Post by Jim W on Oct 26, 2022 8:06:54 GMT -7
A Primer on Transistorized Controllers
February 14, 2022, 01:57 PM
A Primer on Transistorized Controllers
This seems like an appropriate place for this. Some of you may know all of this and more. To some its all new. While I race HO scale cars and my M-Magic controller is designed for HO Scale the ideas presented are applicable for any scale electronic controller. So sit back. read and enjoy this primer on Transistorized slot car controllers.
The British Slot Car Racing Association (BSCRA) website “https://www.slotcarracing.org.uk/control/index.htm” provides an excellent description of what happens inside a transistorized controller. I used their pages as a resource when designing my M-Magic HO scale controller. The following circuit represents a “Positive Gate” transistorized controller using an NPN transistor.
Click image for larger version Name: npn1.gif Views: 0 Size: 4.6 KB ID: 149723
The wire colors in the BSCRA circuit are different than what is found on a standard Parma controller. The UKs Red Wire (Pin L) would correspond to the Parma White (Power) wire. The Blue wire (Pin N) would correspond to the Parma Black (Track) wire. Finally, the green wire (Pin E) would correspond to the Parma Red (Brake) wire.
Electronic controllers are defined as Positive or Negative gate. A Positive gate controller has its white wire connected to the tracks power supply positive terminal. A Negative gate controller has its white wire connected to the tracks negative power supply terminal. Tracks wired for Positive gate is becoming the world standard. Tracks wired for Negative gate is less common but there are still some out there.
The controller circuit shown is for a two-wire controller such as a Trek, Difalco, OS3 or my M-Magic. The brake wire is not part of the voltage divider circuit and is not required to be connected for these controllers to function.
A three-wire controller such as the Ruddock, Theisen or Lucky Bob is similar but different as the lower end of the sensitivity adjustment resistor is directly connected to track supply 0 Volts (Pin E) instead of Pin N. Only the full power contact and the Emitter are connected to Pin N. A three-wire controller uses the brake wire as part of its voltage divider circuit and requires that the brake wire be connected in order to function.
The BSCRA article discusses a controller designed for a 12V track. The same basic circuit works on any track voltage from 8 to 24 Volts. The circuit’s resistor values and ratings will change depending on the track voltage and the type of cars being run.
The controller works by using a voltage divider consisting of the part power band resistors and the sensitivity potentiometer to provide a variable voltage to the transistor. Most controllers add a fixed resistor located between the sensitivity potentiometer and Pin N to raise the starting voltage from 0 volts to a higher value such as 3 or 4 volts. The transistor then acts like a variable resistor to provide a voltage to the car that is approximately equal to the voltage provided to it. The transistors output voltage will be either 0.6 or 1.2 volts lower than the voltage provided to it by the voltage divider circuit depending on the type of transistor used.
A MOSFET (or FET) controller uses a similar voltage divider. The output of the voltage divider is provided to a driver board which adjusts the duty cycle of the MOSFET. The switching outputs of the MOSFET transistor are seen as a variable voltage by the slot car.
The circuit shown above would be considered a five-band controller. The band resistors are usually of the same value (for example 100 Ohms). Using all of the same value resistors will make the controller’s response linear. Different values of resistance can be used to change the controller’s response curve. If you are going to make a non-linear controller, I suggest using a program such as Excel to calculate and plot the voltage dividers output for each trigger position.
A choke consists of an additional resistor in the transistor control circuit that can be bypassed when the choke function is not required. The choke resistor can be fixed or variable. A mush button consists of a momentary contact switch that bypasses the sensitivity potentiometer when depressed.
A controller using a wiper board needs two isolated wipers connected to the trigger. The first wiper contacts the part throttle bands and provides a variable voltage to the transistor. The second wiper contacts the brake and full power bands and is designed to carry full armature current. A wiperless controller requires only a single trigger wiper to connect the brake and full power bands to Pin N (Black Wire).
The circuit shown does not have overcurrent protection. If there is a short circuit (a.k.a. Fault) on the track, then excessive current can pass through the transistor depending on the trigger position. If the current passing through the transistor is above its rating, the transistor will fail.
