View Full Version : 12v Power Supply for mini car computer
02-14-2005, 03:08 AM
I have used the BOE and BSII to build a mini car computer (temperature, compass, etc). Now I want to actually put this in the car!
1) Can I directly hook up a "hot" wire (for example splicing the radio power supply) and ground to the BOE 9 volt battery clips? Will the BOE and STAMP regulate this to 5V?
2) When power supply is removed, and then applied (eg, stopping and then restarting the car), will the STAMP reset and then execute the stored basic program?
I am a bit new to this, so any guidance would be most appreciated.
02-14-2005, 03:59 AM
1. The BOE has an on-board regulator (2940, I believe) which will regulate anything from 5.5 to 16 volts to +5.
2. The BS2 also has an on-board regulator, but the BOE by-passes it, which is OK.
3. The BS2 program is stored in on-module EEPROM. (That's Electrically Erasable PROM (Programmable Read-Only Memory)). As such, yes once loaded with a program, it stays in there indefinitely, ready to re-run on power-up.
4. When NOT using the BOE, you need to wire the positive voltage to 'Vin' of the BS2 module, and ground to Vss. This supplies the on-module regulator, which will then produce +5 on the Vdd pin. If you directly drive the Vdd pin with 9-volts, you will destroy the BS2. When using the BS2 regulator, you only have about 50 mA to work with. The BOE regulator will produce 1000 mA.
02-14-2005, 04:34 AM
so it sounds like my method of soldering the 12v wire and ground to the 9v battery clips on the BOE would work fine. Also, this seems to be the best way to go, given the BOE will give me 1000mA...
02-14-2005, 08:14 AM
Yes, that should work fine. Don't you have a 5.5/2.1 mm barrel connector that would connect to the BOE board directly?
02-14-2005, 08:50 PM
duh. yes i do. That would probably work better - just splice the wires into a male barrel connector....
My BOE board says 6 to 9 volts. Although the regulator might handle 12 volts (or 13.8 when car is running) if you draw too much current the regulator might overheat. (13.8 V - 5.0V = 8.8 Volts which at 1 amp would produce 8.8 Watts of heat in the regulator.
At 9 volts this would be only 4 watts. So assuming the regulator can handle 4 watts, you could draw about 454 milliamps @ 13.8 volts.
This is all just a guess, but it's something to think about.
Check out· the "SX-Video Display Module"
02-14-2005, 11:57 PM
According to National's datasheet on it (www.national.com/ds/LM/LM2940.pdf (http://www.national.com/ds/LM/LM2940.pdf)) max input voltage is 26 volts, even for the 5 volt version. Their high voltage performance chart shows it working fine up to 30 volts.
But, as Bean pointed out, the higher the input voltage the higher the heat generated, and thus the greater the need for an adequate heat sink.