Simple makeshift UPS using SSR switching

[pauldse]

Hi All,

I'm a newbie here and to electronics generally tbh. I'm hoping to design a SSR-switched circuit to switch quickly between a mains-driven 500W 12v supply (utilising an ATX PSU) and a couple of big 12V deep-cycle batteries in parallel to create a sort of UPS, favouring mains but with the option of running purely off battery when in the field and hopefully being v quiet (for use on a film set). The output will power an array of hard-drives and a laptop, plus a second monitor. Could you tell me if I am on the right track with the accompanying schematic, please, or if I've missed anything - as well as how I might work out the values for the T1 R1,2,3 and C1. Also if I have spec'ed the SSR correctly.

Many thanks for any help!

Paul.

 

[chuckey]

500W @12V ~45 A , D3 must be rated for this current, also it will drop .7 V so output on ATX will be 12 V - .7 = 11.3V, power in D3 ~ .7 X 45 = 35W, it needs a heatsink. Likewise D1 and D2 dissipate 35 W between them, hence must be heat sinked. I suspect your SSR will drop volts and so that will have to be heat sinked. You must specify a type number that can handle the 45 A, this will then give you its input current for switching, I believe its normally about 5 -20 mA. I think that you need to put in some hysteresis so if the ATX 12 V starts to sag, the circuit flips over and will not come back until the ATX is back to 12 V - I think this should be adjusted upwards to compensate for the D3 voltage drop, so set it for 12.7 V. Oh yes some form of indication on the ATX that its 12V is good, else the first time you know its failed is when the two batteries run out of puff!
Frank

 

[pauldse]

500W @12V ~45 A , D3 must be rated for this current, also it will drop .7 V so output on ATX will be 12 V - .7 = 11.3V, power in D3 ~ .7 X 45 = 35W, it needs a heatsink. Likewise D1 and D2 dissipate 35 W between them, hence must be heat sinked. I suspect your SSR will drop volts and so that will have to be heat sinked. You must specify a type number that can handle the 45 A, this will then give you its input current for switching, I believe its normally about 5 -20 mA. I think that you need to put in some hysteresis so if the ATX 12 V starts to sag, the circuit flips over and will not come back until the ATX is back to 12 V - I think this should be adjusted upwards to compensate for the D3 voltage drop, so set it for 12.7 V. Oh yes some form of indication on the ATX that its 12V is good, else the first time you know its failed is when the two batteries run out of puff!
Frank

Thank you Frank - you're a gentleman. And in Southampton too - what are the chances? - I'm in Bitterne! Now, great point about the 12v indicator for the ATX; I'm cool about the heatsinks (...) and the voltage drop over D3 and I'll have a go with the component ratings - but I'm lost at the bit about building in hysteresis: could you tell me in layman's terms how is this done please? Does it mean voltage divide at R1 and R2 giving only just enough Vb to trigger with say 12.5V o/p at the ATX? Should R1 be variable to allow this to be set...? Or am I completely off the page here? And am I right in thinking I might find some internal adjustment for the o/p voltage of the ATX that will get it up to 12.7V? Sorry for so many more questions - thanks again for your help. Paul.