W A R N I N G !

W A R N I N G !

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Thursday, March 3, 2011

Fun with power

Since my regulator is coming soon, I had to check how much heat will be coming out of it. So I had to see how much the amp draws and calculate from that.

0.24A. No change whether playing music or not.

Lets make it a quarter of an ampere for easy calculation. The regulator drops 4V. 4V @ 0.25A = ...1W.

That's pretty easy to dissipate with a mints tin. Heck, even 2W would be possible.

Just that the full power dissipation of 12W would be really impossible. Lets hope I'll never come to that. Can that amp draw 3A anyway?

Since the Zhaolu is now open I might as well take a few temperature readings... I've always known its power supply to be inefficient, lets take more evidence of it.

Power draw should have a somewhat linear relationship with temperature rise. In practice, the relationship between rate of heat transfer and temperature may not be linear, but the error is reduced by having the heatsink in the open with the fan blowing onto it - this minimizes the issue of different surround air temperature screwing the calculation.

Power supply power off - air temperature

Power supply on, amp off

Power supply on, amp on

18 vs 25 degree rise.

Output voltage = 15V, input voltage = 14V x √2 = 19.8V, voltage drop = ~5V, current draw = 0.25A, additional power dissipated = 1.25W, additional temperature rise = 7 degrees, thermal resistance = ~5.6ºC/W

Sounds logical. It is actually way lower than what it should be, but I had a fan blowing at it.

Using that thermal resistance, power used by regulator doing nothing = ~3.2W

Sounds logical.

I did remember this bugger to use 7 to 8W idling. 3.2W is for one LM317 at 20V in/15V out. There is also the negative -15V guy and a +5V guy too, so roughly would add up to around there alright, assuming power draw by the DAC + headphone amp circuit to be negligible, which it should be.

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