Isolating mains supply from spikes

Hi,

I have a watchman electronic device recently fried (dead) by a mains voltage spike. The mains domestic electric box being switched off (for some work) and then back on again.

Can I introduce anything to protect sensitive electronic devices from the power surge that happens in the few moments that the power is re-connected ?

More-over I have some home automation going into the house. The low voltage electronics side of this can be powered by microUSB phone charger type power supplies. However I have been recommended to use "isolated power supply". For this. Can I ask a stupid question? Are these the same thing?

In the main switch mode power supplies will not allow spikes through them. The way they work is AC is converted to DC feed into a capacitor and then turned back into high frequency AC transformed down to voltage required and then back into DC the DC voltage will alter the mark/space ration of the low voltage DC to AC bit so regulating the voltage output of the extra low voltage side.

As the switch mode power supply gets older the capacitor used to store the low voltage DC will degrade and voltage spikes can make an already weak capacitor fail this failing can in turn cause spikes on the final DC output. With good quality units a crow bar device will cause the output to be closed down but low quality devices may not have the crow bar circuitry.

If the capacitor is not taken out by the low voltage spike it would likely fail anyway shortly after but because low voltage spikes will cause it to fail just a little earlier than normal they are blamed for the failure of the power supply where likely it would have failed anyway.

With a good power supply the spikes will not affect it unless really large for example an electrical storm and it is likely any device you use will be taken out by an electrical storm anyway.

However you can get sockets both fitted in the wall and as extension leads which will reduce the spikes but just like the capacitor in the power supply the capacitor in the protectors also has a limited life so it's not a case of fit and forget they need replacing at regular intervals for them to be effective.

So to use them means keeping careful records and swapping the module or whole extension lead on a regular basis. Seem to remember recommend swap every two years.

As to if it's really worth all the hassle of extra protection or if you just accept the power supplies will need changing is hard to say. What ever you add will also need replacing be it a UPS or simple filter so in real terms you end up swapping more items as their capacitors fail so it is likely in most cases they are a waste of money.

May as well just accept that any power supply on it's last legs will likely fail when you get a spike just like old light bulbs always seemed to blow just as you switched them on.

kez1 said:

I have a watchman electronic device recently fried (dead) by a mains voltage spike.

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How do you know that was the cause?

kez1 said:

The mains domestic electric box being switched off (for some work) and then back on again. ... Can I introduce anything to protect sensitive electronic devices from the power surge that happens in the few moments that the power is re-connected ?

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It's not necessarily 'a surge'. When electronic devices, and some other electrical things (like light bulbs/lamps) decide that it's time to die, that death very commonly happens at 'switch-on' - it's relatively uncommon for them to just 'suddenly die' whilst working normally. It could be that some of those 'deaths at switch-on' could be avoided by 'soft start' technologies, but that same approach may also cause problems with some equipment.

In general, 'death on switch on' most commonly means that it was going to happen pretty soon, anyway - so, IMO, it's probably not worth expending much time/energy/cost in trying to 'delay the inevitable'.

Kind Regards, John

……unless the electrician fried the Watchman when doing Insulation Resistance testing…. just a thought.

kez1 said:

a mains voltage spike. The mains domestic electric box being switched off (for some work) and then back on again.

Click to expand...

Mains electricity supplies do not spike when you turn them on. They are at (about) 240v, whether they have just been turned on or have been on for years.

I am assuming that you live in an ordinary house in an ordinary street, and do not have an industrial site with its own substation, an induction furnace, or huge electric motors.

The replies above are all correct. In my experience though, there is another reason why electronic equipment fails after a power interruption.

I used to work alongside someone who really knew electronics. I have a degree in the subject but he taught me the stuff you don't learn at university.

In every switch mode power supply, there is a part of the circuit which only ever gets used when power is applied. It kick-starts it into life. In most PSU's I've looked at, there is an electrolytic capacitor involved. Electrolytic capacitors have a shelf life, and it makes no difference how often the device is powered on or off as to when if will fail.

Where I work, if a switch mode PSU fails while powered up, it goes in the bin, as it isn't worth the time trying to fix it. If it goes when power is cycled, I often crack it open, replace this 'start up' capacitor (90% success rate) and the PSU will usually live for another year or two (until a different capacitor needs changed).

Thanks for the replys guys

I had some really strange behaviour with a power supply (from a BBC master 128 so ~25 years old) recently. One of the caps didn't discharge reliablly when turned off (it could end up with anywhere from about 30V to about 200V on it at switch off time). It would start with all the caps discharged but after turning off and back on it would start either very slowly or not at all. Once it had failed to start once it wouldn't start again until the caps were manually discharged.

initially I thought that the charge on the start cap just was some old design that didn't have a drain resistor (since I couldn't initially find one) and on the advice of some guides I found online I replaced the start cap (which made no difference) but eventually after much tracing of the PCB we (my brother actually) found out that there was a drain resistor and that it was open circuit. Replaced the resistor and it worked fine.

thanks plugwash ... I can't remember any micro electronics ... frustrating ... not even the basics ... and life unfortunately is too busy for the kind of time necessary for informed reading ... i'm gonna struggle on though ... thank you for ur response