Hi All,

I need to design a power supply for my pic circuit and i need to know how much current my total circuit uses as i am going to use a DC Plugpack to power the circuit. I need to measure the total current required so i can get the required current rating on the DC plugpack.
I'm not sure where to measure in the circuit to give me the total current. Would i have to measure each of the inputs and outputs on the PIC and then add all the current readings up in series?

The schematic is a xps document so i cant attach it at the moment. Its got some leds, an lcd screen and a couple of piezo buzzers on the outputs.
Thanks

 

without measurments, Your DC plugpack with 250mA will do well if you are doing simple pic circuits, ( switches, LED and even LCD ), and if your loads are higher than that, you can measure the total current by connecting your Ammeter in series on the adapter output, but in this case choose a higher mA output for measurements purposes, then chose the proper for the normal operation.

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Its what your friend in Your mind, what you in your friends mind

 

Get your digital multimeter and set it to measure current, then place it in series with your device. This will show you the current draw while it is operating. However, as aljamri said you should be well within 250mA with what you have described.

 

i forgot that i also have a couple of inputs, a PIR sensor and a keypad. Using the multimeter to measure the current where abouts on the PIC (using 16F870) would i take my measurement to get the total current.

Thanks

 

Well, you connect in series with your power supply ( what ever you are using at the moment ) and this will show you the total.

 

Just for safety though, always start from the highest range and work down with current, or else you will find yourself blowing your fuses on the mA scale rather quickly, as most multimeter's max out at 300-500mA on mA scale

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Spency.

PIC Micro's - Your mind is the limit

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hi,
As the cost of a reasonably specified wall plug mains to dc adaptor is so low,
I would recommend you buy one that will satisfy ALL/most of your future projects.

If you are using logic ic's and PIC's, get a regulated +5Volt capable of supplying at least 2Amp.
Its only about 10VA, its a small adaptor.

If you are using logic ic's and PIC's, and you also wish to power say 12Vdc devices, get a regulated +12Volt capable of supplying at least 2Amp and follow this with a 7805, +5V 1A regulator.

Its only about 10VA or 24VA, its a small adaptor and they cost less than £5GBP.

I would advise that you also fit a 2A fuse 'inline' with +Vdc output in order to avoid a similar psu burnup,
as recently experienced by a forum member.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Measured the current today and the maximum current reading i got was only 45mA a lot less than expected.
I want to use the backlight on my LCD and its a 16 pin LCD. i've used 14 of the pins and i know the last 2 control the backlight. How do i wire it up to use the backlight?

Thanks

 

hi,
IF the backlight is an LED type, the backlight LED's require a series resistor.
The datasheet should give you some details of the LED arrangement.

On LCD's I use, the series resistor is quite a low value, in the order of 10hm:

I would suggest that you start with say 100R and work your way down in resistance value until the backlight is bright enough.
Use a meter to check the current as you determine the resistor value, the current could be in the order of 50 to 150mA.

Be sure to observe the polarity of the LED connections,, A and K.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

do not neglect capacitors recomended for in/out puts of voltage regulators. I once had problems to get PIC work properly, some experts in this forum told me that those cheap Wall Adapters DC output is not pure and PIC may refuse to eat.

__________________
Its what your friend in Your mind, what you in your friends mind

 

Thanks

I bought a 9v DC plug pack and i used the 5 volt regulator with the two capacitors and its working fine at moment outputting the 5v. you have reminded me that i'm going to need to test my circuit with my designed power supply.