I need some help with what I thought would be simple.

I want to control a single digit seven segment display that I have already built. Each segment has 4, .3 amp, 12 V incandescent lightbulbs in parallel. I wanted it to run manually with a push button using a CD4026 IC. I’d like to use a MOSFET to carry the load just under 3 A for each segment of the seven segment display. I believe the current required from each 4026 output will exceed the spec. I therefore would like to use a NPN transistor to receive the signal from the 4026 and It in turn operate each Mosset. I’ve bread boarded it up and I can’t get the thing to work. I used a 2N2222 NPN transistor and a IRF 520 MOSFET . I used a one K resistor between the 4026 outputs and the base of the NPN transistor and then connected the NPN‘s emitter to ground and the collector to the gate of the Mosfet. The source of the MOSFET is grounded, and the drain is carrying or providing the ground connection to each segment of the seven segment display with all bulbs having common connection to the +12 V feed. Can anyone see why this wouldn’t work? I used a 10 K pull down resistor at the gate, but when I power up the circuit, nothing happens when I energize the base or send a signal to the base of the NPN transistor. Is it because the MOSFET gate requires a positive voltage? Anyway, any help would be greatly appreciated. Thank you.

 

Try to draw a circuit diagram and post it here, that should make it more clear what's going on. Also have a look at transistor array ICs like ULN2003.

 

The 4026 is able to drive the mosfet directly. Remove the 2N2222 .

 

Try to draw a circuit diagram and post it here, that should make it more clear what's going on. Also have a look at transistor array ICs like ULN2003.

Yes please do. I believe that I understand where the problem lies, but hesitate to provide an opinion which may not be accurate.

Schematics are the language of electronics.

 

Is it because the MOSFET gate requires a positive voltage?

That is correct.
Configuration from post #1 which is easy to see why it does not work.

All that is required:

 

That is correct.
Configuration from post #1 which is easy to see why it does not work.
View attachment 357446
All that is required:
View attachment 357447

I’ll breadboard it a check it out

 

WHAT voltage is the 4026 operating at?? Even at 15 volts it is still a low current output.

 

WHAT voltage is the 4026 operating at?? Even at 15 volts it is still a low current output.

12volts
Yeah I’m concerned with the amount of current the 4026 can handle. 2 segments for number one and all 7 segments for number 8.

 

12volts
Yeah I’m concerned with the amount of current the 4026 can handle. 2 segments for number one and all 7 segments for number 8.

Not an issue using the mosfet driver.

 

Not an issue using the mosfet driver.

Yep…tested 4 lamps that had a .79 amp load on the mosfet and the current pulled by the charging of the gate is 1.7 ma. Thank you for the help.

 

Yep…tested 4 lamps that had a .79 amp load on the mosfet and the current pulled by the charging of the gate is 1.7 ma. Thank you for the help.

1.7ma seems high, is there a pull down resistor on the gate of the mosfet?

 

1.7ma seems high, is there a pull down resistor on the gate of the mosfet?

Yes…10K, that was a typo should be 1.1ma) I actually thought looking into the workings of the mosfet that there would be a surge while the gate. charged then bleed off to near zero but it holds until shut off then goes to zero

 

A suggestion is to investigate the sink and source current for the CD4026. It might have adequate pull down capability to switch the mosfet off completely enough. Meaning no pulldown needed.