Tuesday, September 20, 2011

Current sources

Many applications require current sources rather than voltage sources. Current source provides a constant current to the output. This current does not depend on the output voltage. An idea current source is a high output impedance device: it will supply current, but does not show in other ways on the load circuit.
When you need a low-current source, using a linear regulator is a typical approach. When you need a high-current source, using a linear regulator is inadvisable, because of the high power dissipation in the series resistor. To solve the wasted-power problem, you can use a switch-mode regulator.

  • 40V current source operates from -40 to +85°C - wide-output-current range sources that accommodate a wide supply and temperature range, based on LM317
  • Current source has high output impedance - composite 10-mA current-source configuration that has a compliance voltage of 5 to 42V, a set-current error of less than 1% and an output impedance of greater than 100 megaohms
  • Digital current source is nonvolatile - Digitally programmable current sources that feature automatic trimming and retain the setting despite power-down cycles are useful in applications such as RF- and laser-communications drivers. This circuit is particularly suited for setting the drive current for the optical pump in widely tunable VCSELs (vertical-cavity surface-emitting lasers).
  • High-speed regulator makes great current source - bandwidth of this circuit measures more than 1 MHz, only the selected MOSFET and the ratings of the input supply's bypass capacitors limit the voltage compliance
  • L200 Regulator Circuit - easy to build a power supply with one single L200 IC, offers a variable current limit of up to 2 A, as well as voltage regulation
  • Precision current sink costs less than $20 - simple active load circuit for current from 1 mA to 1A and voltage from 3 to 40V
  • Programmable current source powers charger - digitally programmable current source capable of sourcing currents as high as 2.55A
  • Simple scheme keeps current drain constant - It is sometimes advantageous to keep the overall current consumption of an electronic device constant. This circuit outptu variable current to 5V load and maintains a constant current of approximately 102 mA in the power supply input.
  • Switching regulator forms constant-current source - When you need a high-current source, using a linear regulator is inadvisable, because of the high power dissipation in the series resistor. To solve the wasted-power problem, you can use a switch-mode regulator. This circuit uses an LM2576 adjustable regulator. It needs only a few external elements and has an adjustable sensing input, which you use for controlling the output current.
  • Variable 3 - 24 Volt / 3 Amp Power Supply - regulated power supply can be adjusted from 3 to 25 volts and is current limited
  • Variable Power Supply - based on versatile L200 voltage regulator, independent voltage (3-15V) and current (10mA-2A) limits

Chargers

Battery information and circuits

    Normal batteries have generally 1.5V per cell voltage (except some Lithium cells which have 3V voltage). The batteries which have higher voltage output are built genrally from many 1.5V cells in series all put inside same "case". Rechargeable batteries have generally somewhat different lower voltages and capacity than their their "only once usable" counterparts. Normal alkalines are 1.5V each, but Ni-Cd batts are only 1.2V each. If you use four of them for instance, you only get 4.8V instead of 6V. You can make up for it by adding a cell for each four cells, so five cells will give you 6V. That's probably why some equipment instructions don't allow them. Another reason could be that NiCd batteries have enormous short-circuit current capability (easily tens of ampreres or more) compared to many traditional batteries (causes greater risk of fire in case of short circuit). NiCd batteries are easy to charge at slow charge mode. Just apply a charging current, typically around 50 mA for 500mah cells, for 12-14 hours to change them. It does not hurt the batteries if you charge the batteries at the current with somewhat longer times (even few times longer) than needed. When batteries are are charged after they are full, the applied power is converted to heat, so NiCd batteries should start to become warm then. Some will say sinking C/10 (50mA for 500 mAh battery) indefinately into NiCd is acceptable but this is not a recommended practice. NiCd batteries can also be charged faster with special "smart chargers" which use controlled higher current to the batteries until they are charged and stop this current when batteries are full (the charger has special sense circuitry for this). Exercise extreme precaution when handling and testing NiCd batteries. NiCd batteries include some amount of cadmium (Cd), which is dangerous material for enviroment. Do not throw NiCd batteries away with your trash. You should bring those batteries to recycling. When discharging NiCd battery packs be cautious about over-discharge. Individual NiCd cells can go to zero volts but in a battery pack letting them drop below 1V each may reverse and ruin a weak cell since they are in series. In some applications NiCd batteries are replaced with higher capacity NiMH batteries. Those batteries have pretty much similar characteristics to NiCd (except that they need somewhat different charger). NiCad and NiMh cell voltages are identical. There is some difference at end of charge and used by sophisticated chargers. NiMh are sensitive to overcharge. NiMh should be preferably be always charged with "smart chargers" designed to charge NiMh batteries. NiMh batteries can also be charged with constant current C/10 (50mA for 500 mAH cell) current, but you should avoid excessive overcharge because this will shorten battery life. Compared to Nicad NiMh are nearly twice the capacity, can be recharged from any level, selfdischarge more tha twice as fast, have about half the full cycle number life, and are not as robust when charging. Many new small gadgets like cellular phones use Li-ion batteries. Li-ion batteries have a high energy density. Exercise extreme precaution when handling and testing Li-ion batteries. Do not short circuit, overcharge, crush, drop, mutilate, penetrate, apply reverse polarity, expose to high temperature or disassemble them. Only use the Li-ion battery with the designated protection circuit (cellular phone batteries usually have this). Abuse of Li-ion can cause "explosion" like happening, because in short circuit the case temperature can get very high and the electrolyte inside Li-ion battery is highly flammable. Car batteries are built are lead acid cells. They have a cell voltage of around 2 volts (means 6 cells in series makes 12V battery). Normal 12V car battery is designed to be quite robust. It can be carged with almost any reasonably current limited constant voltage source of around the nominal voltage of the battery (for example 13.5V for 12V car battery). Car batteries are built for heavy currents but DO NOT deep discharge them, only shallow discharge. If you repeatedly run a car battery down to much below 50% of capacity you will seriously shorten its life. Warnings on car batteries: Car batteries contain dangerous acid hare heavy, so handle carefully. Charging car batteries can generate highly flammabble hydrogen, so it is best to charge those only in well ventilated spaces. Car batteries have very large short circuit current (hundreds of amperes), so do not short circuit them (a fuse near battery terminal is essential for safety to avoid wire fires in short circuit happens). There are also other types of lead acid cells than car batteries. Some are designed for deep discharge use and some for some other applications. Generally lead acid batteries don't like to be discharged below 1.67 volts per cell (10V for a 12V battery) and their full capacity can only be extracted if the load current is something like C/10 or C/20 (where C is the barrery capcity in Ah and resulting current is in A). Short list of most common battery characteristics:
    • Lead Acid: most economical for larger power applications where weight is of little concern (cars, boats, wheelchairs, emergency lighting, UPS systems), low energy density (30-50 Wh/kg), available in low cost versions, cell voltage 2V, voltage limiting rather than current limiting is used for charging
    • Sealed lead acid (SLA,Gelcell): maintenance-free lead acid battery with electrolyte in moistened separators, enclosure is sealed, used for wheeled mobility, typical charge times are 8 to 16 hours, must always be stored in a charged state, 200 to 300 discharge/charge cycles
    • Nickel Cadmium (NiCd): mature and well understood technoogy used in chargeable batteries used in many applications (power tools, two-way radios, video cameras), standard against which other batteries are usually compared, not very good energy density (45-80 Wh/kg), cell voltage 1.25V, life cycle 1000-1500 charges
    • Nickel-Metal Hydride (NiMH): higher energy density (60-120Wh/kg) compared to the NiCd at the expense of reduced cycle life, no toxic metals, used in mobile phones and laptop computers, cell voltage 1.25V, life cycle 300-500 charges
    • Lithium Ion (Li-ion): fastest growing battery system, high-energy density (110-160 Wh/kg) and lightweight, technology is fragile and a protection circuit is required to assure safety, applications in notebook computers and cellular phones, cell voltage 3.6V, life cycle 500-1000 charges
    • Lithium Polymer (Li-polymer): uses a dry solid polymer electrolyte, cell thickness measuring as little as one millimeter, suffers from poor conductivity (high internal resistance)
    • Lithium Ion Polymer (Li-ion polymer): uses a combination of dry polymer electrolyte combined with some gelled electrolyte, ultra-slim geometry, high energy density (100-130 Wh/kg), used in mobile phones, cell voltage 3.6V, life cycle 300-500 charges, promotional reasons most battery manufacturers mark this kind of battery simply as Li-polymer
    • Reuasable Alkaline: Special alkalinen battry which can be charged few times (energy density 80 Wh/kg), cell voltage 1.5V, life cyle around 50 cyled of 50% charge/recharge

