sizing an inverter

troypiggo (Troy) · #1 29-09-2009, 07:04 PM

I'm looking at inverters for when I only have 12V supply, but am a little bit electrically-challenged (or is it electronically-challenged?) and need some help. There are a few questions sprinkled through this post in italics that I need help with.

I already have a little Belkin 140W (300W peak) inverter and have been using that to date, but suspect I'm pushing its limits. Think it was designed for just notebooks mainly.

I'll be getting one from Jaycar. Notice there are 2 pages of them. Some are "pure sine wave" and some aren't. Important for the usage mentioned below? Massive price difference.

So what am I running through this inverter? This is a list of the things that I will probably run off it. Everything else I have already runs off 12V supply.

* Netbook - adapter says 240V 1.2A in, 19V 1.58A out. Which do you use for calculating the watts required? I assume it's the input, but that's 240x1.2=288W isn't it? Way more than the Belkin can take, but it does work fine. 19x1.58=30W may be closer to making sense? I know you can get DC/DC adapters, but I've looked in the past and the plug for this Aspire One doesn't appear to be one of the standard ones.

* MacBook Pro - may start using this more and the netbook less, so probably instead of the netbook. Adapter says 100-240V 1.5A in, 16.5-18.5V 4.6A out. I've had a bit of a look for 12V supplies for these, figuring they'd be readily available, but can't seem to find one. Similar question to above, what are the right watts?

* 2 powered USB hubs. No idea about their requirements, but figure pretty nominal?

* 40D with AC adapter - says 240V 45VA in, 7.8V 2A out. Isn't a VA the same as a W? Does that mean it uses 45W?

Should all of the above be used for the "normal" rating of the inverter as opposed to the surge/peak? How do you know what surge/peak power capability is required?

DavidTrap (David) · #2 29-09-2009, 08:07 PM

Couple of things,

For Macs - there are no Apple 12V adaptors. There is a company in the US who will cut your power supply cable and put a plug in so you can swap between the AC adaptor and a DC-DC adaptor. Google is your friend.

The Mac adaptors are about 85W - if you do some image processing you may go close to drawing this current (do the fans on the Mac start running, if so you are working the processor hard and drawing lots of current)

Your netbook is putting out 38W, but the input is 240x1.2 = almost 300W. This would make it extremely inefficient. I would guess the netbook adaptor would consume 50-60W, allowing for 20% inefficiency of the adaptor.

My USB hub would consume ~20W max. You are right about the camera adaptor 45VA is the new terminology for 45W.

I would guess that a 300W Inverter would do the job for you (assuming you may want to run both laptops), but maybe 500W would be safer & allow for expansion.

Personally, I use a modified sine wave inverter without difficulty. Many will argue that computers need a Pure Sine Wave inverter - my sparkie mate told me that the power adaptors cope quite well with a modified sine wave inverter supplying their 240VAC.

Just my 2 cents worth,

David T

beefking (Nathan) · #3 29-09-2009, 11:10 PM

Quote:

Originally Posted by troypiggo

I'm looking at inverters for when I only have 12V supply, but am a little bit electrically-challenged (or is it electronically-challenged?) and need some help. There are a few questions sprinkled through this post in italics that I need help with.

I already have a little Belkin 140W (300W peak) inverter and have been using that to date, but suspect I'm pushing its limits. Think it was designed for just notebooks mainly.

I'll be getting one from Jaycar. Notice there are 2 pages of them. Some are "pure sine wave" and some aren't. Important for the usage mentioned below? Massive price difference.

So what am I running through this inverter? This is a list of the things that I will probably run off it. Everything else I have already runs off 12V supply.

* Netbook - adapter says 240V 1.2A in, 19V 1.58A out. Which do you use for calculating the watts required? I assume it's the input, but that's 240x1.2=288W isn't it? Way more than the Belkin can take, but it does work fine. 19x1.58=30W may be closer to making sense? I know you can get DC/DC adapters, but I've looked in the past and the plug for this Aspire One doesn't appear to be one of the standard ones.

the output on the power pack for the netbook would be the power it requires - a 12Vdc/19Vdc conversion would result in a fewer losses than converting dc-ac-dc. If you intend to use the AC-DC converter you have, then you need to allow for the input power.

Quote:

* 40D with AC adapter - says 240V 45VA in, 7.8V 2A out. Isn't a VA the same as a W? Does that mean it uses 45W?

well, sort of and, no. The most watts it can draw will be 45W, but if you were to measure it, it may be less. It means that the current required is the same as a 45W load, even if the measured load in W is less.

Quote:

Should all of the above be used for the "normal" rating of the inverter as opposed to the surge/peak? How do you know what surge/peak power capability is required?

yes, all the above ratings should be used to size the normal rating. Surge rating lets the user know how long the inverter can overload for before being damaged e.g. a 300W inverter might be rated at 350W for half an hour or 400W for 15 minutes. I think it's to do with the heat dissipation from the electronics.

Surge ratings may be important for a varying load, one with motor inrush or something that exceeds the normal rating. It would be safe to assume that the loads listed above would be pretty constant. Any inverter sized to cope with the constant load should be able to supply startup currents to your equipment for the short time they'd be present.

