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Everything Else » Yikes, it gets worse, Blew up the Voltage Regulator

November 08, 2012
by Ralphxyz
Ralphxyz's Avatar

So because of the problem I am having with the two black bars I thought I'd try another breadboard.

I had tried a different breadboard and everything worked but I went back to one of my original Nerdkits breadboards.

I just had to add the LED header (which I suspect might be causing problems).

When I powered the breadboard nothing happened the LCD backlight did not even light so I got out my multi meter and started testing.

I was not getting 5v on the VCC rail.

I pulled and reinserted my power lines and when I powered up my power indicator LED started flashing which was strange?

I pulled the power lines again and tested the power input directly and got the expected steady 15 volts. I use a handheld PDA 15v 2 amp switching power supply which I have been using for the past two years.

The power supply tested fine.

I plugged the power back in and tried again, again the LCD backlight did not light and I noticed that the power LED was flashing.

I powered down and removed the mcu and the LCD header.

When I powered up again without the mcu or LCD the power LED flashed four times and then bang the Voltage Regulator blew!!

Luckily I had turned the breadboard to take pictures otherwise the voltage regulator would have blown right into my face.

I have shorted out the + and - and heated up (scorching hot) other voltage regulators but never had one blow like this, plus I am not detecting any shorts on the breadboard.

So any ideas?

Ralph

November 08, 2012
by Ralphxyz
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I found my problem,it was Me!!

I had "quickly" thrown together a header for my power cable:

I reversed the + and - so I had my connections reversed!!

Well that is a good lesson in case anyone wondered what would happen if one reversed their power supply connections, the voltage regulator blows!!

Ralph

November 08, 2012
by BobaMosfet
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Ralphxyz--

May I suggest using a diode in series with your voltage regulator-- this will save you in this exact circumstance. A few extra components could include an LED, that would light if the polarity is wrong.

BM

November 08, 2012
by Ralphxyz
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Hey BM that is a good idea. Would you sketch a diagram. I think I can picture it but a napkin sketch would help.

Ralph

November 09, 2012
by BobaMosfet
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Ralphxyz-

1 diode 1 resistor (1K6 Ohm since your PDA is 15V) 2 LEDs, (RED and GREEN)

I've already helped you by telling you what value the resistor should be. The rest is left as an exercise, and is well within your capabilities :P

BM

November 11, 2012
by Ralphxyz
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and is well within your capabilities :P

Ha, you would not believe how limited I am plus finding/taking the time!!

Ok here is my napkin sketch of a circuit:

Now what diode should I use? The supply is 15 volts 2 amps, and what about the voltage drop do I have to worry about that?

Ralph

November 12, 2012
by BobaMosfet
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Ralphxyz--

You can use a ECG116 Diode (Plastic; .8V FV @ 1A; 600 PRV), or a 1N4004 Diode (Plastic; 1.0FV @ 1A; 400PRV). Any 1N4004/5/6/7 will probably work as each stops a higher level of Peak Reverse Voltage. This still allows 1A of current through to the LM7805.

Your schematic can be simpler using fewer parts. Try this:

R = E/I

R = (15 - 2.2) / .008 R = 1.6K Ohm

We're assuming a nominal voltage drop across each LED of about 2.2V (it will be different, but this should be close enough. The difference in actual voltage will change the current through the LED marginally, but won't exceed 20mA.)

O 15V           O Gnd
|               |
|               |
|   R1    D1    |
+-\/\/\-+-|<----+
|       |       |
|       +--->|--+
V D3        D2  |
-               |
|               |
|               |
O               O

R1: 1.6K Ohm
D1: Red LED
D2: Green LED
D3: Polarity Protection Diode

Hope that helps,

BM

November 13, 2012
by Ralphxyz
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Hey thanks BM, now to get a lesson in "Polarity Protection".

I don't understand your location of D3 versus my location.

Oh wait a moment while I think this through a little more.

I think your location (the correct one) is because of the flow of electrons from - (negative) to + (positive) not + to -.

I was just looking at Ricks Nerdkit board and see that he has a diode for Polarity Protection it looks like he had a problem with USB power using a 1N4001 diode, I should not have problem with 15 volts at 2 amps.

Ralph

November 13, 2012
by BobaMosfet
BobaMosfet's Avatar

Ralphxyz-

No problem. Since USB is +5VDC, a 1N4001 drops 1V, leaving 4VDC to reach the circuit, and not more than 1A (with some derating) allowed, so that may have been his problem.

D3 can be on either rail, doesn't matter. I put it on the positive rail because there wasn't anything there. I use one fewer resistor than your design.

BM

November 13, 2012
by Ralphxyz
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D3 can be on either rail, doesn't matter.

oops now I am confused again.

I have to stop doing everything else (like thinking) and build some circuits to see what is happening with the diode, especially that it can be on either leg, that really throws me (though it is starting to make sense).

I use one fewer resistor than your design.

Yeah I like that, that's experience for you, I would never have thought of that. Now what would reduce the component count would be to use a LED as the D3 diode it would be really neat to use a RGB diode so you would have only one. I don't imagine a LED would handle 1 or 2 amps.

I just googled polarity protection diode makes some interesting reading.

Ralph

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