15 V AC backup supply to serve as power backup for modem.

[pkshima]

Hi,

My ADSL modem needs 15 V AC power supply. It has a wall adaptor that does the step down. So far so good.

But in power cuts, I cant be online. I use laptop so no backup required for a computer.

So I am thinking if I can build a circuit to serve as power backup for the modem using batteries. It just needs 15 V AC so I think a UPS would be an overkill. If it needed DC, I would be all set but the darn thing needs AC so something to convert DC to AC without building a full blown inverter.

Any ideas ?

Thanks in advance.

 

[Nigel Goodwin]

First find out if it does actually NEED AC, many don't, it just goes directly into a bridge rectifier, so you can feed it DC instead.

 

[pkshima]

First find out if it does actually NEED AC, many don't, it just goes directly into a bridge rectifier, so you can feed it DC instead.

Thats an interesting point. So how do I check that. Shall I open the modem up and see the power line is going to 4 diodes. Or shall I just feed 15 dc and see if it still works ? I hope that wont fry it up

 

[Nigel Goodwin]

Thats an interesting point. So how do I check that. Shall I open the modem up and see the power line is going to 4 diodes. Or shall I just feed 15 dc and see if it still works ? I hope that wont fry it up

Best to look first, and ensure that ONLY four diodes (as a bridge) connect to it. Think a bit harder about the DC voltage you need though!

 

[mvs sarma]

Best to look first, and ensure that ONLY four diodes (as a bridge) connect to it. Think a bit harder about the DC voltage you need though!

The O P resides in India and most probably,if he has been using the ADSL modem for last 2 years, it might be HUWAWEI make.
Now that the guarrenty period is over he could very well open it and see. If the model he is using is MT841, perhaps i could do that service for him
If the internal voltages are derived by half-wave rectifier or a bridge complemented by capacitor blocked -ve voltage generator, the AC input (low Voltage though) would be needed. Other wise, he has to improvise a dc-dc converter with inverted output. after that mod perhaps one can use DC supply.
It is a good pint brought forward by PKShima, many laptop users might be suffering. As modding ADSL Modem is not worth it, perhaps one can think of deriving 15V 50/60Hz Ac (squarewave though) can be attempted and perhaps it needs isolationbut low voltage is just sufficient and no need for 230V .

 

[Willbe]

Power it up and measure the voltage across the fat filter capacitor just downstream of the diodes. This is your required DC voltage.

If the DC is near 12v you can test that it works by using a car battery to power the modem (don't run the engine).
For other voltages use flashlight batteries to get to the voltage you need or check if you already have a wall xformer from some other gadget that already puts out the correct DC voltage.

If the modem doesn't work properly with your injected DC voltage it may need AC for some other function.

If it does work on DC and you tell us what current this modem needs [look on the xformer] and for what time interval you reasonably need backup power, somebody here can fix you up with a home-built backup system.

 

[Hero999]

It probably needs 15VAC but it doesn't matter whether it's a sinewave or squarewave.

AC is just required to operate a voltage doubler that gives a bipolar 20V supply.

To run off 12V you need a swtching regulator to get 18V to 24VDC followed by an h-bridge driven by an oscillator to prduce a squarewave.

Alternatively you could go for a single sided push-pull stage with a blocking capacitor but you need an input voltage of 36V to 48V and you need to be sure that the modem draws the same current on both positive and negative cycles so it's a bad idea.

 

[pkshima]

Thanks for the replies Nigel, MVS, WillBe and Hero !

If the model he is using is MT841, perhaps i could do that service for him

Thats indeed the exact modem I have ! If you already aware of its internals, I wouldnt have to open it up then

 

[Hero999]

Don't forget if you don't want to bother with a switching regulator then use two 12V SLAs in seres to power your inverter.

 

[mvs sarma]

Thanks for the replies Nigel, MVS, WillBe and Hero !



Thats indeed the exact modem I have ! If you already aware of its internals, I wouldnt have to open it up then

Let us first work out and arrive at a workable solution,before thanking, PKshima. I was out of station and back just few minutes back
I shall workout and inform our members soon.

 

[mvs sarma]

Hai All,
I have opened up my Huwawei MT841 Modem and it is evident that they used 4 Schottky diodes in Bridge configuration and there after managed to generate other voltages. Now it is clear that we can supply 12 to 15V Dc and it would work.
Hope this info serves PKShima and many others.
Yes. I should have taken few shots and put on net, but for my camera being lent to a friend of mine.

