At 05:04 PM 5/27/2006 -0700, you wrote:

>On May 27, 2006, at 4:48 PM, Spehro Pefhany wrote:
> > You can swap out the diode, in the current position, for a TVS or
> > beefy
> > zener, and the circuit will work as-is. For example, a 1.5KE62A (JEDEC
> > 1N6290A) . The solenoid will drop out faster than if you used a
> > (cheaper)
> > 1N400x across the coil.
>
>Cool... I'll check into that as well.
>
> > If you don't care about the drop-out time, the 1N400x is easier on
> > both the transistor and the diode.
>
>So is it smart in this circumstance to have something on both the
>transistor and the diode?  Or have I misunderstood?

Okay, you can think of these three being in series:
1. the supply, generally with a fat output capacitor
2. the transistor (on or off, with or without a TVS)
3. the coil (with or without a diode)

(view with fixed-width font such as Courier)

                     Diode
                 +-----|<-----------+
                 |                  |
                 |    ___           |
        o--------+----UUU- ---------|------------------------+
                 |                  |                        |
                 |  Solenoid        |                        |
                 | 31R              | C                      |
                 |       .----------|------------------.     |
                 |       |          |    TIP122        |     |
                 |       |          |                  |     |
                 |       |          |                  |     |
                 |       |          +------+-----+     |     |
                 | +     |          |      |     |     |     |
                ###      |          |      |     |     |     |
                ---    B |        |/       |     |     |     |
                 |       |----+---|        |     |     |     |
                 |       |    |   |>       |     |     |     |
       50VDC     |       |   .-.    |      |     |     |     |
                 |       |   | |    |      |     |     |     | TVS
                 |       | 8K| |    |      |     |     |     |
                 |       |   '-'    |      |     |     |   \ - \
                 |       |    |     |    |/      -     |     ^
                 |       |    +-----+----|       ^     |    / \
                 |       |   .-.         |>      |     |    ---
                 |       |   | |           |     |     |     |
                 |       |   | |           |     |     |     |
                 |       |   '-' 120R      |     |     |     |
                 |       |    |            |     |     |     |
                 |       |    +------------+-----+     |     |
                 |       '-----------------|-----------'     |
                 |                       E |                 |
                 |                         |                 |
       o---------+-------------------------+-----------------+ 0V

When the transistor switches off, the voltage at the collector rises from
about 0.8V typically (assuming forced beta of 250, so base current of
6.4mA). If there is a diode across the coil, the voltage rises to a diode
drop above whatever the supply voltage is at the time, and that is your
maximum collector voltage. The current continues to flow at 1.6A decreasing
exponentially toward zero, and most of the stored energy is dissipated in
the copper of the coil. If there is a TVS in there, it will not conduct
unless the supply voltage is higher than the TVS breakdown. BTW, the
50V is a bit close to the 60V breakdown voltage for a TIP120, so a TIP122
would be a better choice for the few pennies more you get 100V
breakdown and (unlike MOSFETs) no worse "on" characteristics.

If there is no diode in there, the voltage rises to the TVS breakdown,
about 62V for the TVS I suggested. It's guaranteed to have less than
85V across it with 18A current, so it will clamp the voltage nicely
for a TIP122. With that option, more of the energy is dissipated in
the TVS, and the magnetic field collapses more quickly so the solenoid
drops out faster. A higher voltage TVS (well within the ratings of the
transistor) will be even faster.

The TVS also protects the transistor if there is a high voltage
transient on your power supply caused by something else in the circuit.

Sometimes it's possible to use a low power C-B zener and cause the transistor
to turn on partially and dissipate the power in the junction, but that's
much harder on the transistor and has other potential issues.
It's sometimes done in power ICs. Some MOSFETs are rated to safely absorb a
certain amount of inductive energy through avalanche, but that tends to be
a rather small amount of energy compared to what an 80W solenoid stores.

So, the diode or TVS would be the same whether you use a MOSFET or darlington.

At 50V (unregulated?) operating voltage and 1.6A there are not a huge number
of logic-level MOSFETs with low Rds(on) and good voltage margin, so you might
need to factor a 10V-12V gate drive circuit into the equation, but the
heatsink savings might be worth the extra cost and complexity.

>Hope this helps,


Best regards,


Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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