Homemade Car Charger

[cliffmoon]

For anyone interested in building a car charger for their zaurus, this is a report on my experience in doing so.  My first attempt was to construct a reference design for a pair of LM7805 5 volt linear regulators in parallel.  Each LM7805 is rated for 1A, therefore I made the assumption that they would be able to handle the 2A draw that the sharp AC adapter is rated for.  The linear adapter showed excellent ripple rejection and load regulation.  However, after extended use the circuit overheated and the linear regulators went into thermal shutdown.

After that I realized that due to the lack of heat sink space and the current requirements of the zaurus, I had to implement a switch mode power supply.  I found a switch mode power supply implemented utilizing a pair of 555 timers.  It takes a minimum of components and can easily be adapted for the purposes of this adapter.  That circuit may be found here.

My adaptation of this circuit is as follows:

[img]http://www.moonpolysoft.com/circuit.png\" border=\"0\" class=\"linked-image\" /]

L1 is a supply choke to filter out ignition spikes and protect other electronics from ripple eminating from the switch mode power supply.  

U2 is a zener diode to protect the 556 and LM411 in case any spikes make it past the supply choke.  U2 is a 5W diode, so it should be able to handle anything short of tractor trailer 24v supply.  

I changed R5 from a fixed voltage divider to a 20k potentiometer.  I found that even 1% resistors were not accurate enough to regulate the supply to within 5% of 5v.  Therefore the potentiometer must be hand trimmed with an o-scope or multimeter.  This also acounts for any difference in the gain of the opamp, therefore different units can be switched out according to availability.  

Finally, I changed out the MOSFET to a ZXM64.  The ZX has an exceptionally low on resistance and a peak current rating of 13A.

If anyone has interest in making one of these, I will post a digikey part manifest and pictures of my board.  Thanks.

[Twisk]

Hi cliffmoon.
Nice circuit.

You might try the TL431 to get your 1.25Vref (V2).

Newbie question: Since I haven't received my SL-5500 yet I'm not clear as to whether you can you use the Zaurus while it is attached to a charger. Any help?

Thanks.

[cliffmoon]

Actually I used an LM285Z for the voltage reference.  The to-92 package was easy to shoehorn onto the limited board space I had to work with.

I have an SL-5600 and it works perfectly well running off of a charger.  I would venture to guess that a 5500 would be the same way.

Thanks for the compliment, however most of the circuit design was scavenged from the page referenced.  The adaptations are fairly standard for introducing a circuit into an automotive environment.

[Twisk]

Yeah, the to-92 pkg is just right.

Thanks for the compliment, however most of the circuit design was scavenged from the page referenced. The adaptations are fairly standard for introducing a circuit into an automotive environment.

Still, it was nice of you to adapt it specifically to the Zaurus.

BTW. What connector fits the charger input on the 5600(5500)? Not proprietary I hope

[omega]

On my  c760 it's the one that comes on cd players, and my zen... about 1.7mm?  get one of those multi connectors..

[V-Man]

Radio Shack size "B" adaptaplug.  Not sure what the actual dimensions are though.  Make very sure that the polarity is correct or you WILL blow the charging circuit in the Z.

[VeeDubb]

I made the assumption that they would be able to handle the 2A draw that the sharp AC adapter is rated for. The linear adapter showed excellent ripple rejection and load regulation. However, after extended use the circuit overheated and the linear regulators went into thermal shutdown.

for anyone else who tries this, i would point out that there is in fact, a far simpler solution that the original poster was just on the cusp of before switching to this highlly complex, albeit, highly functional design.

All you need to do is use a project box with enough room for a heat sync.  that's it.  As he said, two of them in parallel worked very well and provided very stable voltage, something which i might add, is VERY hard to do with the chaotic voltage produced by an automotive alternator.

You can buy such heat syncs at an good electronics supply store.  In my area, the place to go is NORVAC Electronics.  I'm sure there is an analogue of NORVAC in your area.  Radio shack carries a heat sync they reccomend for that particular regulater, but you will need one for EACH of the two regulators, so that means they'll have to be a good distance apart, so more wire, more soldering and generaly more places to screw up.


Oh, and NEVER use the adaptaplug system.  I won't bother getting into all the reasons it's horrible now, but avoid it like the plague.  Just get a properly sized soldered plug and solder it on.  It will be smaller, more attractive and more functional.  And, it will cost about 93% less.

[VeeDubb]

An alternative suggestion.  This will work with either of the two electronic designs.

