Current source design (tricky?)

| /|\ Vd-s

Replace R1 with a schottky diode, and run R2 to ground. Or buy an opamp.

John

i remember the days of using an incandescent lamp! :)

i have some brand new one's still in packages! :)

Heheh- admitted- it is pathetic.

That's the characteristically clever Widlar invention- but I don't think even he would use it at that small an Rs. We can boil down your calculations by defining M=(1+R2/R1), and this relatively large M mixed with small Rs does not make a very good combination. It turns out M=Vgs(Io)/(Vbe-RsIo) so that RsIo comparable in magnitude to Vbe and Vgs(Io) much larger , make M large, 7.5 in this case. So the main drawback is that Io=(Vbe-Vgs(Io)/M)/Rs so that dIo/dT~(dVbe/dT)/Rs which is not a good number, -20mA/oC in this case. Then at even -20oC you have Io increased by nearly 1 amp over the 25oC setting, and a similar decrease at 70oC. That variation is just too wild.

Yeah, well thats a new one to me. i remember seeing some of the first Mac solid state Hi-fi audio gear.. germanium transitor power amps with a incandescent bulb in the bias circuit to help get a better linear response on the on the final stages of the amp! :)

See...

Newsgroups: alt.binaries.schematics.electronic Subject: SED Question: Current source design (tricky?) - LowDropCS.pdf Message-ID:

...Jim Thompson

Why not. I've got about 100 of them.

In article , Jamie wrote: [...]

There used to be a "constant current" tube that was really a specially tricked out light bulb.

I read in sci.electronics.design that Ken Smith wrote (in ) about 'Current source design (tricky?)', on Sun, 13 Mar 2005:

Well, yes, for sufficiently basic differences. A 'barretter' has an iron wire filament in a hydrogen-filled envelope.

I suspect that was a form of short-circuit protection, but I've never seen an amplifier using it.

Yes, thats the device I remember. I had to hunt high and low to find one for a radio I was fixing.

It wasn't a form of "outrageous overkill" that drove my MOSFET selection, although it helped in the issue you raised, but rather an effort to find a part that could better handle 3A * 15V = 30W max, in the event someone used the design with a shorted load.

An apparent overkill is common when one uses modern power MOSFETs, designed for switching use, in linear applications where one wants to reduce the burden on the heatsink. The IRF4905 is rated at 200W (for impossible 25C case temp), but this means it has a nice low 0.77 C/W junction-to-case thermal resistance. After adding in say 0.33 C/W for an insulator, the 1.1 C/W total means we add a modest 33C penalty to the heat-sink's job when dissipating 30W.

And I wanted a part that was easy to get and low cost: DigiKey has 10375 in stock, at $2.36 qty 10. The 55V 74A 0.02-ohm IRF4905S (D2Pak case) is ideal for the 14V automotive industry, $1.88, 1k. The IRF4905 and IRF4905S are on my jellybean-part lists.

I see it. You evaded the problems of working in the triode region by deliberately choosing the appropriate FET, even though it looks like outrageous overkill.

Yes, going for current overkill on these low-voltage drop c-c sources looks the obvious design choice now. It reduces the work the amplifier has to do, which helps in things like transient PSRR, etc.

Thanks.

You should be safe doing this:

View in a fixed-width font such as Courier.

. .. -----------------REGULATOR MODULE------------------- .. | (RM) | .. | +----------------------+---+ | .. | | | | PMOS | 2.5A .. | | I-> | | +-------+ | IL ..VBATT>--------+----/\/\----+---------------------|s d|---->