The BSCRA article indicates that one way round this is to use a transistor with a really high current rating. Upping the transistors current rating will only work if the transistor can overpower the track power supply. In such a case the controller could be unharmed but track voltage would fall to zero until something else fails to clear the fault. It should be noted that the transistors current rating is time dependent and the published maximum rating is much higher than the transistors continuous current rating. Most likely, this dog won’t hunt.
The second (and preferred) solution is to install a fast-acting (non-time delay) fuse on either side of the transistor and full power contact. This option works! A good rule of thumb is to use a fuse that is rated for 20% of the transistors current rating. Despite what some manufacturers advertise, a time-delay fuse, circuit breaker or PTC (a.k.a. Solid State Fuse) will not protect the transistor from a severe fault. There are several articles discussing controller overcurrent protection on the Tech Pages section of my Siberia Racing website “http://siberiaracing.altervista.org/Tech-1.htm”
.
In theory one could use several transistors wired in parallel to get a controller with a really high current rating. This is not likely to work in practice because transistors rarely share current equally when simply connected in parallel, and as they heat up their sharing ability becomes less equal. (They can be made to share current much better by putting a 0.1 Ohm resistor in the emitter lead of each transistor). The higher this series resistor value the better the transistors will be at sharing the load. The downside is that the series resistor addition will increase the transistor’s voltage drop.
The base (B) to emitter (E) voltage drop isn't exactly 0.6 volts or 1.2 volts as stated previously. This voltage will decrease as the transistor’s internal temperature increases. This voltage will increase as current passing through the transistor increases. The B to E voltage drop is not identical in all transistors (not even in a pair of the same part number transistor).
The current going to the transistor’s base should be small compared with the current passing through the sensitivity adjustment resistor. If the base current is significant, then the voltage drop from the + voltage to the transistor will be larger and the voltage drop across the controller will increase as the base current increases. The voltage drop depends on the transistors base current relative to the emitter current. The ratio between these currents is known as the gain of the transistor. Tables of transistor properties can list the gain as “Gain” or as "hfe". The gain might be 1000 or more for a darlington pair transistor and as low as 80 to 150 for a single transistor.
The transistors gain is not a constant. The gain changes as the transistor’s internal temperature changes. The gain also changes with the current passing through the transistor (in some cases reaching its maximum at 5 - 10% of the transistors maximum rated current, and decreasing as current rises. And, as stated previously, the gain is not identical in any two transistors (not even in a pair of the same part number).
These changes mean the controller characteristics change as it warms up. This doesn't seem to cause a practical problem, possibly because drivers are used to compensating for the car characteristics changing during a race and the transistor heating doesn't change the controller characteristics all that quickly.
The driver adjusts the controller to suit the car he is using, which is pretty much depends on how much current the motor needs in each corner. The driver will adjust the controller to suit the current that the motor actually takes and also to compensate for the differences between one transistor and another (even though he doesn't realize turning the sensitivity knob on the controller is doing all those things at the same time).
Like the controller, the car characteristics are also changing during a race. Some examples of these changes are - Tires grip better once as they warm up, armature resistance goes up as it warms up, and the cars handling changes as the tires wear or become dirty. Finally, the power supply voltage may change due to time, temperature or current draw. The maximum changes in voltage are with battery powered tracks without battery chargers. However, some regulated supplies output voltage can change depending on the instantaneous current requirement and as the power supply heats up.
The biggest change that the BSCRA writer measured is on a track where battery voltage dropped from 13.5 volts to 12.5 volts during a race. The drivers didn't notice this change - presumably they just braked a bit later and pushed their controllers down a bit more in corners as the race progressed.
The worst-case voltage change I have ever observed was at the final race of the 1981 Indiana HOPRA season. Track power was by batteries only. No battery charger or power supply was installed in parallel to the batteries. The automotive batteries were used and track power went from 18V or more at the start of the day to probably 15V or less at the end of the main as the batteries faded. I discuss the phenomenon of battery fade or why a 50 Amp-Hour battery may only be a five amp-hour battery when used on a slot car track on the Siberia Racing website Tech Pages. During the race voltage wasn’t measured but it is known that the hot motor cars slowed significantly as the batteries were depleted. My car had a lower power motor which wasn’t as impacted by the power drop. Like the little train that could It just kept chugging along. The car made the main and eventually won the race to make me the 1981 Indiana HOPRA champ.
Regards.
