Total Electronics Novice In Need Of Power Supply Advice

[Paul Short]

Hi All

Newbie here in need of some advice regarding using an UK mains AC/DC adaptor with a device that has had it's power supply wiring chewed through by a rabbit! I'll explain as best as I can, but as the title suggests I am a novice as far as electronics is concerned, so I apologise in advance it none of his makes sense.

Ok, I has a device that needs the following power input:

7.5V - 12V and 0.8A to 0.5A

I have an UK mains AC/DC adaptor that has a switchable voltage supply and switchable polarity. The voltage options are 3, 4.5, 6, 7.5, 9 and 12. The adaptor states that that the amperage output ranges from 300mA to 3.6VA.

I have tested the adaptor polarity switch is set correctly by testing the adaptor on a similar device to the one I want to actually use the adaptor with, that I wasn't too worried about blowing up. I put the adaptor at 12V, which the label of the test device says it needs, and tested and it powers up the device fine.

So now comes using the adaptor with the device I actually want to use. What voltage should I set the adaptor to, bearing in mind the device says it needs between 7.5V to 12V? Also, am I right in thinking that the device will only draw the amperage from the adaptor it needs to function, and if so, would that mean the adaptor is fine to use with the device?

I hope that all makes sense and I would really appreciate any advice and comments you have on this query.

Many thanks in advance.

 

[dave9]

Yes the device will only draw the current it needs which means that you could use a power adapter rated higher (in mA or A) than the device, not lower.

Your power adapter isn't capable of enough current at 300mA. It's probably unregulated, has a multi-tap transformer inside to achieve the different voltage outputs so it will not be able to reach 500mA at 12V, nor 800mA at 7.5V.

12V, 0.5A (or 1.0A) regulated PSU are fairly common. You should not have much trouble finding one to buy inexpensively if you don't have one lying around. The main ease-of-implementation issue would be that the plug is the right size, including the center pin. If you have to solder on a different plug then you might as well just solder on a new cable to the old PSU which the rabbit chewed through.

You should check that the power adapter has a center positive polarity plug that matches the device. That is by far the more common arrangement.

 

[Paul Short]

Yes the device will only draw the current it needs which means that you could use a power adapter rated higher (in mA or A) than the device, not lower.

Your power adapter isn't capable of enough current at 300mA. It's probably unregulated, has a multi-tap transformer inside to achieve the different voltage outputs so it will not be able to reach 500mA at 12V, nor 800mA at 7.5V.

12V, 0.5A (or 1.0A) regulated PSU are fairly common. You should not have much trouble finding one to buy inexpensively if you don't have one lying around. The main ease-of-implementation issue would be that the plug is the right size, including the center pin. If you have to solder on a different plug then you might as well just solder on a new cable to the old PSU which the rabbit chewed through.

You should check that the power adapter has a center positive polarity plug that matches the device. That is by far the more common arrangement.

Many thanks for the reply Dave.

So from what you are saying it doesn't really matter about the amp rating as long as it's higher or at least equal to the higher value stated on the device? But it looks like the adapter I'm using probably won't be good enough for the job, is that correct? Thing is I am using the adapter at the moment, is that likely to cause any problems? I've had to use it as it's for my internet modem, and without that running I'd have my beloved sons whinging at me because they can't play Tom Clancy's Siege online.

Never thought of soldering a new cable on the old PSU, shame as I thought it couldn't be salvaged so binned it, and it happened to be bin collection day today, so I can't even go routing through rubbish to retrieve it.

 

[Paul Short]

I just checked the adapter and it was really hot. I didn't like it so I've disconnected it.

I have found a BT Home Hub PSU lying around, and as the connector seemed to fit perfectly I have decided to use that instead. Can you tell me if this would be ok to use for the time being if I give you the info on the PSU please? The info says the following:

Input 230V ~ 50Hz 300mA
Output 12V = 1500mA

Cheers.

 

[kellys_eye]

I just checked the adapter and it was really hot.

Such adapter usually have a thermal fuse wound into the transformer and when it goes 'open' it renders the adapter totally useless (unless you have the skills to dismantle/repair them). Wise decision to stop using it.

12V 1500mA is fine (same as 1.5A) and, as you already know, the modem will only take what it needs.

 

[BobK]

Do make sure the polarity is correct before plugging it in.

Bob

 

[Paul Short]

Brilliant, thank you for all your comments and advice, greatly appreciated.

Just out of interest, if the polarity was incorrect when plugging it into the modem, and I switched it on, what would happen?

 

[dave9]

^ Either nothing if the modem is polarity protected, or it lets the magic smoke out and stops working... "maybe" blowing a fuse to save itself but then you have to know how to fix that, fuses on small electronics may be a small surface mount chip, not the old school little glass type and are usually soldered on.

 

[Paul Short]

^ Either nothing if the modem is polarity protected, or it lets the magic smoke out and stops working... "maybe" blowing a fuse to save itself but then you have to know how to fix that, fuses on small electronics may be a small surface mount chip, not the old school little glass type and are usually soldered on.

Ok, cheers for that Dave. I've had the modem running for a couple of hours with the BT Home Hub PSU and everything seems to be working fine and is nice and cool to touch so I'm pretty sure it's all safe.

Thanks to all of you for your help in this matter, at least the PSU will be ok until I can source an official PSU for my modem.

 

[dave9]

So long as the PSU you're now using is decent quality and not worn out from years of running hours, there is no reason you can't keep using it indefinitely, unless you need to put it back with the hub to use the hub.

 

[Paul Short]

So long as the PSU you're now using is decent quality and not worn out from years of running hours, there is no reason you can't keep using it indefinitely, unless you need to put it back with the hub to use the hub.

The PSU is actually from an never used Plusnet Hub setup, which is basically a BT Home Hub in disguise. I've always preferred using my own modems and routers so never used ISP provided hardware. Should last me for quite some time which is the main thing.