# Using 5 V adapter to power 12 V cordless phone

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.

• I don't think $5 V × 1 A> 8 W$. – Harry Svensson Dec 25 '18 at 16:33
• 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. – nanofarad Dec 25 '18 at 16:33
• Just get a 12V 1A wall wart. Readily available. Cheap. Problem solved. The only wrinkle may be matching the connector plug. – Phil Freedenberg Dec 25 '18 at 16:55
• Craigslist: power adapter for sale. 5v 1A. ------------ craigslist: WTB power adapter. 12V, 8W. – Harper - Reinstate Monica Dec 26 '18 at 19:58

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.

• 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) – Raool Dec 25 '18 at 18:46
• 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. – Transistor Dec 25 '18 at 19:57
• Thanks a lot for all your help.I will read more information on lead acid battery first, before charging. Have a nice day! – Raool Dec 25 '18 at 21:34

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

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...