Power Dissipation - Op Amp Circuit

Thread Starter

matthej

Joined Oct 10, 2020
35
Hi All,

I want to make sure I am calculating power dissipation in an op amp correctly:

So from the OP400AY datasheet:
Iq = 725uA. I will be operating it at +/-15V.
Lets assume I am driving a 10K load per datasheet up to +/-12V

So Pq =725uA *30V = 21.75mW.
Pd = (24)(24)/10K = 57.6mW - (or is it 12*12/10K)
Total = 21.75+57.6 = 79.35mW

Do I then just divide up the power for each rail to get the current for the +15V and -15V rail assuming its sourcing an sinking the same?

So 79.35mW/15 = 5.29mA for each voltage rail?

Thanks!

 

wayneh

Joined Sep 9, 2010
17,023
Lets assume I am driving a 10K load per datasheet up to +/-12V

Pd = (24)(24)/10K = 57.6mW - (or is it 12*12/10K)
You tell us. Does the load see 24V across itself all the time? The rest of the calculations make sense, and yes the current on the two rails will be equal and opposite.
 

Audioguru again

Joined Oct 21, 2019
2,836
If it plays acid rock at full blast (overdrive)all the time then the power dissipation will be the maximum. If it plays normal music then the dissipation will be much less.
 

Thread Starter

matthej

Joined Oct 10, 2020
35
You tell us. Does the load see 24V across itself all the time? The rest of the calculations make sense, and yes the current on the two rails will be equal and opposite.
I am just trying to calculate absolute worst case. The +/-12V voltage swing across RL=10K is taken directly from the datasheet. I guess after reading this, the load only sees 12V across it worse case and not 24V. Sound right?
 

wayneh

Joined Sep 9, 2010
17,023
I am just trying to calculate absolute worst case. The +/-12V voltage swing across RL=10K is taken directly from the datasheet. I guess after reading this, the load only sees 12V across it worse case and not 24V. Sound right?
I can’t tell without more details. Is this a DC signal or AC? If it’s audio, the average power will be far below the peaks.
 

wayneh

Joined Sep 9, 2010
17,023
Its an AC signal. Just trying to account for absolute worst case.
Well worst case is continuous signal at one of the rails but that should not really happen with an AC signal. Consider what the worst case really is, keeping Murphy’s law in mind. The user might (will) turn a dial to the extreme, or ALL dials to the worst possible extreme.
 
A first order estimate would be to assume a full scale +-12V CW sinewave delivered to the 10K load.
Pwr = Vrms**2/R = (1.414*12V)**2/10K
This would be the pwr dissipated by the load, not the opamp.
 
The load on the op amp depends on the volt
The OP400AY says it has output short circuit protection snd can handle +/-20v supply. If you are only applying +/-12VDC supply, why are you worried about maximum power dissipation? It is impossible to exceed the dissipation limit of this op amp with a power supply within the specified voltage limit.
 
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