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Old 24th Oct 2020, 2:26 pm   #1
Bazz4CQJ
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Default 12VDC power switching circuit

Can anyone suggest the neatest way to make a small switching unit which will alternatively send 12VDC @200mA to solenoid A, then off A and on to solenoid B and then off B and back to A, etc, etc with each 'on' time being 10-20 seconds?

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Old 24th Oct 2020, 2:50 pm   #2
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Default Re: 12VDC power switching circuit

A "power" two transistor multivibrator with the collector loads being the coils (with suitable reverse diodes/capacitors/zeners/resistors)?
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Old 24th Oct 2020, 3:13 pm   #3
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Default Re: 12VDC power switching circuit

Yes... that sounds like a neat solution! I have a bag full of TIP31's left over from some project, which might even do it sitting on a PCB with little or no heat sinks.

Thanks .

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Old 24th Oct 2020, 3:36 pm   #4
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Default Re: 12VDC power switching circuit

A 555 with two transistors should do.
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Old 24th Oct 2020, 3:36 pm   #5
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Default Re: 12VDC power switching circuit

Maybe a TIP31 would need a driver transistor? Maybe something like a BFY50 would do, and without a driver?

Re a 555, I did think about that; not sure where the pros and cons lie.

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Old 24th Oct 2020, 6:19 pm   #6
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Default Re: 12VDC power switching circuit

Quote:
I have a bag full of TIP31's
For 200mA almost any "modern" NPN silicon job will do, no driver required as the second transistor is driven by the first (and vice versa). What is the voltage the solenoids require and the resistance? I do fancy doing a simulation in LTSpice.
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Old 24th Oct 2020, 6:20 pm   #7
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Default Re: 12VDC power switching circuit

Just re read the original post, 12V (dimwit!)
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Old 24th Oct 2020, 6:28 pm   #8
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Default Re: 12VDC power switching circuit

Here we go... (had to use differing capacitors to make it start, LTSpice can be a bit too accurate)
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Last edited by merlinmaxwell; 24th Oct 2020 at 6:30 pm. Reason: added explanation
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Old 24th Oct 2020, 6:40 pm   #9
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Default Re: 12VDC power switching circuit

Many thanks for that. The detail that I have for these solenoids is limited to 12VDC, 1.2W, but in each on sequence, there will be two in parallel, so 2.4W in all.

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Old 24th Oct 2020, 7:10 pm   #10
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Default Re: 12VDC power switching circuit

200mA is nothing: I've got a binful of 1960s Texas Instruments BCY39-type transistors that did such 'driving-reed-relay-coils' stuff in 1970s TXE4 telephone-exchanges.

Drive them from TTL. The biggest issue for me would be back-EMF from the solenoids - not just the voltage-spike when you turn-the-current-off, but also ensuring they de-energize promptly by having somewhere engineered so they can dump the stored magnetic-charge. Simplistically, a 'back-EMF' diode across the solenoid will stop the back-EMF being destructively-reflected into the driver transistor, but that also means extending the current-flow-time in the solenoid and extending the time between the 'cut-off' signal and the solenoid de-energising. A series-resistor inline with the dump-diode can help dissipate the charge, but there's a lot to be said to providing a reverse-driven winding on the coil if you want a 'smart' solenoid-release.
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Old 24th Oct 2020, 9:45 pm   #11
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Default Re: 12VDC power switching circuit

Quote:
Originally Posted by G6Tanuki View Post
The biggest issue for me would be back-EMF from the solenoids - not just the voltage-spike when you turn-the-current-off, but also ensuring they de-energize promptly by having somewhere engineered so they can dump the stored magnetic-charge. Simplistically, a 'back-EMF' diode across the solenoid will stop the back-EMF being destructively-reflected into the driver transistor, but that also means extending the current-flow-time in the solenoid and extending the time between the 'cut-off' signal and the solenoid de-energising. A series-resistor inline with the dump-diode can help dissipate the charge, but there's a lot to be said to providing a reverse-driven winding on the coil if you want a 'smart' solenoid-release.
Thanks for the comments but I'm not expecting the timing to be at all critical; I don't think it would matter even if there was a very brief overlap.

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Old 25th Oct 2020, 12:59 am   #12
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Default Re: 12VDC power switching circuit

I would pull a couple of MOSFETs out of an old computer power supply.
They come with built in diodes and are high enough impedance to work without drivers in a simple multi vibrator.
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Old 25th Oct 2020, 2:55 pm   #13
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Default Re: 12VDC power switching circuit

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I would pull a couple of MOSFETs out of an old computer power supply.
They come with built in diodes and are high enough impedance to work without drivers in a simple multi vibrator.
I think I may have "binned" the last of the old PSU's I had in the junkbox. Can you point at a MOSFET type which would fit the bill?

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Old 25th Oct 2020, 3:32 pm   #14
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Default Re: 12VDC power switching circuit

The good old brain remembered the 4521 CMOS oscillator/divider, they can work at 12V and drive a pair of emitter followers to boost the output in push pull, no need for back EMF protection with a push pull drive. Or stack 20 together (20X10mA)!
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Old 25th Oct 2020, 3:42 pm   #15
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Default Re: 12VDC power switching circuit

Well, the "Chief Designer" is now of the opinion that 200mA may have been an underestimate and so it looks like a design review needs organising but thanks for the inputs as the basic requirement continues.

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Old 25th Oct 2020, 3:44 pm   #16
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Default Re: 12VDC power switching circuit

I remembered because I have one of the things in the photo below, it is used to time the light when you turn the "hour glass" over, 11 minutes. And before you ask it isn't full of diamonds just cubic zirconia, very pretty.
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Old 25th Oct 2020, 4:30 pm   #17
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Default Re: 12VDC power switching circuit

Yes, that is an elegant item. I wonder what the fluid is?
The Chief is now talking about 24V at 300mA, but that could change!

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Old 25th Oct 2020, 5:01 pm   #18
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Default Re: 12VDC power switching circuit

The fluid is silicone oil mixed from high and low viscosity types to give the right slowness of fall, all made from bits left over. A real one is worth/costs umpteen pounds, look up "De Beers Hourglass".

Back on topic, if you could get away with 20V high voltage CMOS will do that.
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Old 25th Oct 2020, 5:06 pm   #19
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Default Re: 12VDC power switching circuit

Quote:
Originally Posted by merlinmaxwell View Post
all made from bits left over.
You made that, damn impressive

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Old 25th Oct 2020, 5:50 pm   #20
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Default Re: 12VDC power switching circuit

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You made that, damn impressive
Only from bits of the original thing left over. All I did was source the Cubic Zirconia stones (remarkably cheap, they worked out at less than a tenner for 2000 of them with postage etc.) and get the oil the right thickness. Oh and it was a present for SHMBO.

We are drifting a bit, back on topic, if 30s is acceptable how about a clock movement, two magnets on the second hand drive and a couple of reed swithes.
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