{"id":187,"date":"2017-08-05T00:45:28","date_gmt":"2017-08-04T23:45:28","guid":{"rendered":"http:\/\/www.burningimage.net\/cattrack\/?p=187"},"modified":"2023-07-22T07:56:58","modified_gmt":"2023-07-22T06:56:58","slug":"gps-cat-tracker-power-source","status":"publish","type":"post","link":"https:\/\/www.burningimage.net\/cattrack\/gps-cat-tracker-power-source\/","title":{"rendered":"Choosing a Power Source"},"content":{"rendered":"

The last thing I need to work out before getting on with the schematic is which battery chemistry I’m going to use. \u00a0The most important thing at this point isn’t necessarily identifying the exact battery I want to use, I need to just know the voltage, such that I can design the circuit to suit.<\/strong><\/p>\n

In order to work out the ideal battery chemistry, I need to work out how much each component draws, both in sleep mode and at its peak.<\/p>\n

<\/p>\n\n\n\n\n\t\n\n\t\n\t\n\t\n\t
Component<\/th>Current Draw in Sleep<\/th>Peak Current Draw<\/th>\n<\/tr>\n<\/thead>\n
PIC 16LF18345<\/td>13 \u00b5A<\/td>1.8 mA<\/td>\n<\/tr>\n
Nano Hornet ORG1411 GPS Module<\/td>18 \u00b5A<\/td>47 mA<\/td>\n<\/tr>\n
CC1125 RF Transceiver<\/td>1 \u00b5A<\/td>47 mA<\/td>\n<\/tr>\n
25LC1024 EEPROM <\/td>1 \u00b5A<\/td>5 mA<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n

This tells me that the total current consumption whilst sleeping will be around 33\u00a0\u03bcA, with a maximum possible peak current of around 100 mA. \u00a0This peak power consumption assumes that we are transmitting at the same time as attempting to acquire a GPS fix.<\/p>\n

All of the components I’ve chosen thus far run from 3.3 V. \u00a0They’ll also run at 3.0 V if needs be, so if I can find a battery with a native voltage of 3.0 V then I won’t need a regulator.<\/p>\n

On paper, a lithium ion battery like the CR2032 looks ideal. \u00a0It’s native voltage is 3.0 V, so no regulator needed, and the capacity is fairly good at around 225 mAh. \u00a0In sleep mode, this will in theory give us over 6800 hours use – well over 9 months! \u00a0However… the maximum current draw from a CR2032 is around 15 mA. \u00a0This is much lower than the 100 mA I may need to supply, so that knocks that idea on the head!<\/p>\n

\"\"<\/p>\n

Wikipedia has a really good page<\/a> with a list of loads of battery chemistries, along with voltage and typical capacities. \u00a0To cut a long story short, I sat on this site for several hours, but didn’t find anything that I liked or seemed to fit my requirements perfectly.<\/p>\n

In the end I’ve settled on using a lithium-ion battery. \u00a0This is the chemistry that pretty much all rechargeable electronics use these days. \u00a0The high energy-density should be perfect for my needs. \u00a0Another bonus with lithium-ion batteries is that they come in hundreds of different shapes and sizes, so I should be able to find one that fits in whichever case I end up using.<\/p>\n

Lithium-ion battery downsides<\/h4>\n

\"\"<\/p>\n

This is the lithium-ion battery from a Boeing 787 Dreamliner. \u00a0I don’t want Button’s collar to end up like this!<\/p>\n

Generally speaking, lithium-ion batteries catch fire whilst charging<\/em>, rather than discharging. \u00a0I don’t plan on adding any charging circuitry to CatTrack – it will be down to the user to switch the battery over every few months. \u00a0I’m also going to ensure that the PCB is in a sturdy plastic enclosure such that the battery can’t be chewed\/scratched or get caught on anything.<\/p>\n

Voltage Regulators<\/h4>\n

Because a lithium-ion battery voltage varies from around 3.5 V to 4.2 V, I’ll need a voltage regulator to regulate the voltage down to 3.3 V. \u00a0There are two basic types of voltage regulator: Linear and Switching. \u00a0Linear regulators are by far the simplest to implement, but the drawback to them is that they are inefficient and can only step voltages down. \u00a0When using a linear regulator, the current that the regulator draws from the input supply is the same as the current that your circuit draws from the stepped-down voltage. \u00a0This means that the larger the difference in input to output voltage, the more inefficient the regulator is, and therefore the hotter it gets.<\/p>\n

For example, if you used a linear regulator to power your 5V circuit from a 12V supply and your 5V circuit pulled 100 mA, the regulator will also pull 100 mA from the 12 V supply.<\/p>\n

Switching regulators are far more efficient, they don’t get increasingly inefficient as the input voltage increases like Linear regulators do. \u00a0The downside is that they are also far more complex and generate a lot of undesirable electrical noise. \u00a0Stepping up a voltage is possible with a switching regulator, which is a huge advantage. \u00a0For example you could power your 5 V circuit from a 1.2 V battery using a switching regulator. \u00a0As is always the way with physics, you don’t get anything for free, so the current drawn from the 1.2 V battery will be around 4.5 times higher than the 5 V circuit draws.<\/p>\n

Because my input voltage (3.5 V to 4.2 V) is very close to my desired output voltage (3.3 V), there is no point in using a switching regulator. \u00a0The tiny efficiency gain at these voltages just isn’t worth the extra circuitry and hassle.<\/p>\n

The\u00a0dropout<\/em> voltage is a key parameter in a voltage regulator. \u00a0This is the minimum difference between the input and output voltage for the regulator to operate correctly. \u00a0I need the dropout voltage to be 0.2 V maximum (3.5 V minus 3.3 V).<\/p>\n

The Microchip MCP1700<\/a> fits my needs perfectly, with a very low typical dropout voltage of 178 mV!<\/p>\n

\"\"<\/p>\n

Power source and regulator sorted!<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

The last thing I need to work out before getting on with the schematic is which battery chemistry I’m going to use. \u00a0The most important thing at this point isn’t necessarily identifying the exact battery I want to use, I need to just know the voltage, such that I can design the circuit to suit. […]<\/p>\n","protected":false},"author":1,"featured_media":197,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[58,57,63,62,17,59,61,64,66,38,65,60],"class_list":["post-187","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-posts","tag-battery","tag-cr2032","tag-current","tag-dropout-voltage","tag-gps","tag-li-ion","tag-linear","tag-lithium-ion","tag-mcp1700","tag-microchip","tag-switching","tag-voltage-regulator"],"_links":{"self":[{"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/posts\/187","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/comments?post=187"}],"version-history":[{"count":8,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/posts\/187\/revisions"}],"predecessor-version":[{"id":215,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/posts\/187\/revisions\/215"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/media\/197"}],"wp:attachment":[{"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/media?parent=187"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/categories?post=187"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/tags?post=187"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}