Scan 100 to 500v at input of PIC microcontroller

[innovetion]

Hi,

I am using PIC18f4550 micocontroller. Right now I am using inbuilt ADC to check input voltage i.e. 0 to 5v.
But now as per my application I need to scan 100V to 500V DC input voltage at ADC. How can I do ?
Please suggest me.

Thanks in advance.

 

[Ian Rogers]

Potential divider!!!

 

[ronsimpson]

I am doing the math in my head so watch out!

1)I want to loose 500mw (1/2 watt) doing this.
2)Connect a resistor from ADC pin to the 500 volts. AND Connect a resistor from ADC pin to ground.
2a)One resister will have 500 votlts (495) across it. AND The other resistor will have 5V across it. The ADC pin will have 5V on it.
2b) 500V .5W 1mA so use 500k resistor AND 5V 1mA so use 5k resistor.
2c) Most small resistors are good for 200 volts or 250 volts so use two 250k resistors. (maybe 3 resistors)
3) OK 500k and 5k or 470k and 4.7k or 510k/5.1k. Use 1% if you can but 5% will work. (resistors)

 

[innovetion]

Thanks for your reply.

Using with resistors is a bit risky when I am using around 500V. Is there any other way ? or direct IC available ?

 

[Ian Rogers]

Why is it risky!!! Is there a possibility the voltage will be higher???

If you calculate the divider so you see 5v at 750v, you will never damage the pic..

 

[ronsimpson]

you will never damage the pic

Look at the data sheet. There should be input protection diodes on the PIC. I think the diodes can handle 10mA. If the resistor voltage divider draws 1mA or 0.5mA then you are very save. If the input voltage gets 10% too high for a time, the voltage on ADC input will be 5.5V but the excess current will be shunted off to the supply. So it is safe. You will not be able to measure any voltage above supply accuracy.
5V=FFh, 5.1V=FFh, 5.2V=FFh

 

[dr pepper]

Standard resistors have a lowish voltage rating, so you might need to put a few in series to be safe.
Further protection like a fusible resistor and an mov might als be a good idea.

 

[Mikebits]

Good point Dr Pepper. He can just go with higher wattage parts. Vrated = √(PR).