It might be worth tracking down a portable power meter (I think there are one's around called "kill-a-watt"s) and just checking how much your setup actually draws - the rated values may be quite conservative.

troypiggo (Troy) · #4 30-09-2009, 06:11 AM

Thanks guys. Been thinking about this overnight some more. Wondering if those fairly high ratings on the notebook adapters are allowing for the case where it's charging the notebook battery as well as running the thing. So if the notebook is already fully charged, it will draw much less?

Still, sounds like I've been pushing the capability of the Belkin and risk it overheating or something. It has a fuse on the back of it, so hopefully that would protect equipment from damage, but still...

Looking at Jaycar inverters rated for 300W, there's one for $85 and the closest one with pure sine wave is 380W and it's $270! Worth that much more, considering that DSLR AC adapter and the notebook adapters both may handle non-pure sine wave type input a little better from what I've read? (The USB hubs don't, though)

Barrykgerdes · #5 30-09-2009, 07:19 AM

Quote:

Originally Posted by DavidTrap

My USB hub would consume ~20W max. You are right about the camera adaptor 45VA is the new terminology for 45W.
David T

Watts =Volts times Amps. This is the pure DC power
VA = Volts times Amps. An AC term meaning roughly the same. The Voltage is the RMS Volts times Amps but if the current and voltage are not in phase (power factor) the actual power will be less

I think this is right

Barry

troypiggo (Troy) · #6 30-09-2009, 10:13 AM

Thanks for the clarification, Barry.

mswhin63 (Malcolm) · #7 30-09-2009, 10:20 AM

I would run laptop directly on batteries instead of an inverter, try:

http://jaycar.com.au/productView.asp?ID=MP3463

I use this a lot for more than just laptops.

troypiggo (Troy) · #8 30-09-2009, 10:48 AM

Quote:

Originally Posted by mswhin63

I would run laptop directly on batteries instead of an inverter, try:

http://jaycar.com.au/productView.asp?ID=MP3463

I use this a lot for more than just laptops.

Thanks Malcom. Couple of questions if you don't mind.

* It mentions 7 attachments incl Apple. Do you know if that would include my MacBook Pro? I'm new to all things Apple and not sure if their connectors are standard or different for each model.

* I see it has adjustable output voltages, but only mentions 3.5A out. As per my OP above, the MBPro adapter has "16.5-18.5V 4.6A out". I guess I could use 17V or 18V setting, but what about the 4.6A?

* How would you run several items off it at the same time? eg my powered USB hub etc. With the inverters I was linking above, I'd just use a power board type thing.

* When you say you run other things of it, not just notebook, what type of things do you run off it? How do you connect them if it's intended for laptops?

mswhin63 (Malcolm) · #9 30-09-2009, 07:44 PM

Blimey, your laptop draws lot of current so I suppose it is not going to work well, to higher risk of overheating. My and my wifes laptop only draw 3.3A.

The PSU(Power Supply Unit) primary design suggests a laptop but like any PSU it will work on anything else. I sell these as I have a whoesale account with Jaycars supplier for a number of Grey Nomad that need assistance with listening system while travelling.

It can be used for anything that has a DC power input as long as it can handle the current.

I think your only alternative is a DC-DC inverter. I am preparing my car with 80AH sealed LA batteries with a 750W inverter to supply other item. I would still use the laptop PSU to supply the laptop as the efficiency will be better.

rally · #10 30-09-2009, 08:17 PM

A DC to DC converter working at its normal ratings is usually about 85% efficient, I am guessing that a 240v Inverter is possibly similar or less and a 240v power supply unit is similar or maybe even worse.

That means you could potentially be wasting 15-25% of your batteries energy in all of these conversions - I dont know what their idel current draws are, but they could be a hidden killer !

If you are going out in the field there may be some benefit to having 240v for other devices, on the other hand there might be a benefit of not having 240v lying around on the wet ground ?
Only you know that.

I chose the DC/DC converter route and made up different leads with pkugs an sockets to suit so they couldnt get mixed up.

Check the voltage requirements of your USB hubs they are more usually 5v DC not 12v DC. (but 12v models do exist - just harder to find).
USB 2.0 Type A sockets are in theory rated to supply up to a maximum of 500mA (1/2 amp- 2.5watts) per socket, but a device has to announce itself as a high current device in order to be able to draw more than 100mA. So the required power for a USB 2.0 hub with say 7 ports can actually be quite high !
Of course if you know for sure that none of your devices draw this much power then you can cut down ?
If its a separately powered device then you are probably quite safe, but if the nly cord is the USB cord then it is also supplying power.

Assuming the Apple power jack is readily available and is just a simple 2 wire, it should be relatively easy to make up your own cable.

