OT: Will These Work?

In message , Derek Geldard writes

What, dropping from 12v to 9V ?

Reply to
geoff
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Thanks for the explanation

But I still don't see why that militates against a "suck it and see approach", unless you agree with those who have warned of overheating. (I can only offer one data point: a Netgear router ran for a couple of weeks from a 9V PSU when its 12V failed, though I do not know how well the 9V was regulated.)

Reply to
neverwas

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Reply to
John Rumm

In message , Calvin Sambrook writes

Probably noisy as hell and could possibly cause the router to lose connection if it's an ADSL modem/router combo. 3Com had a major problem a few years ago with a cheap plugtop SMPSU they supplied with one of their ADSL routers.

I keep looking at them, wondering if I should buy one but then I think of the box full of 'useful' bits n bobs under the stairs.

Depends, plenty of routers use MC34063 switch mode chips (or similar) for their internal regulators, if you drop the input voltage the switch has to stay on for longer and thus it dissipates more power, probably not enough to overheat it but...

For a linear regulator the opposite is true, the lower the voltage the less power it has to dissipate.

Reply to
Clint Sharp

As I had my Silly Scope out for other, much scarier, reasons I took a quick look at the Tesco SMPS running my doorbell. Set to 6V and driving only two LEDs at about 30mA it had about 50mV of essentially unstructured noise. Fastest rise/fall times in the 1nS region so probably no components over

500kHz. I don't know what it takes to upset a router but if any of my guys designed anything which fell over because the incoming power had that sort of noise on it I'd cry.
Reply to
Calvin Sambrook

An ADSL "router"...

ADSL2 uses carriers from around 25kHz to nearly 1.1MHz, ADSL2+ goes up to 2.2ish MHz. MF broadcast stations can reduce the data rate, interference at night from foreign MF stations knocks a good 512kbps to 1Mbps of my link.

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it's only 50mV on the power rail but the bit of wire connecting PSU to router will radiate nicely and ...

Reply to
Dave Liquorice

Would you care to expound upon that last comment?

Reply to
Woody

The power dissipated by a linear regulator is the product of the current through it and the differential voltage between input and output.

If the input voltage is lower, for a given output voltage, less power is dissipated by the regulator.

MBQ

Reply to
Man at B&Q

In message , Woody writes

Ahh, perhaps I wasn't quite clear enough, I should have said 'the lower the *input* voltage to the regulator the less power it has to dissipate' but given the context I thought my meaning would be apparent.

F'rinstance, if you have a 5 volt linear regulator passing 1 amp and an input voltage of 10 volts then the regulator will have to dissipate 5 watts, if you have the same regulator but with a 15 volt input then it has to dissipate 10 watts.

Linear regulators dissipate power as heat so feeding one with a lower input voltage will generate less heat.

Reply to
Clint Sharp

In message , Calvin Sambrook writes

Load it up somewhere close to rated output, preferably with something other than a simple LED or resistive load and check again. I'm not saying it's a bad PSU in the grand scheme of things but that it's a little too cheap to be really good.

I wouldn't expect a noisy PSU to cause the router to fall over unless the PSU was exceptionally bad and you'd probably have problems with anything else you powered from that PSU (perhaps not your doorbell or LEDs though)

ADSL is a fairly special case though, RF interference picked up by the telephone line can cause a modem/router to lose sync, it's not the case that it falls over because of the noise, it's that the ADSL 'signal' gets swamped by the noise, reducing the SNR to a point where it's unusable.

500KHz is almost smack bang in the middle of the band of frequencies ADSL uses so if it's close to the phone line (which is a given because you're powering the router with it) then it could cause problems.
Reply to
Clint Sharp

With the proviso that they need a certain amount of head room to work properly. So a 5V regulator will behave as you describe, but an input voltage of less than say 7V may prevent it from working correctly at all.

Reply to
John Rumm

Exactly where I was going. Most 'normal' (as distinct from low drop-out types) actually require an overhead of 2.3V, plus you have to consider mains variations, so a starting target input of about 8V (possibly even more) is more like it.

Reply to
Woody

Some routers (inc in the Netgear range) have a built in print server. So the higher power rating of the adapter *may* be due to the need to drive the USB port(s) for attached printers.

Midge.

Reply to
Midge

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