I was asked to assist with a project at work. The requirements are a little strange but here they are:
Make a standard LCD monitor mounted on a cart display an image from a workstation located up to 50 feet away.
The cart will be mobile and constantly moving throughout the day.
It has to have its own power (Can’t be plugged in except at the end of the day).
We cannot use a laptop to do it because of some odd security restrictions in the environment where it will be used. Standard mobile devices like tablets are also out because of the security restrictions. The monitor only needs to display a window showing information from a mainframe application and that is it.
Run time needs to be 4 - 6 hours before recharging.
I found a wireless monitor transmitter/receiver package that should take care of the signal.
However, I am having trouble figure out how to power the monitor and the wireless receiver. The monitor may just be a typical LCD monitor in the 15 - 17" range or I could go a little smaller if I had to in order to save power. Would a large UPS (uninterrupible power supply) be able to power a monitor for hours at a time? If so, how much capacity should it have? The physical size of the batteries/UPS isn’t that important as long as it could fit on the bottom of the cart and it doesn’t really matter if it needs to weigh 50 pounds.
Online calculators do not work for this because they always reference whole workstations and all I care about are the monitor and receiver.
Here is APC’s load-based UPS selector. The first 17" monitor I found specs on said 25w power consumption, which gives me results that look pretty reasonable. Doesn’t even require battery expansions, though they’re from the cheap range of products.
I’ve seem battery operated monitors, but nothing bigger than 9". The UPS is a good way to go if it’s batteries can tolerate the repeated charge/dischare. There are also 12 volt monitors available in all sorts of sizes. There are a lot of rechargeable 12 volt workstations meant for daily cycling like that.
Set up an experimental environment. You could insert a power meter between the monitor and mains. Then run the monitor through a typical day and measure the total energy (kWh, joules, etc.) usage. Add a safety factor and see if you can find a UPS that can provide that much energy.
Remember that power, energy, current, and voltage are different quantities! With great power comes great current squared times resistance.
If you know that the receiver will use less energy than a known type of CPU but don’t know how much less, running the calculation using the CPU will yield an energy requirement that could be excessively cautious but at least is known to be safe (better spend a little extra money now than have the unit fail at a critical moment and lose a contract or worse).
It could be done with a UPS, but most alarm when line voltage is removed. It’s usually not hard to disable this function, but you would have to open it up if it’s not software selectable.
I would recommend a small inverter, say 150 – 200W just to be extra safe (you should be using less than 50W even with the receiver) coupled to a deep cycle battery.
At 50W, you would be using a little over 4 ah. For six hours use, you need 25 ah capacity from your battery, but I wouldn’t want to deplete the battery. Luckily, even a modest deep cycle battery should have about 80 ah of capacity.
I don’t think UPS units are designed for frequent use (repeatedly deep-discharging the battery). They usually use standard lead-acid batteries.
If I were you, I’d use a deep-cycle lead-acid battery, commonly sold as marine batteries. If you choose a 12V battery, you can use an off-the-shelf 12V to 115V inverter sold for automotive use. I would keep the charger mounted to the cart, but install a toggle switch to select between charging (battery connected to the charger) and power usage (battery connected to inverter).
If the monitor converts AC to DC internally or through a brick, you’re losing energy converting it from AC to DC via an inverter (inside a UPS or outside) and then converting it back. Depending on how the monitor is built, you might have to open it up and manually disconnect the AC->DC converter and hack in a DC jack. This requires you to know what you are doing, could damage the unit even if you know what you are doing, and will likely void the warranty.
OK, let me tell everyone that a UPS is NOT the right tool for this job!
No UPS that I know of will start up without being connected to 120v. That means that if it shuts down during the day for some reason, you will need to take the cart to an outlet and plug it in to get it running again. Either use an AC/DC inverter, or do everything with 12v DC.
Thanks for the replies so far everyone. I took the advice about not using a UPS to heart and I think I am interested in using a deep-cycle battery with an inverter instead.
I looked at inverters and found this one. Does it look suitable for this use?
The other major concern is neatness because this will be used in a heavily regulated environment. Do I need to get someone to build a box that holds the battery plus the inverter and charger or is there an off-the-shelf solution that just lets you drop a battery into a box that already has these functions?
I doubt that you are going to find an off-the-shelf box that will be perfect. Better to pay someone to make a nice wooden or Polypro box, with mounts for all your equipment.
Do you already have a cart you intend to use for this, one that needs to hold additional hardware? If not, you might look at one of the computer carts used in hospitals. Here, for instance, is one from Rubbermaid. Typically, they have a monitor mount and a box to hold the computer or other electronics. And they are designed to be wheeled around for hours and then plugged in to recharge.
This is what I would do; I can easily make an efficient DC/DC converter to replace the AC/DC power supply (yeah, the warranty will be void, but so what?), removing the original power supply to make room for the new one (if not on a separate PCB, remove the parts and stick the new board on top of it). This would save a watt or two for the monitor power supply and considerably more for the DC/AC inverter otherwise needed (another solution would be to make a high voltage DC/DC converter to supply around 150 volts to the monitor, which don’t care if the input is AC or DC since they use what are basically DC/DC converters, plus a rectifier, which would be a bit less efficient but still more efficient than a 50/60 Hz inverter). I’m assuming that for the enclosure, you can just use a readily available electronics enclosure (I’d use a case from some other electronics myself, the only thing I’d have to buy is the battery).
ETA: Just adding in a DC jack to bypass the AC/DC power supply wouldn’t work in most cases since monitors often have two or more power rails, such as 3.3/5 volts for logic (an amp or two, so a linear regulator to 12 volts would be inefficient) and 12-24 volts for the backlight.
Upon further thought, I will mention that any housing for a lead acid battery should have at least a bit of ventilation. They release hydrogen gas during charge (don’t remember about discharge). It shouldn’t be much, but you wouldn’t want it to accumulate in a small space.:eek: