Quick relay/transistor question

Prune · 9th August 2004, 06:53 AM

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?

Ouroboros · 9th August 2004, 07:04 AM

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.

Prune · 9th August 2004, 07:13 AM

The logic ICs whose outputs I'm using to drive the transistors are CD4023BCN, CD4027BCN, and CD4071BCN. The MOSFETs are driven by BJTs.

jackinnj · 9th August 2004, 12:52 PM

also -- resistors to the bases of the BJT's since CMOS isn't going to source much current --

Ouroboros · 9th August 2004, 12:59 PM

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?

Prune · 9th August 2004, 02:54 PM

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.

9th August 2004, 06:53 AM	#1
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	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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9th August 2004, 07:04 AM	#2
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	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.

9th August 2004, 07:13 AM	#3
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	The logic ICs whose outputs I'm using to drive the transistors are CD4023BCN, CD4027BCN, and CD4071BCN. The MOSFETs are driven by BJTs.

9th August 2004, 12:52 PM	#4
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	also -- resistors to the bases of the BJT's since CMOS isn't going to source much current --

9th August 2004, 12:59 PM	#5
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	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?