Questions about motherboard with DC input

[insz]

Hi there,

I have an Asrock Q1900DC-ITX MB as the base of a NAS that has a DC input jack and accepts 9~19V input.

The computer only draws about 35W (at the wall) at idle, but on boot all hard drives spin up at the same time, and each hard drive draws about 20W on it's own. I have an old 19V laptop supply that's working, but I want to go bigger to add another drive in the future - so I'll need a larger power supply.

I have a ton of Lenovo 90W laptop power supplies, but they are all 20V output.

My questions are around these power supplies, and the DC input on the motherboard. I wasn't able to figure out what the mb uses to regulate the input power. Can anyone tell from the pic on their website - http://www.asrock.com/mb/Intel/Q1900DC-ITX/

I know the default answer to the question of "can I use a 20V input on a 19V device" is no, but based on the 9~19V rating, I'm assuming there's some significant voltage regulation in place and they might be leaving some space for caution.

As a followup, would it be possible to drop the output voltage of my 20V power supplies? I know resistors aren't the most efficient, but if I only need a 1V drop, and after the first 5 second load is ~35-40W, would it be an option?

Thanks for your help.

[ben7]

Re: Questions about motherboard with DC input

Hi there,

I have an Asrock Q1900DC-ITX MB as the base of a NAS that has a DC input jack and accepts 9~19V input.

The computer only draws about 35W (at the wall) at idle, but on boot all hard drives spin up at the same time, and each hard drive draws about 20W on it's own. I have an old 19V laptop supply that's working, but I want to go bigger to add another drive in the future - so I'll need a larger power supply.

I have a ton of Lenovo 90W laptop power supplies, but they are all 20V output.

My questions are around these power supplies, and the DC input on the motherboard. I wasn't able to figure out what the mb uses to regulate the input power. Can anyone tell from the pic on their website - http://www.asrock.com/mb/Intel/Q1900DC-ITX/

I know the default answer to the question of "can I use a 20V input on a 19V device" is no, but based on the 9~19V rating, I'm assuming there's some significant voltage regulation in place and they might be leaving some space for caution.

As a followup, would it be possible to drop the output voltage of my 20V power supplies? I know resistors aren't the most efficient, but if I only need a 1V drop, and after the first 5 second load is ~35-40W, would it be an option?

Thanks for your help.

You could use a diode to drop the voltage.
However, the laptop should be ok with 20V.
No guarantees of course :P

-Ben

[SteveNielsen]

Re: Questions about motherboard with DC input

35w / 20v = 1.75a so you'll need a 2 amp diode. Typically, a silicon diode will have a drop around 0.6-1V, so it may be necessary to use two si diodes in series to drop below 19v, it depends on the diodes in use.

[insz]

Re: Questions about motherboard with DC input

Thanks Ben and Steve for getting back to me, and after some reading it seems like a diode or two would do exactly what I need.

For the first 10 seconds of the NAS powering up, the power draw will be up closer to the 90W mark, before settling down to ~35W (hard drive spinup). 90W/20V = 4.5A, so I was looking for a diode with a higher amp rating.

It seems like when you go larger than about 3A, diode selection changes. I was looking for a plan silicon diode, but I'm finding rectifiers, schottky diodes, zener diodes, etc... Most of the larger amp rated diodes advertise their low forward voltage (which I actually want to keep higher for my needs).

Do you have any recommendations for diodes that would work? So far to me, these seem like the best candidates:

http://www.jameco.com/1/1/5324-stth5...ode-220ac.html 5A 600V

https://www.westfloridacomponents.co...ier+Diode.html 5A 80V schottky (2x)

http://uk.farnell.com/fairchild-semi...201/dp/1467531 5A 100V (2x)

Also, would there be any reason not to do this? I know that with resistors, they produce heat while dissipating power. Is this also true with diodes? If I had two schottky diodes in series between the AC-DC power supply and the power connector on the motherboard, dropping the voltage from 20V to ~18.5V, would they generate heat?

Thanks again for your help

[budm]

Re: Questions about motherboard with DC input

"I know that with resistors, they produce heat while dissipating power. Is this also true with diodes?" Yes.
You will need heatsink for the diode. Look at the spec of the diode to see what the Vf will be at the required current.
if Vf is 1V at 4.5A then the diode will dissipate 4.5W!

[mur]

Re: Questions about motherboard with DC input

I don't think 20v into a 19v device is a problem. It's only 5% difference and motherboards like this usually allow for more than that. I have an asrock AM1H-ITX, which says 19v +- 10% and I power it with a 20v adapter without any problems.

But are you sure that the dc-dc converters on the motherboard are powerful enough to power up 60w of HDDs when spinning up?

[mariushm]

Re: Questions about motherboard with DC input

The board will run just fine with 20v.

Any sane engineer that advertises something as 9-19v input will leave about 10% margin for safety, in case the voltage of the adapter coming out fluctuates a bit with load.

The polymer capacitors used at the input would be rated for 25v and the ICs used on the board should also handle at least that much, the mosfets are probably rated for at least 35-40v (though it's not recommended to drive mosfets with voltages close to their maximums).

The only problem I see with high voltage would be the possibility of extra noise (coil whine) if the switching regulator was optimized to work very well with a narrow range of voltages (let's say 12-19v) and using 20v would lower its "duty cycle" (i think that's the term) too much.

Anyway, if you really insist on making sure the board gets 19v, I wouldn't mess with diodes or resistors. I'd just buy a *1084 (LT1084, LM1084, manufacturer of the day 1084), whatever version is designed for more than 12v input voltage and more than 15v output voltage : http://www.digikey.com/product-searc...=0&pageSize=25

It's a linear regulator that can do up to 5A and has a maximum voltage drop of 1.5v (at 5A load, around 1v at low currents.

A good example would be this: http://www.digikey.com/product-detai...13-5-ND/669183

Basically give it 20v input, configure it using two resistors to output 18.5v-19v and it will do the best it can to keep the voltage at 19v or a bit lower (but never above 19v).
The regulator will dissipate (Vin - Vout) x Current watts, and if that's more than about 1 watt, you need a small heatsink but those are easy to find and you'd need one for diodes anyway (and they're harder to heatsink).