# Solar charging circuit question

Here we have a limiter circuit, pinned to 2.75V.

There are 2 AAA batteries on the right and a 5V solar panel on the left. There is a sensor to determine when the circuit is on or off at P0.6 (high(2.5V) is ON and 0.00V is off), P0.6 is connected to uC.

Here is the problem: When solar panel voltage is rising from zero to 2.75V - everything is good and is working as it should be. However, when the voltage is dropping the circuit is supposed to turn off at below 2.75V and so is P0.6 is supposed to read 0.00V, but P0.6 is continues to read high (2.5V) until the solar input drops to zero. How to improve it to completely turn the circuit it off when the input from solar drops below 2.75V?

1. Disconnect processor pin from P0.6 - does it make a difference? Processor pullup may be driving U7 on.

2. Suggestion - try R16 & R17 = 10K each.
Should work as is but R16 = 10k will turn off T1 better and R17 = 10k will allow U1 to regulate properly - not that it needs to here.

The circuit should work if built as shown - but not quite as you describe.

Call voltage at top right Vbattery.
Voltage at left = V_PVIN Assume battery is present at all times - if not results may be indeterminate.
Assume T1 is driven hard on and has minimal voltage drop (forced beta is very high).

When charging, V at P06 will be very close to Vbattery (via T1) and Vpvin will be a diode drop above Vbattery. Vp06 will remain at Vbattery until U1 turns off, even when V pvin falls below battery voltage.

U1 turns off when R18, R19 divider turns it off = when V pvin = 2.11V with R18 & R19 as shown.

Iv V p06 does not fall to ~= 0V when V pvin is < about say 2V then there is something wrong with how your circuit has been built.
eg say Vpvin = 2V.

V at R19, R18 junction is 2V x R18/(r18_+r19) = 2 x 47/115 = 0.817V. This is below the TLV431 turnoff voltage of 1.25V so U1 MUST be off.
Worst case off state cathode current of TLV431 is < 0.05 uA (!!!) so max V across r16 = V = IR = 0.05 uA x 100k = 5 mV !!! so T1 is off.

So circuit should work.
Change 2 resistors as above - but should be OK as is.

Notes:

The circuitry to the left of and including R15, R14 is a shunt regulator which shunts PV panel current via T2 & R20 when Vbattery reaches or exceeds 2.75V - ie as long as T1 is hard on the shunt regulator action is not affected by V_PVIN, only Vbattery.

The circuitry to the right of R18,R19 inclusive is intended to allow the clamp regulator to sense battery voltage when the PV panel voltage rises above a certain voltage and to disconnect the R15, R14 voltage divider when PV Voltage is low (to stop the divider draining the battery when there is no PV input).

With the values shown U1 should turn on and off sharply at about 2.11V (TLV431 = 1.25V reference. Vtrigger = (R18+R19)/R18 x 1.25 = 2.11V. When V PVPANEL exceeds ~ 2.11V U1 turns on turning T1 on and placing Vbattery across R15 + R14.

Below panel voltage of 2.11V charging will still occur via D2 if V_battery is low enough BUT battery should never be that low.

Sanity check: Ensure R17 is not too large -

• A TLV431 typically needs about 80 uA max to operate - varies with manufacturer - say 100 uA.
I_U1 = I_R17 ~= (Vbat - VbeT1 - Von TLV431)/R17 = (2.8-0.6-1.25) / 100k ~~~= 9 uA = too low. So TLV431 will not be in proper regulation BUT it will still draw enough current to fully turn on T1 (see below) so in this case this should not matter.

• If forced beta drive of T1 is too low then T1 will not saturate and divider voltage will be too low.

I_R17 = 95 uA. I_T1-CE ~= 2.8V/103k ~= 25 uA.
So forced beta T1 ~= 25/9 ~+ 3 = OK. A BC807 with forced beta of 3 will have very very low saturation voltage - well under 0.1V (probably).

• Russell, changing R16 & R17 = 10K each worked, now it turns off at 2.0V thanks! – Jim Dec 18 '14 at 18:54
• Russell, how much extra power will the circuit draw from the AAA batteries when there is no sun because of this change? – Jim Dec 18 '14 at 19:19
• @Jim No extra current draw in no sun condition - because, when it is turned off U1 only draws under 0.05 uA worst case (according to spec sheet) which is unnoticeable. What was probably happening with the 100k x 2 is base leakage current in T1 via R16 was providing a tiny amount of on bias for T1. As the divider T1 supplies only needs about 30 uA then T1 base needs maybe 0.1 uA drive. Wow. Changing to 10k reduces the voltage in R16 and makes the difference. Looks like T1 has a very healthy beta (current gain) - usually a good thing :-). – Russell McMahon Dec 19 '14 at 14:26

Seems to me that your charger is wasting milliwatts unnecessarily into the diode D2, resistor R4 and other components. Why not just connect the solar directly to the batteries and have a zener (+resistor) in parallel with the batteries to eat all the excess power when the voltage reaches 2,75 volts?

That way all the milliamps from your solar would go to the batteries. If someone feels it's a bad idea, please comment.

Edit: At least one bad idea is that if there's no forward diode D2 (or some other circuit for that purpose) and it's dark, then the battery is discharged to the panel. See Russel's remarks below.

• The complexity seen here would not be justified in many cases. The circuit is optimised for micropower operation. The rh TLV431 would be an unnecessary extra in most cases. It turns on the battery reference divider only when sun is present, suggesting that the ~ 30 uA in R14, R15 matters. A zener provides an unsharp knee. A PV panel with a zener will either cook the battery or undercharge it and will partially discharge it after charging. IF energy utilisation is at a premium the arrangement shown gives well defined charging cutoff voltage and essentially zero battery drain when off. – Russell McMahon Dec 18 '14 at 12:06
• R4 shunts a very small % of PV current to keep PV Vout low as the sun rises - so that the battery divider is not turned on until the PV panel is getting into its CC mode. D2 is almost always used - stops back discharge of battery into panel - this is usually small in dark but can matter as light comes up but Vpanel < Vbat. As Vpanel Voc is usually usefully above Vbat a Schottky diode at D2 costs essentially zero charging capability. Except when MPPT is used panels run at well below optimum voltage in close to constant current mode. eg a 12V lead acid battery panel has Vmax power at about 18V! – Russell McMahon Dec 18 '14 at 12:12
• Good points, @Russell. Maybe I have a tendency to always oversimplify. – PkP Dec 18 '14 at 13:07