if i measured right were all over the place between 11 and 196 in ohms

[sd75i]

Does this help at all

[sd75i]

1st pic left. this one right

[Rich_S]

  Sorry i was working for last 4 days.  Ok item 1 was measured in AC and highest reading was .1.  Item2 and 3 if i measured right were all over the place between 11 and 196 in ohms

If you are only getting 0.1 VAC at the transformer, the transformer is dead or you have an issue with your on / off switch. It should be some where in the range of 12 to 16 VAC.

[sd75i]

  Would that be the transformer that I measured first?  Maybe both are out?

[Rich_S]

  Would that be the transformer that I measured first?  Maybe both are out?

You said when you measured Item 1, you only read 0.1 VAC. Item 1 is the output side of your transformer. I cannot tell from your picture if you can take a reading on the input or primary side of your transformer? In basic terms you should have house voltage coming into the power pack. The house voltage is approx 120 VAC. I cannot see if you can read this house voltage at your on / off switch? The output side of your step down transformer (Item 1) should read 16 to 18 VAC. If you are only getting 0.1 volts AC at item 1, you need to check and see if you are getting house voltage at the on / off switch? If you are getting house voltage (approx. 120 VAC) at the on / off switch, then it sounds like your step down transformer has gone out to lunch and won't be back  Hopefully the problem is the On / Off switch? 

Another test you can try, with the power pack unplugged, put your multi-meter on the ohms scale, hold one of your meter leads to one of the prongs on your power packs plug, hold the other meter lead to the other prong. With the power switch turned off on the power pack, you should read infinity. With the switch turned on, you should read a low resistance value, something like 50 ohms. If you only read infinity in both positions, the switch or step down transformer are bad.

Switch turned off


Switch turned on