Steve "Maddman" Medanic
From "HRW Forum
Jim W
Tags: None
February 14, 2022, 01:57 PM
A Primer on Transistorized Controllers
This seems like an appropriate place for this. Some of you may know all of this and more. To some its all new. While I race HO scale cars and my M-Magic controller is designed for HO Scale the ideas presented are applicable for any scale electronic controller. So sit back. read and enjoy this primer on Transistorized slot car controllers.
The British Slot Car Racing Association (BSCRA) website “https://www.slotcarracing.org.uk/control/index.htm” provides an excellent description of what happens inside a transistorized controller. I used their pages as a resource when designing my M-Magic HO scale controller. The following circuit represents a “Positive Gate” transistorized controller using an NPN transistor.
Click image for larger version Name: npn1.gif Views: 0 Size: 4.6 KB ID: 149723
The wire colors in the BSCRA circuit are different than what is found on a standard Parma controller. The UKs Red Wire (Pin L) would correspond to the Parma White (Power) wire. The Blue wire (Pin N) would correspond to the Parma Black (Track) wire. Finally, the green wire (Pin E) would correspond to the Parma Red (Brake) wire.
Electronic controllers are defined as Positive or Negative gate. A Positive gate controller has its white wire connected to the tracks power supply positive terminal. A Negative gate controller has its white wire connected to the tracks negative power supply terminal. Tracks wired for Positive gate is becoming the world standard. Tracks wired for Negative gate is less common but there are still some out there.
The controller circuit shown is for a two-wire controller such as a Trek, Difalco, OS3 or my M-Magic. The brake wire is not part of the voltage divider circuit and is not required to be connected for these controllers to function.
A three-wire controller such as the Ruddock, Theisen or Lucky Bob is similar but different as the lower end of the sensitivity adjustment resistor is directly connected to track supply 0 Volts (Pin E) instead of Pin N. Only the full power contact and the Emitter are connected to Pin N. A three-wire controller uses the brake wire as part of its voltage divider circuit and requires that the brake wire be connected in order to function.
The BSCRA article discusses a controller designed for a 12V track. The same basic circuit works on any track voltage from 8 to 24 Volts. The circuit’s resistor values and ratings will change depending on the track voltage and the type of cars being run.
The controller works by using a voltage divider consisting of the part power band resistors and the sensitivity potentiometer to provide a variable voltage to the transistor. Most controllers add a fixed resistor located between the sensitivity potentiometer and Pin N to raise the starting voltage from 0 volts to a higher value such as 3 or 4 volts. The transistor then acts like a variable resistor to provide a voltage to the car that is approximately equal to the voltage provided to it. The transistors output voltage will be either 0.6 or 1.2 volts lower than the voltage provided to it by the voltage divider circuit depending on the type of transistor used.
A MOSFET (or FET) controller uses a similar voltage divider. The output of the voltage divider is provided to a driver board which adjusts the duty cycle of the MOSFET. The switching outputs of the MOSFET transistor are seen as a variable voltage by the slot car.
The circuit shown above would be considered a five-band controller. The band resistors are usually of the same value (for example 100 Ohms). Using all of the same value resistors will make the controller’s response linear. Different values of resistance can be used to change the controller’s response curve. If you are going to make a non-linear controller, I suggest using a program such as Excel to calculate and plot the voltage dividers output for each trigger position.
A choke consists of an additional resistor in the transistor control circuit that can be bypassed when the choke function is not required. The choke resistor can be fixed or variable. A mush button consists of a momentary contact switch that bypasses the sensitivity potentiometer when depressed.
A controller using a wiper board needs two isolated wipers connected to the trigger. The first wiper contacts the part throttle bands and provides a variable voltage to the transistor. The second wiper contacts the brake and full power bands and is designed to carry full armature current. A wiperless controller requires only a single trigger wiper to connect the brake and full power bands to Pin N (Black Wire).
The circuit shown does not have overcurrent protection. If there is a short circuit (a.k.a. Fault) on the track, then excessive current can pass through the transistor depending on the trigger position. If the current passing through the transistor is above its rating, the transistor will fail.
The BSCRA article indicates that one way round this is to use a transistor with a really high current rating. Upping the transistors current rating will only work if the transistor can overpower the track power supply. In such a case the controller could be unharmed but track voltage would fall to zero until something else fails to clear the fault. It should be noted that the transistors current rating is time dependent and the published maximum rating is much higher than the transistors continuous current rating. Most likely, this dog won’t hunt.
