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I have voltage source of 7v 250mA. Question is, when I apply it to regulator do I get 5v and 1.5A on the output of regulator?Or do I need 1.5A for a linear regulator ? How much current would I need to get 1.5A out when using a buck regulator?

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  • \$\begingroup\$ What regulator are you using? if you are using a linear regulator, (like a 7805) the maximum output current will be the same as the input current i.e. 250mA, if you are using a DC-DC converter the output current will depend on the efficiency of the converter. \$\endgroup\$
    – Icy
    Feb 1, 2016 at 11:19
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    \$\begingroup\$ @autistic - that change doesn't make sense and puts previous answers in jeopardy. Be very careful about making changes like this. Also, what does "Or do I need 1.5A for a linear regulator ?" actually mean - it doesn't make sense to me. \$\endgroup\$
    – Andy aka
    Feb 1, 2016 at 11:29
  • \$\begingroup\$ Andy I didnt see the answers .Perhaps early onset dementia .I could not believe that someone with a reasonable reputation would believe that regulators were more than 100% efficient .I thought that editing the question would be kinder than letting it get pelted with downvotes which are very easy to get here compared to other sites . \$\endgroup\$
    – Autistic
    Feb 1, 2016 at 11:50
  • \$\begingroup\$ @Autistic well it doesn't look like it received any down votes prior to the change and the additions you made still don't make sense. \$\endgroup\$
    – Andy aka
    Feb 1, 2016 at 12:50

2 Answers 2

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Power out can never be greater than power in. You are expecting an output of 5 V at 1.5 A i.e. 7.5 watts. Power in is 7 V x 0.25 A = 1.75 watts.

Conclusion is you can't get what you want from what you are supplying.

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  • \$\begingroup\$ How much current I can get? \$\endgroup\$ Feb 1, 2016 at 11:09
  • \$\begingroup\$ @VladimirDjukic What regulator are you considering? \$\endgroup\$
    – Andy aka
    Feb 1, 2016 at 11:20
  • \$\begingroup\$ Could you do for this regulartor: ti.com/lit/ds/symlink/lm2940c.pdf it is just 1A instead of 1.5A \$\endgroup\$ Feb 1, 2016 at 11:38
  • \$\begingroup\$ @VladimirDjukic that regulator will "steal" 10mA down the GND pin and therefore the maximum current you can output is input current minus 10 mA = 240 mA. \$\endgroup\$
    – Andy aka
    Feb 1, 2016 at 11:42
  • \$\begingroup\$ I will need to check this when I come home, thanks! \$\endgroup\$ Feb 1, 2016 at 12:53
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Considering Vin=7V and Iin=250 mA and if you are using switching regulator, in that case Pin = 1.75 W ( ~ Vin* Iin), So for Vo=5V and considering 100 % efficiency ( which is not possible practically ), you can get 0.35A at output.

Note :

n ( efficiency )= Po/Pin

If 95% efficiency, Po=1.66W and Io=0.33A.

So depending upon efficiency of your regulator, you will get output current capability of your regulator.

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  • \$\begingroup\$ You won't get more current at the output than the current at the input for any linear voltage regulators. \$\endgroup\$
    – Icy
    Feb 1, 2016 at 10:54
  • \$\begingroup\$ @Icy True, but linear regulators were not specified by OP or referred to by Electroholic \$\endgroup\$
    – jms
    Feb 1, 2016 at 11:07
  • \$\begingroup\$ Could you show me calculations, how you get 0.33A? \$\endgroup\$ Feb 1, 2016 at 11:09
  • \$\begingroup\$ He has: Pin = Vin * Iin; Pout = Pin * efficiency; IOut = POut / VOut. \$\endgroup\$
    – Icy
    Feb 1, 2016 at 11:12
  • \$\begingroup\$ I will need to check this when I come home, thanks! \$\endgroup\$ Feb 1, 2016 at 12:53

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