I was unpleasantly surprised at the prices (and limited capabilities) of commercially-available dew controllers, and was already gathering parts for your focuser design, so this looks like another project for my list. But I'm confused about one thing--why is the circuit using a LM7808 to feed +8V into the Arduino, just for it to regulate it down to +5V and then serve as the source for all the other +5V loads? It seems to me (not an EE, nor do I play one on TV) that it would make more sense to use a 7805, and directly supply all the temp sensors, the display, the BT module, etc., from that. I'm sure there's a reason it's designed this way, but I'm having trouble figuring it out. Any thoughts?
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12V of course is required for the dew straps,
looking at the circuit
12V in goes to Diode D2, which has a forward voltage of around 1.x volts
That gives let us round things a bit, a little less that 11V going in to the regulator
So the regulator has around 11V going in to drop to 8v going out to the VIN pin of the arduino.
Basic rule of thumb for regulators is that the input voltage must be around 1.5v higher than the output of the regulator in order for the regulator to work)
If the output of the regulator was applied to the 5v rail, then we have two competing 5v rails - the output of the arduino derived from USB power, and the output of the regulator at 5v. These outputs are never going to be perfectly the same, so its going to be a bit dicey most of the time.
Ideally we want a single source that can handle everything. And it also has to work when a USB cable is provided. But if the USB cable is connected then the Arduino prefers to use the 5V from the USB cable, so we need to make sure it does not.
The output of the arduino at 5V derived from USB power is limited to around 200ma max. We can power the switches, the leds, the nano and input to the mosfets and probes etc as these are less than 200mA,
Now, coming to back to non competing 5v rails, if a 8v regulator is used, the 8V out goes to the vIN pin of the nano, which the nano has an onboard regualtor which drops from (in our case 8v) to 5V which is great. When vIn voltage is around 1.5v higher than 5v, the nano switches from 5V USB power and uses the voltage from Vin to generate 5V - so we do not have two competing 5v sources. only a single 5v reference source produced by the Nano and derived from the 8V at vIN.
As long as the GND on the nano (effectively from USB ground) is also connected to the ground of the 12v source, we avoid ground loops as well.
regards
Robert
Last edit: brownrb 2022-02-10
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
I was unpleasantly surprised at the prices (and limited capabilities) of commercially-available dew controllers, and was already gathering parts for your focuser design, so this looks like another project for my list. But I'm confused about one thing--why is the circuit using a LM7808 to feed +8V into the Arduino, just for it to regulate it down to +5V and then serve as the source for all the other +5V loads? It seems to me (not an EE, nor do I play one on TV) that it would make more sense to use a 7805, and directly supply all the temp sensors, the display, the BT module, etc., from that. I'm sure there's a reason it's designed this way, but I'm having trouble figuring it out. Any thoughts?
Hi Dan
12V of course is required for the dew straps,
looking at the circuit
12V in goes to Diode D2, which has a forward voltage of around 1.x volts
That gives let us round things a bit, a little less that 11V going in to the regulator
So the regulator has around 11V going in to drop to 8v going out to the VIN pin of the arduino.
Basic rule of thumb for regulators is that the input voltage must be around 1.5v higher than the output of the regulator in order for the regulator to work)
If the output of the regulator was applied to the 5v rail, then we have two competing 5v rails - the output of the arduino derived from USB power, and the output of the regulator at 5v. These outputs are never going to be perfectly the same, so its going to be a bit dicey most of the time.
Ideally we want a single source that can handle everything. And it also has to work when a USB cable is provided. But if the USB cable is connected then the Arduino prefers to use the 5V from the USB cable, so we need to make sure it does not.
The output of the arduino at 5V derived from USB power is limited to around 200ma max. We can power the switches, the leds, the nano and input to the mosfets and probes etc as these are less than 200mA,
Now, coming to back to non competing 5v rails, if a 8v regulator is used, the 8V out goes to the vIN pin of the nano, which the nano has an onboard regualtor which drops from (in our case 8v) to 5V which is great. When vIn voltage is around 1.5v higher than 5v, the nano switches from 5V USB power and uses the voltage from Vin to generate 5V - so we do not have two competing 5v sources. only a single 5v reference source produced by the Nano and derived from the 8V at vIN.
As long as the GND on the nano (effectively from USB ground) is also connected to the ground of the 12v source, we avoid ground loops as well.
regards
Robert
Last edit: brownrb 2022-02-10