Overheating regulator

Guys,

I have designed a shield for the MEGA2560. It has the following chips on it (when fully populated) MCP3208 ADC, AT25M02 EEPROM, 3 X LTC1664 DACs. It also has connectors for an RTC, Bluetooth module and an XBee as well as either a 20X4 LCD module or a TFT LED display.

Now, I didn’t want to power all that and the accelerometers I’m connecting to the ADC from the MEGA’s power supply so I used a 5V 1500mA regulator to feed the shield and linked to VIn on the MEGA to the incoming power supply to the board.

At the moment, all I have on-board is the ADC chip with one axis of a GY-61 accelerometer to one channel and the TFT LED display both of which are being fed 5V from my on-board regulator which remains cool. The power supply is a 7.2V 2500mAh RC battery which is well within the MEGA’s recommended input voltage of 7-12V but despite the fact that it isn’t supplying anything other than the MEGA itself, the regulator on the MEGA next to the power supply socket is getting very hot. Any ideas why this might be?

Luckily, I only got 10 boards made as a prototype and I’m thinking that I might use the 5V from my regulator to feed the MEGA also. Any other suggestions?

Ultimately, the board will be inside an enclosure, will be mains fed and remain powered up for very long periods so I need to get this right.

I welcome your input, Steve.

Post the schematic.

Schematic attached

Steve.

vibro-i ADC Board.zip (61.7 KB)

I suppose there is no way for you to measure the input voltage current ? (by cutting one of the wires and putting a meter in series with the 7.2 V (which is probably more like 8.4V if it is a Lipo since 7.2 is the discharged voltage)

There's no schematic.
You are feeding Vin with battery pack? Can't explain the heat.
Feeding the battery pack into the barrel jack? Reverse polarity diode is only rated for 1A, the regulator for 800mA.
Or you are feeding the Mega 5V from the shield into the 5V header? Put a diode across the 5v regulator (cathode to 5V, anode to regulator input) so the regulator is not totally reverse driven.

.zip took a while to appear.
In Eagle(.sch?), use File:Export:Image and create a .png file so everyone can see it. Attach the .png.

Battery pack tests at 7.85V I’ll try to measure current tomorrow.

The voltage is supplied to the shield, not the MEGA’s barrel jack. It is supplied to the VIn pin of the MEGA from the shield.

Please see .png schematic

Steve.

Is the LCD display backlit?

You're tying the outputs of your shield's regulators to the outputs of the Mega's regulators, putting them in parallel. The load sharing between them will be unpredictable. It's entirely possible that most of the load is coming through the Mega's regulator. A backlit LCD is a power hungry beast and will probably overload it.

Cut the traces connecting to the 5V and 3.3V pins on your shield, leave Vin and GND connected.

You’re tying the outputs of your shield’s regulators to the outputs of the Mega’s regulators, putting them in parallel. The load sharing between them will be unpredictable. It’s entirely possible that most of the load is coming through the Mega’s regulator. A backlit LCD is a power hungry beast and will probably overload it.

How can you tell the "shield " 5V is shorted to the Mega 5V ? I see the mega pins and I don’t see any distinction between one 5V and the other 5V so I suppose you could assume that, although, to be honest, I am shocked that anyone designing a shield would not know better.

Jiggy-Ninja:
Is the LCD display backlit?

You're tying the outputs of your shield's regulators to the outputs of the Mega's regulators, putting them in parallel. The load sharing between them will be unpredictable. It's entirely possible that most of the load is coming through the Mega's regulator. A backlit LCD is a power hungry beast and will probably overload it.

Cut the traces connecting to the 5V and 3.3V pins on your shield, leave Vin and GND connected.

I don't think so, the mega's 5V pin is net "5V" whereas the regulator output is on net "+5V" - it
looks like the Mega isn't getting any power in fact, is its phantom powering via I/O pins?

raschemmel:
How can you tell the "shield " 5V is shorted to the Mega 5V ? I see the mega pins and I don't see any distinction between one 5V and the other 5V so I suppose you could assume that, although, to be honest, I am shocked that anyone designing a shield would not know better.

My apologies to CM_Guy, I misread the schematic. I did not notice the (far too) subtle difference in name between the +5V net powering the shield and the 5V net coming from the MEGA connector JP16. I thought they were the same.

I my defense though, the RTC Connector JP7 is connected to the MEGA's 3.3V regulator, not the shield's. Is this deliberate, or a mistake?

MarkT:
I don't think so, the mega's 5V pin is net "5V" whereas the regulator output is on net "+5V" - it
looks like the Mega isn't getting any power in fact, is its phantom powering via I/O pins?

Good catch. I don't see the SUPPLY net connected to any of the MEGA's connectors.

I don't understand why the Mega's Vin pin is not on JP16.

You wouldn't bring 5V in on Vin to the Mega, you'd bring it on 5V on the ICSP header or 5V on the power header, J16.
I'm with MarkT, no power connection to the Mega, just Gnd. Connect +5V to 5V, put a diode from 5V to the Mega Vin pin.