This little charger was designed and built in a day because I could not find anything commercially available to do the job. And that job was to charge very small packs at their C/10 rate. Specifically, 8 new 150 mAh NiCads needed to be charged before becoming part of a pack, and the really nice peak detect charger sitting on my desk only goes down to 100 mA charge rate. That, and wanting to be able to slow charge my existing 270 mAh packs decided the range on currents that needed to be included. This little charger fills those requirements to a tee and has even more to offer.
Knowing a little about electronics and chargers can be a dangerous thing, so this design is as simple as possible, consistent with good practice. For a constant current source thats variable with low parts count, the LM317 is hard to beat, so this chip was selected. In order to use this regulator as a current source, the resistor is placed in series with the load and then the ADJ pin is connected. This causes the device to regulate the current until 1.25 volts is developed across the resistor instant current source. By changing the value of the resistor you change the current according to the formulae A=1.25/R. Could not be simpler than that. There are, however, two things to be aware of: 1) your resistor is in the load side of the charger, generating heat at the rate of 1.25 X Amps, and 2) there is a voltage drop of 3 volts required across the LM317 in order for it to do its job. Neither of these poses a problem in this design, because the currents are very low (10 to 300 mA) and the LM317 is good for 37 volts (or 42 in the HV version). Thus, the number of cells that can be charged is only limited by the input voltage. In order to get the variable output currents we could use a rotary switch, but instead a dip switch was chosen, as they are much smaller and cheaper. The input power on my charger is a wall wart, rated at 300 mA and 12 volts, but any DC supply up to 30 volts could be used. The circuit design follows.
As you can see there is no provision for automatic cutoff. This is not a NOT a peak detect charger just a constant current source. At a latter date this may change, but for now its KISS. The LM317 has current limit and thermal shutdown but let's NOT test this out the resistors are going to be burning long before any of this cuts in. On my charger there are actually three outputs: OUT1 and OUT2 go to the "N" cell holders, but my 4 pin Deans connects directly to the bussed side of DIP1. R9 and R10 make the current split between the two holders, as long as there is a battery in each if there is not, the entire output current would go into the single cell that is actually in the circuit.
The picture really says it all, but some elaboration is in order. Normally a fancy EDA tool would be used to lay this out, but its so simple that I just didnt bother. Note that one side of all the resistors connects to the output leg of the LM317 and then to one of the switches in the DIP pack. All of the pins on the other side of the DIP pack then connect to the ADJ leg of the LM317, and this is also the output that connects to the battery you want to charge. To select a current, you turn on a switch, and thats it. Remember to turn off any other switches that may have been turned on from a previous charging session because the currents will sum: i.e. add together. Here is the component and back views of the charger.
Parts list and choosing values for the resistors:
All part numbers Radio Shack
ID Part Part Number Required IC1 LM317 #276-1778 1 Rx Resistors #271-308 (100 various values) 1 DIP1 Switch #275-1301 1 PCB PCB #276-149A 1 C1 0.1uF #272-135 1 PS Power Wall wart, find one J 1 CN Connect Connectors you need / want 2
Current for resistors in the package.
R Current (A) Current (mA) 10 0.1250 125 47 0.0266 26.6 100 0.0125 12.5 150 0.0083 8.3 220 0.0057 5.7 470 0.0027 2.7
There is also the freedom to add two resistors together to get a desired current. For example, if you wanted a 15 mA channel you can get very close by putting a 100 Ohm and a 470 in parallel that will supply 15.1 mA pretty close. In my charger there are four 27 mA channels, two 12 mA channels and one each 8 and 6 mA. Feel free the chose the ones you would like. Put your selections in R1 through R8, R9 and R10 are used in my charger to split the current to the two "N" cell holders, but are out of the circuit for the deans 4 pin plug. All my larger packs use this type of plug, as do my other chargers, so when a different plug comes along I make a pigtail to convert from 4 pin deans to the desired connector.
Building this charger is very easy and could be laid out any number of ways. Ill pass on some tips of how I built mine.
- Place and bend the leads on IC1 as shown. Then solder in place.
- Place DIP1 and solder just two pins on opposite corners.
