7805 regulator question

[Ken]

There was not enough explanation on the spec sheet for me to
understand its full workings: How high an input voltage do I need to
ensure 5 volts out? E.g., will it output 5 volts if the input voltage
is 5.5 volts? What volage figure tells me the minimum input voltage?

Also, if the DC in is clean, will the DC out be clean?

Ken
(to reply via email
remove "zz" from address)

 

[Robert Monsen]

Ken wrote:

There was not enough explanation on the spec sheet for me to
understand its full workings: How high an input voltage do I need to
ensure 5 volts out? E.g., will it output 5 volts if the input voltage
is 5.5 volts? What volage figure tells me the minimum input voltage?

For a 7805, the datasheet recommends 7 to 20V.

Also, if the DC in is clean, will the DC out be clean?

The point of the regulator is to 'clean up' the output. However, if the
input dips below 7V, the output will reflect that.

One tip in the datasheet is to use a capacitor on both the input and the
output. A 0.33uF on input, and a 0.1uF on output is customary.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[John Fields]

On Sat, 18 Dec 2004 04:47:05 -0500, Ken <cprstn54zz@att.net> wrote:

There was not enough explanation on the spec sheet for me to
understand its full workings: How high an input voltage do I need to
ensure 5 volts out?

---
According to the data sheet at:

http://www.fairchildsemi.com/ds/KA%2FKA7805.pdf ,

7V
---

E.g., will it output 5 volts if the input voltage
is 5.5 volts?

---
Not likely.
---

What volage figure tells me the minimum input voltage?

---
Look for "Dropout Voltage" on the data sheet and add that to the
output voltage.
---

Also, if the DC in is clean, will the DC out be clean?

---
Yes, unless you get the regulator to oscillate. Take a look at Figure
5 or Figure 8 on the data sheet for recommended input and output caps.
---

--
John Fields