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Old 25th Oct 2020, 6:04 pm   #21
Refugee
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Default Re: 12VDC power switching circuit

Have a look at these two searches.
The gates are voltage driven at 12 volts mostly so they will work in a simple multi vibrator.
If your loads are DC you can drive them directly without relays.
Those starter buttons on modern care drive the solenoid directly with a FET.
Electric power steering is done with a stepper motor driven with FETs so they are jelly bean products.

https://uk.farnell.com/webapp/wcs/st...6877,110169483

https://uk.farnell.com/webapp/wcs/st...6877,110169483
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Old 25th Oct 2020, 8:11 pm   #22
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Default Re: 12VDC power switching circuit

OK, thanks for that. I think the way forward is to breadboard a MOSFET multivibrator based on the worst case assumptions that I need to run with 24V and at least 300mA. That will work with some solenoids that are available from my junkbox, whereas the 12V 200mA are on route from China, so won't be here for a while, and may be slightly under powered anyway.

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Old 25th Oct 2020, 11:39 pm   #23
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Default Re: 12VDC power switching circuit

MOSFETs may need a pull down resistor and voltage limiter zener on the gates.
The main diode is built into the FET though.
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Old 25th Oct 2020, 11:55 pm   #24
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Default Re: 12VDC power switching circuit

And you may also need to force one to start before the other.
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Old 28th Oct 2020, 8:32 pm   #25
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Default Re: 12VDC power switching circuit

I had a similar project involving four solenoids

https://youtu.be/PIwZuu1fK-c

Ended up with an ATtiny85 microcontroller driving 4xTIP41C transistors (and a bit of code)
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Old 28th Oct 2020, 9:26 pm   #26
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Default Re: 12VDC power switching circuit

Interesting, but simplicity is the keyword here. I have this feeling that one transistor and a relay could be made to do this. I vaguely recall making a unit which provided variable delay windscreen wiper control back in the 70's, when single speed wipers was standard. Unijunction transistor?

Two transistor in a multivibrator seems OK; I'm waiting for the solenoids to arrive.

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Old 29th Oct 2020, 1:08 am   #27
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Default Re: 12VDC power switching circuit

Bazz-I remember the wiper delay circuit ( without any components). It was basicly ( for positive earth cars) , a pair of PNP transistors . The first used a RC network on the base to switch it on, with the second providing current to operate a PO2000 RELAY . One contact drove the wipers, the other shorted out the capacitor in the RC network and released the relay ,allowing for the RC network to re charge.
I designed a modified version of this circuit to drive our TRS cable test circuit , using cold cathods when the ATE timer circuit was taken out of service.
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Old 29th Oct 2020, 2:24 am   #28
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Default Re: 12VDC power switching circuit

OK, I've actually found the two circuits, which appeared in Elektor, September 1976. Just so that we stay on topic, these are circuits with variable delay switching for any application.

The first is a 555 which operates a relay. The second is a 2N2646 unijunction transistor (still available) which triggers an SCR, which switches on the load, but that circuit expects the load to switch itself off at some point in order to cycle.

As I want to cycle about every 10-20 seconds for long periods of time, the idea of using a relay in the year 2020 sounds a bit "clunky", but it would be a pretty sure-fire place to start. The relay would certainly allow easy swopping between solenoids of either 12 or 24 volts, which would be a very nice to have feature to begin with.

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Old 29th Oct 2020, 8:30 am   #29
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Default Re: 12VDC power switching circuit

The 555 can drive 200mA and work at up to 18V, that should drive a fair sized relay, the push pull output doesn't need a back EMF diode.
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Old 30th Oct 2020, 7:05 pm   #30
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Default Re: 12VDC power switching circuit

The back EMF from a relay coil is indeed an issue, and for many years a flywheel diode was recommended, and it was possible to buy relays with built in diodes. But certainly in some fields, the relay manufacturers have moved away from this and instead have a resistor. I don't know the detailed reason why but it's somehow related to the opening characteristic of the contacts where a diode affects the time or speed of opening and ends up causing earlier contact wear.

