default

2013-06-30 Turn On A Device With A Normally Closed Switch

Electronic Components
on June 30th, 2013 by - No Comments

DSC_0399Often a project has to be turned on or off by a switch that has only one set of contacts, and those contacts are normally closed.  When the project needs to be activated, the switch has to go from normally closed to open.  One common example of this is a window or door switch that is magnetically operated.  When the window or door is closed, the magnet pulls the switch closed, so most of the time the switch conducts current.  but you want the switch to activate the project – alarm for example – when the door or window is opened.

To make matters more complicated the alarm is battery operated, so you want it to draw very little current when the windows and doors are closed.  So somehow you need to invert the switch’s action, but it has to me done with a very tiny current flowing through the switch most of the time.  This is when you need to use a few transistors to invert the switch’s current.

Operation

The switch Sw1 is located at the door or window.  If the a;arm is hard wired then there will be a two conductor cable from the switch to the circuit.  Sometimes the alarm is operated remotely through a radio frequency, so the switch would be on or very close to the transmitter.  The wire from the switch to the circuit may pick up AC hum and radio frequency interference, so the C1 capacitor bypasses these frequencies to the negative, thus preventing false turn-on.

The R1 resistor furnishes a very small current, only about 40 microamps, to the switch while it is closed.  When the switch is closed, the voltage at the base of Q1 will be near or at zero volts.  No current will flow through Q1, and thus no current will flow through Q2, and there is no 9V at the output of the circuit.

When the switch is opened, the current from R1 will then be diverted through the base of Q1, turning it on.  The current gain of Q1 may be a few hundred, so the current from the collector to emitter may be a few hundred times 40 microamps, or several milliamps.  The collector voltage of Q1 drops to near zero, and nearly 9 volts is across R2.  R2 will allow about 2 milliamps to flow through the base to emitter junction of Q2, turning it fully on.  The full 9V is then available at the output.   The gain of Q2 may be more than 100, so at least 100 mA should be available at the output.  But 9V batteries don’t have the ability to supply heavy current, so a more typical load  might be 40 mA maximum.

This same circuit can be applied to bigger batteries with more current, but higher power transistors may have to be used.  The Q2 is critical, it has to be turned fully on so that there is very little voltage drop and very low power dissipation.

I my case, I am going to use a switch to remotely tell me if the garage door is open.  I thought that I could put the switch at the point where the door stops when it’s fully open.  This can then be a simple normally open switch, closed when the door is fully open.  But what happens if the door is opened part way, but never reaches the end of its travel?  The door would be open but there would be no indication of being open.  Therefore I decided that I should put the switch at the bottom of the door, where it will be activated as soon as the door is slightly opened.  The normally open switch is held closed by the closed door, and then opens as soon as the door opens.  This is why I had to come up with the circuit I have described.

 

default

2013-06-29 What Happens When Two JTs Interact With Each Other?

Joule Thief SMPS DC-DC
on June 29th, 2013 by - No Comments

I was thinking of Paul’s Joule Thiefs, which use a CdS photocell to shunt the base bias current to negative during the daytime, so the JT stops flashing and saves the battery.

But then I thought what would happen if I took two of these JTs, and put the photocell of the first one near the LED of the second one.  Then put the photocell of the second one near the LED of the first one.  The photocell of each is being influenced by the light of the LED of the other.  Will this tend to synchronize the  two so that they will both be on the same frequency?  Or will the photocell’s response be so slow that they will not synchronize?  Or will they try to synch up, but never quite synch, and instead interact with a beat frequency that is visible as a slow or fast change in brightness?  This should be an interesting experiment.

One thing is likely to happen: the room light will probably interfere with the interaction.  It may be necessary to keep the room dark to avoid this.  Another thought occurred. There may be a critical point where the two JTs are stable, but very sensitive to any changes in the ambient light.

It may be possible to get them to also interact with small changes, so if someone waves their hand over them and causes a shadow, will it cause a disturbance that propagates from one to the other?

This looks like an  experiment that could burn up a bit of ones time, but might have some interesting results.

A Different Interaction

DSC_0402S4I built two very similar JTs on the same ground bus.  Both positive leads were soldered together.  The transistors were PN2222A, as were the 1k resistors.  The LEDs were both blue.  The coils used the same core, and with three strands of 30 AWG solid enameled wire about 9 inches long wound trifilar.  One winding was separated for the feedback (base) winding, and the remaining two were soldered together for the primary (collector) winding.  I wound one of the JTs with a few more turns, so one coil measured 590 uH, the other measured 690 uH.  I thought this was better because the two would run at different frequencies.