    General battery information

    General charging circuits

    Battery chargers for NiCd batteries

    • Battery charger indicates rate of charge - a single LED indicates whether the battery charger s delivering a trickle charge or a fast charge, cricuit designed to charge 2-14 cells
    • Constant Current Nicad Charger - The schematic for this charger is pretty simple. You can charge from 1 to 20 +/- nicads at a constant current of from 20 to 200 ma +/-.
    • Make Your Own Simple Rx/Tx Battery Charger with Peak Detect - This circuit is designed to peak charge Rx and Tx batteries. It's programmed for a C/2 charge rate for 250mAh and 500mAh batteries (charge currents of 125mA or 250mA). It'll charge Rx from 12V at the field, Tx from a 15V supply (like a car w/engine running). Use a 15-18V supply to charge at home (you should be able to find wall cubes with this rating). It is based on a Maxim IC, the MAX713.
    • Ni-Cd Batteries Charger - A very basic circuit that takes approximately 12-15 hours of charge AA-batteries at 50mA charging current.
    • Nicad Battery Charger - uses a single transistor as a constant current source
    • NiCd or NiMh battery charger - This charger can be used for AAA, AA and Baby C batteries. This battery charger is based on MAX712/MAX713.

    Li-ion battery chargers

    • Charge Li-ion batteries from ac line voltage - converts energy from 120V ac to a regulated voltage or current as necessary to charge two Li-ion cells in series
    • Supply derives 5 and 3.3V from USB port - This circuit derives its power from a USB port and produces 5 and 3.3V supply rails for portable devices, such as digital cameras, MP3 players, and PDAs. The circuit allows the port to maintain communications while, for example, charging a lithium-ion battery. IC2 boosts the battery voltage, VBATT, to 5V, and IC3 buck-regulates that 5V output down to 3.3V.

    Lead-acid battery cargers

    Combined power supplies and batery chargers

    Battery discharging circuits

    Battery backup circuits

    Battery protection

    Battery status sensors

    Normal battery chargers

    Bascially all types of alkaline cells can be recharged, although the battery manufacturers discourage this (dangers of overcharging and battery leaking). You will need a special charger for charging alkaline cells (normal NiCd chagers are not suitable for this). There are some special alkaline cells nowadays which chan be charged better than normal alkaline cells. The best practice is not to discharge completely the cell or battery but rather to give a short charge often. Do not attempt to recharge a totally discharged cell or a cell showing even the slightest sign of damage.
    • Alkaline Charger - This circuit was specifically designed to recharge alkaline cells. It will take around 1 day for a discharged AA cell or 9V battery and up to several days for a large D type cell to carge.

    Other battery circuits

Solar panels and circuit for them

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