Battery Charging

[NZ0I]

The LiIon / LiPo charge device that we are using in the receiver and the transmitter provides a charge current that maxes out at 500 mA. All of the batteries that we are considering have a high capacity and a safe charge rate of well over 1A. The 2600 mAh cell that I am using while developing and testing the receiver will easily charge overnight after a full day of running the Control Head and Digital Interface boards. But there are times (like when the battery needs to be charged) that the USB power will not supply enough current to power the receiver. And for the transmitters we'll probably want an even higher capacity cell, and it will take longer to charge.

A faster charge will require more current than a USB can provide. We would want to add the possibility of attaching an external charger (like a wall wart), capable of providing 6V at 1A or more. That will also mean adding a power jack to the transmitters, and receivers too. 

Some suitable charge-regulating devices: 

   https://www.digikey.com/products/en?keywords=MCP73837T-FCI%2FUNCT-ND

   https://www.digikey.com/products/en?keywords=MCP73871-2AAI%2FML-ND

Another bonus for the transmitters is that adding an external power source (e.g., a 6V SLA battery) to greatly extend their transmit time would be easily done: simply attach the external battery to the charging jack. Most folks will probably want to do that for the homing beacon transmitter, which runs continuously. It isn't yet clear, though, exactly how much current the transmitters will draw. 5V at 1A is 5W of power supplied by the charge circuit, and the transmitters might put out more than 1W on two separate bands simultaneously. We will need to keep an eye on the transmitters' power requirements if we want to ensure that they can run off of an external charge source.

 

[Patrick R. Sears]

The second choice has a pad on the bottom of the chip.  Hummm...

I love the idea about the ability to have an additional external power source.  But how would that work exactly.  The modifications I'm planing to do to our current 80m transmitters would cause the plug inserted for charging to disconnect the main board.  The main board has an on/off switch, so don't truly need to do that I guess.  But if the jack needs to be able to both recharge the batteries and power the main board, the internal batteries would be in parallel?  Or would there be a circuit that somehow determines if the power coming in at the jack should be used to recharge the batteries or disconnect the batteries and power the transmitter?  Maybe there is a simple way I could modify the current 80m transmitters to do the same?

As an aside, the extra jacks (back-ordered) for the 80m transmitters just arrived yesterday, happy day  :  )

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[Patrick R. Sears]

Ah!  Got it.  That's what the MCP73837 does!  I thought it just handles how to recharge batteries.  But it looks like it also takes care fo switching between power supplies.  Very cool.

On 04/12/2017 09:08 AM, NZ0I wrote:

[Charles Scharlau]

Regarding pad-on-the-bottom parts:

After Jerry suggested placing a thru-hole beneath the pad area, for pad-on-the-bottom parts, I've come to accept them as reasonable choices for hand soldering. In fact, two of the components we have spec'd right now have bottom pads. But by placing a plated thru-hole (or two) beneath the part, and applying some liquid flux, it isn't so difficult to solder to the pad from the opposite side of the board.

The other thing that I found challenging was soldering to the "pinless" QFN parts: those components that have a small metal corner along the bottom edge for soldering to. But by making the solder pad generously long, and adding a little liquid soldering flux to the board and component, it is not a difficult matter to flow the solder along the PCB pad up onto the vertical metal pin surface on the side of the component. It is rather like soldering a tiny chip resistor or capacitor, in that the solder flows up and onto the vertical pin surface. But the key is to have nice long pads for the PCB footprint, allowing the soldering iron to transfer heat and solder readily onto and along the pad.

Speaking of nice long pads, I've found oversized pads to be key for all hand soldering of surface mount parts. I would rather hand solder an 0805 resistor to a generous pad-size footprint, than attempt to install a 1206 part onto a standard footprint. The 0805 is much more easily soldered, provided I can readily place the soldering iron tip onto the footprint's pad surface.

I've been using this stuff: https://www.digikey.com/products/en?keywords=SMD291ST8CC-ND. It seems to work quite well for spreading the heat and flowing the solder to where it belongs.

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[Charles Scharlau]

Regarding adding a switching supply to your existing transmitters:

CW transmitters are not particularly vulnerable to the noise generated by switching power supplies; unlike sensitive receivers that are detecting and amplifying uV-level signals. Simply by using standard recommended filter components, you are unlikely to encounter any noise problems in your transmitters caused by a switching supply.

A power supply is a nice stand-alone module that can be added as an auxiliary board. Some pre-built switching supplies are available from SparkFun and AdaFruit: https://www.adafruit.com/product/2465?gclid=CjwKEAjwoLfHBRD_jLW93remyAQSJABIygGp1LZkfc4CKZJxkN-zP0FBe76nOKJfFyzSgMpjq_VrTBoCgyXw_wcB

The 5V supplies could supply the 5V needs of your transmitters, and they could make the 3.3V linear regulators operate more efficiently by allowing them to drop only 1.7V across them to bring 5V down to 3.3V.

Your bigger problem is 12V, or whatever "high" voltage you are using for the final amplifier FET. Although pre-built 12V switching modules are available (buck or boost) they are fairly expensive. But you might be able to find an OSH Park board design (openly available) that you could purchase and populate at lower cost. 

If you want to switch to using LiIon/LiPo batteries (or any other battery below 12V), then you will only need a boost supply. You could probably take the tested design we are using for the receiver (or the untested one for the transmitter) and adapt it to your needs, but you'd need to design your own auxiliary PS board.

[Patrick R. Sears]

Thanks for the info Charles.  I'm definitely going to do something.  Not sure what yet.  Currently, the battery pack puts out 23+ volts when fully charged.  The LM317 brings that down to 8 V to get 1W out into a 50 ohm load.  I don't have the data here, but basically, a pot next to the LM317 controls its output voltage which then changes the power put out by the transmitter.  I don't remember how high the voltage needs to be to get 5W out.  5W was kind-of my design goal max output power.  The transmitters are currently all set to about 8V output at the LM317.

[Charles Scharlau]

If you want to use your existing battery packs, then you will need a buck converter. A linear regulator, such as the LM317, will provide a variable voltage drop across itself in order to provide a regulated voltage at its output. The voltage drop across the LM317 results in heat within the regulator. If the LM317 is dropping 24V down to 12V then the same amount of power going to your transmitter is also being lost as heat in the regulator (50% power supply efficiency). At 8V output, the LM317 is dropping 16V across it, and is therefore dissipating twice the power being delivered to the transmitter. That is 33% efficiency.

An average switching supply should be 80% efficient, and a well-designed one could be better than 90% efficient. So you should greatly extend the transmit time on a full charge, and reduce the amount of heat generated, by going to a switching supply.

If someone has made a buck converter available at OSH Park, it might not require much more than changing a couple resistor values to adjust it for 12V output. Switching supplies generally use a voltage divider to set the output voltage level, and can be adjusted (over a limited range) simply by changing a resistor or pot... similar to the LM317 in that regard.

[Gerald Boyd]

For the 2nd unit I see that there is a demo board for $26.

I used a DDS chip that had a pad on the bottom on another project. I put a plated through hole on the board that was large enough for the solder tip to flow through. 

I know a guy out in California that turned a toaster oven into a SMT reflow oven. There are lots of YouTube videos on that subject. He soldered BGA devices using it.

Having a higher capacity charger can help both the homing beacon and anything else that we want to change quick for a fast turnaround. Maybe good for training camps etc.

I don't see having an external supply as a big issue. Already do that for charging the drone.

The black plastic bricks are plentiful on the surplus market.

Jerry

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