With 1.5V power connected, both blue LEDs lit up brightly.  I measured one at 31 kHz, and the other at 36 kHz.  I put the o’scope on one. I connected a 1 nF capacitor (to block DC) and a 10k pot in series, and then to the collectors of both transistors.  As I varied the pot, I could see the pulses interfere, but I didn’t see any variation or ‘galloping’ in the brightness of the LEDs.  This was what I was looking for: a visible pulsing if the LEDs, sort of like a candle flickering.

I disconnected the cap and pot and connected a red LED between the collectors.  It lit up, a bit dim, and caused a drop in brightness of the blue LEDs. It looked to me as if the two JTs were trying to synchronize when I coupled them together.  But the slippage was too fast to be visible as pulsating light.  I’m going to have to think about reducing the difference in their frequencies.  Perhaps I should add the CdS photocells and try to get coupling in the way I talked about at the beginning of this blog.

Update Jul 4 – I removed turns on the one with higher inductance so that it would be about the same as the other.  I checked the two with the scope as I varied  the resistor.  At certain points, I saw the waveform gallop or blur, but this must be too fast to see because the LEDs looked like steady brightness.

I brought a neodymium magnet close to the core of one toroid, and it changed the waveform on the scope, but I didn’t see any change in the LED brightness, just steady light.  They seem to be independent, must be because this is Independence Day!  I’m still thinking about this, but for right now, I think I’ll do something else.

default

2013-06-28 Brick Wall or Sidewalk To Nowhere

Other Off Topic
on June 28th, 2013 by - No Comments

Have you ever had one of those days when you think everything’s going along smoothly, and then suddenly you run into a brick wall?

The Sidewalk to NowhereP1030723BrickWall

default

2013-06-27 When And When Not To Use A Joule Thief

Joule Thief SMPS DC-DC
on June 27th, 2013 by - 11 Comments

I have often seen the conventional Joule Thief circuit used in situations where it is a poor choice – there could be better choices that would do the job.

The Joule Thief or blocking oscillator circuit transfers a low voltage, high current input to a higher voltage, lower current output, with a low efficiency and high losses.  It’s a very simple circuit but has serious deficiencies.  The typical Joule Thief’s input voltage will be 1.5VDC and the output load will be one LED (or 2 or more LEDs in parallel).  The output voltage will be below 5VDC.  The parts numbers I have given are for through hole transistors; if you want to use surface mount parts you can also find equivalents for them.

A Joule Thief with a 1k resistor and a small transistor, known as a ‘small signal’ transistor will put out up to 100 milliwatts to a standard 5mm diameter white or blue LED.  The power can be reduced by increasing the base resistor from 1000 ohms to 3300, 4700, 10000, or more ohms, and the power to the LED will be reduced along with lower battery current and longer battery life.

If you use a 2N4401, PN2222A, BC337-25, the LED will get about 20 mA or 66 milliwatts with a fresh 1.5V battery.  If you use a 2N3904, BC547 or similar transistor, expect to get less than 50 milliwatts, maybe only 30 or 40.  The LED will not be as bright.

The Joule Thief circuit puts a heavy demand on the transistor for high current at very low voltage.  It has a difficult job driving a single LED.  If you want to drive a larger LED or two or more standard LEDs with more than 100 milliwatts, then you will need a higher current transistor specially made for switching very high current at very low voltage.  Some common ones are 2SC2500, 2SD5041, KSD5041, ZTX1048A, NTE11, SS8050.

Solar Photovoltaic Cell And Other Low V Sources

One example of a low voltage source to which a Joule thief might be applied is a solar cell, or solar PV cell.  The typical single solar cell puts out a maximum of about a half volt no load and somewhat lower, maybe 0.45 volts with a load.  The typical silicon BJT (bipolar junction transistor) takes more than this to start, so using a regular transistor is not a solution.  The best solution is to put at least two of the PV cells in series to get a higher voltage, with three or more giving the better efficiency.  Even so, using a Joule Thief will lower the efficiency to about 50%, which is excessively high loss.  It would be best to use several cells to get 3, 6, or more volts.  For best efficiency it is best to do the conversion as close as possible to the cells and run the higher voltage to the load, which saves on the heavier wire that would otherwise be needed.

