💬 Effective Solar Supercap Boost Charger for Small Solar Panel
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@ncollins That AVX-BestCap looks like a great find. I love the compact form factor of it. Looking at the datasheet though, I worry that the self-discharge of 5-10ua might be a bit much: http://catalogs.avx.com/BestCap.pdf
That's high enough that it might consume either all or at least much of the microamps produced by the 3722-9L (assuming just a single cell is used) when there's light available, and the leakage would continue overnight in the darkness to eat into whatever had been accumulated.On the other hand, the all aluminum caps at that same 50mF capacity are huge and for that reason not such a good fit either for a sensor that aspires to be compact.....
@neverdie With all of your energy harvesting approaches, how are you handling unforeseen restarts? Almost all of my harvesting sensors have the same issue: if they drop below 1.8v (depending on BOD) they can't recover without manual intervention. Only one of my nodes has ever recovered and I think it's because the solar panel produced enough current to push through the MCU and radio startup.
My thought was to use the MIC2778 supervisor ic (datasheet), which has programable hysteresis, and a mosfet to wait for supercap/battery to reach a voltage high enough to handle a cold start.
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@neverdie With all of your energy harvesting approaches, how are you handling unforeseen restarts? Almost all of my harvesting sensors have the same issue: if they drop below 1.8v (depending on BOD) they can't recover without manual intervention. Only one of my nodes has ever recovered and I think it's because the solar panel produced enough current to push through the MCU and radio startup.
My thought was to use the MIC2778 supervisor ic (datasheet), which has programable hysteresis, and a mosfet to wait for supercap/battery to reach a voltage high enough to handle a cold start.
@ncollins Yes, that should work. Since I disabled BOD in order to sleep at very low currents (100na for the atmega328p), I built my own in hardware using a voltage detector. So, if voltage drops below 1.8v, it cuts off the power. When it goes above the hysteresis point, it restarts.
If you leave BOD enabled, I think it may do this work for you, and then you wouldn't need more hardware. But then your sleep current would be higher. So, it's a tradeoff as to how you want to do it.
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@ncollins Yes, that should work. Since I disabled BOD in order to sleep at very low currents (100na for the atmega328p), I built my own in hardware using a voltage detector. So, if voltage drops below 1.8v, it cuts off the power. When it goes above the hysteresis point, it restarts.
If you leave BOD enabled, I think it may do this work for you, and then you wouldn't need more hardware. But then your sleep current would be higher. So, it's a tradeoff as to how you want to do it.
@neverdie yeah, the fixed hysteresis of the internal BOD (328p, NRF51) isn't wide enough to handle startup for my nodes. I'm guessing it's due to the high ESR and corresponding voltage drop of the supercaps I've been favoring.
I also didn't realize how chatty mysensors can be during start up. My latest nodes have MY_PASSIVE_NODE enabled, skip presentation, and use dumb sleep.
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@neverdie yeah, the fixed hysteresis of the internal BOD (328p, NRF51) isn't wide enough to handle startup for my nodes. I'm guessing it's due to the high ESR and corresponding voltage drop of the supercaps I've been favoring.
I also didn't realize how chatty mysensors can be during start up. My latest nodes have MY_PASSIVE_NODE enabled, skip presentation, and use dumb sleep.
@ncollins I forgot to mention that I use a load switch, controlled by the voltage detector, to turn on and off the atmega328p. I don't drive it directly from the load switch. You could use two different voltage detectors to get a much wider hysteresis, or you could use a hysteresis chip for a more elegant solution. I know for sure that the voltage detectors draw a minuscule amount of current. I haven't looked into the hysteresis chips enough to know how they would compare along that dimension, but I'm guessing they would be similar.
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@neverdie yeah, the fixed hysteresis of the internal BOD (328p, NRF51) isn't wide enough to handle startup for my nodes. I'm guessing it's due to the high ESR and corresponding voltage drop of the supercaps I've been favoring.
I also didn't realize how chatty mysensors can be during start up. My latest nodes have MY_PASSIVE_NODE enabled, skip presentation, and use dumb sleep.
@ncollins Here's the load switch that I've been using: http://www.ti.com/product/TPS22860 I like it a lot. I think it, or something similar, should be in everyone's tool box.
