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We ae making our first installation and plan to use 320mA power supply. Is this enough for 60 products or do we need 640mA? And what happpens if the power supply is to small? |
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I would go for 640mA in that case. 320mA or 160mA is in my opinion only suitable for main-line and backbone. If the power is insufficent you will have a power drop and the devices might behave according to power failure. It´s cheaper to do it the right way at start. |
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I agree with Fender. Even if the most of KNX devices consume only 5 mA (320 mA / 5 mA = 64 devices), don't forget that the KNX specifications said that a KNX device may consume up to 10 mA on the KNX bus. It is the reason why manufacturers are selling 640 mA power supplies. Reliability is a main goal of a KNX installation. The assurance of proper operation involves adherence to specifications. |
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insert all the devices in ETS. then go to DIAGNOSTICS==>CHECK PROJECT. there you'll see estimated total power consumption. if you find that ETS didn't find data for some devices, you check technical specifications for that devices, and calculate them in final number. if total power consumption is near the limit (I usually look that it's not more than 3/4 power of power supply) then it's better to use stronger power supply. other thing to watch for is length of the BUS cables. if you have very long lines (more than 200-300m from PS), better to use stronger PS, because you can expect some voltage drop in long lines. I agree with your first sentence. This is a good practice. But the voltage drop is only due to the length of the bus and the power of the devices (U = R x I). the voltage will be the same with a 320 mA power supply or with a 640 mA... of course, only if the current does not reach the 320mA limit.
(Aug 09 '10 at 10:25)
cthulu
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yes, the current limit is the thing i'm talking about. i once had project which wasn't planned by me. electricians wired the loop through the floor, and made bus line break in the same room where bus line started too. on some floors i had 60 or less devices, but on some i had max 64 devices. there were lots of zennio Z38 displays (20mA average current consumption). at the end of the line they were constantly restarting because voltage has been dropping as the last few have been turning on and off. I had to cut the line on half and reconnect other end of the line to create two branches of same line. the problem was simply solved by that, so it's easy to see that voltage drop in BUS cable was causing this... sorry i had to write another answer, but it was too long to make a comment. |