 

[neon]

Hi,

My ADSL modem needs 15 V AC power supply. It has a wall adaptor that does the step down. So far so good.

But in power cuts, I cant be online. I use laptop so no backup required for a computer.

So I am thinking if I can build a circuit to serve as power backup for the modem using batteries. It just needs 15 V AC so I think a UPS would be an overkill. If it needed DC, I would be all set but the darn thing needs AC so something to convert DC to AC without building a full blown inverter.

Any ideas ?

Thanks in advance.

it amy say AC but believe me it end up being DC. probably 9v wich with a battery and an external swich can do it if power fail. measure the modem volts and current and duplicate that with batteries

 

[ericgibbs]

it amy say AC but believe me it end up being DC. probably 9v wich with a battery and an external swich can do it if power fail. measure the modem volts and current and duplicate that with batteries

In thats the case ie: all 'dc' why is it the ones I have converted from mains to battery require 'ac' for the -V drivers.?????

 

[pkshima]

Hai All,
I have opened up my Huwawei MT841 Modem and it is evident that they used 4 Schottky diodes in Bridge configuration and there after managed to generate other voltages. Now it is clear that we can supply 12 to 15V Dc and it would work.
Hope this info serves PKShima and many others.
Yes. I should have taken few shots and put on net, but for my camera being lent to a friend of mine.

I think now I can thank Thanks a lot for a complete solution and also for the knowledge lacking which I was afraid to open the modem myself. Now I have the knowledge as well as confidence to open the modem if I feel like

rep ++

 

[mvs sarma]

I think now I can thank Thanks a lot for a complete solution and also for the knowledge lacking which I was afraid to open the modem myself. Now I have the knowledge as well as confidence to open the modem if I feel like

rep ++

Incidentlly, I could clean up some dust that settled inside. These are not new to me as I was doing equipment servicing while in service.

Perhaps you are meant for tomorrow and you would create wonders and get the applause for that PKShima jee.

 

[neon]

In thats the case ie: all 'dc' why is it the ones I have converted from mains to battery require 'ac' for the -V drivers.?????

BELIVE THIS NO PHONES ,IPOD,PC will run on plain AC all of them eventualy works on DC DC DC DC DC. The line is AC BUT ALL ELECTRONICS RUN ON DC.

 

[mvs sarma]

BELIVE THIS NO PHONES ,IPOD,PC will run on plain AC all of them eventualy works on DC DC DC DC DC. The line is AC BUT ALL ELECTRONICS RUN ON DC.

I appreciate the level of confidence, you hold. Try to see the erstwhile external dial-up modems powered by similar AC output step down adopters.

i will give you a schematic where you would agree that it would not work on DC DC ....

Please don't take things for granted.

 

[ericgibbs]

BELIVE THIS NO PHONES ,IPOD,PC will run on plain AC all of them eventualy works on DC DC DC DC DC. The line is AC BUT ALL ELECTRONICS RUN ON DC.

DC yes, but in case you are NOT aware, there a two polarities of DC , POSITIVE and NEGATIVE.

I suggest you Google voltage polarity and see what you can learn.

EDIT: I think also Googling Rectification would put you on the right track.

EDIT: to make it easier to follow, a diagram of a Earlier Modem Line Driver stage.

 

[neon]

DC yes, but in case you are NOT aware, there a two polarities of DC , POSITIVE and NEGATIVE.

I suggest you Google voltage polarity and see what you can learn.

EDIT: I think also Googling Rectification would put you on the right track.

EDIT: to make it easier to follow, a diagram of a Earlier Modem Line Driver stage.

If you are nitpicking then sugest a both polarity from a reference point politicaly CORRECT.

 

[neon]

DC yes, but in case you are NOT aware, there a two polarities of DC , POSITIVE and NEGATIVE.

I suggest you Google voltage polarity and see what you can learn.

EDIT: I think also Googling Rectification would put you on the right track.

EDIT: to make it easier to follow, a diagram of a Earlier Modem Line Driver stage.

Voltage is not born with polarity WE the designers users assign polarity. Current flow assign polarity not the presence of a voltage which can be +/- depending on current flow. Want to know more?