# How to calculate power supply for 100x loads @ 3V 700mA

I'm currently designing a setup using 100x loads that requires a 3V 700mA supply ea. The loads are connected in parallel with the main power supply. I'm a bit doubting about my power supply dimension and I don't think I'm calculating this right... But, it should be something like 3V 70A? Is that right? The thing is that 70A is a bit scary and I don't really want to play with such big current. I would like to have an opinion.

• Do they all need to have exactly the same supply voltage, e.g. can you use multiple supplies? Apr 23, 2014 at 3:35
• Your calculations are correct. You not only need to worry about the size of the power supply but also the size of the wires to carry 70A with minimal voltage drop. You'll be much better off splitting them over several power supplies, as @Ignacio Vazquez-Abrams suggested. Apr 23, 2014 at 4:14
• What kind of loads? What are you hooking up 100 of? It may make a big difference as to the specs of the supply you need. Will they all draw 700ma at the same time? Apr 23, 2014 at 4:17
• They do need to have the same power so yes same supply voltage, so having multiple supply is not an option they might fluctuate ... ok let say i got voltage drop using a 70A power supply, but when the wire hits the parallel network, all the load will be fed by the same amount Iin/100, right ? They do need to be powered by the same supply but not especially at exactly 3v 700mA, it could be 800mA or 600mA ! @DoxyLover
– Waz
Apr 23, 2014 at 4:20
• The loads are thermoelectric cooler (TEC) and they will all be connected together on the same power line and need to have the same current draw in order to be at the same temperature one from the other ! @Grant
– Waz
Apr 23, 2014 at 4:23

Your calculation is correct - 0.7A x 100 = 70A.

Power is not that scary - 70A at 3V is just 210 Watts.

Some large PC (and server) power supplies are able to deliver such currents.

However you should split your power supply to few power supplies.

Easiest way to built such power supply will be traditional transformer.

Transformer price is determined by power, not current. You can use transformer with few 7A windings (split to 10 circuits), use separate rectifiers at every block. That will reduce short-circuit current and significantly improve fire safety.

You should use fuses at all outputs or split whole power to 10-25 blocks with separate fuses, because 70A will melt thin wires when they will be accidently shorted or damaged.

You can build cheap unregulated power supply like this:

Unregulated means that voltage may vary/drop under diffrent load conditions.

Peltier cells not require very stable voltage and you can stay with unregulated supply.

Fuses are required at every transformer secondary.

• Thank you so much for the suggestions @Kamil ! Would you mind giving me a very basic schematic for me to understand a bit more how this should look like ?
– Waz
Apr 23, 2014 at 10:37
• I've added schematics. I have additional question. If this is for huge thermoelectric device made of 100 peltier cells - why you dont want to connect these devices in series instead of paralell? Apr 23, 2014 at 12:22
• That's awesome ! Again thanks a lot ! I'm quite unsure yet if I should have them in series or parallel ... according to the discussion above, it seems that having them in series would be easier for the power supply design, but maybe be more tricky to control the h-bridge. But now If i look at your design it look quite possible to do a split of the main in 10x supply, powering 10x devices.
– Waz
Apr 23, 2014 at 23:33

70A is correct , but not too scary. 10kA can be scary. Thats what your household can supply near a substation worst case , with a screw driver short circuit. Vaporized copper melted on my glasses , did that when I not too carefully used a screwdriver to pry off a staple for 120Vac line that I was relocating near a fuse panel.

The latest CPU uses more than this too.

Consider using a few magnetic breakers, busbars and welders cable for distribution, if it needs to be flexible..

What's the startup current 500A?
;)

Off the top of my head, a Murata 70A 3V supply should be around $1~2/Watt but a 500W$50 PC x4 =$200 will supply 20A each on the 3.3V rail. With some shopping with might get some old 15A units free and I'll let U figure how to drop 0.3 V with 0.2A (1.5Ohm) using nichrome wire from some old toaster for free using short strips for each feed wire. But I suspect your boss can sign a PO for$250. Stay cool. use Ohm's Law , use fault protection and make it look like a million dollar test jig with LED's.

• The only problem now that I know my calculation are good. Is that it seems, 70A 3V is not a common power supply and for the few that can throw 70A there are very expensive :/
– Waz
Apr 23, 2014 at 8:00
• Depends on your creative skills, does peltier care about ripple? 70A@3V=42.8 mOhms and in a full bridge PSU and for T=8.3 ms interval If C=5x T/R for reasonable ripple = 1Farad so forget about 10k uF
– user40708
Apr 23, 2014 at 20:20

How about this option, which will allow switching hot/cold direction and also allow you to use more economical PSU, wiring and H-Bridge.

Wire the units up in 25 sets of 4. Each set would have 4 elements in series, and then you wire the 25 sets in parallel.

Or you could do it the other way around -- 4 sets of 25. Each set of 25 wired in parallel, with the 4 sets in series. I think the first arrangement might be better as it's easier to troubleshoot a dud element.

This will allow you to use 12V at only about 17A, which is well within the range of an inexpensive PC power supply. For 17A you can use wire intended for house electrical wiring. And the FETS you need to H-bridge the whole thing can be much more modest too.

Finally, if you're not sure that 12V exactly is the right voltage, you can instead get readily-available generic "12V" PSUs that have trimming adjustments that allow going up or down a little from the nominal voltage.

• not a bad idea :) I'm gonna add it to the list of possible setups. Thank you !
– Waz
Apr 23, 2014 at 23:46