Alt-BEAM Archive
Message #04356
To: "'beam@sgiblab.sgi.com'" beam@sgiblab.sgi.com
From: Wilf Rigter Wilf.Rigter@powertech.bc.ca
Date: Tue, 8 Jun 1999 18:14:45 -0700
Subject: [alt-beam] Re: SE questions not found in the FAQ.
Hullo Bob,
The D1 works this way:
Charge when bright, discharge when dark.
When the light is bright the cap starts to charge through the 1N4001 diode.
This is a 1A diode with a slightly lower Vf (ie at 100ma) than small signal
diodes like the 1N4148 so maybe a 1N4148 is ok too. When the cap is charged
above 2V, the voltage will be high enough to turn on the 2N7000 if the
output of the solar panel drops significantly! When a shadow, dusk, etc
falls across the solar panel, the output voltage will drop depending on the
load setting of the 150K pot. The minimum setting is of course useless and
it is generally adjusted for 10K and up. When the voltage across the solar
panel collapses, the voltage across the 1N4001 will change from -0.6V to a
voltage which is the difference between the cap voltage and the voltage
across the pot. As long as that is greater than ~2V the 2N7000 will turn on
and connect the cap to the load. Without feedback the switching will be
soft and waste power. The D1 will reset when the light level increases.
The PM1:
Charge until it reaches the 1381 trigger level then discharge. Reset after
the 1381 cap (0.47uf) is discharged (very slowly) through the 1381 and
reversed biased 1N4001.
The PM1 should work a little better since the threshold is well defined and
the 1381 provides some "snap action". While the main cap C1 charges, the
0.47uf on the input of the 1381 charges through the 47K resistor and the
1N4001 diode. This 47K is required to bypass the load which may be high
impedance or have several diode drops in series internally when turned off.
That way the voltage on the 1381 cap is about 100mV lower than the main cap.
When the voltage on the 0.47uf cap crosses the 1381 trigger level the 1381
output goes high and the 2N7000 turns on. The 0.47uf across the load is
probably not needed but may be used to filter motor noise.
The 1N4001 is quite leaky and will eventually discharge the 0.47 cap below
the 1381 reset threshold. A 1N4148 has very low reverse leakage and the
circuit may take a long time to reset.
Both of these circuits are simple but nasty designs and will shortly be
obsolete.
Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
tel: (604)590-7493
fax: (604)590-3411
> -----Original Message-----
> From: Bob Shannon [SMTP:bshannon@tiac.net]
> Sent: Tuesday, June 08, 1999 4:15 PM
> To: beam@sgiblab.sgi.com
> Subject: SE questions not found in the FAQ.
>
> I have a few questions on the D1 and PM1 solar engine schematics found
> at:
>
> http://www.beam-online.com/Robots/Circuits/circuits.html
>
> This is the second version of the 'D1' schematic I've seen. Something
> looks wrong here.
>
> First, if I adjust the 150k pot to its minimum resistance value, the cap
> will never charge.
> And if the gate of the 2N7000 is really directly connected to the
> negative side of the solar
> cell, what is this pot supposed to be adjusting anyway? Simulated
> capacitor leakage?
>
> Maybe the gate of the 2N7000 is supposed to go to the wiper of the pot?
>
> Then there is that 4001 diode. Should it really be between the cap and
> the solar cell?
> Your going to loose a full .7 volts here alone.
>
> Shouldn't that really be a 1N5817, or at least a 1N4148, and place it
> between the source
> of the 2N7000 and the negative connection between the cap and solar
> cell?
>
> Using a high-side PNP switch is a lot more efficient, and much more
> minimal than the D1 engine
> and rare 2N7000 fetlington.
>
> As drawn, I can beat this circuit in 3 parts, all from Radio Shack.
>
> There must be an error here somewhere, or I'm missing something big.
> Does anyone have a detailed theory of operation for this circuit?
>
> Now, about that PM1 drawing...
>
> What is the reason for having the .47 uf cap and 47K resistor across the
> load being controlled
> by the PM1?
>
> Clearly (as drawn) we need to get current through the 4001 diode to
> charge the .47 cap on the
> 1381's input. (so this wont trip at the 1381's preset voltage?)
>
> So why not place the 47 k resistor between the positive supply and the
> input of the 1381, with
> the .47 uf cap to ground.
>
> Could we then eliminate the 1N4001 completely, or is this needed to
> discharge the .47 uf cap?
> (there is nothing like this needed to discharge the .22 caps in a
> photopopper!)
>
> If so, lets really slash and burn, and simply connect the input of the
> 1381 to the positive side
> of the main capacitor, and be done with it! (we have eliminated 4 parts
> from the circuit.)
>
> Does anyone understand why the published circuits are shown this way?
>
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