VOH drops when driving the load

[jerryd]

ElectroTech forum,

I'm using a CD4026BE decade counter/7-segment driver
to drive a seven segment display.

The datasheet for this part specs VOH at 4.95 volts min
with VDD of 5V.

I measure 5V VOH with no load but as soon as I use this
output drive a segment through a 330 OHM resistor the
voltage drops to about 2.7 volts measured to ground.

It drives the segment OK but I also want to use this to
drive the input of a CD4082BE Quad AND gate. Unfortunately
the datasheet for the 4082 specs the VIH at 3.5 volts min
with a VDD of 5V.

Measuring just across the resistor I get .99V and just
across the segment diode I get 1.7V. The datasheet
doesn't say anything about load or no-load conditions
so I don't understand I why I don't get the full 5V VOH.

Any suggestions?
jerryd

 

[ljcox]

You should re-read the data sheet.

According to the data sheet that I have,

Voh = 4.95 Volt if the output current is < 1 uA.

If you want to drive a 330 Ohm resistor, you need the 74HC series as it has a much better drive/sink capability.

The 4000 series cannot source or sink 15 mA & maintain Voh or Vol

 

[Diver300]

That sounds normal.

https://focus.ti.com/lit/ds/symlink/cd4026b.pdf

That shows typical output levels of 4.6 V with a 1 mA load, or 2.5 V at 3.2 mA. That is just about what you are getting.

The 4000 range are not capable of producing a lot of current, especially with a low supply voltage of 5 V.

You need some other type of amplifier to boost the signal. You could amplify the current so that nearly the full 5 V is available with more current, but you might get the brightness of one segment different from the other ones.

You could amplify the voltage with something like a 74HCT1G04 https://www.nxp.com/documents/data_sheet/74AHC_AHCT1G07.pdf

That would give the full 5 V to drive the 4082 as long as the input is over 2 V.

 

[ljcox]

If you look at the data sheet posted by Driver300 above, you will see that it recommends using transistors to drive the 7 segment display.

See page 5 under Fig 14.

You could use a Darlington array such as the UNL2003. Data sheet attached.

 

[jerryd]

Electro Tech forum,

Thanks for the replies. I understand the problem now.

The 4026 is driving the 7 segment display just fine it just
won't also drive the 4082 Quad AND gates. Can I replace the
4082 with a TTL Quad AND that only requires 2V VIL?

Or would increasing VDD to 9 volts do it?

jerryd

 

[ljcox]

I suggest you post a diagram of what you're trying to do.

A picture is worth a 1000 words.