T100MD-1616+ and EXP4040 over heat problem

[wec]

Hi,

Your PLCs are good. We just finished a project using one T100MD-1616+ with two EXP4040 expansion board. TBASIC is great! It makes programming much easier.

However, there is one problem I found so far. The voltage regulators on each board (the one without heat sink on the PLC and expansion boards) are very hot. The whole system runs at 24V. I checked the J1-12v jumper. It is removed.

I was wondering why there is no heat sink on the voltage regulator. Let me know how to solve this over heat problem.

Regards,

[support]

A linear voltage regulator works by consuming away the power arising from the difference between input and output voltage. This extra power turns into heat and therefore it is quite normal for the voltage regulator to become somewhat hot but not excessively so (this may be a bit subjective but as long as the casing does not go above 60 degree C it should not be alarming).  However, if there is some damaged component on the PLC then it is possible that the PLC draw far more current than normal and that can lead to overheating of the regulator.

Can you measure the current consumption of the PLC (without the I/O, LCD or expansion board)? You can measure it by connecting a mA meter between the 24V power supply and the PLC's 24V terminal. It should draw between 110 to 150mA which is normal. But if it measures above 200mA then something may be wrong with your board (such as a burnt compoment) which would require inspection/repair.

Some of the voltage regulator did not have heat sink because the natural convection is sufficient to carry away the heat.

[wec]

I checked the current. It is less than 100ma. The temperature is 75 degree C.

Let's do some calculation here:

It should draw between 110 to 150mA which is normal

The Power Dissipation of the 7805 on the EXP4040
=(24-5)x150/1000=2.85Watts.

According to the datasheet of the 7805http://www.national.com/ds/LM/LM7512C.pdf, on page 5 "Maximum Average Power Dissipation", the maximum power dissipation without heatsink under 25 degree C is only 2 Watts.
2.85Watts is over the limit of 2 Watts.

I managed to put a heatsing to the 7805, the temperature is now 60 degree C.

Please let me know if I am wrong.

[support]

T100MD+ consumes about 110mA to 150mA with all I/O off. That is the current consumption in the 5V logic section but there are three stages of step down so your formula isn't right.

For EXP4040, when all I/Os are OFF the logic section consumes only about 20mA and hence the power disspation by the 7805 is (24-5)*0.02 = 0.38W, well within the package power disspation.

When some of the I/Os are turned ON then the incoming current will go up accordingly (approx 3 to 5mA per I/O) but this additional current goes to the 24V section and does not cause the 5V regulator to heat up.