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Topic: [SOLVED] L7805 regulator problem (Read 11303 times) previous topic - next topic

cjdelphi



a  800ohm-1k resistor from the 7805 to the base of the TIP31, and hook it up like like that, then feed the relay and whatever you need to that to take a load off the 7805...

polymorph

I still don't see a mention of a heat sink on the 7805. It'll be moot if you use a SMPS regulator, but you should know that a TO-220 package cannot dissipate much power without a heat sink. With everything powered from the 7805, and 12V feeding it, you have almost 1.5 times as much wattage burning up the 7805 as the entire circuit is dissipating.
Steve Greenfield AE7HD
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cjdelphi


What kind of heat sink do you have on your 7805?  What package style is it?

Agree and repeat, driving the relay directly from the ATTiny is going to result in a dead IO pin.



Um, I think there *WAS* mention of a heatsink....

polymorph

The only mention of a heatsink was a suggestion to use one, and someone (before me) asking if he was using one.
Steve Greenfield AE7HD
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alnath


Ok so now I'm trying to drive the relay with a logic level fet but it won't switch on. I'm using a PSMN022 (http://www.nxp.com/documents/data_sheet/PSMN022-30PL.pdf) because its the only logic level laying around my house. I have it hooked up with the gate connected to the attiny pin, the drain going to the relay, and the source connected to ground. That's the right way to hook it up correct? Is this just the wrong type of fet?


Should work .
If you have a multimeter, check the voltage on the gate (I hope the tiny isn't damaged)
I would put a 120 Ohm resistor between the output pin and the gate, and a 15k or 22k between the gate and GND .
And again, a heatsink on the 7805

mwhite494

Okay so I went and bought a standard heat sink designed for TO-220 packages for the 7805. Now it doesn't get nearly as hot and I can put my finger on it for as long as I like. So I've sourced the problem to the way I was driving the relay, which is what alnath pointed out earlier. I've confirmed this because if I manually change the wires of the circuit instead of using the relay, circuit works just fine. Now I'm trying to use a logic level mosfet to switch on the relay. Here's the updated schematic:



Note a few changes, I'm now using a 12V relay instead of a 5V relay, so the positive end of the coil is directly connected to 12V, and coil resistance is now 320 ohms instead of 55.
So when the tiny outputs HIGH on pin 0, it should send a 5V to the gate of the mosfet connecting the coil to ground. But no matter what I have in the gate the source and drain are always connected. I'm probably using the mosfet in the wrong way, can someone see whats wrong?

mwhite494

Mistake on the diagram the part # for the relay is actually QUAZ-SS-112D not QUAZ-SS-105D

cmiyc

Quote
But no matter what I have in the gate the source and drain are always connected.

That might imply that the MOSFET is blown.  When the gate is connected to ground, DS should be a relatively high (open) resistance.  Don't leave the gate floating and make measurements on DS.

Have you verified the MOSFET isn't blown?

Also keep in mind that you were driving the IO pin beyond its absolute limits.  It is also possible that the IO pin is damaged already.  Maybe use a different pin.
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mwhite494

Yea I think the mosfet may be blown, can I just use a tip120 transistor in place of it? The IO pin still outputs 5v when on so I think it might be alright.

mwhite494

I also think I damaged the tiny, if I try to use the function digitalWrite() on any of the pins the circuit stops working. So for now I'm just going to install a toggle switch instead of using a relay, and eventually I'll just replace the tiny. Thanks for the input everyone, it helped a lot.

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