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I'm working on an android controlled input/output/gauges manager for cars based on a pic24 micro controller. I'm really not an electronics expert so I had someone working on the schematics but he has no time anymore.

I previously asked a question here about the outputs schematic that he designed: Mosfet as switch for high output current and it turns out the schematic was crap!

I've found automotive high side drivers for a fair price and I think these would do the job perfectly. I want 5 outputs to drive various loads (leds, solenoids, dc motors, etc) and turn them on/off from the pic micro controller.

So I've chosen a single channel driver (for bigger loads) and a 4 channels driver (supports 12A so I would think that equates to 3A per channel?). http://www.st.com/content/ccc/resource/technical/document/datasheet/cd/1f/d3/43/b8/2d/48/0b/DM00157127.pdf/files/DM00157127.pdf/jcr:content/translations/en.DM00157127.pdf

Here is the schematic that I think would be ok for both drivers. Is 1w rating enough for the zener diode and is 1a enough for the ground diode? I think I can share them between the 2 drivers, is that correct?

enter image description here

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  • \$\begingroup\$ You cannot just stick 12A through these devices and not do any calculations about power consumption. Think about one channel passing 3A through a typical on resistance of 140mR. 3*3*0.14 = 1.26W. You have four channels running, that's 5.04W. Then look at the thermal resistance when you have a big copper heatsink and you've really layed out the board well of 26.8 degrees C. That's a temp raise of 135 degrees! If you really do want 12A you cannot use that four channel part. \$\endgroup\$ – DiBosco Jul 27 '17 at 7:40
  • \$\begingroup\$ Thanks for pointing out thermal resistance. I hadn't even though of that... But doing the calculations with P = A x V, that means that without a heatsink, the quad channel driver supports only 0.09A per channel!! Is that right?? 4.47W = 0.37A x 12V. 4.47W x 26.8C per W = about 120 C + 30 (Ambient) = 150 C (Min shutdown temp). 0.37/4= 0.09A!!! Is my calculation wrong? That seems ridiculously low... \$\endgroup\$ – MadMister Jul 28 '17 at 0:07
  • \$\begingroup\$ I * I * R is the calculation you should be using. Each channel is 120R. So add up all the channels' power consumption. I think it should be a lot higher than 90mA, but it will be loads less than 12A. Those datasheets' headline figures are so misleading when you can't in practice put a heatsink on devices like that. \$\endgroup\$ – DiBosco Jul 28 '17 at 6:19
  • \$\begingroup\$ 120mR, sorry, not 120R! \$\endgroup\$ – DiBosco Jul 28 '17 at 14:47
  • \$\begingroup\$ With that equation, I get 2.83A per channel (very acceptable)! 4.47w max / 4 = 1.12W per channel. 2.83 x 2.83 * 0.14 = 1.12W. This is a HUGE difference compared to using P = A x V... Can you explain when to use one equation or the other? It makes no sense to me that equations give such different results. I don't get why one is dependent on voltage and the other one not at all? Like I said, my electronics knowledge is limited so maybe I'm missing something?? \$\endgroup\$ – MadMister Jul 29 '17 at 0:10

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