[SOLVED !] ZX4120 Vbat wiring


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Thread: ZX4120 Vbat wiring4892 days old

  1. #1
    dbrb2
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    Default ZX4120 Vbat wiring

    Hi,

    I'm trying to wire up a ZX4120using a 1.2V NiMh cell as shown in the datasheet (page 11)

    The choice of R1, R2 is, we are told, specified by desired charging rate. I can see that this is so for R1 (between the diode and battery) but not R2 (between diode and Vbat). R1 sets the current into the battery, but the choice of R2 seems arbitrary - even unnecesary...

    For R1, we have:

    Vdd = 3.3v
    Diode drop ~ 1.7v
    So voltage across R1 ~ (3.3-1.7)-1.2 =0.4v

    The battery capacity is given as XAh

    So for a charging rate of 0.1C, R= 0.4 / 0.1X

    For a 600mAh battery this gives about 7 ohms

    Any thoughts?

    Cheers,

    Ben

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  • #2
    david
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    Hello Ben,
    Why do you want to use a 600mA.H battery for back-up?
    The battery only has to supply enough for 8k of protected SRAM and a RTC circuit - total drain is typically 4uA.
    A common type of back-up battery is a 6-10mA.H button cell.

    I agree that R2 appears redundant but a low value resistor is still recommended.
    Also consider the situation where you may replace the cell with the circuit running - the LED will not drop 1.7V with only the back-up current as a drain and you risk over-voltage on the SRAM/RTC which is limited to around 2V max.
    The baseband micro normally runs with a 1.8V core.

    Cheers,
    David

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  • #3
    dbrb2
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    Hi,

    I realise 600mAh is a tad excseeive, but iut was the lowest NiMh cell I could find, and is pretty tiny anyway. Hopefully replacement of the cell will not be needed for a long time, and also hopefully the charging current will always be enough to ensure no overvoltage. Unfortunatley if I am to have a rechargeable backup, I can see no way around this potential problem. I could perhaps mount the backup cell underneath the main cells, such that it is not possible to remove one without first removing the other...

    I'm using the ZX4120 breakout board made by crownhill, which has a 1K resistor on it already in the place of R2 - I think I'll leave this as the only resistor and not add any extra of my own on the board.


    Cheers,

    Ben

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  • #4
    dbrb2
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    Aah... I've got hold of a far more sensible capacity battery - a 20mAh NiMh button cell.

    However, I was wondering if anyone had any advice on voltage drops across LED's.

    I am using the LED to ensure my 3.3v supply can charge the battery and not risk damaging the GPS unit. However, as the previous poster pointed out, if the charging current is too low, the voltage drop will be smaller, and I risk frying my
    unit.

    Datasheets never show in much detail of voltage drop vs. current for low currents - only the overall trend is shown. Any thoughts on how much current has to be drawn from an LED to get a given drop? Obviously colour has an impact here as well (E=hf etc)

    Ben

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  • #5
    david
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    Hi Ben,
    If you are using 3.3V as the charging source and have a red LED in series then you will always have some small charging current to ensure the LED drops the usual 1.65-1.7 for a red LED. Check the maximum continuous charge current for the cell and set up the series resistor to apply this. The IxR drop through the resistor will be around the same as the LED. e.g. 3.3V source, 1.65V LED drop, 0.2mA charge current gives R=8.2K.
    If the charging source (3.3V) is turned off when the unit is not being used you are faced with a usage duty factor. The SRAM/RTC draws say 5uA continuously , 24/7 whereas the charging only happens when the unit is on, perhaps 1 hour per day (maybe). So roughly you have charge at 1 hour x 0.2mA vs discharge at 24 hours x 5uA. If you only use it 1 hour every week then it will never hold up. Keep it in mind.
    BTW I believe the ZX4120 has been discontinued in favour of the ZX4125. The latter is a 6 layer pcb and better suited to an on-board passive patch but would show little difference for an active antenna.

