T&L Publications

How much can the 555 timer handle on the #3 pin output as in m.a <p>thanks in advance
Myp71
Ryan

Couple hundred. Unless you're talkin' about the CMOS version, 7555; I don't remember, think it's 100 max...

The data sheet (Signetics, 1985) claims that it can source or sink 200 mA. It also claims that the maximum power dissipation is 600 milliwatts.<p>The tabulated characteristics are not that optimistic.<p>Assuming a 5 volt supply, 25 centigrade:
Output low, sinking 8 milliamp, output voltage less than .4 volt.
Output low, sinking 5 milliamp, output voltage less than .35 volt.
Output high, sourcing 100 milliamp, output voltage greater than 2.75 volt.<p>There are some curves that show the output voltage increasing for sink currents greater than 5 milliamp, leveling off at about 2 volts for 100 milliamp sink current. This represents 200 milliwatts of dissipation in the integrated circuit. The voltage drop when sourcing 100 milliamp is also about 2 volts, which will also cause 200 milliwatts of dissipation in the chip.<p>The 600 milliwatt dissipation rating is probably based on a 150 centigrade junction temperature. Signetics does not give thermal resistances for this part, but most of the temperature rise is usually between the case and the ambient environment. When dissipating 600 milliwatts, this part will probably be very hot. This is not recommended on a continuous basis. Even at 100 milliamp, the part may run warm.<p>The 100 milliamp rating is probably stretching the truth a little bit. Perhaps 10 or 20 milliamps is more realistic. For more than this, I would add some type of buffer amplifier.<p>[ May 18, 2003: Message edited by: stephen ]</p>

I would like to retract my comment regarding positive efforts of teaching electronics and it's state of affairs. I am reminded to be more careful not to post opinions that could bring up difficulties. I am truly sorry.<p>[ May 18, 2003: Message edited by: dimbulb ]</p>

dimbulb and I are fine. Sorry for the hiccup in the thread.<p>Hey, anytime you want to increase the output capability of a 555, you can add a simple one-transistor inverter (grounded emitter, 10K ohm resistor in series with the base and 555 output, 1K collector resistor to whatever voltage you need. You'll have good pull-down ability but not so good pull-up. But most applications that we have use pull-down simply because it's easier to implement a turn-on function that way.<p>Dean<p>[ May 19, 2003: Message edited by: Dean Huster ]</p>

Having been on the "seeking end" of information quests, and so many times wishing that a gifted person would jump in with an appreciated answer, I suspect that most magazine and forum contributers simply enjoy lending a hand. This and the prospect of discussing electronics, which is generally unappreciated (and found boring) around our own coffee tables...<p>
<p>[ May 19, 2003: Message edited by: gadgeteer ]</p>

What is going on here I just asked a simple question.