The power supply (PSU) for my modem router (3com) and although under warranty I am having problems getting 3com to provide a replacement. The PSU is rated 12VDC 1000ma. To solve the problem I have bought what I thought was a suitable replacement with a nameplate rating
12VDC 1000ma. I have measured the open circuit voltage of this replacement as 16.8VDC which makes me very nervous. The open circuit voltage of a colleagues PSU (Belkin), also rated as 12VDC 1000ma, is
12.1VDC.
Can someone give me advice whether I can use the 'replacement' PSU without fear of damage to the router?
I understand that PSUs come as 'regulated' and 'unregulated' I assume I have an 'unregulated' PSU and that the full load voltage is likely to be 12VDC as rated. Does this make sense?
His is probably a regulated one whereas yours is unregulated (as you suggest below).
It'll probably be OK but I must admit I'm not sure that I'd risk it. It depends how much the 3com router will cost to replace really.
Yes, sort of, but it depends how much power your router *actually* takes whether it will load the PSU down to close to 12 volts. Re-reading your post I've changed my mind, I think I probably would risk it, I thought originally you said the 3com PSU was 100mA and the replacement was 1000mA but in fact they're both the same.
The only issue is whether the 3-com PSU was regulated, if it was then the router may not be too happy with an unregulated supply.
Yes. You really need a regulated one. However, if the 3Com router is of low value to you and you want to risk it, it might work. On the other hand, it might damage it. My guess is 60% works fine, 30% doesn't work, 10% causes damage.
As a matter of interest, what is wrong with your 3Com power supply? I also have one with which I have a problem!
Ever since my ADSL line was upgraded from 512k to 1M, my 3Com wireless router wouldn't work reliably - and gave up after being on for about 30 minutes. The symptons were that the noise margin would plummet, CRC errors would go sky high, effective throughput would be less than dial-up - and then it would refuse to synch altogether. 3Com replaced the router under warranty - but the new one was exactly the same! [I have another, different make, router and a PCI modem - both of which are ok - so I knew it wasn't the line].
Then, I read in uk.telecom.broadband about factors affecting noise margin - including noisy power supplies. I duly powered my router with an old power supply which I had in my bits drawer - and the router works perfectly - with a much higher noise margin than it *ever* reported when using the 3Com PSU.
The supply I'm using *is* regulated, and is rated at 1000mA. It gets quite hot, so I imagine that it's running close to (or even above!) its limit. Also, when it's been on for a few hours, the router's synch light goes into a continuous fast blink - but it still works! The basic difference is that my supply contains a tranformer plus rectifier - whereas the 3Com jobby is a switched power supply - which is prone to generate a lot more electrical noise. The 3Com supply is rated at 1.25 amps - but the router itself says 1A max.
You could load your supply be connecting a couple of car sidelamp bulbs in parallel to it. These are nominally 6w each - so 12w at 12v should draw 1A. If the voltage when powering these bulbs is much closer to 12V, I think I'd give it a go.
If it's unregulated then the voltage under load will vary according to that load.
The problem also is that if you use a regulated supply where a non regulated type is needed, the regulator *inside* the device won't work given that they need a slightly higher incoming voltage to operate correctly. This may, or may not, matter.
Thanks very much for all your very useful comments.
With respect to 'Set Square' post, my 3com wireless ADSL router has worked without problems although there seems to be a body of opinion that it is not a 'good' product. The problem with the PSU was it simply failed, that is no output.
As a follow up, today I received an email from 3com to say they have agreed to provide a new PSU.
The replacement PSU I have now found has a 'problem' in that the plug does not fit the router, or any other device I have (3) with the same OD socket. From visual inspect it appears that the ID of the plug is too small to allow the centre pin to enter.
The source of the PSU advises this PSU is in common use with a range of CCTV equipment that I find puzzling. I was of the understanding these plugs are standard in so much the OD of the plug determined the ID for the centre pin, am I wrong here?
Yes, I've got several - all with the same OD but different ID's! If there
*is* any standard, maybe the different pin sizes are for different power ratings - to ensure that you don't overload weedy supplies by connecting them to high current consuming devices?
I have found, rather been told the answer to my question that being the centre pin of the power receptacle fitted to a device requiring a 'regulated' power supply is made larger than one that will accept an unregulated supply.
However, I've just found one that you can have for your very own!
formatting link
5V, 0.7A.
formatting link
no load, 6.2V, with 0.2V ripple. At 10 ohms load, 3-4.5V. Yes boys and girls, that's 1.5V ripple on a USB hub PSU. There is no input filter capacitor...
On 14 Sep 2005 22:40:57 GMT, Ian Stirling scrawled:
I thought the idea of a powered hub was that you got the full 500mA available to each port to overcome problems that a motherboard may have with supplying a full 2A just for the USB? If so, then that PSU isn't suitable for a 2 port hub, let alone 4.
Always an optimist me, well, not really but just this once I was, never mind......
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