I need to power an RFID from 24 V DC voltage. The RFID requires max. 9 V and max. 120 mA. The RFID suggests a 7809 linear voltage regulator. I thought too much and I want to use a 7809 because if I use a switching regulator (10$) for 24 V to 12 V and then 12 V to 9 V with 7809, it costs a lot.
If I go with 24 V to 9 V, (24 V - 9 V) * 120 mA = 1.8W,
If I go with 24 V to 12 V and then 12 V to 9 V ,(12 V - 9 V) * 120 mA = 0.36 W.
It will take ~22000 hrs of operation to realize the $8 savings from going to the switching regulator vs going for the linear regulator and that's okay for me.
- I made this circuit; someone suggested to me to use a 82 Ω, 2 W resistor. If I use that resistor, would I still need to use a heat sink? If I add that resistor (82 Ω), is power dissipation = (10.2 V - 9 V) * 120 mA = 0.24 W ? (I used a 75 Ω resistor for the RFID part)
- I read somewhere (I couldn't find the link) that says you need to put in 3 different output capacitors for clear power, is that correct? Are these capacitors okay? And all these capacitors are ceramic, should I use electrolytic ones?
- I found this heatsink özdisan.comdatasheet, this heatsink has only graphic and it says that at 2 W the rise in temperature will be 20°C, 5°C from junction to case, plus another 5°C for the TO220 case to heatsink, resulting in 30°C isn't it wonderful?
I have another heat sink farnell.com, it doesn't have a graphic but it says Thermal Resistance is 14°C/W , how can I compare which is better? I couldn't get its Thermal Resistance: 14°C/W . Thank you.