DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
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INTRODUCTION
Sheet #: D7p
Title: POWER SUPPLY AND
Updated by:
Updated:
First Issued:
Originally
Compiled by:
Page:
VOLTAGE CONTROL
Richard Napper, MMR
October 1999
October 1956 (D7g.1)
Kenneth Mortimer
The use of selenium rectifiers in power supplies for
1 of 7
model railroading use has passed away. Selenium
rectifiers are no longer even made today. Today's power
sources use solid state electronics with transistors and integrated circuits. These advanced
techniques allow the model railroader unsurpassed control of their locomotives, which are not
obtainable with past methods.
There are two types of power supply design in general use: linear and switching. Although
switching power supply design is usually less costly and more efficient, it does not lend itself to
locomotive control because the switching supply must have a minimum load of approximately
10% of its rated output current (load) before it will even begin to operate. Since there are times
when there will be no load on the locomotive throttle, switching power supplies can not be used
in this capacity. This leaves us with linear power supply design for use as throttles.
All power supplies have their output capacity rated in Volt-Amps (VA). This rating is the total
available power from the power supply, including all outputs. This will include not only the
variable DC output, but also the fused DC and fixed AC outputs in commercially built power
packs. A typical rating may be 18VA. At first glance one would say the Power Pak would provide
12VDC at 1.5 Amps. (12 x 1.5 = 18). Remember that the 18VA is the total of all three outputs, so
the actual power available to run the locomotives may be as low as 12VDC at 0.5 Amp. (6VA).
Keep this in mind if you decide to purchase a commercially built power pack.
Construction of your own power supply can provide the model railroader with the features that
are desired or needed for a particular situation. Just as each model railroad is unique, a custom
built power pack may better fit a given style of locomotive handling.
In the most basic form, all linear power supplies utilize a power transformer, rectifier circuit, and
capacitive filter circuit. In addition, the power pack must have either a throttle circuit, or regulator
circuit to control the locomotive speed. Choke or inductive filter circuits could also be used, but
their superior filtering is not needed in the power pack. The choke adds both major cost and
weight when considered along side the capacitive filter circuit. Inductive filter circuits will not be
used for model railroad power supplies. Capacitive filtering is less expensive, and provides the
necessary ripple reduction desired in the power supply. Capacitive filtering does have its own
disadvantage caused by the short, high current pulses which must flow through the rectifiers and
transformer as the capacitor is charged to replace the current lost to the load when the rectifiers
are turned off.
RECTIFIER CIRCUITS
Fig 1: HALF WAVE
Fig 2: FULL WAVE
CENTER TAP
Fig 2: FULL WAVE BRIDGE
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7b
D7s
Title: POWER
ELECTRICAL
SUPPLY AND
FUNDAMENTALS
VOLTAGE
CONTROL
Page: 2
Issued:
Aug.
of 71954
RECTIFIER CIRCUITS - continued
Three types of rectifier circuits are available for consideration in the power pack. They are half
wave, full wave center tap, and full wave bridge circuit. Fig. 1 shows the half wave rectifier circuit
which should really not be considered because of its extremely high current spikes and poor
filtering. It is only worth using if you need ½ watt or less of power. Since a single locomotive will
use anywhere from 3-6 watts, you can see why the half wave rectifier circuit is not really useful in
model railroading.
Fig. 2 shows the full wave center tap rectifier circuit. Since two diodes are used, you obtain two
current pulses for each cycle which doubles the ripple frequency from 60 to 120 HZ. This reduces
the amplitude of the surge current in the rectifiers (diodes) and transformer while increasing the
circuit filtering effect without an increase in capacitor size. Since all diodes pass current in only
one direction, their output is pulses of current that have a AC component (ripple) riding on top of
the DC current. The purpose of the capacitor filter is to increase the DC current while reducing the
AC (ripple). All power supplies will have some amount of ripple, but the capacitor filter will reduce
it to a negligible amount. This rectifier circuit requires a center tapped secondary transformer
winding. That is why it is called full wave center tap (FWCT).
