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Old 9th August 2004, 07:53 AM   #1
Prune is offline Prune  Canada
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Question Quick relay/transistor question

Is it OK to have power at the emitter/collector of relay driving BJTs and motor driving MOSFETs if the bases and gates are connected to unpowered logic?

I ask because I keep my 5 V logic unpowered in standby (both Vdd and Vss at ground), except for the standby on/off controller and remote control reciever, but the unregulated 12 V going to the relays and volume motor is not turned off by a relay since I assumed current wouldn't flow through with a lack of signal at the base/gate. This way I don't have to worry about adding another relay to turn off the 12 V. Could something get fried or not work right in such a setup?
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Old 9th August 2004, 08:04 AM   #2
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Everything should be OK. I suggest you add a pull-down resistor (10k or so) between the gate and source of the switching FET to ensure that you don't get the possibility of a floating gate.

Although HCMOS outputs will go to a maximum of a diode drop above ground when no power is applied, other families like 74LCX have outputs that go high-impedance with no power applied.
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Old 9th August 2004, 08:13 AM   #3
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The logic ICs whose outputs I'm using to drive the transistors are CD4023BCN, CD4027BCN, and CD4071BCN. The MOSFETs are driven by BJTs.
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Old 9th August 2004, 01:52 PM   #4
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also -- resistors to the bases of the BJT's since CMOS isn't going to source much current --
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Old 9th August 2004, 01:59 PM   #5
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That's true. Do you really need to use CD4000 series at 5V? It's just that it really doesn't source much current at all at 5V. Can't you use standard 74HC parts?
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Old 9th August 2004, 03:54 PM   #6
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Quote:
Originally posted by jackinnj
also -- resistors to the bases of the BJT's since CMOS isn't going to source much current --
Yes, I used near-maximum resistances that still toggle the relays.
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