Teensy 3.0 with NiHM voltage range

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nyarlathotep

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I'm interested in running my project off NiMH battery packs. Four nominal 1.2V NiMH batteries yields an easy 5V, but..

Fully charged NiMHs output around 1.4 volts, supposedly some start as high as 1.45 volts. So we're looking at 5.6-5.8 volts fully charged, which exceeds the Teensy 3.0's 3.7-5.5 volts range.

I suppose a simple voltage divider circuit is the easiest way to achieve this, well maybe just a resistor. Is it perhaps best to go with a voltage regulator though, given that my packs should mostly hang out around 5 volts?

I briefly looked into voltage regulator boards when considering building this to support both NiMH and LiPo packs, which are nominally 3.7 volts per cell but range from 4.23 volts down to 3 volts. I found that inexpensive regulator boards like the LM2596 want 1.5 volts more than their output voltage though. I could obviously design the entire system around two cell LiPos packs (7.4V, 6-8.4V) and six cell NiMH packs (7.2V, 6-8.7V), but then the WS2811 strips would need voltage regulation too.

As an aside, I'd ideally want the system to shut off around 4V because a four cell NiMH battery pack incurs some risk of polarity reversal below about 1 volt per cell, some voltage regulators do this.
 
5.8 volts is probably fine.

Just be sure not to try charging them while connected, because they can get to about 2V per cell if driven with a high charging current.
 
Alright, thanks. I presume equipment that runs and charges simultaneously must switch completely from battery power to the charger current when charging, probably just a couple transistors and diodes to do that.
 
Your Teensy 3.0 schematic has a 500mA fuse between VUSB and VREGIN. I suppose placing my own 500mA fuse between the battery and the Teensy's VIN makes good sense, yes?

Bourns Resettable 500mA Fuse has 0.41 ohms of resistance, or 1.17 ohms 1 hour post trip. I've no clue what current the Teensy 3.0 draws usually, probably not much since the 74HCT245's VIN takes power separately. In any case, 500 mA * 0.41 ohms = 0.205 volts, or 0.48 volts 1 hour post trip, so this fuse cannot screw anything up, even post trip. I'll try doing it this way.
 
The Teensy 3.0's VREGIN is actually powered from VIN or USB via a (schottky ?) diode, so the actual voltage the IC sees is at least 0.3 V below the input. Therefore with 5.8 V in, you'll still be within the IC's spec. In any case, while not recommended, a few 0.1 V above spec, especially when not at an elevated temperature (i.e. 105 ºC) will be OK.
 
Ahh, wonderful explanation, thank you. So 5.8 V is morally the exact limit where I'm *probably* not exceeding the CPU's limits. And anything above that starts to get real iffy real fast.

I haven't yet built my battery packs so I haven't yet tested this. I'll verify that the battery packs aren't going above 5.8 V before I do so. Also, my power supply has no setting between 4.5 V and 6 V. I haven't even considered trying it with 6 V, well it gives above 4.5 V when set to 4.5 V.
 
If you are only worried about exceeding the Teensy's voltage, you could add your own diode in series with the pack to drop the V by 0.6 V. Or for optimal range, use an LDO (e.g. LP2950 or LP2951) set to 3.3 V. These can take 30 V in (don't do that, you'll have power dissipation problems), and are available in TO-92 or DIP packages.
 
I'll read up on diode's that drop the voltage only a small amount, thanks.

As for LDOs, the LP2950-50 that do 5 V out want at least 5.5 V incoming. An LP2950-33 that spits 3.3V out should presumably be wired to the Teensy's 3.3V pins, correct? According to the schematic there is no difference between them. This would leave no voltage on VREGIN. Is that okay?
 
Ahh, you've addressed this question before :
- Yes, I could run the Teensy off the 3.3 V pin, but reversing polarity or exceeding 3.8V will destroy it.
- I could more safely run the Teensy off with 3.3 V on VIN just fine too, but running additional stuff off that sam 3.3V line might fail.

I'll pick up both a 3.3V LP2950 and the diode and work something out, thanks.
 
I'm still in the sketching phase, but I'm planning to run NiMH instead of LiPo because I can run boost converter and avoid the futzing that comes with extracting 3.3v out of a battery that can range either side of the output voltage. Drop in switching regulators are pretty slick these days., and it can save money especially on board cost if you need to dump any significant amount of heat due to regulators.Also, there's a pretty slick polarity protection scheme than can get you less than a diode drop, with a pretty low parts count of vanilla components. Try Googling "geofex polarity protection"
 
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