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Monday, December 23, 2013

Low cost Step down Converter with Wide Input Voltage Range

The circuit described here is mostly aimed at development engineers who are looking for an economical step-down converter which offers a wide input volt- age range. As a rule this type of circuit employs a step-down converter with integrated switching element. However, by using a more discrete solution it is possible to reduce the total cost of the step- down converter, especially when manufacturing in quantity. The TL5001A is a low-cost PWM controller which is ideal for this project.

Low-cost Step-down Converter with Wide Input

The input voltage range for the step-down converter described here is from 8 V to 30 V, with an output voltage of 5 V and a maximum output current of 1.5 A.

When the input voltage is applied the PWM output of IC1 is enabled, taking one end of the voltage divider formed by R1 and R2 to ground potential. The cur-rent through the voltage divider will then be at most 25 mA: this value is obtained by dividing the maximum input voltage (30 V) minus the saturation voltage of the output driver (2 V) by the total resistance of the voltage divider (1.1 kΩ ). T1 and T3 together form an NPN/PNP driver stage to charge the gate capacitance of P-channel MOSFET T2 as quickly as possible, and then, at the turn-off point, discharge it again.

The base-emitter junction of T3 goes into a conducting state when the PWM output is active and a voltage is dropped across R2. T3 will then also conduct from collector to emitter and the gate capacitance of T2 will be discharged down to about 800 mV. The P-channel MOSFET will then conduct from drain to source. If the open-collector output of the controller is deactivated, a negligibly small current flows through resistor R2 and the base of T1 will be raised to the input voltage level. The base-emitter junction of T1 will then conduct and the gate capacitance of T2 will be charged up to the input voltage level through the collector and emitter of T1. The P-channel MOSFET will then no longer conduct from drain to source. This driver circuit constructed from discrete components is very fast, giving very quick switch-over times.

Diodes D2 and D3 provide voltage limiting for the P-channel MOSFET, whose maximum gate-source voltage is 20 V. If the Zener voltage of diode D2 is exceeded it starts to conduct; when the forward voltage of diode D3 is also exceeded, the two diodes together clamp the gate-source voltage to approximately 19 V.

The switching frequency is set at approximately 100 kHz, which gives a good compromise between efficiency and component size.

Finally, a few notes on component selection. All resistors are 1/16 W, 1 %. Apart from electrolytic C1 all the capacitors are ceramic types. For the two larger values (C2 and C5) the following are used:

C2 is a Murata type GRM21BR71C105KA01 ceramic capacitor, 1 µF, 16 V, X7R, 10 %;

C5 is a Murata type GRM32ER60J476ME20 ceramic capacitor, 47 µF, 6.3 V, X5R, 10 %.

D1 (Fairchild type MBRS340T3) is a 40 V/3 A Schottky diode. Coil L1 is a Würth WE-PD power choke type 744771147, 47 µH, 2.21 A, 75 mΩ.

T1 (BC846) and T3 (BC856) are 60 V, 200 mA, 310 mW complementary bipolar transistors from Vishay.

The TL5001AID (IC1) is a low-cost PWM controller with an open-collector output from Texas Instruments.

Author : Dirk Gehrke Copyright : Elektor

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Sunday, May 5, 2013

Build a Converter VGA to BNC Adapter

There are monitors which only have three BNC inputs and which use composite synchronization (‘sync on green’). This circuit has been designed with these types of monitor in mind. As can be seen, the circuit has been kept very simple, but it still gives a reasonable performance. The principle of operation is very straightforward. The RGB signals from the VGA connector are fed to three BNC connectors via AC-coupling capacitors. These have been added to stop any direct current from entering the VGA card. A pull-up resistor on the green output provides a DC offset, while a transistor (a BS170 MOSFET) can switch this output to ground. It is possible to get synchronisation problems when the display is extremely bright, with a maximum green component.

In this case the value of R2 should be reduced a little, but this has the side effect that the brightness noticeably decreases and the load on the graphics card increases. To keep the colour balance the same, the resistors for the other two colors (R1 en R3) have to be changed to the same value as R2. An EXOR gate from IC1 (74HC86) combines the separate V-sync and H-sync signals into a composite sync signal. Since the sync in DOS-modes is often inverted compared to the modes commonly used by Windows, the output of IC1a is inverted by IC1b. JP1 can then by used to select the correct operating mode. This jumper can be replaced by a small two-way switch, if required.

