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I'm using a L7805C voltage regulator , to power an SMPS circuit. But its getting heated after 10 min or so , even if I've put a heat sync.

SMPS IC is used to convert +5V to -9V.

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    \$\begingroup\$ What current is being taken by the SMPS? What dc voltage is fed to the 7805 input? \$\endgroup\$ – Andy aka Apr 23 '15 at 10:12
  • \$\begingroup\$ Voltage input to the Voltage regulator - 12V (two wheeler battery). Voltage input to SMPS - 5.08V \$\endgroup\$ – Akkukkaje Apr 23 '15 at 10:14
  • \$\begingroup\$ Current to SMPS is required. Clearly the voltage will be 5V because it's a 5V regulator. \$\endgroup\$ – Andy aka Apr 23 '15 at 10:15
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    \$\begingroup\$ Why are you powering a switchmode with a linear regulator? \$\endgroup\$ – pjc50 Apr 23 '15 at 10:31
  • \$\begingroup\$ I used to power it the same way earlier, without any problems. \$\endgroup\$ – Akkukkaje Apr 23 '15 at 11:06
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A 7805 will always produce waste heat because it is a linear regulator. This means that current out = current in and, given that it might regulate 7V down to 5V, the power dissipated is 2V * current out. If the current out is 0.5 amps then the power dissipated is 1 watt.

If the voltage in is 12V and the current out is 0.5 amps then power wasted as heat is 3.5 watts and this will fry a 7805 without a heat-sink.

Work it out with the formula: -

Power dissipated by 7805 = \$(V_{IN} - 5V)\times I_{OUT} \$

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