powering a pic

[mactack]

Hi,

I figure this may be a very dumb question since I can't find an answer by myself but here it goes.
I'd like to power a pic16f690 with a 12V, 300ma wall wart and a 78l05. Datasheets for 78l05 regulators say the max current output is 100ma but reading with a multimeter gives me over 200ma (can't find out exact value since I can only set it to 200ma or 10a). Now, the pic datasheet says max input current for vdd is 250ma.

Now, should I just put a resistor after the regulator, find a another wall wart or just not worry about it?


Thank you for your patience

 

[Oznog]

There is no way your PIC16F690 is itself consuming 200mA. The spec sheet will say what it consumes at that frequency, it's gonna be like 10 or 20 mA. The 250mA limit on Vdd is a maximum load that you could create without damaging the pin. Like a hypothetical worst-case where you had the max load for each pin and each port exactly maxed out while the core is operating at max freq and with all the peripherals operating. That might exceed 250mA but regardless, the PIC itself is only consuming a few tens of mA and external loads are taking the rest through PIC output pins.

Your meter reading is wrong, you've got something shorting out the reg output, or the reg is connected wrong. If you mix up Vdd and Vss the PIC will probably draw large currents and was destroyed in the first moment you turned on power.

Don't be discouraged if you did though, anybody here worth their salt has already blown a PIC at some point.

 

[Matt(Pic progger)]

Just another little nugget for you, don't try to draw 100ma from a 100ma 78L05 (TO92 package), it will heat right up, and probably nuke it'self.

This spec is a bit like the P(eak)M(usic)P(ower)O(utput) you get on cheap Hi-Fi's where 5W (RMS) suddenly becomes 30W PMPO.

I suppose if you heat-sink the 78l05 then you may get away with it, but I wouldn't recomend it. I always try to not run these regs above 50% of the rated current, even then they get pretty warm!

And yes, I have killed a few PIC's by getting these little buggers back-to-front Note the larger 78L05 (500MA/1A version) has pin-out's the other way around to the TO92 package (100MA) version.

Hope this helps

 

[mactack]

Does this mean I could give 300ma to the pic and it'll draw what it needs?

And thanks for the tip on the regulator, I guess it's connected wrong because it does not heat up at all.


Thanks

 

[Nigel Goodwin]

Does this mean I could give 300ma to the pic and it'll draw what it needs?

You don't 'give' it anything, it takes what it needs and no more (as does almost any circuit) - the power supply just needs to be capable of providing the maximum power required, or anything above that.

Imagine yourself as the circuit, and the battery as a BIG bottle of whisky - the entire bottle might get you falling down drunk (or even dead!), but you don't have to drink it all!.

 

[Matt(Pic progger)]

Imagine yourself as the circuit, and the battery as a BIG bottle of whisky - the entire bottle might get you falling down drunk (or even dead!), but you don't have to drink it all!.

lmao, nice analagy, now where is that Vodka (don't like whisky).

On a more serious point, if your regulator was not getting hot, then it's probably a short circuit before the regulator, unplug the PIC and see if you are still drawing that much current. The PIC will only take 2-3MA (with no LED's etc on) and the reg about the same, you shouldn't see more than about 10MA being used by a 78l05 and a PIC (assuming nothing else is running from the circuit).

So your adaptor of 300MA, as you sujested, will give (a reserve) of power and the PIC should only take a small amount of this.

Take heed from Nigel though, and see where messing with PIC's will get you!

Hope this helps..... and "Cheers" Nigel!

 

[mactack]

Thanks guys!

 

[lord loh.]

If you have a 7805L or 78L05 or L7805, they have a 100mA limitation... you could go for the regular LM7805 with no L prefix or suffix (or infix ) and it could deliver as much as 1A. And do use a heat sink.

The current drawn depends on the resistance. Your supply needs to be a constant voltage supply and must have the capability to maintain the voltage across a range of current... As Nigel said, the voltage is give....and the current is taken depending on what the device needs...

 

[Oznog]

Oh wait do I understand that you shorted the + and - output of the reg with a current meter?
Not supposed to do that. This doesn't measure anything useful and can damage the power source and/or your meter. A current meter is used to measure current going from a source to a load, the meter is not supposed to be the load!

 

[amindzo]

hi,
you won't have any problem. i powered my pic with the regulated power sully that can get 1A and i had no problem.

 

[mactack]

Oh wait do I understand that you shorted the + and - output of the reg with a current meter?
Not supposed to do that. This doesn't measure anything useful and can damage the power source and/or your meter. A current meter is used to measure current going from a source to a load, the meter is not supposed to be the load!

yeah... it's what I did, not really knowing what I was doing I didn't want to connect the pic before checking tension and current. What would be signs of damaging the meter? complete/absolute breakdown? de-calibration? both? worse?

Thanks

 

[philba]

hope your meter is ok. you place the current meter in series with the load. you were measuring the current draw of your meter. I would bet no real damage was done if you didn't leave it on for long.

tension - you mean voltage? you can measure that with the dmm in DC mode on the output terminals. it should read close to 5V.

you can test the Current Meter by using a resistor as a load. hook the current meter + to +V of the regulator, metter - to the resistor and the other resistor lead to the regulator ground. If you use a 1K resistor you should see around 5 mA on the meter. use ohms law (I = V/R) if you have a different value resistor.

if all is good, connect the current meter + to the +V output of the regulator and the - of the meter to the +V input of your circuit. connect regulator gound to circuit ground. power it up and if it reads more than 50-70mA, pull the plug fast.

 

[kchriste]

Your meter's probably Ok because the 78L05 should have limited the current to around 250ma. See page 7 of data sheet:
https://www.electro-tech-online.com/custompdfs/2006/07/LM78L05.pdf
And look at the chart called "peak output current"