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I'm making ECG device and for that I need a dual power supply.

I decided to use 3 x 9V batteries and below there is my scheme:

enter image description here

unfortunately I encountered a problem. I created that power supply using that scheme but after connecting to the output small computer fan, the RPM (and voltage) gradually decreased and finally stopped rotating. construction was hot but after a couple of sec it started to work again but with the same result. where is the problem ? virtual ground ? maybe I should make scheme with 4x 9V batteries ? like this :

enter image description here

greetings

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    \$\begingroup\$ "I decided to use [...] 9V batteries" Well there's your problem. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 14 '15 at 0:23
  • \$\begingroup\$ yeah but I had a reason for that. This is a medical device and I want it to be safe (completely cut from external power supply) and mobility \$\endgroup\$ – Melus Mar 14 '15 at 0:29
  • \$\begingroup\$ So then use a LiFePO4 pack. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 14 '15 at 1:02
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    \$\begingroup\$ You should add more information. Specifically: what was hot? The regulators? Where did you connect the fan? How long is "gradual"? Over 10 seconds? 20 seconds? 1 second? When it's dead, what's the voltage/temperature at the regulators and batteries? How much current is the fan drawing? \$\endgroup\$ – iAdjunct Mar 14 '15 at 1:14
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Did you happen to use the "heavy duty" 9 volt battery? They have an amazing capacity of only around 50 mAh! So if you put any real load on them, their voltage will drop quickly and they will likely heat up as well due to high internal resistance which is tripled when put 3 in series. Take away the load and the voltage will go back up some.

The same will happen to a lesser extent with alkaline 9 volt which have a capacity of around 300-400 mAh.

You would be better off with lithium batteries, but if you want to keep it simple, consider AA, or C, or D alkaline in series.

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In the past you would have been right to use 9V batteries.

These days it would be better to use a high capacity D-cell pack of say 4 cells and then a low ripple switching boost converter and a low ripple supply inverter. There are ICs that do this and have voltage regulation built into them. It will require extra components and design but will reduce the cost of batteries and offer the option of a single rechargeable battery if desired, quite possibly less heat dissipation in the PSU circuits and a cheaper single pole on.off switch.

The links on the page below will send you on one path of discovery.

http://www.linear.com/products/switching_regulator

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