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Simple Power Source Switching that's faster than a mechanical relay...

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I'm trying to implement dual power source for a 12VDC 60W ATX PSU module which normally only has one 12VDC input. I'd like to have a 12VDC input and a battery input for a SLA battery. The PSU can handle voltages from 8 to 15VDC (i hacked it XD) so a straight connection from the battery to the input will work. I tried implementing this with a SPDT relay by tying all the grounds together and having battery VCC in relays N.C and 12VDC adapter in N.O. Turned out the relay is way too slow at switching the load to another power source so the PSU powers down the computer when 12V adapter is removed and battery takes over. I was able to fix it with a massive bank of capacitors, but I don't really like that so maybe there are some better solutions? perhaps making the transfer really fast with mosfets, yet still having the sources isolated somehow?

Here's a little schematic what I currently have:

Short Circuit:
Mosfets give much faster swicthing than relays, like 10~100ns vs 1-10ms. But there always remains a deadtime

To keep things simple, you can use a couple of or-ing diodes in parallel with the switching elements.
This will limit the voltage drop to 0.5-1V below the lowest feed during the switchover.

how would the circuit look like?

A few years ago I was interested in a similar problem.  That was, "How fast does a computer Uninterruptible Power Supply (UPS) have to be when switching over to battery backup, to be considered uninterruptible?"

According to ATX Power Supply Spec. 2.2 (section 3.2.11), an ATX power supply must have a Voltage Hold-up Time of 17ms.  That means that if the AC power Fails completely, the ATX power supply must maintain the proper voltage ouputs under full load for at least 17ms.  Now, it's not coincidential that one 60Hz AC cycle is 16.6ms, and one 20Hz AC cycle is 20ms.  In essence, this specification is saying that the ATX power supply's main filter capacitor must be able to supply operating voltage under full load for one AC cycle, until it's voltage drops so low that the computer will malfunction.

I measured the mechanical relay response time during switchover using my oscilloscope and the results are as follows:

Switchover from AC to battery backup took my mechanical relay 5 ms.

Switching from battery backup back to AC line voltage took my mechanical relay 9ms.

So my conclusion is that a mechanical relays are quick enough for Uninterruptible power supplies, given that a filter capacitor can hold a charge for at least 17ms while supply full load voltages. I know this doesn't solve your problem, but I thought I'd mention my experiment nonetheless.

Did you take a picture of a device that can take screen shots? why?


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