Backup power supply 1

[christinesoler]

I am working on an application that requires the addition of a CHEAP! backup power source when the main power is down.

Requirement: When main power is disconnected I need to supply a piece of equipment with at least 20 seconds of continous power (80-240V, 0.35 amps minimum) so that it energizes.

The easy solution (some kind of UPS backup) is expensive per customer.

Is anybody aware of any circuit I can design or component (anything that can hold the main power charge for at least 20 seconds after loss of power) that I can use for this application? Or is my only solution to find a cheap backup power supply??

Any advice would be greatly appreciated.

 

[IRstuff]

Not sure how cheap you're talking here, but short of buying a UPS, you could buy a 12VDC sealed battery and a 120 VAC automobile inverter that would certainly handle the power for 20 seconds. The cost of those two items is about $40-$50.

Better yet, if you could directly connect to the battery, you'd slice off the inverter cost of about $30. At that point, your battery could be somewhat smaller and might only run $10-$15

The only other alternative is to use capacitors to keep only the most critical operations running and allow the rest to go down. TTFN

 

[christinesoler]

"The only other alternative is to use capacitors to keep only the most critical operations running and allow the rest to go down."

Would I just add the capacitor to the main power line (in series with L1)of the device I am interested in powering up?

Any suggestions on what type and size of capacitor would provide me with 20 seconds of the main power(80-240V)after power loss?

thx for your input!

 

[IRstuff]

The capacitor would need to be on the DC part of the power distribution.

Again, you might need to triage the system and determine if there are parts that can be allowed to go off.

The basic rule of thumb is that the amount of capacitance required is such that the removal of the stored charge will not cause the voltage to drop by a certain amount. To wit:

if you're allowed a 0.5 volt drop, then C=integral(current)/delta_voltage. Example: 100amps for 20 seconds and 0.5 volt delta ==> 4000 farads, which is a not insignificant amount of capacitance.

Note that this is not an external appliable solution; you'll need to make cuts and jumps to use this.

You could potentially use the same idea at the other end of an inverter, but it's not clear that it would that much cheaper than a battery. TTFN

 

[STABY]

Hi,
Please tell me what kind of critical equipment you are trying to protect for "brownouts".
What kind of wave shape needs: sine wave, square wave, stteped, so we can suggest some solution.
Has the equipment a power supply, if yes, try to work on the DC stage.
Jorge

 

[mdonner]

The load power consumption and nature (AC,DC, motor,coil,lighting, power supply) will determine the type(s) of backup system.
Bare in mind that the capacitor solution may be more expensive than UPS (depending on the required power).
For small (<1 KW) the battery/car-inverter looks the best.
For higher loads there are several solutions.
If the applicatin is on some printed circuit board, a small battery may be embedded in the circuit (such as RAM retention applications).

 

[dydt]

hi...
hi...
if you can sneak in inside the power supply of your critical load then you can use a simple battery back up..say the heart of that circuit is being supplied by a 12VDC power supply...you can get a 12Volt battery charger and a stand-by battery and tap to the + and - of the power supply....but you have to put other protection like reverse polarity, an SSR, etc...

i tried this once years ago in a basketball game wherein a the game clock was backed up a DC battery..

is this possible in your application?

dydt

 

[christinesoler]

The power source needs to be as follows: 115V AC (coming from wall AC) but it can be anywhere from 80-240V AC and a minimum of 0.35 amps. The device I am energizing is a unique valve system. It needs to be powered up for at least 20 seconds after loss of power. My power source is not DC.

thx!

 

[melone]

Is it loss of power or turning it off? Are you concerned that the supply will be accidently unplugged or are you worried about a &quot;normal&quot; power down routine?

 

[christinesoler]

Both scenarios would apply.

 

[IRstuff]

Then you need to do a cost-benefit analysis. What are the consequences of not having the back-up? If your customer is unwilling to balance the cost, the nso be it.

Otherwise, the battery and inverter or a cheap UPS is your only option. TTFN

 

[melone]

Do you have some sort of &quot;brains&quot; in your setup, i.e. micro, PLC, etc.? If so, you can take care of the normal power down sequence with a automated power down routine. Otherwise, you are kinda stuck with a UPS type solution.

 

[peebee]

Can you drive the valve solenoids from DC? (probably)
Can you drive the controls from DC? (good question)

If so, maybe convert the whole thing to DC and put in a 120V DC station battery & charger.
-------------------
Here's another thought, maybe you could buy or build a small motor-flywheel-generator set, this might take care of you, especially if frequency is not important.
-------------------
Here's another thought, I think there's some new power correction devices out there that might do what you want, I think they're essentially small UPS's but with capacitors instead of batteries for energy storage. They are designed to do about what you're looking for, very short term ride through. I have no idea of the pricing on these. My Square-D rep was pushing these maybe 6 months ago, maybe try calling Square-D. Sorry, I completely forget their name for these things.
--------------------
But I still think your best, cheapest, and easiest solution would be to run out to CompUSA or check E-Bay and buy the smallest cheapest UPS system you can find on there. You'll be in business in 5 minutes for like $500 (or maybe less) and you'll get 20 or 60 minutes of backup time. The time you've probably already spent looking for a solution, and the time it would take to implement any other solution, would far outweigh the cost of a cheapie UPS system unless you and your boss are working for free.

 

[IRstuff]

Compusa lists this Belkin UPS at $59.99:

http://www.compusa.com/products/product_info.asp?product_code=50143141&pfp=BROWSE

Belkin's website:

http://catalog.belkin.com/IWCatProd...h=/Root/PowerProtection/UPS/UPSDesktopUsage/#

TTFN

 

[STABY]

Hi,
I think the best solution is change the valve or only the solenoid, some companies offer the chance to change only this part, so you can install a 120 VDC valve and back-up it with 10 small lead-acid batteries in serie and a simple charger to mantain charged this small battery bank.
Please check with ASCO or others brands of valves.
Jorge.

 

[mdonner]

I would take STABY's idea (replace ac coil with dc coil/valve) a step further and use a capacitor for back-up: 0.35amps X 20sec = 7q
then c = 7q / 80v < 0.1F rated for 500vdc. (this may end up as a bank of several capacitors bigger than a battery bank).
You will need a series blocking diode paralleled with a charging resistor.
This eliminates the batteries maintenance.
The supply to the coil will be a bridge rectifier, fed by the original ac supply to coil.

To stick with the AC coil and avoid system modifications, you can use the car-inverter solution (rated for 100w at least) with a battery/capacitor bank, and a dc power supply to feed the inverter from wall outlet (and walla! - UPS).