VGA to BNC adapter PCB layout

This switch should be mounted directly onto the PCB, as any connecting wires will cause a lot of interference. The PCB has been kept as compact as possible, so the circuit can be mounted in a small metal (earthed!) enclosure. With a monitor connected the current consumption will be in the region of 30 mA. A 78L05 voltage regulator provides a stable 5 V, making it possible to use any type of mains adapter, as long as it supplies at least 9 V. Diode D2 provides protection against a reverse polarity. LED D1 indicates when the supply is present. The circuit should be powered up before connecting it to an active VGA output, as otherwise the sync signals will feed the circuit via the internal protection diodes of IC1, which can be noticed by a dimly lit LED. This is something best avoided.

Resistors:
R1,R2,R3 = 470Ω
R4 = 100Ω
R5 = 3kΩ3

Capacitors:
C1,C3,C5 = 47µF 25V radial
C2,C4,C6,C7,C10 = 100nF ceramic
C8 = 4µF7 63V radial
C9 = 100µF 25V radial

Semiconductors:
D1 = LED, high-efficiency
D2 = 1N4002
T1 = BS170
IC1 = 74HC86
IC2 = 78L05

Miscellaneous:
JP1 = 3-way pinheader with jumper
K1 = 15-way VGA socket (female), PCB mount (angled pins)
K2,K3,K4 = BNC socket (female), PCB mount, 75Ω

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Wednesday, March 27, 2013

Valve Sound Converter

‘Valve sound’ is not just an anachronism: there are those who remain ardent lovers of the quality of sound produced by a valve amplifier. However, not everyone is inclined to splash out on an expensive valve output stage or complete amplifier with a comparatively low power output. Also, for all their aesthetic qualities, modern valve amplifiers burn up (in the full sense of the word!) quite a few watts even at normal listening volume, and so are not exactly environmentally harmless. This valve sound converter offers a cunning way out of this dilemma. It is a low cost unit that can be easily slipped into the audio chain at a suitable point and it only consumes a modest amount of energy.

A valve sound converter can be constructed using a common-or-garden small-signal amplifier using a readily-available triode. Compared to using a pentode, this simplifies the circuit and, thanks to its less linear characteristic, offers even more valve sound. For stereo use a double triode is ideal. Because only a low gain is required, a type ECC82 (12AU7) is a better choice than alternatives such as the ECC81 (12AT7) or ECC83 (12AX7). This also makes things easier for home brewers only used to working with semiconductors, since we can avoid any difficulties with high voltages, obscure transformers and the like:the amplifier stage uses an anode voltage of only 60 V, which is generated using a small 24 V transformer and a voltage doubler (D3, D4, C4 and C5).

Since the double triode only draws about 2mA at this voltage, a 1 VA or 2 VA transformer will do the job. To avoid ripple on the power supply and hence the generation of hum in the converter, the anode voltage is regulated using Zener diodes D1 and D2, and T1. The same goes for the heater supply: rather than using AC, here we use a DC supply, regulated by IC1. The 9 V transformer needs to be rated at at least 3 VA. As you will see, the actual amplifier circuit is shown only once. Components C1 to C3, R1 to R4, and P1 need to be duplicated for the second channel.

The inset valve symbol in the circuit diagram and the base pinout diagram show how the anode, cathode and grid of the other half of the double triode (V1.B) are connected. Construction should not present any great difficulties. Pay particular attention to screening and cable routing, and to the placing of the transformers to minimise the hum induced by their magnetic fields. Adjust P1 to set the overall gain to 1 (0 dB). The output impedance of 47 kΩ is relatively high, but should be compatible with the inputs of most power amplifiers and preamplifiers.

For a good valve sound, the operating point of the circuit should be set so that the audio output voltage is in the region of a few hundred millivolts up to around 1.5 V. If the valve sound converter is inserted between a preamplifier and the power amplifier, it should be before the volume control potentiometer as otherwise the sound will change significantly depending on the volume. As an example, no modifications are needed to an existing power amplifier if the converter is inserted between the output of a CD player and the input to the amplifier.

http://www.ecircuitslab.com/2012/04/valve-sound-converter.html

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