.. | | Rs 100m | | | | | | .. | / / | | | g | | .. | 2k 1% 2k 1% | / +-------+ | .. | / / | 2k | | .. | \ \ | / | | .. | | | | \ | | .. | + +---+ | | | 220 | | .. | | | | +-||---|---+---/\/\----+ | .. |Vref ---/ | | | 22n | | | .. | // \ -+ / | | | | .. | --- 200\---+ | | .. | +------------|------|+/ === | .. | | | |/ | 100n | .. | | | LM741 | | | .. | / / (NTE941)+------+ | .. | 2k 1% 2k 1% | | .. | / / | | .. | \ \ | | .. | | | | Vref | .. | +------------+---------+ IL= ---- | .. | | Rs | .. -------------------|-------------------------------- .. N . .. .. calibration: disconnect load, . .. jumper TL431 K-A to short, . .. adjust 200R pot for 0.000V at '741 (+)-(-) . .. w/ batt applied. . .. . .. TRANSIENT PROTECTION . .. .. FAST BLO +--RM---+ .. 5A | | .. VBATT>---~-----+--------+------------BATT IL---->>--+ .. | | | | | .. +---+ |+ +---N---+ | .. 24V | - | === | | .. TVS |/^ | |35WVDC | | .. | - | |1000U | +-----+ .. +---+ | | |LOAD | .. | | | +-----+ .. | | | | .. +--------+----------------+ | .. | --- .. / /// .. 22 1W .. / METAL .. \ OXIDE .. 1n4007 | .. +---|

Fred Bloggs wrote: scratch that- slight scale factor problem: View in a fixed-width font such as Courier.

. .. ----------------------REGULATOR MODULE------------------- .. | (RM) | .. | +---------------------------+---+ | .. | | | | PMOS | 2.5A .. | | I-> | | +-------+ | IL ..VBATT>--------+----/\/\---------+---------------------|s d|---->

.. | | Rs 100m | | | | | | .. | / / | | | g | | .. | 2k 1% 2k 1% | / +-------+ | .. | / / | 2k | | .. | \ \ | / | | .. | | | | \ | | .. | + +---+-/\/\-+ | | | 220 | | .. | | | 1k | | +-||---|---+---/\/\----+ | .. |Vref ---/ | | | | 22n | | | .. | // \ -+ / / | | | | .. | --- 100 200\---+ | | .. | +-----/\/\-+------|------|+/ === | .. | | 910 | |/ | 100n | .. | | | LM741 | | | .. | / / (NTE941)+------+ | .. | 2k 1% 2k 1% | | .. | / / | | .. | \ \ | | .. | | | | Vref | .. | +-----------------+---------+ IL=----- | .. | | 10xRs | .. ------------------------|-------------------------------- .. N . .. .. calibration: disconnect load, . .. jumper TL431 K-A to short, . .. adjust 200R pot for 0.000V at '741 (+)-(-) . .. w/ batt applied. . .. . .. TRANSIENT PROTECTION . .. .. FAST BLO +--RM---+ .. 5A | | .. VBATT>---~-----+--------+------------BATT IL---->>--+ .. | | | | | .. +---+ |+ +---N---+ | .. 24V | - | === | | .. TVS |/^ | |35WVDC | | .. | - | |1000U | +-----+ .. +---+ | | |LOAD | .. | | | +-----+ .. | | | | .. +--------+----------------+ | .. | --- .. / /// .. 22 1W .. / METAL .. \ OXIDE .. 1n4007 | .. +---|

It kinda makes one wonder just where thermionic technology would be today, if it had the same $$$ spent on it as Si did.

I read a neat paper a few years back in an IEEE professional comic, wherein it was shown that Si cant beat TE at high power levels, due to the operating temperature ranges. If only we could *actually* buy static induction transistors, thyristors etc. *sigh*

Cheers

In article , Terry Given wrote: [....]

It is starting to look like SITs are just one of those things that never make it into general use. The IGBT has eaten up some of its market but not that much. The one device that really seems missing from the spectrum is something with a Vgs(off) of about -2V and that can handle 600V and.or

In article , Fred Bloggs wrote: [....]

I suggest adding a capacitor between the points I've marked "A" and "B" in the modified drawing below. I suggest the value as 22nF. Imagine a positive step arrives on the VBATT of the existing circuit. The output of the LM741 will only step positive by 1/2 that amount so the other 1/2 appears as a step in the Vgs of the MOSFET (ignoring its capacitance).

Unless I've over looked something, the added 22nF will reduce the step to near zero and match the time constant of the other 22nF so the whole effect is brought near zero.