Delivering "clean" power to the Ultimate GPS Breakout v3

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foodini
 
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Delivering "clean" power to the Ultimate GPS Breakout v3

Post by foodini »

I have a half-dozen of these things in various projects, but I've always had some odd issues with lock getting lost, the serial comms disappearing, etc. Nothing too major, but it does seem to be more prevalent when I'm on battery.

I'm currently working on what will be an open-source hardware project with this module (and a Feather M0 AdaLogger) at its heart. Before I turn an unsuspecting public loose on my power incompetence, I wanted to ask what is the best way to avoid having the board brown out? It's sharing power rails with an LSM9DS1 9-axis accelerometer and a ms5611 barometer. Of the three, the GPS is the major power hog. I've no idea how you're supposed to calculate capacitors to use in these situations and wanted to see what the general wisdom for this device is.

Thanks...

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mikeysklar
 
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Re: Delivering "clean" power to the Ultimate GPS Breakout v3

Post by mikeysklar »

@foodini,

A 1000uF capacitor would be a good place to start for power smoothing to prevent brown out. Also making sure you have plenty of battery capacity in the design to avoid low voltage cutoff. Are you wiring the GPS breakout direct to the battery or going through a Feather Adalogger which will have 500mA max current limit on its voltage regulator.

foodini
 
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Re: Delivering "clean" power to the Ultimate GPS Breakout v3

Post by foodini »

I'm using a batter connected to the Logger - a 150mAh, for the moment, but have 'em up to 2000mAh that I could use. When I connect the batter through my ammeter, I see occasional spikes to 450mA, but I can't believe that's true. The meter updates every 500ms or so, so maybe there's something it how it works that it's not being accurate. I will probably hook up a very small resistor in series w/ the battery so I can connect an oscilloscope to either end of the resistor and get a better idea of the current draw at a much higher update frequency.

I have to power the GPS from the AdaLogger because I need a way to power everything down on a power switch (using the EN pin on the Logger). I could put the switch between the battery and the Logger, but then I'd have to be certain that the power switch was on for charging to work.

foodini
 
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Re: Delivering "clean" power to the Ultimate GPS Breakout v3

Post by foodini »

I've dropped a 1ohm resistor into the connection between the battery and Logger and connected a 'scope to either end of it. I do see spiking to about 250mA for the entire circuit, but no hint of anything as bad as 450mA. This is all with only a 47uF cap on the GPS.

I'm a little concerned about putting a 1000uF cap in the circuit. It will be guaranteed to be discharged when the circuit starts up, so it'll be drawing power as fast as it can, potentially browning out the rest of the circuit while its trying to start. Should I be worried about this?

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adafruit_support_bill
 
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Re: Delivering "clean" power to the Ultimate GPS Breakout v3

Post by adafruit_support_bill »

I do see spiking to about 250mA for the entire circuit,
Batteries in general tend to be very stable power sources - as long as you stay within their rated operating parameters. The 150mAh LiPo is rated for a maximum discharge rate of "1C' which would be 150mA. So it is not surprising that you might see some voltage sag - or even a dropout when you exceed that by a rather wide margin.
https://cdn-shop.adafruit.com/product-f ... th_PCM.pdf

If space is at a premium, you are probably better off going to larger cell with a higher discharge rating instead of adding a big cap. The 350mAh cell in the store is rated for 1.5C (525mA).
https://www.adafruit.com/product/2750
https://cdn-shop.adafruit.com/product-f ... 150906.pdf

foodini
 
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Re: Delivering "clean" power to the Ultimate GPS Breakout v3

Post by foodini »

Since the OP, I'd already moved to a 500mAh, simply because I knew I was going to need the thing to run for many hours on a single charge.... and the 150 wasn't going to do it.

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adafruit_support_bill
 
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Re: Delivering "clean" power to the Ultimate GPS Breakout v3

Post by adafruit_support_bill »

If you are powering through the Adalogger, you have the further restrictions of the on-board voltage regulator. Try powering the GPS directly from the battery.

foodini
 
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Re: Delivering "clean" power to the Ultimate GPS Breakout v3

Post by foodini »

If I power the GPS directly from the battery, I can't power it off without disconnecting the battery or I could wiring the power switch between the battery and the logger. But if I did that, the device would fail to charge if I forgot to turn the switch on while it was on USB... and would discharge if I forgot to turn it off when I disconnected the cable. I've already abandoned the idea of powering the peripherals through a transistor that was switched on by the logger, but the xistor was enough of a voltage drop that it magnified the brownout issue. Everything seemed to work most of the time, but getting rid of the xistor does seem to have gotten me closer to a stable.

I considered using a relay to power the peripherals, but my application involves a certain amount of vibration, which could potentially cause very brief disconnects in a relay.

Thanks, everyone, for the ideas, by the way. I'm chasing them all down as much as I can!

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adafruit_support_bill
 
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Re: Delivering "clean" power to the Ultimate GPS Breakout v3

Post by adafruit_support_bill »

If I power the GPS directly from the battery, I can't power it off without disconnecting the battery . . . .
If you power the GPS directly from the battery, you can determine whether the power supply issue lies with the battery itself or the intervening circuitry. Once you determine that, you are in a better position to address the root cause.

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