Re: 250mA voltage clamp



Walter Harley wrote:
"Fred Bloggs" <nospam@xxxxxxxxxx> wrote in message news:439C89D3.9000006@xxxxxxxxxxxxx


The interesting question is how he plans to limit the I/O differential to 2V with 35V in and 24V out- looks like 12V to me, and at 1/4 amp makes for 3W. Sounds like someone is confused, again.


It's a voltage clamp. When the input voltage is above 24V, the differential will be Vi - 24V. When the input voltage is below (24V + Vdropout), the differential will be Vdropout. I want Vdropout to be less than 2V, preferably less than 1V. (Though there is no requirement that Vdropout be precise or constant.) I'm sorry if that confused you.

I could simply use a LM317 as a preregulator. According to the data*** its dropout voltage (at room temperature or higher, at less than 500mA) is around 1.7V. But I don't know whether to trust it to remain stable around and below dropout, which is why I asked about that.

A voltage clamp is just a simple emitter follower with a zener from base to GND. The TIP41 with a 24V zener will work well. The trick is to keep the power transistor base drive low impedance and somewhat regulate the zener current against your 18V to 35V input swing. You can use something minimal like this:
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..
..
..                             V+
..                             |
..               .-------------+
..               |             |
..               |             |
..             [270]           |
..               |             |
..               |             |
..               +------.      |
..               |      |      |
..               |     [47]    |
..                >|    |    |/
..         2N2907A |----+----| TIP41
..                /|    |    |>
..               |      |      |
..               |      |      |
..               |      |  +   '-----> OUT
..            [1K/1W]   Z
..               |      A 24V [1N4749]
..               |      |  -
..               |      |
..               '------+
..                      |
..                      |
..                     ---
..                    ///
..

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