- Select and write down the resistors you want to use, and what order they go in. Bend all the resistor leads so they fit in the rows of holes shown. Select the top one and trim one lead to about 0.4" long. Place this resistor with the short leg on the side close to the DIP switch. Bend the sort leg over in the direction of the DIP switch and make sure it reaches the DIP pin, but not past it. Once it's right, use it as a template to cut all the others. Put R2-R8 in the board and solder to the DIP pins. Trim the other leg on the resistors to about 0.1" above the board surface. Now place R1 in the board and solder just the DIP side. Take the lead from the other side and bend it flat on the board touching or close to all the other resistor leads AND the OUT pin on IC1. Trim the end as required so its neat. Solder this lead to the other resistor leads first and then to the OUT pin on IC1.
- Place C1 on the PCB and solder. Do not cut the leads; instead, bend the top one over so it connects all the far end DIP pins. Solder all these connections. Take a left over resistor lead and connect the same pin on the capacitor and the ADJ pin on IC1 (the middle one). Take the other lead and bend it over leaving enough length so you can conveniently connect the (-) power supply connection and any output connectors you want.
- Now check all the joints and make sure everything that should be connected is and that nothing is shorted. I missed the adjust connection the first time around and could not figure out why there was always about 250 mA output. After looking at it the non-connection was found and fixed.
- Now to test it. Turn all the switches OFF and connect the power supply and feel the LM317 and resistors (notice there should be no load connected at this time). Neither it nor the resistors should get warm at all. If anything gets warm, remove the power and check your work. Once all is well, short the output connector and recheck for any heating. If there is none, turn on the switch that supplies the least amount of current that is larger than 10 mA (remember the chip needs at least this load to work correctly.) Assuming you are using a 12 volt supply and something less that 50 mA, it's going to be hard to feel any heat being generated so if you don't all is still well. If you are using a wall wart with an adjustable output voltage, the lowest usable is 4.5V anything less and the regulator can't work. Now to check the current, use any voltmeter with an Amps range. Connect the leads to the proper points on the meter and select the highest range (most likely 20A), short the output though the meter, and watch. There should be no reading on a 20A scale; if there is, immediately remove power and check everything again. If you have a nice auto ranging meter it will show you the current being delivered and it should be very close to the calculated value. If you have a manual range meter you may have to move connections for the next lower amp range. Try it there and look for a reading; continue until you get a reading on the meter. If all is well and the meter reading is within 5%, you are all done.
- Remember this charger has no automatic shut off, you must remove the battery in order to stop charging. Note that if you leave a battery connected without input power it will slowly get discharged by the LM317. Now, go charge some batteries so you can fly again.
This is a very simple little charger (and discharger, by the way) that is meant to do one thing well. You won't be disappointed in how well it works; the LM317 will take care of you for years to come. So far the only batteries charged by my charger have been NiCads, but there is no reason NiMH could not be charged this way. How about some enterprising soul posting something about charging these? NEVER use this charger for lithium batteries of ANY sort.
- Cheap, about $18 Canadian (Im a Canuk, eh!) + connectors and wall wart, and there is still a bunch of parts left over. At a real electronics store this would be more like $10, so US about $7. Lets see who can build it the cheapest no cheating, only new parts J .
- Excellent construction notes. J
- Nothing smoked yet J .
Please post any comments to me on the E-Zone Discussion Groups so everyone else can gain from your insight as well. I usually keep up on the sailplane, slow flyer and modeling science discussions; my user name on the E-Zone is pfg. If there is enough interest, a kit of parts or even assembled chargers could be made available.
|Nov 25, 2004, 12:56 PM|
i made this simple charger but instead of using an array of current resistors i used only one (5 ohms) to give me an output of 250mA. however, when i tested it using only one cell, the output current is only 110mA. when i added another cell, output current went down to 50mA.
my batteries are AA nimh/s, 2300mAh, 1.2volts. current was tested by putting in series a milliammeter. my intention for making this charger was to charge household nimh/s at C/10.
what did i do wrong? thanks.
|Feb 17, 2005, 02:32 AM|
Joined Feb 2005
hi!i'm a new member here & im about to make this charger by jbourke. im planning also to use only a 5-ohm resistor to charge my ni-mh 2100 mAh, 1.2volts. have u found remedy to your problem?kindly forward to me at firstname.lastname@example.org in case i'l meet d same prob...tnx!
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