faulty 7805? (and pot/adc questions)

[Dakta]

Got a few questions for a few problems I have, some of them may be quite silly but I'm getting nowhere fast

Essentially, i've written and tested some code for a basic PID controller, which works fine when tested on a matrix multimedia v2 development board, using the onboard lcd to give me visual feedback to make sure the values are what im expecting. Everything was good, 4k7 pots gave me the right voltage sweep and since the sensor I will be using for feedback gives a voltage range of 0-5v I wanted to use a pot which could output the same range, so the setpoint and feedback voltages would share pretty much exactly the same voltages for the same values (i.e the voltage output of the variable resistor would be the voltage the controller would try to get as feedback). Plumbing in a 4k7 resistor as mentioned earlier worked rather well, lcd gave me 255 at full rotation and zero when turned down.

Anyway, first problem with developing a stripboard version of the circuit - the pic's are giving out a lot heat and burning out (they wont program afterwards). I checked for shorts, couldn't find any (yet), buttt.....the 7805 voltage regulator seems to be giving out 7 volts according to my multimeter...something that shouts that somethings not right. The input is pretty much dead on 12v. Is this toast?

And secondly, my pot isn't giving the right voltage sweep either, gives between 5-7v depending on rotation. I assume this could be related to the 7805 problem, or, not to disrespect myself but I could just be plain ignorant regarding what values I should be using. The pot is linear, before you ask! (and i've used the same pot succesfully on my development board, using a screw terminal).

Maybe it needs an inline resistor?

anyway, sorry about the rather silly questions, just wondered if there's any suggestions?

cheers

 

[colin55]

Start by using a battery of the correct voltage.

 

[Dakta]

I'l give it a go. By comparison the 7805 on my experiment board gives out almost 5v dead on, so I certainly think the 7805 is now suspect.

 

[upand_at_them]

It's possible to wire a 7805 to give you 7V. That's because what it actually does is produce a 5V difference from pin 3. If pin 3 isn't at ground then you're not getting 5V. Got a circuit diagram of what you're doing? There shouldn't be any resistors on the 7805.

 

[Nigel Goodwin]

Sounds like he's not fitted the capacitors on the 7805?, so it's oscillating and providing the wrong voltage.

It takes a LOT to kill a PIC though, has he perhaps fitted the PIC the wrong way round?, it's easily done, and the PIC gets red hot (as the protection diodes shport out the supply).

 

[Dakta]

Nigel had it spot on - I'm ashamed to say I messed up with a capacitor. (sorry!) It was there on my drawing but was it there on the board? nawwww.... amateur mistake...christ... (i've been staying up till the early hours doing these projects so thats my excuse)

The pic's do seem to be dead (fitted the right way). All the ones I tried now go red hot whenever you try and do anything with them (even program them in a seperate willem programmer). A fresh pic doesn't get hot though the only one I had available doesn't have a CCP module so I need to buy a stack of new pics to get my project running, (or code my way around it thanks to Nigels tips from the past).

There is one last question now the project seems to be running, and it's a simple question really.

If I want a pot fed 5v to give out a range of 0-5v, what value would I want to look for, or does it depend?

I've plenty of 4k7's pots I used on my experiment board but i've a feeling they've got an additional resistor on there in line with it. I appreciate this is probably the king of all newbie questions, but there has to be an answer somewhere (or I might just get a 1k resistor and try it).

Cheers btw, somethings finally gone right today!

 

[Electronworks]

LOL! I've been caught by this - even after 20 years in the industry. Forgot to fit the input cap to a 7805 and it gave out 7V and killed my PICs too.

Happens to the best of us

 

[Dakta]

aye, bit of a bummer isn't it when it happens.

Ah well, do you know of anywhere where I can look into the sizing up of the right pots? (input, is 5v i'd like a range of 0-5v or as close as possible).

At the moment i'm getting 4-5v depending on pot position, so I'm guessing i've got the values waaaay out.

 

[Dakta]

Nevermind...seems ive blown the pot too LOL

 

[Electronworks]

You've blown the pot too?? Wow, that does take some doing. You can use any pot you like to give you a 0-5V output. Connect one end to 0V, one end to 5V and the wiper will give you 0-5V as you turn it.

The only 'variable' you need to think of is the current the pot takes. The higher the resistance value, the less current it will take. This will have no effect on the wiper voltage though, so unless you are designing a low power circuit, it does not matter what value you use. 4k7 is ideal

Dont connect any series resistors.

On the subject of your blown PICs, try just connecting the Vcc and GND pins then see if they get hot. it could be something else in your circuit that is causing them to get hot. If they still get hot with just Vcc and GND, they are probably blown :-(

 

[Dakta]

Yeah, I think I must have had a short which blew both the pic and pot. Not sure if it might have had anything to do with the missing capacitor issue I initially had though.

It does actually look like i'm back on track. Is there anything I can use, like a sealant or something over a finished stripboard circuit to insulate it from foreign objects because this will be spending some time in the passenger seat?

does look like i'm more or less sorted though...thanks to you all