555 question

[ljcox]

Does anyone know if the Discharge output can be connected to a voltage higher than Vcc?

I have 2 data sheets & neither gives any info on this issue.

The Disc output is an open collector, so I cannot see why it could not be connected to a voltage higher than Vcc provided that it is less than or equal to the max Vcc.

For example, if Vcc = 8 Volt, could the Disc be connected to say 12 Volt?

Any assistance will be appreciated.

 

[k7elp60]

The discharge pin(7) is an open collector so it can be connected to a higher voltage thru a resistor. The critical thing is not to exceed the dissipation of that open collector transistor.

 

[ljcox]

Thanks for the reply.

I thought that would be the case, but there is nothing in the 555 data sheet I have that indicates that it is possble for the Disc output to exceed the Vcc.

Whereas, the data sheet for the 74LS07 clearly indicates that the open copllectors can withstand up to 30 Volt.

However, the DS for the 74LS05 which also has open collectors states that Vcc max is 7 Volt but makes no comment on the collector capability.

So I assume the collector voltage must not exceed 7 Volt.

 

[ericgibbs]

hi Len,
Checked my Texas manual for the LS05, I would agree 7V on the open collector output is an absolute maximum.

I agree with k7elp, the 'discharge' pin should work upto a max of 15V.

E.

 

[MrAl]

Hi,

This really depends on the technology used to make the 555. Using the bipolar process it might work to allow voltages higher than +Vcc on the discharge pin, but in the CMOS versions of the 555 it's definitely not a good idea at all. There's a thing called CMOS latchup that might kick in, plus there are protection diodes that run from each and every pin to +Vcc so any voltage higher than +Vcc is going to try to forward bias that protection diode. If the diode burns it's hard to tell how much of the chip it damages and thus how much longer the chip is going to operate before it fails entirely.

 

[k7elp60]

Thanks MrAl,
I seem to forget some things on occasion. Real good points, especially about the CMOS version.

 

[rumpfy]

lj,
The disc pin is for the purpose of discharging the timing capacitor. It is not intended as a general purpose output pin. The pin is designed to handle high discharge current from the timing capacitor. if you need a high voltage drive function then use an additional switching transistor.
The datasheet for the 555 gives a maximum vce of 18 volt. The vce of the disc pin would have to be 18 V max.
Going outside the manufacturers ratings can cause trouble.
Bacchus Marsh, australia.

 

[ljcox]

Thanks for the replies.

I wanted to avoid the need for an external BJT as there is no point in using one if I don't have to.

Rumpfy. I have no intention of exceeding the max Vcc. My intention is to supply the main part with 8 Volt & use the disc BJT for level shifting to a 12 Volt level.

It's a pity that the data sheet does not cover this possibility.

k7elp60. I would not do it with the CMOS version given that the 74HC05 does not allow > Vdd on the open drain outputs.

I'll let you know what happens once the person I'm helping (on another forum) tests the configuration.

 

[jpanhalt]

Here is an application note for the NE555 that is no longer available from its manufacturer:

https://www.electro-tech-online.com/attachments/555an-pdf.52952/

It gives a nice discussion of the discharge transistor (Q14). On page 6 (see: "Note") it mentions that in the NE555, the discharge current is internally limited to 35 to 55 mA. There was a discussion awhile back on ETO in which others mentioned passing much higher currents than that. Depending on which version of the 555 you use, it may be worth nothing that possible limitation.

John