    Cheers,
    David

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  • #6
    dbrb2
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    Thanks david. I checked the voltage drop, and at a chargin current of 0.1C the current was high enough to give a voltage drop of 1.7v, which is good (even at 0.05C it was OK - C is 20mAh)

    As for the ZX4125, I did look at it - it comes in some variations that have a built in patch and WAAS/EGNOS as well as auto detect of an external antennae.... I was drooling at the thought!

    Unfortunately other than the manufacturers website, I could find nobody selling this new model. However, the interface appears similar, so I could always upgrade at a later data if the oportunity arose.

    Cheers,

    Ben

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  • #7
    david
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    Hello Ben,
    The interface is the same and as mentioned, if you are using an external active antenna there is little to pick and choose between them.
    The product is based on the Nemerix chipset NJ1006 (RF) and NJ1030 (Baseband) and both should be "A" type silicon by now. These are quite low power and allow tracking at 1Hz update rate with only 22mA total current drain - impressve! Actually that's for the ZX4125 so yours should be less - perhaps around 19mA.
    Do watch the interface from the NMEA output to a Pic if running off 5V - you may need a grounded base level translator.
    Looks like the charging circuit will do the job.
    Good luck.

    Cheers,
    David

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  • #8
    dbrb2
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    Yep - I noticed the voltage problem, so the PIC and GPS unit are both running at 3v3. The only 5v component is the compass, but it communicated with the PIC via a 3.3v I2C bus.

    I am using an active antenna, so as you say performance should be similar. One of the variations of the 125 supports WAAS I think, which while nice is not absolutely needed.

    I'll post here when (if) I get the whole thing working. My boards have arrived from olimex, so maybe it will be done sooner rather than later, touch wood. Then it becomes a software project...

    I have most of the software written, but some still left to do :-)

    Cheers,

    Ben

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  • #9
    dbrb2
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    Oh, btw David, is your calculation for charging capacitor value not incorrect?

    You say the voltage across the resistor is 3.3-1.6v, but in fact it is (3.3-1.6)-1.2

    Where the extra 1.2 is the voltage of the battery being charged...


    Ben

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  • #10
    david
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    Hi,
    You're absolutely correct - I did forget to subtract the battery voltage.
    This means there is very little voltage overhead to play with and the R value will be quite small. Fortunately when those little batteries discharge the internal resistance climbs quite high so they are somewhat self protecting. The bad news is that the high resistance makes it harder to get current in when the battery is flat.
    Nemerix typically use 2.8V on the RF section, 1.8V on the I/O and 1.2V for the logic core. I think the NVRAM (as it's called) is a 8K section of SRAM with a separate supply pin for main/battery supply.
    Ferro-magnetic RAM and do away with the battery?

    Cheers,
    David

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  • #11
    dbrb2
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    :-)

    No joke - I've got an HP calculator from the 60's with a whole board full of magnetic core memory! The thing works well, but does nothing my credit card sized one doesnt, and takes up a whole desk... Not exactly compact though, and one suspects the compass on the board might be thrown slightly depending on what data you had written!

    Cheers,

    Ben

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  • #12
    david
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    Hi,
    How cool is that - a HP calculator from the 60's in working condition!
    What about a photo?

    Cheers,
    David

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  • #13
    dbrb2
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    I seem to have lost my photos, and the calculator itself is 70 miles from here...

    but here are some photos of the same model:


    http://www.hpmuseum.org/hp9100.htm


    It works well - I also have the printer module for the top, but the rubber on the roller has corroded and gone very sticky - almost liquid -and gunked the roller mech up. I have the maintenance manual, but never got around to trying to fix it.

    It is as accurate as my hand calculator, but to square 2 takes, I thibk, 8 key strokes :-)

    Ben

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  • #14
    david
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    Hi,
    That's really nice - and it doubles as a room heater too I guess. Let's not knock it. There's some very old space craft out there that are still operational and have similar technology on-board.

    Cheers,
    David

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