constant current source

Yes.

Yes, it will work fine. But, the 555 uses quite a bit of current (3 ma to 7.5 ma, depending on the supply voltage) just to light the chip. And, it requires an extra outboard transistor. I would think you could do better depending on the degree of regulation you need.

Doesn't Maxim make some precision current sources for low power aps???

Regards,

M

The trigger current is 0.5uA, so there is enough margin with this value, but non-linearities will start popping up with less. If you do not need the

200mA output current capability, but are happy with 10mA source or 50mA sink, you can use the LMC555, a CMOS version, which has only 10pA input current on that pin. It will consume much less current and work at frequencies up to 3MHz.

--
ciao Ban
Bordighera, Italy

Yes. I assume you're talking about figure 13. Just take care, as Ban said, to use a CMOS version of the 555 with low timing currents.

I disagree, for two reasons. First, the Early effect (most usefully expressed as a slight change in Vbe vs Vce) is no worse at low currents than at ordinary currents, and second, the voltage drop across the emitter degeneration (current-setting) resistor overcomes the slight Early effect. The transistor current source will work as well as before, after scaling the design to low currents.

Now, if you were using a simple two-transistor current mirror, without emitter resistors, that'd be another matter.

Not necessary, and the large variation of Vgs introduces problems.

--
 Thanks,
    - Win

Kell, . Yes. Make sure that the Re is large enough so that the voltage across it is >>Vbe, the Base-Emitter voltage. This ensures that Vbe and variations in Vbe with temperature will have minimum effect on the collector current. . Regards, Jon

Okay I can use a bigger cap so the chip doesn't load it and affect the timing. Looking for some accuracy here. Just need to know the tolerance. I have 1uF dip tantalum caps with K printed on them, meaning 10%. My 4u7 dip tantalums have an A printed on them. Does that mean anything about the tolerance?

And another thing: if anybody wants to suggest an alternative constant current source, while keeping the headroom down to 3 volts.

One could use a bipolar transistor biased like that for currents down to the nanoamp region. However, it is a poor "constant" current source (or sink if one uses an NPN) due to the Early effect. Use a JFET (good to about 30V, the max rating i have seen), or a Depletion MOSFET (good to 500V).

You need to worry at least as much about leakage current as tolerance, if you're trying to make an accurate long-time-constant ramp based on charging a capacitor with a constant current source.

You might instead consider a microcontroller (or other counter mechanism) and a D/A converter.

ISTR a thread on this group a few years back discussing the various alternatives.

If you have to buy the transistor, I would get the 2N3906. It has twice the beta, 60 min at 100 uA. Also, somebody suggested using two transistors in a current mirror. You might want to check that out; it will give you more headroom.

For accuracy, at the 1 uF level, I would use some kind of plastic dialectric cap.

Tam

kell wrote: [1 of only 19 Usenet postings (posted from Google Groups)]

No one has mentioned it yet, but you don't include context when you respond.

Some threads that you may find useful:

't-read-this-at-Google-don't-see-Usenet-the-way-you-do Follow the link in that post and follow the link in THAT post.

The **Show options** stuff will prove useful.

Grise puts a fine point on it here:

He may be a bit strong with his rhetoric--but not by much.

't-read-this-at-Google-don't-see-Usenet-the-way-you-do

Okay I went to the links and saw the advice to click show options and then click reply; I'm trying that now to see how it works. Does this give you the context you need? I never used a newsgroup until Google started its groups thing. I actually did not know Google leaves newsreaders in the dark until you told me.

.

. That's better.[1] Actually, I read on Google as well. I just didn't want you to form a bad habit.

. Therein lies the problem. A newbie who posts from Google often does it wrong for weeks then gets defensive when corrected (usually by someone in a bad mood.)

[1] You don't have to re-post everything from the previous post; feel free to snip out as much from the automated blockquote as you wish . I only leave enough to show what part I'm referencing ("context"). See above.

Expanding on what Homer said, if you get a newsreader and use it a bit you may like it better than Google Groups.

I have Gravity, but only use it rarely e.g., responding to a poster who cross-posts to more than 5 groups (and I don't know which one he reads) or when Google doesn't archive a particular group on the Groups line.

Generally, I prefer reading from my browser.

kell said

If you really want to enjoy USENET you might consider a real newsreader. Most consider newsgroups-via-Google a pretty poor interface. YMMV

I'd recommend XNEWS.... but there are many options.

: :Expanding on what Homer said, if you get a newsreader and use it a bit :you may like it better than Google Groups. :

: JeffM

I forgot to include the major point I was trying to make: Even if you go back to Google Groups after trying a newsreader, you'll have an appreciation of how others see Usenet.