A question for electronic gurus:
I was thinking about getting another Oncore or similar module and putting together a battery powered GPS module. Basically, because I'm a cheapskate and I'm not going to pay over $100 for a gps module for my pocket pc when I can make one for $40. So my question is related to power, how can I power an Oncore-like unit with batteries? I could get 4 AAs together for 6V, but what do I do to regulate that to 5V efficiently? I don't think the 7805 setup I have in my car would be very kind of batteries. Would the more efficient switching regulators be my only option or is there something else I can do to make that 6V go down to 5 (something cheaper)?

I'm no EE, but I'll bet that hooking the 6v battery pack directly up to the 5v unit would work fine. Anyone want to refute me on that?

I wouldn't advice it since the datasheet says Vcc of 4.75 to 5.25VDC.

Probably the best solution would be to use a DC/DC-converter of some sort.

Stig

you could try running it on 4.5 volts.

Have you tried RatShack or a hobbystore? I bet you could easily find a 4.8 volt (4 x 1.2V cells) battery pack intended for some sort of small eletrical gadget. Then just add a 5V DC transformer (walll wart) to charge it. You could even build your own from loose 1.2volt cells.

-Mike

Yeh, aren't all standard NiCad rechargable batteries 1.2v not 1.5v (which is normally a problem - but not for you) so 4 would give you 4.8v purrrfect

Cheers, Sim

OK this is from an EE here....

First, I would definitely not run straight off 6v. The electronics will probably have absolute max ratings of around 6v. Good chance of blowing things up.

A low dropout (LDO) 7805 might be nice running off 6 Volts. A standard 7805 needs around 1-1.2V higher on it's input to work effectively. It's a linear reg so it becomes less efficient the higher input voltage you have. You're only dropping a volt or so, so a LDO linear reg off 6 Volts is probably the simplest and best solution.

As for running off 4.8V you may start to have issues when the battery starts running down. The device may work down to 4.5V but you're pushing it.

A regulator of some sort is the best way since it also provides some protection at connecting batteries the wrong way round etc. You have to consider that these surplus modules have power supply pins with essentially zero protection.

Do you have any current or power consumption specs?

That sounds simple enough I'll have to get one of those Low dropout 7805s. Makes my job easy, thanks.

Perhaps an even simpler way is to use 2 silicon diodes in series with the 6V battery pack, they will give you the 1v drop you need. Something like two 1N4001's should do it.

Still not real good practice. You're relying on a diode voltage drop of 0.5-0.6V. In reality the drop with vary depending on the current drawn through them. At low currents, the drop might only be around 0.2V. The datasheet for the diode will have a graph of forward voltage drop vs current.

Also if the battery voltage starts to drop so will the output voltage. LDO regulator won't have this problem until the battery voltage goes below the minimum input voltage.

Do you have any current or power consumption specs?

< 0.9 W at 5 V at 25°C with active antenna drawing 20 mA

Stig

OK call it 1W. One 1W at 5V is 200mA. 1V drop at 200mA is only 0.2W for the regulator to dissipate. Easy for a 7805 with minimal (if any) heatsinking and will still be quite efficient - around 85% efficient in fact which is about the best you will get.

For reference a 1N4004 diode with a 200mA current, the forward voltage drop is 0.8V. So one wouldn't be quite enough and two would be way too many.

IIRC, the Oncore has an antenna power up transient of 100mA. I'm not sure if that's factored into the 0.9W figure quoted or not. Probably best to assume not, and allow for a 300mA worst case current draw.

The Oncore is TTL level CMOS. That means that the pins are not protected in any way, and as such are susceptable to damage from overvoltage conditions (and static).

allow for a 300mA worst case current draw

Still only 0.3W dissipated which would easily be dissipate by a TO220 (package) 7805.