If we have 3 PCBs connected in series (i.e, negative terminal of one is connected to positive terminal of the next PCB), and this is connected to a 36 V DC supply, can we assume the voltage drop across each is 12 V DC?
Application: Have a solar panel with a very-high voltage DC (~200V) and high-current (~30A) and trying to see if I can get away by connecting multiple Bitcoin hash boards in series without expensive buck converters?
Assuming that when you say "connect in series" you mean "connect each board's one and only power input in series with the next ones one and only power input", then all of the above, plus:
Typically, the power supply on a PCB is referenced to the circuit's local ground and things just get weird if you offset that local ground from the rest of the world -- think plugging a USB cable into the board and having sparks fly and smoke come out.
There may be some bizarre application where you'd want to do this, but it would be very difficult to draw just the right amount of power for a board without starving the rest of the boards of either current or voltage.
can we assume the voltage drop across each is 12V DC?
No you can't because each PCB may appear as a different load resistor and, the PCB with the highest resistance between its power terminals will have proportionately a higher voltage across it.
If you have any active device on that PCB, no you can't (active device include diodes, transistors, IC, regulator or basically anything that isn't a resistor, capacitor or inductor).
In addition to what others have said...
You seem to misunderstand the nature of voltage supplied to PCBs. You're treating them as if they create the voltage drop like series resistors.
They do not.
I used to design BiCMOS circuits. Parts of those circuits were heavily bipolar (in every sense of that word...). Bipolar transistors are not like field effect transistors (FET). We depended on the supplied voltage so we could step it down using diodes and VBE drops as part of the circuit.
In other words, the supply voltage was required by the circuit, not created by the circuit. For example, for those circuits that used a 5VDC supply, you could not put a 100-ohm resistor on the supply line and hook it up to a 6VDC source, then expect 5VDC to appear at the true positive input for the circuit. What was more likely to happen is 2.2VDC and a wholly inoperative circuit.
If you chain those PCBs together as you suggest with the voltage capacity of those panels, I expect them to burn up. I don't know if you need the expensive buck converters — but at the very least you need an appropriate transformer so you can hook all the PCBs together in parallel.
If the resistance in the PCB's is the same then yes, otherwise no. The PCB resistance will be probably close to the same, the contact resistance will vary more.
Putting the boards in series will be possible if the outputs are isolated and the boards draw equal power, otherwise no, loading variation will cause different voltage drops across the devices and could cause problems or operation beyond recommended voltage and current values.
Many devices will need to usefully employ varying amounts of power under varying conditions. There are at least four ways a device with varying energy needs may accomplish this:
A device may be designed so that it will pass as much current as necessary to extract the necessary amount of energy per second from the applied voltage, so long as the voltage is above a certain minimum below which the device will shut down.
A device may be designed so that it will pass as much current as necessary to extract the necessary amount of energy from a certain nominal voltage, with degraded or unreliable operation if the voltage falls below that, and with any excess power resulting from excess applied voltage being converted to heat.
A device may be designed so that it will pass a certain amount of current, and drop as much voltage as necessary to extract the required amount of power.
A device may be designed so that when a voltage is applied within a certain nominal range, it will draw an amount of current that will be sufficient to satisfy its maximum energy need, and will dissipate a variable amount of unused energy as heat.
The first two kinds of devices are much more common than the last two, but only the last two may safely be wired in series as you describe. Then a number of devices are wired in series, it's not possible for any of them to pass any more current than the others. If some in a series chain reduce their voltage drop when they're receiving too little power, while other devices increase their voltage drop when they're receiving too much power, the result will be that the devices which were receiving too much energy will get more, and those that weren't getting enough will get less. If, however, devices in a series chain that are getting insufficient energy increase their voltage drop, while those that are getting too mcuh decrease theirs, and if the supply for the chain reduces output voltage when the current draw is above nominal and increase it when it's below nominal, however, things will fall into a balanced equilibrium state.
