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I am trying to power a 12 V 8 W cordless phone using a 5 V 1 A wall adapter. I am using a step up DC/DC boost converter MT3608 for boosting output from 5 V to 12 V.

Theoretically it works - when I connect adapter to boost convertor input and setting potentiometer to 12 V by measuring output through multimeter - it shows 12 V.

But as soon as I connect the cordless the voltage drops to 2-3 V and cordless does not power on. So its not working on load.

I also have a 7.4 V li-ion cells which powers on the cordless fine - using the boost convertor.

My question is the DC coming from adapter different than DC coming from battery?

Also I noticed that the adapter indicator light is constant when charging a basic (non smartphone) phone battery but it pulsates when connected to step up convertor and cordless powered on. This made me more confused.

Please help me understand this behavior of adapter and what's wrong in what I am trying to do?

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    \$\begingroup\$ I don't think \$5 V × 1 A> 8 W\$. \$\endgroup\$ – Harry Svensson Dec 25 '18 at 16:33
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    \$\begingroup\$ A 5V 1A wall adapter cannot provide the 8 W that the phone requires, regardless of any boost converter that you are using. As a result, the adapter is limiting its output current and the voltage sags through. Please try a 2 A or higher adapter instead. \$\endgroup\$ – nanofarad Dec 25 '18 at 16:33
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    \$\begingroup\$ Just get a 12V 1A wall wart. Readily available. Cheap. Problem solved. The only wrinkle may be matching the connector plug. \$\endgroup\$ – Phil Freedenberg Dec 25 '18 at 16:55
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    \$\begingroup\$ Craigslist: power adapter for sale. 5v 1A. ------------ craigslist: WTB power adapter. 12V, 8W. \$\endgroup\$ – Harper - Reinstate Monica Dec 26 '18 at 19:58
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Theoretically it works -

No it doesn't. In theory 5 V x 1 A = 5 W maximum from your PSU. Your phone needs 8 W and you'll lose another 1 or 2 W in the voltage booster. It doesn't work in theory and, as you've found, it doesn't work in practice.

... when I connect adapter to boost convertor input and setting potentiometer to 12 V by measuring output through multimeter - it shows 12 V.

That's the open-circuit or unloaded voltage.

My question is the DC coming from adapter different than DC coming from battery?

Yes. The battery has a much higher current limit.

Also I noticed that the adapter indicator light is constant when charging a basic (non smartphone) phone battery but it pulsates when connected to step up convertor and cordless powered on. This made me more confused.

Why the confusion? Check the manual for the adaptor and you'll probably find that it's indicating over-current fault.

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    \$\begingroup\$ Hi, Thanks so much for clarifying it. It now makes complete sense to me. Also, can i ask whether this arrangement- (5v 1a adaptor and a dc booster)- can be used to charge a 6v 4.5ah lead acid battery- if i increase voltage to 7v for charging battery -- (Its printed- 7v for cyclic use on battery) \$\endgroup\$ – Raool Dec 25 '18 at 18:46
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    \$\begingroup\$ It possibly could but you need to ensure that it doesn't get overcharged. Read up on lead-acid battery charging to gain more understanding first. \$\endgroup\$ – Transistor Dec 25 '18 at 19:57
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    \$\begingroup\$ Thanks a lot for all your help.I will read more information on lead acid battery first, before charging. Have a nice day! \$\endgroup\$ – Raool Dec 25 '18 at 21:34
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1. Theoretically it works:

Theoretically it is not correct, the power of adapter is not equal to the load power.

2.when I connect adapter to boost convertor input and setting potentiometer to 12 V by measuring output through multimeter

No load condition.

3. But as soon as I connect the cordless the voltage drops to 2-3 V and cordless does not power on. So its not working on load.

Obviously it will not, since the adapter is unable to provide the power required by the load.

4. I also have a 7.4 V li-ion cells which powers on the cordless fine - using the boost convertor.

The battery what you are using is having an high current delivery. that is required by the load.

5.Also I noticed that the adapter indicator light is constant when charging a basic (non smartphone) phone battery but it pulsates when connected to step up convertor and cordless powered on. This made me more confused.

Yeh, most of the Boost converters works on PWM technique.. which is going to use a switch and an inductor and capacitive network to change and discharge continuously to boost voltage... in this case the converter is going to draw current from the adapter in pulses (non-continuous).

Try the following...

Go with an readily available 12V, 1A adapter

or

Try an higher current rated adapter at 5V like 5V, 2A ... which is 10 Watts... since no boost converters are 100% efficient.

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  • \$\begingroup\$ Hi, Thanks so much for clarifying it. It now makes complete sense to me. I just have a doubt-- the pwm technique iis it harmful for adapter - i mean will it short circuit, or are the adaters designed for such behaviour. Also, can i ask whether this arrangement- (5v 1a adaptor and a dc booster)- can be used to charge a 6v 4.5ah lead acid battery- if i increase voltage to 7v for charging battery -- (Its printed- 7v for cyclic use on battery) \$\endgroup\$ – Raool Dec 25 '18 at 19:44
  • \$\begingroup\$ Yeh you can ... need to design an auto cut-off circuit. and the charge current should be less than 20% of AH. so in your case < 1A \$\endgroup\$ – Satish Singupuram Dec 25 '18 at 20:40
  • \$\begingroup\$ Hi Satish, The battery says max current 1.35amps. So i guess it should be safe. Also, could you help on whether the pwm technique does not harm the adaptor or does it? When charger indicator light is not solid... \$\endgroup\$ – Raool Dec 25 '18 at 21:39
  • \$\begingroup\$ It will effect so, please regulate the current to 0.5A or less.. so that you can reduce the effect on adapter, please add an high value of electrolytic capacitor between the adapter and boost converter. If 12V adapter is available please go with that. \$\endgroup\$ – Satish Singupuram Dec 25 '18 at 21:46
  • \$\begingroup\$ Thanks Satish, I will buy a new adaptor. That's the safest way to go. \$\endgroup\$ – Raool Dec 25 '18 at 23:38

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