Fig. 3 is the full wave bridge rectifier circuit. This circuit has all the advantages of the FWCT
circuit, and does not require a center tapped transformer winding. The diode count is doubled over
the FWCT circuit. This circuit needs a higher secondary voltage than is needed for half of the
center tapped circuit above, but the total secondary voltage is less than the FWCT circuit. We will
later see that the bridge circuit requires double the secondary winding current than the FWCT
circuit.
CALCULATING POWER SUPPLY RATING
In order to construct a useable power pack you must decide what output rating you want, and work
backwards from the output to the input. Let us say we desire a power supply that can provide 012VDC at 1.5 Amps (18VA) all of which is available to run the locomotives.
All power supplies will use a power transformer with
a dual primary winding input. The input rating is
120/240VAC that can be used in the USA and
overseas.
We call the power supply linear because all circuit
assemblies are in series from input to output. Fig.
D shows the linear relationship, and shows us that
Fig 4: LINEAR POWER SUPPLY
there are three voltage losses between the
transformer secondary and the desired output
voltage (Vout). Vreg is the voltage drop across the voltage regulator circuit, and it is usually about 3
volts. Vripple is the voltage drop due to the ripple voltage in the output. Let us set this at 5% of the
output voltage peak. 5% of 12VDC is 0.6volts. Vrect is the voltage drop across the diodes used in
the rectification circuit. This value is usually 1.25 volts. Vac is the RMS value of the transformer
secondary winding voltage. It will be the full secondary voltage for the bridge circuit, and must be
doubled for the FWCT circuit.
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7b
D7s
Title: POWER
ELECTRICAL
SUPPLY AND
FUNDAMENTALS
VOLTAGE
CONTROL
Page: 3
Issued:
Aug.
of 71954
CALCULATING POWER SUPPLY RATING - continued
The formula to calculate the secondary transformer voltage is:
The secondary winding voltage is 33.5VAC centered tapped for the FWCT rectifier circuit.
For the bridge rectifier circuit, we must do the same calculation except that the Vrect will be twice
as much because there are two diodes in the circuit at all times. The secondary winding voltage
will be 17.99VAC for the bridge rectifier circuit. As previously stated the voltage for the bridge
circuit is slightly more than ½ the voltages for the FWCT circuit. We now have half of the
necessary specifications for the transformer secondary winding. We now need to find the current
rating, lac is the secondary winding current. Idc is the output current of the power supply.
For the FWCT circuit: Iac=1.2 x Idc therefore Iac=1.2 x 1.5Amps so Iac=1.8Amps
For the Bridge circuit: Iac=1.8 x Idc therefore Iac=1.8 x 1.5Amps so Iac=2.7Amps.
The minimum specifications for the transformer secondary are:
For the FWCT circuit: 33.5VAC Center Tapped @ 1.8AMPS. VA=60.30
For the bridge circuit: 17.99VAC @ 2.7AMPS. VA=40.57
Notice that the bridge rectifier circuit is a little more efficient in its conversion than the FWCT
rectifier circuit. The bridge circuit does require that the transformer secondary winding supply twice
the current required from the power supply output. Our supply current output is 1.5Amps. But the
secondary winding must supply almost exactly twice that amount, just under 3.0Amps. This is the
defining characteristic of any bridge rectifier circuit.
These calculations give you the minimum specifications for the needed transformer for our power
supply. Transformers are readily available with 16 or 18VAC no center tap, or 36VAC with a center
tap. Secondary winding current is available up to 100Amps. so current rating is no problem. For
our use in this example, purchase a 36VAC CT transformer with a secondary current of 2amps.
For the FWCT rectifier circuit, and use a 18VAC transformer with a secondary current of 4Amps.
for the bridge rectifier circuit.
