power supplying method for boards around a room. - Page 1
Forum section: electronics / beginners

author: Pedram
author_rank: Regular Contributor
timestamp: August 14, 2014, 05:12:30 pm
content: Hi,For a project, i need to place 30+ little sensor boards around a 200m room.They are communicating to a PC using RS485.for supply voltage. which method do you recommend ?1:Multiple 220v->5V@750ma power adapters.2:Single >12V@25A power supply, multiple local (Switching or linear?) regulators.3:Single 5V@25A power supply.Priority:1. Price2. Efficiency3. Fail safe4. Repairabilityalso note that the room is very humid (>99%)

author: rob77
author_rank: Super Contributor
timestamp: August 14, 2014, 05:21:34 pm
content: i would go for a singe 48V supply and local switching regulators. 48V because of lower loses in the cables, and it's legal almost everywhere to use "signal" cabling to carry low voltage DC power.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 14, 2014, 05:26:41 pm
content: How are you handling the RS485 wiring and connections?Are these plug-in or hard-wired?Daisy-chain (serial)? or "Star" (individual runs to each sensor)?I would consider using ordinary Cat5 cable/RJ45 connectors for both data + power.I would send out 12V and have a small 3-terminal regulator at each sensor board for local regulation down to 5V.A single power supply is probably cheaper than multiple small supplies. Especially if you consider running mains power out to 30 local power supplies.A single power supply is probably more efficient than multiple small supplies.We don't know what are the implications for YOUR system of individual failures vs. complete systemic failure?Eliminating 30 power supplies out at the sensor boards makes them MUCH less likely to need repair.

author: Pedram
author_rank: Regular Contributor
timestamp: August 14, 2014, 05:43:33 pm
content: QuoteSingle twisted pair(from POTS line) / daisy-chain.Quoteone night complete failure cause several thousand dollars damage( loosing temperature and humidity co2 oxygen etc ... )complete failure is acceptable up to one hour.(use redundant power supply? )the room is some sort of greenhouse.

author: rob77
author_rank: Super Contributor
timestamp: August 14, 2014, 05:52:59 pm
content: ok. if it's a green house with high humidity, then 24V (or even 12V only) is the legal limit i thinki would suggest to run a power bus around the greenhouse (in fact daisy-chain) with higher gauge wire and have local switching regulators to minimize the loses.regarding the redundant power - i would use 2 power supplies feeding the power bus through a diode and of course each supply connected to a different phase of mains (if 3phase mains is available) to further lower the risk of outage.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 14, 2014, 08:36:02 pm
content: I would use Cat-5 cable so you can run data and power together.I see no benefit to running separate data and power cables.12V is a convenient voltage, and will tolerate 4V droop down to 8V even with ordinary 5V regulators at the sensors.I would use a car battery as a power supply, with a charger to keep it charged from the mains supply.That way you have an automatic "UPS".It would seem simple enough to add an alarm feature if any of the sensors fail, or the mains power fails (for lights, heating/cooling, or whatever?)If you have connection to the phone system (or an inexpensive WiFi dongle) you could add email or text notification of alarm situation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2014, 11:05:19 pm
content: You neglected to mention the current draw... how can you forget the current draw or power consumption when your very question is regarding power?

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 14, 2014, 11:15:23 pm
content: Pedram asked about a couple of different 25A options for 30 sensors, so I was assuming roughly 1A per node.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2014, 11:21:01 pm
content: Quote from: Richard Crowley on August 14, 2014, 11:15:23 pmBut why 25A at 12 or 5V? That just sounds like ripping an ATX supply to me.Also, what's the average vs. peak current draw, and does it vary at all? Example: I should think a tiny passive sensor need not draw much current at all, but an RS485 transmitter will dump 50mA easily, but only when transmitting.Tim

author: pickle9000
author_rank: Super Contributor
timestamp: August 14, 2014, 11:35:04 pm
content: If one or more of the sensors is running an element they could really suck the juice. In that case 2x25 amp supply located at the center of each supply line would be best.

author: Pedram
author_rank: Regular Contributor
timestamp: August 15, 2014, 03:06:47 am
content: boards themselves consume 100mA each, but there are some relays... solenoid valves .. LEDs etc etc ... so 700mA per node is the average consumption and 0.7*30=21A . so 25A power supply to be on the safe side.Quoteusing a car battery is a pretty good idea

author: Pedram
author_rank: Regular Contributor
timestamp: August 16, 2014, 05:02:05 pm
content: i drew a block diagram . is it ok ?