If additional cells are not possible, then the best way to up-convert a half volt is to use  high current MOSFET transistors.  A germanium transistor can be used to get enough voltage to start up the circuit, then the operation can be done with the MOSFETs.

Higher Supply Voltage

If the supply voltage is higher than 1.5V, then the Joule Thief may not be the best solution.  The conventional Joule thief wastes about half of the power, so if you can use the supply voltage directly without the Joule Thief,  it can double the battery life.  If the supply voltage is above 3.5 volts, then the LED may be connected directly to the supply with just a simple current limiting resistor.  This will save batteries, it will save electronics and the LED will do just as good a job of illumination.  The best supply would have three 1.5V cells in series for 4.5V, or four rechargeable cells for 4.8 to 5 volts.  Each LED would need a resistor that drops about 1V at 20 milliamps, or  50 ohms.  Close values commonly available are 51 ohms or 47 ohms, 1/4 watt.  When the battery voltage drops  because of multiple LEDs connected to the same battery, this resistance may have to be reduced to maintain the 20 mA through the LEDs.

If the supply voltage is above about 6 or 7 volts and you use a coil with a 1 to 1 turns ratio, the supply voltage will be too high for the transistor.  It is necessary to reduce the number of turns on the feedback winding to half the number of turns of the primary.  The supply voltage can then be up to about 12 to 14 volts DC.  But obviously the total voltage across the LED(s) would be higher than 3.3 volts, so to conform to the rule that the supply voltage must be less than the total voltage across the LED(s), there must be at least 5 LEDs in series.  If this sounds confusing, just remember that if the supply voltage is greater than the  total forward voltage of the LEDs, they will light up without the Joule Thief circuit, and since the LED(s) connect to the supply through the coil winding with very low resistance, excessively high current will flow through the LED(s), and this will damage or destroy the LED(s).

Alternative Circuits

Some JTers use a two transistor circuit that is mistakenly called a Joule Thief, but is more like an astable multivibrator, but the two transistors’ loads are not equal.  The first transistor only drives the second transistor. This circuit does not have the coil voltage fed back to the base, so the voltage limitation of the emitter to base junction is not a concern.  The one that uses a NPN for the output transistor and a PNP to drive the output is usually the better choice (scroll down to the 1-Cell Boost Circuit schematic here).  But two NPNs work, it’s just that the NPN driver needs a low value resistor to supply the base drive current to the output transistor, where the PNP driver supplies the current, so no resistor is needed – but one is often used.

This circuit can be rearranged so that both transistors drive the coil, and the load is more evenly distributed.  The coil uses the two windings of the conventional JT, but both of the transistors’ connectors are connected to them.  An example of the schematic can be seen here.  This one shows the extra turns on the coil, but it doesn’t have to have those, the rectifiers could be connected to the collectors.

DSC_0409S5Update Jul 10 – I put together two LED circuits that both operate from a 5V supply.  The first was three LEDs in parallel with 82 ohm current limiting resistor for each LED.   The second was a Joule Thief with three white LEDs in series.

For the three in parallel the V drop across each LED was 3.34V, leaving 1.66V across the resistor.  The LED current was 20 mA so 1.66V / 0.02 equals 83 ohms.  I chose 82 ohm resistors, so the total current for the three LEDs was 60 milliamps.  The efficiency of the three LEDs was 66.8 percent.

For the Joule Thief with the same LED current, the supply current was 76.8 mA, and the efficiency was 50 percent.

The conclusion is that the resistors are more efficient and use less current than the Joule Thief.  The choice is obvious: the Joule Thief is not the best choice for this circuit.

default

2013-06-26Germanium Joule Thief Uses a 2N404

Joule Thief SMPS DC-DC Transistors
on June 26th, 2013 by - 2 Comments

P1030763This is one of those Joule Thiefs that uses an old germanium 2N404.  But I got them from a German seller on eBay, and I have a suspicion that they were made within the last decade or so, probably by one of the makers in the former soviet bloc countries.  I’ve measured true 2N404s and they don’t have high gain like these do.