Regarding a circuit, a 555 would work but bear in mind they have a funny little quirk, in that the output is totem pole (that is active high and low) and during change of state both transistors are on at the same time albeit very briefly. Not only can that cause them to be quite generous creators of interference, but on a low impedance supply the dissipation can be surprisingly high. The first time I smelled 'that' smell was when I used a 555 for my own design of intermittent wiper control, which was fine on a power supply but lasted just a few minutes in a car before emitting all of the magic smoke.

Oh, and I'd also say a FET was a good bet for driving relay coils. Lower dissipation generally (depends on RDSon of course) and the intrinsic body diode clamps the back EMF (but consider the first paragraph contardiction here).
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Old 30th Oct 2020, 8:45 pm   #31
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Default Re: 12VDC power switching circuit

Thanks for that info; I'll search out a relay that doesn't need much current. The relay option is good because it will be a while until I know whether I can work with 12V solenoids or have to use 24V ones.

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Old 30th Oct 2020, 8:47 pm   #32
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Default Re: 12VDC power switching circuit

Thanks for that info; I'll search out a relay that doesn't need much current. The relay option is good because it will be a while until I know whether I can work with 12V solenoids or have to use 24V ones.

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Old 31st Oct 2020, 8:25 am   #33
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Default Re: 12VDC power switching circuit

The intrinsic diode in a MOSFET doesn’t clamp the back EMF, it’s in the wrong place and the wrong way round. However, MOSFETS cope with it by avalanche breakdown.
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Old 31st Oct 2020, 9:25 am   #34
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Default Re: 12VDC power switching circuit

Do you need any transistors or relays at all? I'm thinking a pair of contacts linked to each solenoid, cross-coupled and with RC to slug the timing might be all that's needed.
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Old 31st Oct 2020, 1:45 pm   #35
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Default Re: 12VDC power switching circuit

Quote:
cross-coupled and with RC to slug the timing
Rather large capacitors would be needed I think.
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Old 31st Oct 2020, 2:06 pm   #36
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Default Re: 12VDC power switching circuit

There are many, many ways of doing this, with all sorts of mixtures of advantages and disadvantages. You have to first choose one and then do what it takes to make it work. So maybe you didn't choose the optimum one, but that's not a problem so long as it's a viable one. Not choosing leaves you stuck, dithering, on the starting line.

You need to be orderly in making your choice.

Make a list of things the switch must do:

XX volts worst case
YY Amps worst case
How fast must it switch? How much time could it take to switch over?
How much voltage drop is allowable?
What power source is available to it?
What control signal does it get? (voltage, current capability)

Then do a list of things it must not do:

Delay too long
Take too long in switching
Take too much power
Unreliability
Excessive cost
Excessive size
Excessive heat

And plenty of other things you can think of.

In this way you build up a spec for the thing. Specs are important. Without one you could create the ideal thing and not be aware.

Only once you've got these things down can you start designing. There's no point running fast if you hadn't first decided on the direction. It looks boring writing a spec, but it's an important step that can save you a lot of waste and anguish.

Wish I'd done it every time

David
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Old 31st Oct 2020, 7:10 pm   #37
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Default Re: 12VDC power switching circuit

David, post#32 spells out where I'm at. I've found some 24V components in my junk box, but I' hoping the 12V ones (still on the Silk Road) will be adequate when they get here. Close the thread if you wish; thanks to all contributors.

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Old 1st Nov 2020, 2:22 am   #38
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Default Re: 12VDC power switching circuit

No, I'm not angling to close it. I think I'd have gone for a simple multivibrator driving a pair of power MOSFETs with a bit of care to protect the gates if run at 28v. It would mean no moving parts other than the solenoids.

I was just making the point that there is an awful lot of ways of doing this and almost any of them can be made to work. It's the sort of thing that can get me mired in indecision. Sometimes tossing a load of coins is the right answer

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Old 1st Nov 2020, 3:15 am   #39
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Default Re: 12VDC power switching circuit

Yes indeed, and that's very much why I posted the thread on what is not a very complex problem, but just as a way of getting a range of options on the table.

It's very different from the days when I had to go stand in front of a rather 'abrasive' Technical Director every month, and be publicly grilled on matters of Project progress and spend, though I suspect "my projects" moved much faster back then .

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