DIODES
The diodes used in the power supply must be able to pass the current in the power supply and
withstand the PIV (peak inverse voltage) applied to it when the diode is turned off. It does not hurt
a thing to use ratings that are 50% greater than required for the diodes. Vpeak=VAC x 1.414 For
the FWCT circuit Vpeak=18VAC x 1.414 Vpeak=25.45VAC, the same is correct for the bridge
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7b
D7s
Title: POWER
ELECTRICAL
SUPPLY AND
FUNDAMENTALS
VOLTAGE
CONTROL
Page: 4
Issued:
Aug.
of 71954
DIODES - continued
circuit. Diodes come with a PIV of 50V, which would work, but you would be better using diodes
with a PIV rating of 100V. For the FWCT circuit, the diode must carry a current of at least 2Amps.
but a rating of 3Amps. would give a much better safety factor. For the bridge circuit the minimum
current rating would be 4Amps. however, 6Amps. would be safer.
For the FWCT circuit: Diodes should be 100PIV @ 3 Amps.
For the bridge circuit: Diodes should be 100PIV @ 6 Amps.
FILTER CAPACITORS
Now let's calculate the necessary filter capacitor size for each amp. of output current.
Iload=output load current, deltaV=peak to peak ripple voltage
The output current of our power supply is 1.5AMPS., so the required filter capacitor is 15,000mfd
@25WVDC. The voltage rating of the capacitor will insure that it will safely handle the voltage
applied to it.
Since the bridge rectifier circuit has
a lower VA requirement, we will
use it for our final configuration,
Fig. 5.
Such as circuit will provide us with
the necessary output power
12VDC @ 1.5 AMPS. Our circuit
will provide us with highly filtered
DC with little ripple. If you wish to
have pulse power for your
locomotives add the switch in Fig.
Fig 5: BASIC POWER SUPPLY
G. With the refined motors used in
today's locomotives, pulse power is really not necessary, and it causes motors to overheat. I do
not recommend using pulse power in today's power packs.
VOLTAGE CONTROL
To be able to use the power
supply as a throttle, we need to
add a voltage control circuit to
the basic supply of Fig. 5. The
very simplest type of throttle
control we could add would be
the series rheostat to limit the
current drawn by the motor.
The style control is the poorest
type for motor regulation, and
will still have the problem that
the rheostat may not stop all
Fig 6: POWER SUPPLY WITH PULSE POWER
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7b
D7s
Title: POWER
ELECTRICAL
SUPPLY AND
FUNDAMENTALS
VOLTAGE
CONTROL
Page: 5
Issued:
Aug.
of 71954
VOLTGAGE CONTROL - continued
motors due to the use of very low current motors in use today.
The first step up in throttle control would be a single transistor/potentiometer. The transistor must
be able to pass the full rated power output of the power supply without overheating. A power
transistor should be used that is mounted on a large aluminum heatsink, use an insulating kit
between the heatsink and the transistor. A 2N3055 power transistor can handle ten amps. of
current at over 40 volts, so it would be a good choice for the power transistor. The potentiometer
is a 10K ohm, 2 Watt, linear taper resistor. Fig. 7 shows the resulting circuit. This circuit will
provide you with a basic throttle control over your locomotives, but the regulation is still not very
good. The potentiometer must be turned about halfway up before the locomotive will start to move
limiting the throttle response to only half of the potentiometer travel. We need finer throttle than
can be provided with the single transistor control.
Fig 7: POWER SUPPLY WITH THROTTLE CONTROL
We can increase the throttle control by providing a larger current gain than is available in the single
transistor. In this way a very small change in potentiometer rotation will control a much larger
current change in the motor giving us better throttle response. By adding additional transistors to
the circuit, we can obtain our control. We shall use a special type of transistor called a Darlington
pair, which really is two transistors, cascaded for us in one case. We will use the TIP120 NPN
Darlington pair transistor. Examine Fig. 8 for the changes. The potentiometer has been changed
to a 5Kohm value, but you could just as well use the 10Kohm-pot from the previous circuit. R1 is a
current limiting resistor used to save the two transistors from a shorted output, I would suggest a
value from 4.3 - 4.7Kohms, ½ watt. Since both transistors have the same collector voltage, you
can mount both of them on the same heatsink for better thermal regulation. Either insulate both
transistors from the heatsink, or insulate the heatsink from ground. Although this throttle is still
pretty simple, it will provide you with good locomotive control using few parts.
