Modifying an inductive cell phone charger for 6V output

[sjwaste]

Hi folks, I wasn't sure where else to ask this question, but I was hoping for a little bit of help.

I'd like to take an inductive cell phone charger like the Palm touchstone setup, but modify it for use with a baby monitor. We have a motorola monitor that takes a 6V 500mA charger, and it's sort of a pain because it's not standard USB, so if we're downstairs or something, we generally have to remember to take the charger with us. I suppose I could just buy a second one, but I'd like to just plop this thing down on the kitchen table and have it charge.

So given that I have a 5V touchstone and coil, would it be incredibly difficult to modify that for 6V? I know next to nothing about electronics, but I'm handy enough with a soldering iron and could probably figure it out if someone were to provide a starting point. I'm guessing that the inductive coil generates AC output and there's some kind of rectifier (3 diodes?) to step it down to 5VDC. Is it as simple as adding a resistor to get 6V?

 

[ussfletcher]

You would need the other side of the inductive charging loop to begin with. The way these chargers work is to induce current in a coil of the device to be charged. The baby monitor doesn't have one.

 

[sjwaste]

You would need the other side of the inductive charging loop to begin with. The way these chargers work is to induce current in a coil of the device to be charged. The baby monitor doesn't have one.

I understand that Sorry, I should have mentioned, I'm talking about using both halves of the charger. It's very easy to take the charging coil out of the back case of the Palm Pre, and it's the basis for a lot of other inductive cell phone charging mods - basically anything else that uses USB charging, since the output voltage is correct.

I'd be putting the coil in the bottom of the baby monitor's case. What I don't know how to do is modify the output voltage from 5V to 6V.

 

[PottedMeat]

i've seen a palm touchstone taken apart before and i don't believe the receiver is a simple coil & rectifier, it had a PIC controller. My guess is that you'd have to find the controller's output voltage sampling pin and adjust a voltage divider across the output appropriately to fool it into thinking 6V is 5V.

edit: something like this:

http://www.ifixit.com/Teardown/Palm+Touchstone+Charger+Teardown/810/1

-doesn't seem like there's a good view of the receiver circuit.

another option would be to attach a 'boost converter' to the receiver circuit output and adjust it to 6V, they're about $5 on ebay.

something like this (there are a lot like it):
http://www.ebay.com/itm/Adjustable-...474?pt=LH_DefaultDomain_0&hash=item43ae9a6792

 

[stormkroe]

I hate to be the pusher of bad habits, but did you try it unmodified? 5V is really really close to 6V, for most things that loose tolerance will still work. More than likely it will power the monitor for operation, but won't charge worth a crap.

 

[sjwaste]

i've seen a palm touchstone taken apart before and i don't believe the receiver is a simple coil & rectifier, it had a PIC controller. My guess is that you'd have to find the controller's output voltage sampling pin and adjust a voltage divider across the output appropriately to fool it into thinking 6V is 5V.

edit: something like this:

http://www.ifixit.com/Teardown/Palm+Touchstone+Charger+Teardown/810/1

-doesn't seem like there's a good view of the receiver circuit.

another option would be to attach a 'boost converter' to the receiver circuit output and adjust it to 6V, they're about $5 on ebay.

something like this (there are a lot like it):
http://www.ebay.com/itm/Adjustable-...474?pt=LH_DefaultDomain_0&hash=item43ae9a6792

Thanks! That booster might be what I need. I've been looking at the Qi-compatible chargers and I think I'm going to try for one of those instead, since the Nexus 4 I have coming will use that. I bought a cheap Blackberry replacement door for $5 to take the guts out and see how it was built. I think I can fit that into the baby monitor case along with the booster without an issue.

I hate to be the pusher of bad habits, but did you try it unmodified? 5V is really really close to 6V, for most things that loose tolerance will still work. More than likely it will power the monitor for operation, but won't charge worth a crap.

I may try this first. What are the ramifications, though? Is it something as simple as the monitor not powering up? It has a battery pack, probably 4 1.5V NiMH cells, which is why it's a 6V power adapter. If I feed that 5V, am I going to damage anything or will it just not power up?

 

[stormkroe]

A lot of things will work fine with a %16 voltage defficiancy, but the batteries will only partially charge with it. Many components only have %10 tolerances anyway, and the designers know that people are going to run them until the batteries are completely dead, during which time it will be running on low voltage (unless it has cut out circuitry which I doubt).

Look on the bright side, if they quit working it will force you to buy nice new ones

 

[mindless1]

Do you know for certain that this induction charging system even generates 5.0V, or is it only being assumed because the alternative charging via USB is 5.0V? Also, it may not support charging at 5.0V at whatever rate the baby monitor allows current flow, as the charger circuit is purpose built for charging <=4.2V Li-Ion at a constant current, I would presume.

There are too many variables involved. First is it an unregulated or regulated (switching) 6V/500mA charger for the baby monitor? How long does it take to charge and does it have some form of charge termination?

Second we'd need a confirmation of the battery pack type and charging circuit. Odds are it is NiMH or NiCd but those are both 1.2V nominal cells that peak around 1.45V each at the highest trickle charge you'd want to maintain to preserve good cell life.

Odds are, that if it has 6V input it has cells in a battery *pack* that at most reaches 5.6V at a standby full charge level. The point is this would power a circuit not only designed to operate at that topped off 5.6V but certainly continue to work as the battery pack drains down to (probably at least) 4.0V.

Even if it doesn't work like this which would be odd, it won't cause damage to be undervoltage, just turn it off and try something else.

While PottedMeat may be correct that some voltage sense (or current sense depending on integration level) pin may need a voltage divider or resistance adjustment, I think it may be easier to just ditch the rest of the circuit and put rectification diodes directly after the coil and see what current that can put into the battery pack.

We would then be concerned about charge termination but there's that concern already unless the baby monitor were using a 3.7V->4.2V peak Li-Ion cell, otherwise the charge profile and termination detection is different for the two battery packs so the circuit modification probably requires more than just boosting 5V to 6V, or else you'll need to monitor the charge state and manually terminate the charge every time to avoid cooking the battery pack, or at least stressing it towards reduced lifespan and capacity.

 

[Modelworks]

Some of the inductive chargers use a pulsed method, they do not generate a constant current in the device, but generate large pulses 5-10x what the powered device voltage may need. The reason for this is the loss in the power transfer increases with distance, one way to increase the received power is to send more power . In the pulsed chargers there is a charge pump capacitor system at work that generates the DC voltage. Usually these parts are made for one voltage and not adjustable without doing something like removing the ground from the regulator and adding a part to put its reference point at a different voltage.

 

[NeoPTLD]

You might be better off modding the baby monitor itself.
6v 500mA wallwart puts out about 6v under full load. Good chance it puts out like 9v unloaded.

Many of these ~6v devices use 7805 to generate 5v internally. If you bypass the 7805, it can be fed directly from a cell phone charger, which generally put out a well regulated 5v. It is one of the most common power regulator in digital devices. 7805 depends on having a source voltage that is at least a few volts over 5v.