The LED is white but looks bluish in this picture.  It’s still bright even though it’s running off a depleted AA cell, with only 0.87V left in it.  The resistor behind the coil is 1k.  By the time the LED dims to nothing, the AA cell will be below 0.2V, probably 0.1V or maybe less.  That’s DEAD!

default

2013-06-25 High Current Power Supply Uses LM317

Meters and Test Equipment Power supplies
on June 25th, 2013 by - 9 Comments

When I work on a Joule Thief I need a power supply that will go all the way down to zero volts.  Kirk pointed me to a high current power supply (6 to 8 amp) that puts out zero to 30V and uses a LM317.  I have built a power supply that uses a LM317 and goes to zero volts, but it has some very nasty problems (here is a picture of it).  Kirk’ss power supply uses a different approach to get the output to zero volts.  The designer put three diodes in series with the output of the LM317 to drop the voltage before it gets to the two power transistors.

I believe that every power supply should include the D6 and D10.  These protect the power supply.  C8 and C9 reduce the output impedance.  The fuse in the output protects the PS from overcurrent.  But I think the fuse should come before C8, not after.

Deficiencies

There are some deficiencies, one serious.  I think t he two 4700 uF filter caps at the output of the bridge are not enough to give good filtering at 6 to 8 amps.  Somewhere way back in time I read that a good rule of thumb is you need 8300 uF for each amp of current.  But that may have applied to audio amplifiers, not regulated power supplies.  I would add at least two more.

Another is that at very low voltage and very high current output, the power transistors have to dissipate over 150 watts, so a very large heatsink is required.  The power transformer secondary has a center tap, and it would be simple to add a range switch that uses only half the secondary when the PS is on the low voltage range.  This would cut the dissipation in half.

The most serious deficiency is the output is open loop – there is no feedback to keep the output equal to the output of the LM317, which is capable of very good regulation.  As the current changes from 0 to 6 or 8 amps, the voltage drop across the three diodes, the transistors and the two 0.1 ohm resistors could add up to a volt or more.  The drop across the two 0.1 ohm resistors will be 0.4 volts at max current.  These drops are not compensated by the LM317, so this power supply does not have good regulation.  And this is especially true at low voltages, where the regulation may vary from 3V at no load to 2V at full load.  That’s a 50 percent loss.

Second PS

The second power supply link that Kirk sent is similar to the first one above.  It uses six 2N3055 power transistors in parallel for a huge amount of current – up to 20 amps.  It also is open loop – the output will vary depending on the load.

Oh, one more thing – I forgot to mention another very big gotcha.  Both of these power supplies use a power transformer that is very expensive.  Mouser wants more than sixty dollars for a 24VAC, 10 amp transformer, and that doesn’t include the shipping cost – they’re very heavy.  About the only solution for the average experimenter is to modify a MOT (microwave oven transformer), by removing the secondary wire and then rewinding it with some heavy copper wire.  It shouldn’t take that many turns.  There is more info on MOTs online, so check to see how others have done it.

IP ‘borrowing’ – Also, there are some electronics websites that plagiarize other electronics websites.  Down in the lower right corner of the schematic you will see qsl.net/ON6MU which is where this schematic apparently originated, but the URL says electronics-diy.com.  I am not accusing this website of plagiarism ( ON6MU could’ve given permission), but there are many electronics websites that steal the schematics of other websites, put their name on it, and offer it as if it was their original schematic.  I have often seen newer websites that have schematics identical to websites that have been online for more than a decade and the newer website has erased the original name from the schematic, and put their own name on it.  I vote with my feet: I don’t patronize these websites when they come up in a search.  These websites depend on your eyeballs to sell the adverts that are ever present, and the more hits they get, the more they make.  So please don’t patronize them once you have found out about their shady reputation.

The second link has the website as a ‘watermark’ across the whole schematic.  They put this on the schematics to try to prevent others from stealing their schematic.  Problem is it’s very easy to remove – takes me less than a minute to change the contrast in Irfanview and eliminate the watermark.  Funny thing though.  I’ve found that the websites that try to protect their images with watermarks are most often the ones that have stolen the schematics of others.

Another reason to avoid these websites is once they make the copy of the original, any later additions or corrections are lost, so if you use the copy rather than the original. you may be getting an inferior copy with errors or omissions.

default

2013-06-23 Very Low Power Joule Thief Part 2

Joule Thief SMPS DC-DC
on June 24th, 2013 by - No Comments

An earlier blog about very lowpower Joule Thiefs is here.