If you are removing the PSU's then I recommend that you fuse all lines appropriately for their purpose, otherwise you are bypassing an important safety mechanism.
I recommend that you fuse all 12v lines anwyay inless they are already fused

Hope that helps

blueskies123_89 (George) · #11 30-09-2009, 08:18 PM

Hi, sorry to bump into the thread from the middle of no where, but I'm looking for an answer to the same problem. I'm thinking inverters from car batteries are the way to go, heres one for sale on ebay for $31...

http://cgi.ebay.com.au/Car-DC-12V-to...d=p3286.c0.m14

It seems so much cheaper than than the Jaycar unit linked previously, has anyone had a go with one of these?

mswhin63 (Malcolm) · #12 30-09-2009, 08:26 PM

A 300W inverter is a little more than double the price plus 220V not Australian 240V. It will most likely work better designed for people that are world travellers.

I personally wouldn't trust this unit but it quite possible to work. I am almost reluctant to purchace stuff from EBay anyway.

Barrykgerdes · #13 01-10-2009, 07:38 AM

Something that appears to have not been mentioned. What sort of load at 240 volts do you have. A 12 volt battery (car type or in a car) will quickly become discharged with any serious type of load. Don't get caught with a flat car battery at a remote viewing site.

I looked at this sort of problem and bought a "quiet" 2 KVA generator $600. It operates at the end of a 50 metre cable with a cover over it and won't be heard past 30 metres. It runs everything all night on about 5 litres of petrol.

Barry

cfranks (Charles) · #14 01-10-2009, 11:10 AM

I have a couple of '12v to multiple voltage' inverters from Jacar and drive all of my equipment from a 12v battery, 75AH just increased to 150AH. They provide 19V for my Aspire One and 16V for my Gemini. Straight 12V for most of the rest.
Since I have them available for my Radio Control hobby, I use a 2 cell LiPo battery (7.4v) for portable power to my 40D. Excellent batteries but you need a special charger otherwise they explode

. Finally a 12V to 5V regulator for the USB and Serial Port Hubs. Big tangle of cables when connecting and disconnecting in the dark!!
Charles

blueskies123_89 (George) · #15 01-10-2009, 11:22 AM

Quote:

Something that appears to have not been mentioned. What sort of load at 240 volts do you have. A 12 volt battery (car type or in a car) will quickly become discharged with any serious type of load. Don't get caught with a flat car battery at a remote viewing site.

Well, a laptop (specifically my laptop) needs 19V at 4.74A, equating to about 90W. Plus a few accessories I don't think it will ever get near 150W... So an inverter around that range should do it. Does anyone know how many Joules a car battery can hold? That should give a good indication of the expected hrs theses things can power...

beefking (Nathan) · #16 01-10-2009, 03:35 PM

car batteries are designed for multiple small discharges of short duration i.e. lots of discharging to 95% of capacity then recharging. They do not cope well with being regularly discharged to deep levels - that's why you usually need a new car battery after you've flattened it a few times by leaving the lights on.

if you get a battery closely sized to your load, make sure it's a deep cycle one, or the life time will be... lacking.

Barrykgerdes · #17 01-10-2009, 06:33 PM

Quote:

Originally Posted by blueskies123_89

Well, a laptop (specifically my laptop) . Does anyone know how many Joules a car battery can hold? That should give a good indication of the expected hrs theses things can power...

If a car battery had any decent "jewels" they would not be a problem to discard!

Baz

:s crewy:

mswhin63 (Malcolm) · #18 01-10-2009, 08:34 PM

I would suggest not to use the car batteries unless you have a dual system and they are Deep Discharge.

Yu wouldn't want to go to a dark sky only not be able to restart the car. Serious Grey Nomads have dual batteries for just the purpose.

I will configure my sealed lead acid in the back of the car so I can get access to them along with unloading. I also have a small solar panel that I will use for recharging possibly while I am driving to dark sites.

troypiggo (Troy) · #19 01-10-2009, 10:00 PM

Quote:

Originally Posted by Barrykgerdes

Something that appears to have not been mentioned. What sort of load at 240 volts do you have. A 12 volt battery (car type or in a car) will quickly become discharged with any serious type of load. Don't get caught with a flat car battery at a remote viewing site.

I looked at this sort of problem and bought a "quiet" 2 KVA generator $600. It operates at the end of a 50 metre cable with a cover over it and won't be heard past 30 metres. It runs everything all night on about 5 litres of petrol.

Barry

I have a 75Ah deep cycle marine battery. Wouldn't use my car battery. I'd rather get another. Or a gen has been suggested by others also. No idea how to size one of those to meet my requirements. Yet.

mswhin63 (Malcolm) · #20 02-10-2009, 12:07 AM

The best way to size is work out the current draw for the equipment then the time expected to use it for. eg: 5A over 10 hours = 50AH I would the select the 80AH battery or higher. Solar charging is good to top up the batteries if you stay over a couple of days.

Generaly lighting now-a-days can be so easily done with high intensity LED's which draw the same current as low power LED's but generally double the power due to higher forward voltages to give almost 10 times the brightness.

For others munts and other equipment need to be taken into consieration to find the right AH.

As far as the inverter add the max power consumption to give the minimum power inverter maybe add 100W or more.

As I am typing this message outside in the freezing lovely night sky I am using a 20AH battery with a 750W inverter. Works well.

Be careful though if your inverter is too high for the power used it may not switch on. Some inverters require power draw to get them operating.