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 Author Topic: 7805 IC Regulator outputting 9v.  (Read 1085 times) 0 Members and 1 Guest are viewing this topic.
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Edison Member
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 « on: November 08, 2012, 10:50:07 pm » Bigger Smaller Reset

ok so what's going on here?....    whatever the Ground Voltage is determines output voltage?..

0v = 5v out
4v in =  9v out?

Is this simply differential? ....

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Tesla Member
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 « Reply #1 on: November 08, 2012, 11:28:11 pm » Bigger Smaller Reset

Yes, the regulator give you 5V above the reference pin voltage.  If you ground the reference pin the output is 5V.  Since the 4.3V Zener is acting as a 4.3V regulator you should get 9.3V on the output.
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 « Reply #2 on: November 08, 2012, 11:58:18 pm » Bigger Smaller Reset

He is feeding some back with R1 so he could be getting 9 volts
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the land of sun+snow
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 « Reply #3 on: November 09, 2012, 12:46:48 pm » Bigger Smaller Reset

R1 provides bias current for the zener, so it gives a stable voltage = 4.3V. There is probably not enough current
flowing out of the v.reg reference pin to properly bias the zener into its stable-voltage region without R1.

Quote
0v = 5v out
4v in =  9v out?

This makes no sense. You can't get 5V out for 0v in. To turn on the ckt properly, you need Vin at least 2V
higher than Vout. [D1 does nothing during normal operation, so don't confuse with that].

If OP reading something other than approx 9.3V [+/5%] out, then he has it wired wrong.
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 « Reply #4 on: November 09, 2012, 09:29:56 pm » Bigger Smaller Reset

i'm not reading any voltage since i've not built one!

i want to know how this works, not telling you what it's doing!
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 « Reply #5 on: November 09, 2012, 09:33:13 pm » Bigger Smaller Reset

All those regulators do is to hold its output at a given voltage above its adj / gnd pin. For 7805, the output pin is 5v above adj / gnd.

So if you raise the potential on the adj/gnd pin, you will raise the output voltage.

Assuming of course there is sufficient input voltage.

This is a popular way to expand the output voltage of those puny regulators.
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the land of sun+snow
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 « Reply #6 on: November 09, 2012, 10:51:46 pm » Bigger Smaller Reset

Quote
i'm not reading any voltage since i've not built one!

i want to know how this works, not telling you what it's doing!

Ah, now it finally dawned on me what you meant by simply differential. As others indicated, the result isn't
differential, the zener voltage and the v.reg voltage superimpose, ie they are both in the same direction
being in the positive direction w.r.t. ground potential. It's 4.3V + 5V, not 5V - 4.3V.
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 « Reply #7 on: November 10, 2012, 03:21:42 am » Bigger Smaller Reset

got it - thanks
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