I experimented with a very low power Joule Thief using a SS9014 high gain, low noise NPN transistor, a red high brightness LED, a T231212T toroid with two 12 inch (300mm) lengths of 30 AWG (.25mm) solid enameled wire, and a 1k resistor in series with a 100k pot – the 1k was there to prevent the resistance going to zero, which would put the battery directly across the base to emitter and causing damage.  I put a 68 pF disk capacitor in parallel with the pot; without it the circuit wouldn’t oscillate.

With the pot set at minimum, the red LED was very bright; with the pot set at maximum, the LED was still bright.  The battery current was still too high, more than 8 milliamps.  This indicated that the S9014 was indeed very high gain – 600 or more.  It also indicated that the 100k was too low, so I proceeded to increase it.  I removed the 100k pot and soldered in a 470k pot.

I powered it up, and the battery current was a lot lower, but still more than what I wanted it to be, which was about a half milliamp or less.  So I knew my job would be to get an even higher resistor and put it in.  I removed the 470k pot and soldered in a 1 meg resistor.  Now I was getting down below the half milliamp point.  The battery current measured about 375 microamps, or about 3/8 of a milliamp.  The red LED wasn’t very bright, but it was clearly visible.

default

2013-06-22 Increasing The Power Of A Zener Diode

Diodes Transistors
on June 22nd, 2013 by - 2 Comments

I did a search of my blog and I couldn’t find this topic, so I think that I blogged this ‘way back in my late, great watsonseblog.  So I’ll go over this again.

Sometimes it’s necessary to use a Zener diode for overvoltage protection.  It can also be used as a shunt regulator, but nowadays that’s very inefficient and violates the principle of “green and eco friendly”, so I’ll stick to overvoltage protection.

When a power transistor is driving a variable load with an inductive component, the collector voltage can rise to excessive voltage and damage the transistor.  In order to protect the transistor, the designer can connect a zener diode from collector to emitter.  When the voltage gets as high as the zener’s breakdown voltage, the zener conducts and dissipates the excessive voltage.  This works fine for the typical zener diodes, which come in half watt and 1 watt sizes.  But what happens if the zener has to dissipate more than a half watt or 1 watt?  One way is to put two or more zeners in series.  For instance, a 2N3055 power transistor can handle several amps, and is rated at 60 volts collector to emitter.  I could put six 9V, 1 watt zener diodes in series, and across the emitter to collector.  The six zeners could handle up to 6 watts.  If more power is needed I could connect nine 6V zeners for a total of 9 watts, and other combinations of other voltage zeners could have higher dissipation.

But why do we need to use high powered zeners, when we have a more  than 100 watt transistor right there??  Instead of connecting the zeners collector to emitter, we connect them collector to base.  When the collector voltage rises, the zener starts to conduct, and a small amount of current through the zener to the base turns on the  transistor, where a much larger current goes through the collector to emitter.  In effect, the transistor amplifies the power of the zener.

As a example, we connect a 9V, 1 watt zener from the collector to the base of a 2N3055.  We connect a 1k resistor from the base to the emitter, to prevent any leakage current through the zener from turning on the 2N3055 before the voltage gets to 9V.  As the collector voltage reaches 9V, the zener conducts, and up to 100 milliamps through the zener  is amplified to 3 or more amps through the 2N3055.  That’s up to 30 watts of power, and the 2N3055 must be on a heatsink to keep it cool.

This should work with a transistor that is being driven by a driver circuit.  But when the zener conducts, it is overriding the driver, so the design will have to take that into account.  Also there may be a problem with the high voltage and high power in the transistor, which has a “SOA” or safe operating area, that must not be exceeded.  Exceeding this typically results in a transistor that is permanently damaged.

An alternative is to use a second power transistor with the zener between its collector and base, and 1k resistor between its base and emitter.  Then its emitter and collector are connected to the emitter and collector of the first transistor.  When the voltage rises, the zener conducts and most of the power is dissipated in the second transistor.  I have heard this called an amplified zener, and some other similar names.  But it’s a way to save money on expensive high power zener diodes.

default

2013-06-21 Warning – AC Adapter For Brother P-Touch Labeler

Power supplies Security and Safety
on June 21st, 2013 by - 2 Comments

There seem to be a zillion pitfalls out there just waiting to entrap the unwary buyer.  Take for instance my dilemma.