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 16, 2014, 05:36:31 pm
content: You might want to do some more careful circuit design for a "float charger" depending on the chemistry of your battery.It is not clear what is the purpose of that resistor? It looks like both the load and the charge current must pass through that same resistor.Is that a 40A fuse? What is it for? If you are sending power out over many separate circuits, then each of those circuits should be fused separately.A single, central fuse like that won't really protect anything.

author: Pedram
author_rank: Regular Contributor
timestamp: August 16, 2014, 06:23:36 pm
content: Quote from: Richard Crowley on August 16, 2014, 05:36:31 pmthe resistor is just for limiting the current. i know it's not efficient in any how. but it's the simplest way.as i'm going to use 'One Hung Low' brand 12(13.5V actually) power supply i really don't want to overload it. the uC disconnects the load when the battery drops to 9V . so when the charger connects back --> 13.5 - 9 = 4.5V and R=0.5 so it will be charged at ~9 amps initially.and the fuse hmmmm just in case of short circuit etc .... car batteries can supply up to 800A on short circuits. without fuse it destroys everything ...

author: SL4P
author_rank: Super Contributor
timestamp: August 17, 2014, 12:30:13 am
content: I don't think anyone other than the OP mentioned 99% humidity.My vote is for a low-voltage AC supply with local (low-curent) rectifier / regulators - at each node in a hi-potted blob. You'd use inductive transfer (effectively a transformer) of the AC at low currents - and eliminate as many points of contaminant /water entry as possible. Inductive data transfer could be addressed in the same interface as required.Nodes are virtually disposable, but some smart packaging could work around that at slightly increased cost.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2014, 02:25:06 am
content: The nice thing about a greenhouse is, it may be humid, but the temperature should be fairly consistent. Daily swings will be your enemy, and condensation will form as, in the morning, ambient temperature rises around still-cold objects. If you can afford enough heat dissipation to keep the enclosure temperature above that swing, you technically won't need sealed enclosures. If the enclosure is rather large, the dissipation will be rather large though, and therefore ampacity on the distribution lines.Still, it wouldn't hurt to have some goo on the boards to keep moisture out. Potting is probably not practical with the atmospheric sensors. NEMA or IP66/67/whatever sealed enclosures would be handy, given a suitable method to seal the sensors against the enclosure.Tim

author: SL4P
author_rank: Super Contributor
timestamp: August 17, 2014, 03:17:29 am
content: +1 Quite right.A good IP67 or conformal coating will avoid some issues - but any exposed conductors, or connections that are not hermetically sealed -willfail over a modest period of time with atmospheric cycling.

author: Pedram
author_rank: Regular Contributor
timestamp: August 25, 2014, 09:15:44 am
content: ok. as this project also need 220V Voltage and UPSes are expensive. i want to use an inverter plus some dumb switch to switch it on in case of power cuts.( i have the inverter )here is the schematic i drew:K5 is the inverter turn on switch. i will connect it to 555's output ( like other relays with the ULN)VBAT is 13V from lead acid batteryV8 is 8V (7808)VCC is 5V (7505)(power supply section not shown in this sheet )U3A is UV protection for batteryU3B is for power cut detectionU5 is a 10sec latchalso as i want to read the voltage in the software i used a transformer.is there anything wrong with this design ? any suggestion ?i need mains for some small valves and fanes. so switch delay(up to 500ms) is not important.( inverter turns on 100ms after shorting the switch )

author: Kjelt
author_rank: Super Contributor
timestamp: August 25, 2014, 10:38:14 am
content: Redundant (server grade) dual power supply?Car battery would not be my personal choice since it is a chemical device capable of delivering enormous amounts of current (hundreds of amps) in case of shorts/failures and can be the cause of fire (make sure you have written clearance and are allowed to use it in your setup, otherwise the insurance company might not pay when something happens). The battery also needs to be replaced in a few years time. My burglary alarm has a maintenance free lead-acid battery and it is replaced every 5 years. So keep that in mind.For safety make sure every single power cable has its own slow fuseat the powersupply, at around 1,5 to 2 times the max. current it is supposed to draw, so in case of short or other failure in the cable only one node will shutdown and the cable will not sustain extra damage due to the short.

author: Pedram
author_rank: Regular Contributor
timestamp: August 25, 2014, 10:41:52 am
content: Quote from: Kjelt on August 25, 2014, 10:38:14 amand in case of power cut .. ?

author: Kjelt
author_rank: Super Contributor
timestamp: August 25, 2014, 10:50:32 am
content: Quote from: Pedram on August 25, 2014, 10:41:52 amIf that is really a hard requirement then I would definitely choose a UPS which are not that expensive anymore ($150) AND they are safety approved (no insurance hassle) and the UPS maintains the battery life / replacement which you should otherwise build yourself and test extensively.

author: rob77
author_rank: Super Contributor
timestamp: August 25, 2014, 04:03:40 pm
content: Quote from: Kjelt on August 25, 2014, 10:50:32 amUPS is pointless (and any other short-term backup as well). if it's a greenhouse which is able to tolerate 1 hour outage, then it's absolutely pointless to install a $150 UPS which will keep the electronics alive for extra 15 minutes.a fairly big diesel generator is the solution - the greenhouse is able to tolerate the few minutes of outage till the generator is started , so no UPS is needed at all.

author: Pedram
author_rank: Regular Contributor
timestamp: August 25, 2014, 04:12:21 pm
content: Quote from: rob77 on August 25, 2014, 04:03:40 pmmeh... 5min of 220v backup supply is ok. i just want to close refresh dampers and maybe some latched valves. i have the inverter so i think using it is the cheapest way.i think i can rely on room isolation during power cut to maintain the temperature and humidity . so i think there is no need to run air conditioner compressor ,humidifier or the heater with backup supply.