SparkFun Forums

[jpwsutton]

Hi again!

I am now working on a charging circuit for my GM862 and I was wondering if I have the right Switching Regulator. I have an DE-SW050 from Dimension Engineering (http://www.dimensionengineering.com/DE-SW050.htm) which can apparently output up to 1A. After checking the GM862 Battery Application note, I saw that the Max current was 1A, but the recommended current was 400mA. The Regulator will be connected to a 12v line in a car.

Does anyone know if I will need protection for the regulator from the cars power supply? And will I need any additional components for the charging circuit?

Thanks,

James

[MichaelN]

Always use plenty of protection on automotive project, or it may not last a day. I'd suggest a fuse followed by a beefy Tranzorb / TVS diode (which needs to clamp the voltage below 30V UNDER SPIKE CONDITIONS).

As for GM862, you should provide links. I can't see any mention of charging in the Sparkfun product pages for GM862...

[jpwsutton]

Thanks, Would this kind of Diode be the right one? http://parts.digikey.com/1/parts/719758 ... e30ca.html

And I got the information from the Telit GM862 Battery Note: http://www.glyn.co.nz/downloads/documen ... ote_r0.pdf

Thanks,

James

[MichaelN]

Would this kind of Diode be the right one? http://parts.digikey.com/1/parts/719758 ... e30ca.html

No, that is not suitable, as it would allow the voltage to exceed 30V under clamping conditions (check the datasheet).

I suggest a nominal 18V device instead, and a "1500W" device instead of the smaller "600W" package you've chosen (as it can handle much higher peak loads). Something like this:
http://search.digikey.com/scripts/DkSea ... E18AGOS-ND
Make sure you include a fuse, both for protection against reverse-polarity connection, as well as abnormal supply situations. You might also need to add a filter (an inductor and a couple of capacitors) to reduce the interference the switching regulator will send back into the car's 12V power system. In the past I've observed interference with the car radio etc when this was not done.

And I got the information from the Telit GM862 Battery Note: http://www.glyn.co.nz/downloads/documen ... ote_r0.pdf

As per that document, "Current limitation ... needs to be implemented from the external power
supply/charger...". I'd suggest adding a resistor in series. The value would depend on the desired charge current, but a value of (say) 2.2 or 3.3 ohms is probably OK. This isn't ideal, but it would save having to implement a more complicated constant-current circuit.

[jpwsutton]

Hi,

Sorry for the slow reply, I just finished my last exam yesterday for the year so I'm now catching up on my project!

I think I understand..


Does that seem right? Or do I need to put filter caps on the other side of the Regulator as well?

Thanks,

James

[MichaelN]

No, you want to put the TVS diode like this:


It would be a good idea to add a filter capacitor as shown (or even 2: a ceramic cap in parallel with an electrolytic cap), to help smooth the voltage, and assist the TVS diode in absorbing voltage spikes.

[jpwsutton]

Ahhh, I see! Thanks for fixing my diagram!
Just one small question, how do I work out the values for the capacitors? I've googled around, but I cant seem to find a definitive way of working it out :S

Thanks!

[MichaelN]

how do I work out the values for the capacitors?

Normally you should follow any capacitor recommendations from the regulator datasheet closely. In this case, they claim none are required, so it won't be too critical. Perhaps 100uF electrolytic in parallel with a ceramic cap of a few hundred nanofareds. Larger caps won't hurt; just make sure their voltage rating is adequate - preferably 25V or more.