Continue to Site

# Measuring 140V DC on RA0 on a PIC Micro

Status
Not open for further replies.

#### gavo4444

##### New Member
Hi All, i am currently using a voltage divider to bring the voltage down from 140V DC to +5V on the pin of RA0 in a pic micro. My resistors are getting very hot (100W resistors). Is there perhaps another way of measuring 140V DC on a the pin of the micro?

What are the resistor values?

Use a high resistance voltage divider then use an opamp follower to buffer it to an impedance the PIC A2D likes (around 2k load on opamp output).

Hi All, i am currently using a voltage divider to bring the voltage down from 140V DC to +5V on the pin of RA0 in a pic micro. My resistors are getting very hot (100W resistors). Is there perhaps another way of measuring 140V DC on a the pin of the micro?

Use a high resistance voltage divider then use an opamp follower to buffer it to an impedance the PIC A2D likes (around 2k load on opamp output).

Like this:

OpAmp has to be capable of pulling its output pin close to 5V rail. You must use a "rail-to-rail" OpAmp.

#### Attachments

• Divider.gif
44.3 KB · Views: 197
Last edited:
Hi Mike

47K R1 and 1K for R2

Hi Mike

Many Thanks what would be the divider resistor wattages?

The only one that counts is the 100K. Look at the blue trace on my plot

i will then put both a 5W then it should run cool

do you know of any voltage regulator to operate at that voltage as well down to around +12V?

Hi Mike

Many Thanks what would be the divider resistor wattages?

i will then put both a 5W then it should run cool

I would use 1/4W or 1/2W metal-film 1% resistors.

do you know of any voltage regulator to operate at that voltage as well down to around +12V?

Do you want to convert 140VDC to 12VDC?

Make R2, 3k3, a standard value.

Use R1 180k and R2 1.2 k. Add 220k series resistor at A/D input.

Use R1 180k and R2 1.2 k. Add 220k series resistor at A/D input.

This totally goes against the datasheet for minimum input impedance for the input.

For example, on a 18F1320 ........

Microchip Datasheet said:
The
maximum recommended impedance for analog
sources is 2.5 kΩ.

My resistors are getting very hot (100W resistors)
47K R1 and 1K for R2
If those really are 100W rated resistors (wow!) then you have something wired incorrectly.

Please all check the thread dates - this one is four years old!! - Moderator.

Have I really been on here that long ? Actually I've been on since 2007

Status
Not open for further replies.

Replies
9
Views
3K
Replies
34
Views
934
Replies
2
Views
436
Replies
146
Views
11K
Replies
5
Views
815