Fig 8: POWER SUPPLY WITH DARLINGTON PAIR
Sheet #: D7b
D7s
Title: POWER
ELECTRICAL
SUPPLY AND
DATA SHEET
FUNDAMENTALS
VOLTAGE
CONTROL
Page: 6
Issued:
Aug.
of 71954
© NATIONAL MODEL RAILROAD ASSOCIATION
ADJUSTABLE VOLTAGE REGULATORS
Over the years, advancements in electronic component design have now provided us with a
complete voltage regulator circuit in a single package that looks like a transistor. These are called
adjustable voltage regulators. They can provide regulation down to 1.25 volts, and up to 40 volts
on the high end. They require a few external parts before the regulator will function correctly.
These regulators can control currents as low as 0.1Amp., 1.5Amp., 3Amp., or 5amps. One such
manufacturer is National Semiconductor Corporation. The 0.1Amp. Regulator is the LM317LZ, the
1.5Amp. Regulator is the LM317T/K, the 3.0Amp. Regulator is the LM350, and the 5.0Amp.
Regulator is their LM338. All of these regulators are fully short circuit proof, and overheat proof. In
both cases the regulator will just shut itself off until the problem has been corrected.
Fig 9: POWER SUPPLY WITH ADJUSTABLE VOLTAGE REGULATOR
We can use the LM317T regulator for our power supply. It will provide the full 1.5Amps. current
without any problem, but you might use the LM350, 3.0 Amp. Regulator to give yourself a safety
margin. Our power supply circuit now becomes the circuit in Fig. 9. Capacitor C1 is added at the
input to the regulator, LM317T, and it should be a 0.lmfd @ 25WVDC Tantalum. C2 on the output
should be a 1mfd @ 25WVDC Tantalum capacitor. Our regulator needs the 240 ohm ¼ watt
resistor between the output terminal and the adjust terminal with our throttle potentiometer, 5Kohm,
installed as a rheostat between the adjust terminal and ground. The LM317T or LM317K regulator
must be mounted on a heatsink with an insulator kit. The LM317T is in a TO-220 transistor case
while the LM317K is in the TO-3 case. See the pin outlines Fig 10 below for both types.
Metal tap
is also Vout
Vadj
Vin
Vout
LM350/LM317K
TO-220
LM317T
Fig 10: PIN OUTLINES
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7b
D7s
Title: POWER
ELECTRICAL
SUPPLY AND
FUNDAMENTALS
VOLTAGE
CONTROL
Page: 7
Issued:
Aug.
of 71954
ADJUSTABLE VOLTAGE REGULATORS - continued
These regulators will provide you with the best throttle control of one locomotive or a whole lash-up
within their current limits. Remember that these regulators will not turn totally off, the lower output
is 1.25Volts. This should not cause you any trouble because locomotives do not start to move on
less than 3-5Volts. The locomotive motors will not overheat on 1.25Volts.
The power transformers in all of these power supplies provide the necessary isolation from the
120VAC house circuits. The primary winding of the transformer is at 120VAC potential, and you
should build these power supplies in either a grounded steel or aluminum case or a fully insulated
case made from high impact plastic. In North America, the standard wall outlet has three
terminals. The smaller slot is the HOT terminal, the larger slot is the NEUTRAL, and the "D" slot is
the GROUND. You must use a 3 wire grounded plug if you build the supply in a metal case, or use
a polarized two plug if you use the plastic case. No exceptions, this is for your own safety!
All ON/OFF switches, and fuses MUST be installed in only the HOT wire!
NEVER put these devices in the NEUTRAL or GROUND WIRE.
The three wire power cord will be color coded with BLACK or BROWN for the HOT wire, WHITE or
BLUE for the NEUTRAL wire, and GREEN OR GREEN/YELLOW for the GROUND wire. Wire
your transformer primary winding as shown in Fig. 11.
Fig 11: TRANSFORMER CONNECTION DIAGRAM
The power supply provided here is an excellent basic power pack. You can add reversing loop
switches, direction switches, and Volt/Amp meters for a complete supply. Advanced designs
include momentum and braking circuits, and Digital DCC controls. There are many books and
magazine articles that will provide you with the sophisticated power supply designs.