microcontrollertransistors
I see lots of examples of using low voltage (like from a microprocessor) to switch 12v. I want to do the opposite. Is that okay to do, or should I use a relay? What other components will I need?
The project is supply power to an AtTiny85 when a 10-16v (headlight on a motorcycle) turns on. I will be using a LM2937 (or you can recommend another) to provide power the 3.3v source.
This will drive a relay from a micro-controller pin: -
The relays you have described are fine and putting a contact in live and neutral is fine too. Just make sure you select your final one to switch 10A and check to see it is rated for the voltage and has a good gap coil to contacts (or is specified as being suitably safe).
The transistor in the circuit has to be able to supply the relay coil current so it may need to be rated at 200mA depending which relay you choose. Voltage rating 20V or above. If you are using a "hungry" coil and it needs more than 100mA you might want to reduce the 10k to a 2k2 to make sure the transistor turns on properly.
If the relay doesn't have an in-built diode protection circuit then you'll need to add a diode to the circuit across the relay. It's normally not conducting with the anode at the collector of the transistor. 1N400x type will do.
The conditions for saturated operation and the various HFE line are various operating conditions. If you operate the transistor with 1V VCE and carrying 50mA you will see a minimum (worst case) HFE of 60. For saturated operation you need to guarantee that any transistor will work in a circuit and usually it is recommended to operate as if the worst case HFE was 10. If you look at the data sheet for saturated operation they show various conditions where they put 1/10 of the current into the base as the collector current (defined by the circuit). This is often referred to as a "forced hfe". The graph at the bottom of page 3 shows this also with forced HFE of 10.
In your circuit you have R3 as 180 ohm which will require ~30mA if Q2 is saturated. Taking the forced HFE as 10 we require 3mA into the base. This will therefore require 1.8k ohm resistor for R2. Similarly since Q1 will be expected to saturate with the current from R2 (~3ma) it needs at least 0.3mA from R1. R1 would need to be about 18k or lower.
The Forced HFE of 10 is a maximum and is not critical. You could use 5 or 15 for this circuit with no problem. A disadvantage of having a higher current into the base is that it consumes more power in the base resistor and also that the higher current will result in more stored charge in the base of the transistor which will take longer to dissipate and slow down the turn-off.
I would also put resistor from the base of Q1 to ground (maybe 10K) to assist in turn off and ensure that any leakage across the switch S1 does not cause Q1 to incorrectly turn-on.
kevin
Best Answer
Use a buck switching regulator (output 3v3) connected to your headlight wiring. No need for a switch. Don't use a linear regulator for this because it might dissipate too much power and burn (load dependant).
Buck regulators are near enough 90% efficient at transferring power so it will burn very little power. One word of warning is that with any regulator you should rate its input to withstand at least 42V. 42V is a recommended limit for surges in normal automotive environments although a 60V capable regulator will be better and, there are a few. Maybe this TI one: -