The second (and preferred) solution is to install a fast-acting (non-time delay) fuse on either side of the transistor and full power contact. This option works! A good rule of thumb is to use a fuse that is rated for 20% of the transistors current rating. Despite what some manufacturers advertise, a time-delay fuse, circuit breaker or PTC (a.k.a. Solid State Fuse) will not protect the transistor from a severe fault. There are several articles discussing controller overcurrent protection on the Tech Pages section of my Siberia Racing website “http://siberiaracing.altervista.org/Tech-1.htm”
.
In theory one could use several transistors wired in parallel to get a controller with a really high current rating. This is not likely to work in practice because transistors rarely share current equally when simply connected in parallel, and as they heat up their sharing ability becomes less equal. (They can be made to share current much better by putting a 0.1 Ohm resistor in the emitter lead of each transistor). The higher this series resistor value the better the transistors will be at sharing the load. The downside is that the series resistor addition will increase the transistor’s voltage drop.
The base (B) to emitter (E) voltage drop isn't exactly 0.6 volts or 1.2 volts as stated previously. This voltage will decrease as the transistor’s internal temperature increases. This voltage will increase as current passing through the transistor increases. The B to E voltage drop is not identical in all transistors (not even in a pair of the same part number transistor).
The current going to the transistor’s base should be small compared with the current passing through the sensitivity adjustment resistor. If the base current is significant, then the voltage drop from the + voltage to the transistor will be larger and the voltage drop across the controller will increase as the base current increases. The voltage drop depends on the transistors base current relative to the emitter current. The ratio between these currents is known as the gain of the transistor. Tables of transistor properties can list the gain as “Gain” or as "hfe". The gain might be 1000 or more for a darlington pair transistor and as low as 80 to 150 for a single transistor.
The transistors gain is not a constant. The gain changes as the transistor’s internal temperature changes. The gain also changes with the current passing through the transistor (in some cases reaching its maximum at 5 - 10% of the transistors maximum rated current, and decreasing as current rises. And, as stated previously, the gain is not identical in any two transistors (not even in a pair of the same part number).
These changes mean the controller characteristics change as it warms up. This doesn't seem to cause a practical problem, possibly because drivers are used to compensating for the car characteristics changing during a race and the transistor heating doesn't change the controller characteristics all that quickly.
The driver adjusts the controller to suit the car he is using, which is pretty much depends on how much current the motor needs in each corner. The driver will adjust the controller to suit the current that the motor actually takes and also to compensate for the differences between one transistor and another (even though he doesn't realize turning the sensitivity knob on the controller is doing all those things at the same time).
Like the controller, the car characteristics are also changing during a race. Some examples of these changes are - Tires grip better once as they warm up, armature resistance goes up as it warms up, and the cars handling changes as the tires wear or become dirty. Finally, the power supply voltage may change due to time, temperature or current draw. The maximum changes in voltage are with battery powered tracks without battery chargers. However, some regulated supplies output voltage can change depending on the instantaneous current requirement and as the power supply heats up.
The biggest change that the BSCRA writer measured is on a track where battery voltage dropped from 13.5 volts to 12.5 volts during a race. The drivers didn't notice this change - presumably they just braked a bit later and pushed their controllers down a bit more in corners as the race progressed.
The worst-case voltage change I have ever observed was at the final race of the 1981 Indiana HOPRA season. Track power was by batteries only. No battery charger or power supply was installed in parallel to the batteries. The automotive batteries were used and track power went from 18V or more at the start of the day to probably 15V or less at the end of the main as the batteries faded. I discuss the phenomenon of battery fade or why a 50 Amp-Hour battery may only be a five amp-hour battery when used on a slot car track on the Siberia Racing website Tech Pages. During the race voltage wasn’t measured but it is known that the hot motor cars slowed significantly as the batteries were depleted. My car had a lower power motor which wasn’t as impacted by the power drop. Like the little train that could It just kept chugging along. The car made the main and eventually won the race to make me the 1981 Indiana HOPRA champ.
Regards.
Steve "Maddman" Medanic
From "HRW Forum
Jim W
Tags: None
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