254RE: [beam] D1 Solar Regulator was Re: Need help with the SIMD1
- Apr 26, 2000Hi Travis,
This circuit was build and tested yesterday, so hasn't got a long track
record but it is based on earlier designs used by Jim Mullins in some of his
wonderful creations. In my opinion, this is the most reliable of my solar
circuits but feedback from builders will be most welcome.
The solar cell charges a 1F capacitor through a 1N34A Germanium diode to a
maximum voltage of 5.5V. While the charging current flows through the
diode the voltage at the cathode (stripe) is about 100mV negative with
respect to the 0V line. This negative voltage is applied through a 100K
resistor to the base of a 2N3904 NPN transistor Q1 and holds that transistor
off. This cuts off the base current for the 2N3906 transistor (PNP) Q2 and
the output of the regulator will be zero volts.
Rapid switching is very important in this type of circuit because a circuit
that is half ON draws power, draining the cap, but performs no useful work.
On the SIMD1/ Solar Regulator, the output snaps on and off.
At the end of the charging cycle, when the light on the solar cell
decreases, the negative terminal of the solar cell starts to become more
positive than the 0V line The base of the NPN Q1 must be at about +500mV
(positive) with respect to the emitter which is connected to the 0V line,
before it turns on and turns on the rest of the regulator. That usually
happens in the evening but can be simulated by cupping your hand in front of
the solar cell.
When Q1 turns on, the PNP transistor 2N3906 - Q2 receives base current and
it starts to turn on. The regulator output voltage at the collector of Q2
increases to about + 2V when the red LED starts to turn on and to supply
current to the base of NPN transistor 2N3904 - Q3. When Q3 turns on it
"robs" base current from Q2. This in turn controls the base current for Q2
and the regulator output will stabilize at +2V. The 10K resistor from the
regulator output to the negative terminal of the solar cell provides
positive feedback to the regulator turn on by loading the solar cell down so
that it's output voltage drops even more and the regulator "snaps" on. Note
that the red LED is used for reference voltage only and does not actually
The output voltage at the collector of Q2 remains at 2V while the voltage on
the main capacitor can range from a full charge at 5.5V to 2.1V at the end
of the discharge cycle. If no load is attached to the regulator the cap
voltage will drop very slowly because of leakage and a small amount of
current required for the active regulator (<50uA). When a HC chip like a 240
or 14 is powered from the 2V regulated output, the current for that chip is
also very low.
If the HC chip has a LED connected to the output which the same type of LED
that is used for reference, then the current will be limited by a small
voltage drop on the HC driver output. Since the regulated voltage is
constant the brightness of the LED is also constant.
When the voltage on the 1F cap drops below 2V, the regulator reference LED
turns off and the base currents of Q1 and Q2 increase discharging the
remaining charge on the cap and turning any attached flasher circuit rapidly
off. At some point the voltage of the solar cell even in dim light is higher
than the remaining charge on the cap and if there is sufficient light
(usually in the morning) the charging cycle repeats all over again. If the
sun is bright and the solar cell was shielded by your hand, then exposing
the solar cell to the bright sunlight generates enough power to turn the
regulator off and force the circuit back into the charging cycle.
Hope this helps explain the operation.
> <<D1_SOLARREG.gif>> -----Original-----
> From: Travis D. [SMTP:beam_bot@...]
> Sent: Wednesday, April 26, 2000 3:01 PM
> To: email@example.com
> Subject: Re: [beam] D1 Solar Regulator was Re: Need help with the
> I CAN'T WAIT TO TRY THIS CIRCUIT OUT...I REALLY HOPE
> IT WORKS GREAT!!! One question though, How exactly
> does it work?
> Thanks Wilf,
- << Previous post in topic