current regulator/splitter

my knowledge of electrical engineering is minute, and someone might know the answer off the top of their head to what i assume is an incredibly simple situation.
i have six cameras requiring 12v 150mA each with its own LCD that needs 12v 200mA.
I have adapters that are 12v 350mA / am able to find many others in much higher amperage, but realize that i do not know how to dictate what current goes where. i figure that there has got to be some way to link these guys up in parallel if i have a higher amperage adapter, and then regulate the amperage at a more individual level somehow, but my experiments (and dabbling for a few hours on the internet) have led me nowhere in terms of figuring out how to make some sort of current gate/limiter/regulator.
essentially, i want to a) be able to get these to work because right now i am afraid of blowing them with well more amperage than they require b) to be able to consolidate them onto one wall-wart essentially.
thanks for entertaining what i assume is something you cover the first week of an intro engineering course.

The best solution is one power supply since all your devices use the same voltage. These devices will use the current they need and only that much.

Think of it this way, your home has 120VAC distributed though out (exception you may have 240VAC special). When you have a device that requires power you just plug it in. If that device should have a fault, it would trip a ,"Circuit," breaker. In your case the AC/Dc power supply would shut off without issue, no damage to anything else.

This is pretty much your situation as you have described. Who would want six AC/DC modules and where the heck would you plug them in? Also it appears your adaptors at 350mA isn't enough for the camera and the display, besides you don't want to run the adaptor at even close to max ratings. Conservatively you should have 50% margin for reliability.

So it looks like you really should consider replacing all of those adaptors with one 12V power supply that can supply say 50W. Those are readily available online (I have bought a 60W).

Though you will need to have all 6 DC cords connected to a common point. If that's a bit unwieldy, you could opt for two 25/30W AC/DC adaptors, same concept. That would allow you to set them up in separate locations if that's what you wanted.

There is no need to regulate amperage in these circuits. You can connect them all in parallel to one source as long as the total amp load does not exceed the supply. 6 cameras at 12v 150 mA require 12v 900 mA and 6 lcd will require 12v 1200 mA . So you need a total of 12v 2100 mA or more simply 2.1 amps. The loads are resistive so they with limit the amperage flow themselves. Now I presume these are security cameras and it is very likely that you will have long runs of wire to get that power to them and that can cause losses. To prevent a low flow problem you will probably want to make the power supply a little larger. At least 2.5 amp would be the smallest I would consider with long wire runs. The good news is that if you find a larger one at a good price you can use it. Due to the resistive load, if you find a supply that is 10 amps at a price you like, don't hesitate to use it. The down side of what you are doing, if it is security cameras, is that now there is one spot that can be disabled to kill all your cameras. You may want to split the load into two circuits of 3 cameras each and use two power supplies so that if one fails or is disabled the other has the chance to still work. Think about aiming the cameras so that they overlap in a way that each camera is in the field of view of another camera. In that way if anyone disables a single camera their picture will be on another camera.

Must have come up with the same idea at the same time in different parts of the world. ;~)

thanks all for the quick responses. that is more or less the project. it is an interactive installation with ir cameras running a facial recognition/eye tracking patch to affect the environment where the individual is not looking. not too worried that someone will sabotage, but good to know/think about.
the longest run is around 15 feet, and if they are self regulating in terms of current and i can use a 2.5 amp adapter that is amazing! so much easier than i thought. i was ready to break out the laser on some pcbs if i had to consolidate things in a dense manner.
i will likely be back on here loads in the future now!

Here in the 21st century, it is very useful to have at least a basic knowledge of electricity !-)
I would recomment that you read the electricity section in a school physics textbook (for age ~16) to bring yourself up to speed. This will give you the ability to carry out such basic calculations as these.
Or there are many sites which provide free basic courses in electricity and electronics, here are a couple:

www.learnabout-electronics.org/index.php
- Detailed descriptions of components and principles, with good diagrams, animations, & fault-finding techniques

http://www.hobby-hour.com/electronics/ - Teaching from basic principles

http://www.electronics-tutorials.ws/dccircuits/dcp_1.html - Teaching from basic principles

But always come back for help and confirmation of any concepts and ideas here.

One thing to consider, though, some wall transformers ["wall warts"] (and power supplies in general) are not regulated. Typically, the power supply shipped with a particular device is matched to that device, such that the output voltage will not exceed the maximum voltage the device can handle. For instance, a power supply rated as 12V 350mA will deliver around 12V when a device draws from it 350mA but for lower currents the voltage will be higher--much higher in most cases. An unregulated supply that is rated at higher current, will typically have a low current (or open circuit) voltage that is even higher.

So, that needs to be taken into consideration. To be safe, use a regulated power source.

I would say that generally speaking ,"Wall warts," are regulated. Let me qualify that statement. The power supply is WELL (within 2%) regulated internally but does not make up for the voltage drop in the DC cord.

The device it was intended for, let's say a camera, takes into account the drop in the DC cord, enabling it to operate properly.

When you purchase a power supply, simply refer to the specs and it will describe the voltage and current numbers. For instance it may state the voltage at minimum current and the voltage at max current.

I believe we all agree 100% that reading the specs on the device before you buy is a must to make sure: A) it works as intended, B) It will not damage your equipment

If you have any doubts about compatibility, don't buy it unless you can get clear concise answers to your questions/concerns. We all know this of course.

Very good point regarding regulation & intended use!

There are wall transformers that are regulated, but most aren't. I have one right here that has an open circuit voltage of around 18V yet it is rated as 12V at 200ma. This is typical of most wall transformers and even some desk-top transformers. At least that's true here in "The States".

Earl, if you are currently in the USA, I challenge you to get your hands on at least 3 non-switch-mode "wall warts" and test their open circuit voltage and compare it to the voltage printed on the case. I think you will find that, in most cases, the OC voltage will be quite a bit higher than the rated (under load) voltage -- especially for high current devices (e.g. 500ma or more). If you're in some other country, then perhaps regulation is the norm, there.

In fact, there are three types of these wall plug power supplies with DC output voltage (that I know of):
1. Just a transformer with rectifier and filter cap.
2. Transformer/rectifier/filter cap + a linear regulator.
3. Switch mode converter/regulator.

In fact, even a lot of those "battery eliminators" with the switchable voltage are not regulated (at least not the cheaper ones).

Correction on that 12V 200ma wall wart open circuit voltage: 15.3V
I have another one rated as 12V 700ma that has an open circuit voltage of: 17.4V

The only expeiance ive had when going out to calls like this is when devices are parralled up like this i.e all from one main supply is : If one or two or more cameras develope a fault on the internal power supply whch is regulated then it can draw in excess of your total combine currents consequently shutting all other cameras down Or if intruder cuts the supply so although in parralel sounds ok If fault a fault condition occurrs on one or above occurs all may left inoperable

Thats not good from a security point of view


My suggestion is build a decent supply to start with that includes battery backup and incorperate 6 individual feeds which are regulated via Lm7812 fused if nessecary

Mark Harrington

how it works

Ya'll got off track, the question was about amperage regulation not voltage regulation.....

Follow the thread, my friend