Any way to get 19 volts from a PSU?

A

awdark

[H]ard|Gawd
Joined
Mar 22, 2003
Messages
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I purchased a laptop from ebay and didn't actually read it... :rolleyes:

So I was wondering if you guys can think of a way I can get a 19v 1.9A power source.

I can find plenty of 12v power supplies but thats worthless for my application.

Edit:
Not enough amps is worse than not enough volts correct? I found a psu from my printer, 19v .54amp, 5v .3A, 27v .47amp.
 
Gotta have the same voltage, at least the same current. With that being said, I've successfully used old 18V deskjet power supplies in a situation like this. They're usually quite a bit cheaper than a proper replacement power supply. Just be sure to verify the polarity of the plug.
 
Yup, you need at least the spec'd current capacity (amps). Sometimes you can get away fudging the voltage a bit, because there's usually some regualtion onboard, but it's always safer to match it if you can.

If you don't find anything in your parts bin, I'd get a surplus psu like this one for $15 rather than trying to hack something together.
 
Whatsisname said:
You could use two power supplies to produce 24 volts then use a voltage divider.
Click to expand...
So can you just tie them together (his case) 0.54A @ 19V
Code: 
19v ------\_________ 38v______/\/\/\/\/\/\/\_____ 18v
19v ------/			18.5Ohms
Can you combine PSUs in that fashion? I'm looking at that wondering how the voltage would add up. The current makes sense (even though it's still not enough using printer psus), but the voltage doesn't seem to add up in my head. Also, that would make for a pretty high wattage resistor. :(
 
You'd have to put the psu's in series to sum the voltages; parallel sums the current but the voltage stays the same. The reference to the resistor divider was directed at using two pc type psu's in series (12+12=24v). If you have multiple 19v supllies (as in your illustration), just put them in parallel to bump up the current capacity; the voltage will stay 19v.

I don't think a resistor divider is a good solution because the voltage will fluctuate as the current demands change (no regulation; it's dependent on a fixed set of operating parameters), and it's a very inefficient method that will produce a lot of heat and would probably require a hefty resistor. That's one reason switchmode supplies are popular for higher power.

I still think the $15 supply I linked above is the best bet for a safe, reliable solution.
 
Ah yeah thanks for the advice. That $15 is quite reasonable and I think ill just go ahead and order that. Would probably reduce the risk of creating magic smoke and arc welding parts together unintentionaly. :p
 
you know you can get 24v by using a dc/dc converter. That's what happened on the digg case VFD. Input 12v --> output 24v xxxMA. I'm sure there's one out there that will output 19v

I wouldn't bother with a brick. They're huge.
 
Qtip42 said:
I wouldn't bother with a brick. They're huge.
Click to expand...
?

I'm not sure I see the benefit or reduced size of a hacked 12V --> dc/dc 19V to power a laptop vs the standard power brick they come with. I wouldn't calssify them as 'huge', in fact I'm not certain you could find a supply with that power capacity all that much smaller.
 
Whatsisname said:
You could use two power supplies to produce 24 volts then use a voltage divider.
Click to expand...

i don't know what's wrong with hardforum people...

you can't use a voltage divider to regulate voltage (well.... almost never)... there are very very few instances where it would work...

first.. how much current is going to be drawn.. also.. can you garuntee that the current draw n wuill be constant.. because if the current changes on a voltage divider the voltage will change.. so you have to garranty that the current will change almost nothing over time.. in most applicatiosn that's not true...

to regulate voltage properly you can use a 19V 5W zener diode.. or you can get an adjustable LM1084 voltage regulator.. and use a pot to get the 19V you want...

I would never use a voltage divider for voltage regulation.. even if i knew the current draw will be constant... resistor don't have any load or line regulation.. you just don't do it like that... even if it does work in few instances
 
ThirtySixBelow said:
So can you just tie them together (his case) 0.54A @ 19V
Code: 
19v ------\_________ 38v______/\/\/\/\/\/\/\_____ 18v
19v ------/			18.5Ohms
Can you combine PSUs in that fashion? I'm looking at that wondering how the voltage would add up. The current makes sense (even though it's still not enough using printer psus), but the voltage doesn't seem to add up in my head. Also, that would make for a pretty high wattage resistor. :(
Click to expand...


No.. that is not the correct way to get 38V.. you will most likely fry something like that (like.. you will have a 99% chance they will fry.. because both power supplies will not be the same exact voltage.. there will be smoe error)..

you have to put them in series.. not parallel..
 
agent420 said:
You'd have to put the psu's in series to sum the voltages; parallel sums the current but the voltage stays the same.
Click to expand...

not exactly.. the problem with that set up is that there are no resistors in front of the two separate 19V sources.. only then would that setup keep the voltage the same (something above 19V.. but below 38V), and the current sum...

remember both supplies will not be 19V.. there's gonna be some voltage difference in reality.. so there will be voltage difference on a small piece of wire with almost no resistance.. V = IR say's that then you will have a SHIT load of current going frmo one source to the other..

something will fry.. the wire wil probably get hot and melt.. or the power supplies could get busted.. taht configiration just is not correct.. you would need two resistor.. one in front of each source.. to create a votlage summer.. and like i said.. in taht case.. depending on the ressitors you will have a voltage between 19V and 38V.. but again.. that voltage is not regulated.. so it will be no good for this application.. if the summed voltage is bigger than .. then perhaps you can regulate with a zener or linear regulator...

the easiest thing to do.. is just to the put the sources in series... and they will add.. no resistoras needed...
 
Qtip42 said:
you know you can get 24v by using a dc/dc converter. That's what happened on the digg case VFD. Input 12v --> output 24v xxxMA. I'm sure there's one out there that will output 19v

I wouldn't bother with a brick. They're huge.
Click to expand...


you could use a BOOSTER but then you have to make sure it can output at least the minimum current...
 
Your best bet is to use a DC-DC convertor or get a purpose-designed 19V power supply. Period. Using two ATX PSUs in series is NOT a good idea, since DC ground is often connected to AC ground in a computer case. And putting them in parallel won't do you any good. (for those looking to use two PSUs to get more current capacity, the trick is to put a power diode on the + voltage of each PSU).

That said, you *might* be able to use a single ATX PSU for the job--laptop batteries typically provide somewhere between 10 and 14 V, and the laptop is usually fine with that. It all depends on what kind of voltage regulation circuit the laptop has inside of it.

In any case, my advice is to just get the right adapter in the first place. No muss, no fuss. It might be more expensive, but it'll save you a lot of time and heartache.
 
Mohonri said:
Your best bet is to use a DC-DC convertor or get a purpose-designed 19V power supply. Period. Using two ATX PSUs in series is NOT a good idea, since DC ground is often connected to AC ground in a computer case. And putting them in parallel won't do you any good. (for those looking to use two PSUs to get more current capacity, the trick is to put a power diode on the + voltage of each PSU).

That said, you *might* be able to use a single ATX PSU for the job--laptop batteries typically provide somewhere between 10 and 14 V, and the laptop is usually fine with that. It all depends on what kind of voltage regulation circuit the laptop has inside of it.

In any case, my advice is to just get the right adapter in the first place. No muss, no fuss. It might be more expensive, but it'll save you a lot of time and heartache.
Click to expand...
Good advice, but on the point of putting two power supplies in parallel using power diodes, the problem is that there will be a 0.7V voltage drop across the (silicon) diodes and this can be a show stopper.
 
agent420 said:
?

I'm not sure I see the benefit or reduced size of a hacked 12V --> dc/dc 19V to power a laptop vs the standard power brick they come with. I wouldn't calssify them as 'huge', in fact I'm not certain you could find a supply with that power capacity all that much smaller.
Click to expand...


Shame on me, I missed the part that said "power a laptop" hehe.....you're right.
 
Rondu said:
Good advice, but on the point of putting two power supplies in parallel using power diodes, the problem is that there will be a 0.7V voltage drop across the (silicon) diodes and this can be a show stopper.
Click to expand...
Yup, it could be. The problem is that we don't know enough about the laptop's power circuitry to reliably "hack" it. Hence my suggestion to just get the right adapter in the first place.
 
You may want to check out MPJA. If you can't find what you're looking for there, I'd be surprised. :)
 
Buy an 18V laptop supply... there's plenty of good links posted here, like the above one.

You'll thank yourself whenever you bring the laptop somewhere else with the charger, you'll be carrying an actual AC adapter instead of two stacked power supplies, power resistors, circuit boards, etc... if you hack something up :D
 
Yeah, I did get that sony adapter should be coming soon :rolleyes:

I had just hoped for a quick and easy power replacement but sometimes you just need to spend that money.
 
gee said:
You'll thank yourself whenever you bring the laptop somewhere else with the charger, you'll be carrying an actual AC adapter instead of two stacked power supplies, power resistors, circuit boards, etc... if you hack something up :D
Click to expand...
idk... seems like a good way to pick up chix at a lanparty :p
 
agent420 said:
idk... seems like a good way to pick up chix at a lanparty :p
Click to expand...
the lan guy to girl ratio at most lan parties is almost alway way out of whack. you'd need a hefty SRC to handle that.



SRC= sausage regulator circuit.
 
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