Put all the guts into the smallest plastic project box it will fit in, and put a socket on the other end.  To prevent stupidituy from ruining your Z, you should a different size than the charger plug for the Z.

Then you can have a car plug with a long cord and a small barel plug to fit whatever you use (AVplugs actualy work realy well for this) and then you could have a battery box with a short cord that you could load up with AA's or C's or whatever you want.  Just be sure you use a big enough battery box to supply the minimum input voltage for whatever regulation circuit you use.  6AA's will power a Z for an incredibly long time, and 6C's will run for days on end.

[waalkman]

Just a few questions,

what's that 1.25 volt V2 reference?
Isn't that trim pot going to creep over time?
Why didn't you go with one LM7805 & a transistor to increase current?

Inquiring minds and all that...


John

[Twisk]

what's that 1.25 volt V2 reference?
Isn't that trim pot going to creep over time?
Why didn't you go with one LM7805 & a transistor to increase current?

Don't even get me started on power supplies. One afternoon I sat down to design a little 2-15v 0-10A variable bench supply.

Two months later I had a totally hand build doosey, with all the bells and whistles (Remote Sensing, Constant Current and Constant Voltage Modes, Dual LCD displays... and 100% duty cycle.) Talk about obsessing. What can I say. I love power supplies - or maybe it's just POWER that I love!!

As mentioned earlier, the 1.25V is a Vref for the op amp. LM285Z works, like cliffmoon said.

YES, the trim pot will creep; over time, over temperature, etc. In actual fact, the fluctuation in Vout (at the power plug) is not as critical as some might think. Nothing Sharp makes for the consumer market is that finicky, and definitely nothing after-market.

Going the LM7805->High hFE Transistor route works nicely. I like the 2N3055 (beefy). But I think what cliffmoon was trying to do was keep the heat-size-weight to a minimum.

Final comment: use tantalum caps wherever possible!

[omega]

Are tantalum caps not really really really dodgy?

[Twisk]

Are tantalum caps not really really really dodgy?

Actually, no, if you mean dodgy like risky or unpredictable. No, not at all, in fact just the opposite. In small current apps they resolve to a very high accuracy and are superior in stability. I've spec'd them in many designs that are now in use all over the world.

Just out of curiosity, where did you hear that, or where did the question come from?

[waalkman]

Dual LCD displays... and 100% duty cycle.) Talk about obsessing. What can I say. I love power supplies - or maybe it's just POWER that I love!!

Power supplies are pretty easy to come by around here  But I know what you mean, I'm currently enamored with Z80's  

Care to post your circuit?

As mentioned earlier, the 1.25V is a Vref for the op amp. LM285Z works, like cliffmoon said.

You might try the TL431 to get your 1.25Vref (V2).

Yup, you sure did. I just missed it.

YES, the trim pot will creep; over time, over temperature, etc. In actual fact, the fluctuation in Vout (at the power plug) is not as critical as some might think. Nothing Sharp makes for the consumer market is that finicky, and definitely nothing after-market.

People sure seem to be having trouble with it though. And what's up with having to set the voltage in the service menu?

https://www.oesf.org/forums/inde...?showtopic=5835

Going the LM7805->High hFE Transistor route works nicely. I like the 2N3055 (beefy). But I think what cliffmoon was trying to do was keep the heat-size-weight to a minimum.

Actually I was more surprised that the two 7805's would play nice together! The switcher design is *much* better when it comes to size & heat (which makes it the right choice for cars).

Something that my partner told me is that spikes on the 12 volt system can go as high as 80 volts! Something to keep in mind...

Final comment: use tantalum caps wherever possible!

Always good advice


Thanks!

John

[waalkman]

Are tantalum caps not really really really dodgy?

Nope, they would be the cap of choice for accuracy & stability.

http://www.cabot-corp.com/cws/businesses.n...7&entry=product


This is an example of the type of caps that we use:

http://www.capacitor.com.tw/high1.htm

itty-bitty things  


When one blows, it's a significant event.    


John

[cliffmoon]

Potentiometer creep is kept from blowing up the z by the 1N4734A 5.6v zener diode, if the output voltage goes over 5.6v the worst that will happen is the zener will blow and short circuit the output section.  This will consequently blow the input fuse.  The circuit will need repair but the Z will be ok.

The ac adapter puts out 5.2v unloaded, so an absolute maximum of 5.6v shouldn't be enough to destroy the Z's internal charging circuitry.