# Checking my calculations

Discussion in 'General Electronics Chat' started by edmundopt, Sep 1, 2013.

1. ### edmundopt Thread Starter Member

May 4, 2011
60
0
Hello

I have been reading some material about IC thermal power calculations, but since this is my first time, please someone correct or confirm the following :

For a load of 360 ohm @ 12 volts, with 4 of 7 darlingtons active(worst case scenario), using ST ULN2003A (package DIP16) and the datasheet of June 2012.

each pin will take a load of 12/360 = 0,0333 amp.
according to datasheet at 100mA the V(ce) is of 0.9volts, so P = 0.9 * 0,0333, P = 0,02watts

for the total of the four pins, P = 0,08 watts.

Using the Tjmax of 150ºC , and RthJA 70ºC/W, and a maximum Air temp of 70º(read somewere that was the industrial value)

Qmax = (150-70)/(70) , Qmax = 1,14 watts.
Sp everything is fine by my calculations.

2. ### WBahn Moderator

Mar 31, 2012
23,556
7,208
Are the Darlingtions being used as switches and being turned on hard, or are they being used more as an amplifier, such as an emitter-follower configuration?

3. ### WBahn Moderator

Mar 31, 2012
23,556
7,208
I've got to put in proper units, otherwise it is difficult for me to follow because units are so engrained in me.

Load current: IL = 12V/360Ω = 33.3mA

Vce = 0.9V (is that typical, or max? use max)

P/device = (Vce*IL)/device = (0.9V*33.3)mA/device = 20mW/device

Max of 4 devices at the same time

P = (20mW/device)(4 devices) = 80mW

Pmax = (150°C - 70°C)/(70°C/W) = 1.14W

I don't know where using 70°C as the ambient value comes from. I've never heard of it, but it may not be too unreasonable if it is trying to look at worst case design for components in a hot equipment cabinet, for instance). That's a pretty hot cup of coffee, which seems like it would be a pretty safe max to use but not excessively so. It probably also adds in some safety margin so that you can just use Tjmax directly.

So other than the question of whether you are using the Darlingtons as a switch and whether you are using the max Vce voltage, your calculations look good. And you aren't coming close to pushing the thermal limits, so even if your actual Vce was 9V you would still be safe.

4. ### edmundopt Thread Starter Member

May 4, 2011
60
0
thank you very much, I will post units in the next posts, they are being used as switch, switching four G2rl 12v relays from ONRON.

So far I still haven't tested the circuit, and I am still searching information in order to calculate if the internal diodes of the ULN2003A are enough.
Do you have any recomendations, maybe also place schottky diode for each relay ?

5. ### #12 Expert

Nov 30, 2010
18,076
9,676
Here are some circuit drawings in case they clear up any doubts, and a PDF about why something more than a simple diode is sometimes used. I also think you have mistaken the commercial temperature limit (70 C) with, "room temperature" which is often characterized as 25 C.

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