I am installing a VIA-EPIA mother board in my car. This device needs at
least 12 V to operate (at a max 9A). The problem that I am going to
encounter if I power the unit with ACC is that when I start the car the
voltage will drop to sub 12V due to the starter motor.

I've been looking at using a battery isolator with a small batter (
http://www.power-sonic.com/1282s.html ) but my concern is two fold
1) two differen't types of battery in the car - people say this is bad
2) 13.5 V fully charged - .7V from the diodes in a normal battery isolator
is getting close to not powering the motherboard.

Keep in mind that the 2nd power source only has to supply the unit while the
starter is being cranked. Are there better isolators out there that don't
have a voltage drop or that might be better for this situation? Should I
just use a diode and a big capacitor ? Any other suggestions?

I am installing a VIA-EPIA mother board in my car. This device needs at
least 12 V to operate (at a max 9A).

You can get ITX cases with smaller power supplies. 9A is a little on the
high side.

The problem that I am going to
encounter if I power the unit with ACC is that when I start the car the
voltage will drop to sub 12V due to the starter motor.

I've been looking at using a battery isolator with a small batter (
http://www.power-sonic.com/1282s.html ) but my concern is two fold
1) two differen't types of battery in the car - people say this is bad
2) 13.5 V fully charged - .7V from the diodes in a normal battery isolator
is getting close to not powering the motherboard.

Keep in mind that the 2nd power source only has to supply the unit while
the
starter is being cranked. Are there better isolators out there that don't
have a voltage drop or that might be better for this situation? Should I
just use a diode and a big capacitor ? Any other suggestions?

I also have an EPIA in the car. I simply diode isolated a capacitor and
relay. The purpose of the diode was twofold: isolate the cap and also drop
down a little from the ~14.5 volt input. The point of the relay is to
provide a high coil resistance so you don't have to get a gigantic
capacitor. The relay coil draws only a couple hundred milliamps, whereas
the computer supply draws 5 amps.

I'm not sure that I understand where the relay fits in, and how it helps.
Isn't your capacitor size requirements based on how long it takes you to
start the car and how many amps your power supply can provide?

I already have a 12V regulator before my 12VDC-DC power supply.

In article ,
"Jadz" wrote:

I am installing a VIA-EPIA mother board in my car. This device needs at
least 12 V to operate (at a max 9A). The problem that I am going to
encounter if I power the unit with ACC is that when I start the car the
voltage will drop to sub 12V due to the starter motor.

I've been looking at using a battery isolator with a small batter (
http://www.power-sonic.com/1282s.html ) but my concern is two fold
1) two differen't types of battery in the car - people say this is bad
2) 13.5 V fully charged - .7V from the diodes in a normal battery isolator
is getting close to not powering the motherboard.

Keep in mind that the 2nd power source only has to supply the unit while the
starter is being cranked. Are there better isolators out there that don't
have a voltage drop or that might be better for this situation? Should I
just use a diode and a big capacitor ? Any other suggestions?



A switching boost regulator can maintain a 12V minimum even if the input
drops to a few volts. I'm not sure where you'd buy one, though.

Can the motherboard run off 30V? You could pull that from the primary
side of an audio amp's switching power supply.

I'm not sure that I understand where the relay fits in, and how it helps.
Isn't your capacitor size requirements based on how long it takes you to
start the car and how many amps your power supply can provide?

I already have a 12V regulator before my 12VDC-DC power supply.

The capacitor size requirement is based on three things: time, allowable
voltage drop, and current draw.

The capacitor size requirement is based on three things: time, allowable
voltage drop, and current draw.

....which is exactly why I don't see how a relay would help to reduce the
size of capictor needed in this situation.

...which is exactly why I don't see how a relay would help to reduce the
size of capictor needed in this situation.

Perhaps I wasn't clear (that wouldn't be a first).

Its purpose is to reduce current draw. Why? Because a radio shack 10A 12V
relay has a coil resistance of about 350 ohms. That means less current draw
(I = 12/350 = approx. 30mA). It also has a dropout voltage of about 7 volts
or so (you'll have to double check this number). This means that the
capacitor, as it discharges, can drop to 7 volts without the device shutting
off. Your computer input would probably shut off by the time you reached 10
volts or so. So both of these aspects allow you to use a much smaller
capacitor. I used a few caps in parallel, because I wanted to keep my
computer on for 10 seconds so that it would auto shutdown before losing
power. You probably won't need 10 seconds.

Ah, you are only using it as a time delay not to supply power to the actual
unit. My power supply has a safety built in that will shut the unit off
below 12V. So even if the increased resistance of a relay keeps the power on
for a longer time my power supply is still going to cut the juice.

Ah, you are only using it as a time delay not to supply power to the
actual
unit. My power supply has a safety built in that will shut the unit off
below 12V. So even if the increased resistance of a relay keeps the power
on
for a longer time my power supply is still going to cut the juice.

No it won't. You run a wire from 12v cont. to the switched part of the
relay. That supplies your 10 amps. The coil is run by the remote wire of
your head unit, and that's where your diode isolation and capacitor goes.

Mark,



I misinterpreted your reply, as I thought it related to the thread topic. I
had thought your capacitor was allowing you to run the PC at the same time
as you were starting the car.



The question I had originally was how to sustain a 12v cont. at 12V while
starting the car (the starter drops the whole system voltage). To do this it
would seem I would need somewhere around 8F of capacitance, which isn't
going to happen any time soon.



Instead I'm thinking of using a small gel battery and an isolator.

I misinterpreted your reply, as I thought it related to the thread topic.
I
had thought your capacitor was allowing you to run the PC at the same time
as you were starting the car.



The question I had originally was how to sustain a 12v cont. at 12V while
starting the car (the starter drops the whole system voltage). To do this
it
would seem I would need somewhere around 8F of capacitance, which isn't
going to happen any time soon.

Ah, I see what you're trying to do here. I misread your original post. I
don't really understand why you have the computer on before the car anyway.