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@ncollins Here's the load switch that I've been using: http://www.ti.com/product/TPS22860 I like it a lot. I think it, or something similar, should be in everyone's tool box.
@neverdie said in 💬 Effective Solar Supercap Boost Charger for Small Solar Panel:
@ncollins Here's the load switch that I've been using: http://www.ti.com/product/TPS22860 I like it a lot. I think it, or something similar, should be in everyone's tool box.
Awesome, I'll order a few. Appreciate the info, thanks.
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@neverdie yeah, the fixed hysteresis of the internal BOD (328p, NRF51) isn't wide enough to handle startup for my nodes. I'm guessing it's due to the high ESR and corresponding voltage drop of the supercaps I've been favoring.
I also didn't realize how chatty mysensors can be during start up. My latest nodes have MY_PASSIVE_NODE enabled, skip presentation, and use dumb sleep.
@ncollins said in 💬 Effective Solar Supercap Boost Charger for Small Solar Panel:
@neverdie yeah, the fixed hysteresis of the internal BOD (328p, NRF51) isn't wide enough to handle startup for my nodes. I'm guessing it's due to the high ESR and corresponding voltage drop of the supercaps I've been favoring.
I also didn't realize how chatty mysensors can be during start up. My latest nodes have MY_PASSIVE_NODE enabled, skip presentation, and use dumb sleep.
You could either add a capacitor to get it over this hump, or you could just have the atmega328p check it's voltage level at startup. If it isn't high enough, it goes to sleep and keeps checking upon subsequent wake-up's until it's high enough to support radio transceiving without crashing below 1.8v.
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@neverdie With all of your energy harvesting approaches, how are you handling unforeseen restarts? Almost all of my harvesting sensors have the same issue: if they drop below 1.8v (depending on BOD) they can't recover without manual intervention. Only one of my nodes has ever recovered and I think it's because the solar panel produced enough current to push through the MCU and radio startup.
My thought was to use the MIC2778 supervisor ic (datasheet), which has programable hysteresis, and a mosfet to wait for supercap/battery to reach a voltage high enough to handle a cold start.
@ncollins What I tried in the past, and which I rather liked, was to first charge up, say, a 100uF capacitor and use that to power the 328p when voltage threshhold is reached and to then also redirect the incoming solar charge current into a supercap for bulk collection That way, in a cold start scenario, the MCU powers up very quickly--nearly instantly with larger solar cells--rather than having to wait hours for a large supercap to fill up. 100uF is enough to power numerous transmissions.
All this comes at the cost of more circuitry though, so I can understand if someone would prefer to omit this feature to keep things simple.
Maybe we should start a separate thread for this kind of discussion?
The only downside I see to the hysteresis chips is that they consume more current, especially when you consider the amount of current consumed by the voltage divider circuit. In the case of the MIC2778, it recommends the total resistance of the voltage divider circuit be no more than 3megaOhm. At low voltages, though, I suppose that's not much of a problem. Still, for comparison, some voltage detectors might consume a mere 350na in total.
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@ncollins What I tried in the past, and which I rather liked, was to first charge up, say, a 100uF capacitor and use that to power the 328p when voltage threshhold is reached and to then also redirect the incoming solar charge current into a supercap for bulk collection That way, in a cold start scenario, the MCU powers up very quickly--nearly instantly with larger solar cells--rather than having to wait hours for a large supercap to fill up. 100uF is enough to power numerous transmissions.
All this comes at the cost of more circuitry though, so I can understand if someone would prefer to omit this feature to keep things simple.
Maybe we should start a separate thread for this kind of discussion?
The only downside I see to the hysteresis chips is that they consume more current, especially when you consider the amount of current consumed by the voltage divider circuit. In the case of the MIC2778, it recommends the total resistance of the voltage divider circuit be no more than 3megaOhm. At low voltages, though, I suppose that's not much of a problem. Still, for comparison, some voltage detectors might consume a mere 350na in total.
@neverdie Yeah, I think starting a dedicated thread is the right move.
That sounds a lot like what I'm trying to do. Prioritize load (boot/transmission) power, then dump into a reserve. This make sense when light is strong, but when solar isn't producing, how do you get power from the reserve to the load capacitors. Would you boost it?
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Continuing the discussion on a new thread here: https://forum.mysensors.org/topic/10748/solar-energy-harvesting-for-wireless-motes
