Hi
Suppose you have a wireless (RFM12b) battery powered (one coin cell) Tiny84 mini-board (a simple sensor node). This board could also be powered using external 3.3 power supply. Suppose this board can work for 1 month in a 10% dutty cycle.
Now, in order to protect things from reverse polarity, I decided to add a diode to the begnning of the circuit power track. According to the theory about diodes, they have zero resistence (idealy) in the right polarity and infinite resistance in the reverse polarity...
My question is, does that diode has a significative impact on energy consumption (it is on the beggining of the circuit and not in the 10% dutty cycle zone. It will be always operating)
I decided to add a diode to the begnning of the circuit power track.
What does it mean?
In general, diode does dissipate power, setting in series with battery it hold 0.4 - 0.7 V, which may be accounted for 15 - 25 % overall consumed power, as current the same and voltage 3V "in the beginning"
The diode will dissipate (waste) power when any current if flowing through it, small but still a loss. So unless the risk of accidental polarity is high I would not include it if your trying to maximize battery duration. Once the correct polarity is known a 'polarized' connector is a good way to deal with accidental reverse polarity.
If you're feeding a linear regulator, then the double diode waste is irrelevant.
To the OP. Diodes drop a relatively fixed voltage (usually .3V to .6V) depending mostly upon the type of diode and what it is made from. This multiplied by the current determines the power being dissipated (wasted) by the diode.
To the OP. Diodes drop a relatively fixed voltage (usually .3V to .6V) depending mostly upon the type of diode and what it is made from. This multiplied by the current determines the power being dissipated (wasted) by the diode.
Correct, if one looks at typical diode datasheets the voltage drop is not constant but rather increases with increased forward current. See figure 2 graph curve. http://www.diodes.com/datasheets/ds28002.pdf
Lefty
If you want reverse polarity protection without wasting power in a diode drop, a good way to do it is to put a diode across the voltage input so that it shorts out and blows the fuse if you power it in reverse. Otherwise, the diode does nothing except sit reverse biased leaking a miniscule amount of current.
afremont:
If you want reverse polarity protection without wasting power in a diode drop, a good way to do it is to put a diode across the voltage input so that it shorts out and blows the fuse if you power it in reverse. Otherwise, the diode does nothing except sit reverse biased leaking a miniscule amount of current.
Yes, that method is popular and pretty standard for mobile Ham Radios.
alexe100:
My question is, does that diode has a significative impact on energy consumption (it is on the beggining of the circuit and not in the 10% dutty cycle zone. It will be always operating)
It has low resistance but the voltage will drop by half a volt or more when it goes through it. The drop depends on current - see your datasheet, there'll be a graph of voltage drop vs. current somewhere.
That voltage drop has an associated power loss, yes. OTOH power loss depends on how much current is flowing. If your Tiny84 is asleep then it will be microwatts.
Thanks to all of you.
Yes, the main aim is to save energy. So, as Lefty said, it is better to not include de diode.
(Sorry about this kind of question. My background skills are in computer science field not in electronics field and it is more interesting the opinion of you than try to read data sheets and then dont know how to interpret the numbers...)