The bozo at work borrowed the Brother P-Touch labeler and brought it back without the AC adapter – apparently he lost it.  So we put eight AA cells in the battery holder and it has been running on those for a short while.  But they got a whole big box full of fiber optic patch cords, probably more than a hundred, and these had to be labeled sequentially, 1 through 100 or more, with the same number on a label on each end.  We got through most of the job, but then the AA cells were giving out, they were just about dead.  Well, I’m not going to go out and buy 8 AA cells, probably for more than ten dollars U.S., because I can buy a new AC adapter for less than ten dollars on eBay.  So I found a P-Touch labeler in another department, and got the information from its AC adapter.  It said, in part:

Brother Switching Adapter Model AD-24

9VDC 1.6 Amps Output, 100 – 240 VAC input

Polarity:  Center contact is Negative (this is important!!)

So I got on eBay and searched or Brother AD-24 Adapter and came up with many sellers, some selling it for less than 6 dollars, many with free shipping.  One of them looked promising, even though it didn’t look like the original.  I examined the pictures, and found that it was center contact positive, backwards from the original.  If this one was used in the labeler, it would most likely damage the labeler, and would not work in any case.

I started looking through others, and found that either they didn’t show the polarity, or the polarity was wrong, which obviously is unacceptable, because the wrong polarity could damage the labeler.  I did find a few that looked exactly like  the original, and had the Brother name on the label, but generally these were twenty dollars or more with shipping.

Right now, the clueless sellers who are representing that their adapters are suitable as substitutes are creating a dangerous situation where the uninformed buyer is almost certain to damage their equipment.  I think that eBay should require the AC adapter sellers to have a link in their web page that warns the seller about this dangerous situation.  This should apply to all adapters, not just those for labelers.  This warning should also apply to any replacement device that may have a potential to damage the equipment.  This is VERY IMPORTANT: The damage could be so severe that the equipment might not be repairable and the equipment could cost hundreds or thousands of dollars.

Another piece of information that the buyers should be made aware of is the “Efficiency Level” that is also on the adapter’s label.  I did a web search and came up with this document.  I will have to do a lot more reading about this before I get a good understanding of what it means.  But from the articles I have read, I can say this much: if the replacement power supply is lower efficiency than the original, it could cost you more for your electric bill.  These “wall warts” are usually left plugged in 24/7, so if they waste power even though the equipment is not turned on, then this can add up to a substantial amount and affect your electric bill.  It would be helpful if the consumer chose a replacement that is at least as efficient as the original.

There is one other point that may be unimportant in a low power device such as a labeler or battery charger, but gets more important as  the power get higher.  Most of the power that is wasted in the AC adapter appears as heat that escapes out to the ambient air.  This heat may have to be removed from the ambient air by an air conditioner, so you pay more than once for the wasted heat.

 

default

2013-06-20 Reusing Laptop Batteries

Electronics
on June 20th, 2013 by - No Comments

In an email, Kirk said that he has found that ‘dead’ laptop batteries often have only a single cell that’s bad.  I have torn apart some Dell laptop batteries, and found that they have a PC board that has some kind of battery management capability.   I also found that the five or six cells are often the same size as the lithium rechargeable cells commonly known as 18650.  This number is the size of the battery in millimeters.  However, the 18650 cells may have a button on one end signifying that it’s the positive end.  The cells in the laptop battery may have no button, and are typically spot welded to metal straps that connect them in series.  These straps may be peeled off to remove the single cells.  Then the cells can be recharged and tested to find if they hold a charge.

A couple of warnings

First off, the battery management board may keep in its memory how many times the battery has been charged.  Even though the cells may seem to have some life in them, the management may have shut down the battery because it reached a certain recharge limit, and what life that’s left in the cells is very limited.  So don’t expect to get much from the cells.

Another point that I want to strongly emphasize is that these cells are lithium cells, and have been known to explode or catch on fire if they are charged too fast or overcharged.  That is another reason why the battery management board is used – to monitor the charging.  So I highly recommend that the correct charger be used to recharge these cells.  I bought mine from Dealextreme.com for under ten dollars U.S.  It will also charge RCR-123 cells.

I have seen lithium rechargeables charge up to more than 4.2 volts.  On discharge, they may fall to as low as 3 volts.  So it may be necessary to use a DC to DC converter to get a stable output.

© RustyBolt.Info/wordpress
CyberChimps