Photoresistor triggered solenoid, need help

The IRF510 has a typical RDS,ON of 0.5 ohm at VGS=10V for a P=2.5^2*0.5=3.0W and a drive level loss of 1.25V or 10%. The 2N3055 will have P=IB*VBE+IC*VCE=0.25*1+2.5*0.15=0.625W with a drive level loss of

1%. Assuming you have a choice of 2x 2N3055 or 2x IRF510, then the parallel combo of the 2x FETs still has a total P=1.6W and 5% drive level loss. RS has 10W 10, 50 and 100 ohm wirewounds he can series-parallel for 38 ohms.

Just use this:

Rat Shack also sells a TIP120 which he could drive directly from the 470 on the output of the 741 to operate the solenoid.

Ed

I built the following circuit from T.Wilkins:

+12V o---------+-------+-----+ | | | | | | | C | | O | | I | | L | [Cds] | | | | | | +-|>|-| | | | +--+ | | | | |/ | Q2 +--| | 2N2904 | |> | | | |/ Q1 +-| 2N3055 | |>

R1 [330] | | | gnd o----------+--------+

It seemed to be working, switching the solenoid on and off when I covered and uncovered the CdS with my finger. When I taped the CdS to the monitor and began the test however the 2N3904 Transistor fried out. Is there a better replacement for that transistor that I could just pop inplace of the burnt out one?

I copied the circuit from the original post and thus copied the same typo, doh! Q2 is the 2N3904 which fried.

You seem to be igoring the power loss from your 5W resistors, itself about the ~3W of the (single) IRF510 solution.

--

John Devereux

I already said that the overhead is about 10%, the above comparison was between components in the load circuit path. It's just a dumb hobby circuit.

He'll also need to use more than just the one transistor, for the 2.5A that the solenoid draws.

And add a little positive feedback, to give it some hysteresis, so it doesn't oscillate.

Thanks, Rich

Woo ha! I replaced the friend 2N3904 with a TIP3055 and everything is working. I'm getting near constant response times as well which is great. Thanks again to everyone for all the help.

Glad it finally works! The dissipation was still too great for the 2N3904. A TIP3055 as a driver may be a bit overkilled but if it's fine this way, why not!

Tim

You can try using another 2N3055, a TIP31, or TIP41 for Q2, all available at RS.

You don't look so bright using that slipshod thing. At a minimum you can do something like this: View in a fixed-width font such as Courier.

. . . +12V . | . .----------+--------------------------+----. . | | | | . | | | S . | | K S . | | 555 A S . [2.7K] ---------- | S . | | V+/RST | | | . | | CNT|-N/C '----+ . | | | | . | | | .--[100]-+-[100]--. | . +-----|THRESH | | | | |/ |- . | | OUT|-+--[100]-+-[100]--+---| |- . +-----|TRIG | |> |- . | | | 4X 1/2W 2N3055 | . | | GND | | HEATSINK . CdS ---------- | . | | | . | | | . '----------+-------------------------------' . | . --- . /// . . . .

You can use a TIP120 available from radio shack.

Ed

All of these Darlingtons are cooking at over 2-2.5W Pd.

The guy was just asking something "simple"... He may be quite satisfied with this design and, from this, he may be interested to go further and experiments with more elaborate designs.

But using the ubiquitous 555 which have the drive capability and also hysterisis is clever.

Tim

Yep - you can't use the 120 without a heat sink, unless you keep its dissipation low (< 2W).

Ed

Yup. The 120 needs a heatsink, or you need to keep its dissipation under 2W.

Ed