Quote:
Originally Posted by Barrykgerdes
Hi Rowland
I have been watching your efforts for some time but I don't think you understand the relationship between what you have and what you need. Your drawings look great but they really do not solve the problem of getting 7A out of 3A.
Firstly you have a basic power supply that can deliver 12 v at 12.5amps. If this is a standard off the shelf regulated power supply from Jaycar or other supplier you won't need anything else because it is designed to deliver 12v at 12.5a , and you can supply everything off it on a single bus. This is the way we do it in the field.
If it is not a standard off the shelf, eg home made and you will need to regulate it and and that will depend on the actual design as any sort of regulator will need at least 2 or 3 volts in excess of the 12 volt to handle the regulator, and a regulator capable of supplying the full rated load current.
Your stated load requirement is 12v at 7A. There is no way that you can get that from a 12v 3A regulator. I think you have envisaged limiting the current drawn by the items connected to the bus so that they won't draw their rated current. This just won't happen as the item will draw the current it requires at 12volt or not work properly. Yes you can put a current limiting device in the lead to the item but all that will actually happen is the voltage across the device will drop to the point where it will not operate.
Think of it this way. If the item draws 3A at 12v it presents a theoretical resistance of (R= E/I) of 4 ohms. If you limit the current drawn to say 1 A , the voltage available across the unit will be 4 volts (same formula). Of course as you reduce the voltage on the device it will actually draw less current and some point of equilibrium will be met with the 1A. However I doubt if you could get any usefull performance out of the device under this condition.
If you want to isolate each load to avoid interference you will need to increase your main supply bus to at least 15V and run each item off its own secondary regulator. This is hardly worth the trouble as modern commercial devices designed to operate off 12v DC will have some sort of input filtering and any item in it requiring a stable voltage will have a secondary regulator. The worst interfering device you will likely have is a dew heater with a pulse width modulated regulator. These can be noisy so this will be best served with its own simple supply.
Barry
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Barry. You'll be pleased to know that I am not going to run the dew heater with PWM. I've taken your advice on that one - too noisy for this setup. The 12v power supply is moving on in favour of a better one. Variable voltage and very low output ripple. Much more flexible for future requirements as well.
Just to be clear, my intention was to power the bus with 12v 12.5A, with the appliances pulling at max, approx 7A. That is, 3A, 1, 1, 1 and 1A, plus a bit for LED lighting. I toyed with the idea of using linear regulators to each appliance, to clean up the supply and limit current, but I'm tossing that idea now and opting for the simpler option of feeding directly from the bus, using a better power supply.
I know I cant get 7amp from 3amp.
Rowland