Running a cordless drill from an AC transformer

[9fingers]

It's got me completely puzzled, I'm afraid. Are you suggesting that my 2 way switching should be at 240V? If so, I'd rather go out and buy a commercial lifter as don't have the skills necessary to safely go playing around with 240V. DaveL has supplied the 2 way microswitches, so I suspect they are properly rated for the job in hand. Therefore I'm not sure what problem I would be avoiding by doing the on-off switching on the mains side of the transformer.

The advantage that I see is that your power supply with be switched on only when you need the TV to be moving. Your solution might well achieve this too.

There are multiple ways to control this device and you must continue with one that you understand and feel comfortable with.

Bob

 

[stevegruk]

Mike,

Did you get this sorted? Do you need anymore help?

 

[Mike G]

Well....sort of. The lift mechanism and controls work, but there is too much movement in the system, and not enough room in the cabinet. I will be treating this effort as a prototype, and building a more powerful (18V) set-up from steel in due course. The mechanical switching of direction via the over-centre device works flawlessly, but is too big, and in a vulnerable place, and proved very difficult to adjust. The new one will have a similar device, but almost certainly located along the threaded rod so that adjustment will just be a matter of moving it left or right a bit. Another big issue was control of the cables. I had a number of accidents where the moving lift platform fouled a cable.

So, this exercise has moved quite a way down my priority list, but will get done one day.

 

[9fingers]

I too had noted the huge overcentre mechanism.

My thought was that at almost every toggle switch has its own over centre capability inside and is minute compared to yours. I certainly like/prefer something located on the screw ram itself as that should be much more responsive and easier to adjust.
What you have done is a great effort Mike :eusa-clap: and hopefully my suggestions are seen as constructive rather then critical.

Bob

 

[Mike G]

......hopefully my suggestions are seen as constructive rather then critical....

:lol: Indeed, although I tested your patience more than once.

 

[9fingers]

......hopefully my suggestions are seen as constructive rather then critical....

:lol: Indeed, although I tested your patience more than once.

Apologies if it showed. Would not have been my intention Mike

Bob

 

[Mike G]

Not at all, Bob.

 

[stevegruk]

Hi Mike,

If you fancy doing some DIY electronics there is this very good value evaluation board which will drive a motor in both directions up = to 35A - which should be fine. This would replace all the power switches and reversing mechanism you have now.


https://uk.farnell.com/stmicroelect...al-board-high-bridge-motor-driver/dp/2797977

You would need a bit of logic (or a micro if would want to get into programming) to sense if the platform was at the top or bottom, latch this as you pressed the pushbutton and drive the INA and INB inputs to the H bridge board. to drive up or down.

The logic could be done with two little micro switches - which only see low voltage low current - and a dual flip flop IC such as a 74HC74 very cheap. You would need a 5V linear voltage regualt such as 7805 a few capaciots - again ceaps, some 'Veroboard' and soldering skills. The push button would clock the flip-flop which would latch the current position (i.e. top or bottom) and drive the correct input INA or INB. Upon reaching the top or bottom the ‘opposite’ micro switch would clear the flip-flop circuit to switch off the drive and set the D (Data) input to the flip flop ready for the next operation of the pushswitch. An altenative to using microswitch is an opto or magnetic sensor or magnet and reed swtich.

 

[Sheffield Tony]

Hi Mike,

If you fancy doing some DIY electronics there is this very good value evaluation board which will drive a motor in both directions up = to 35A - which should be fine. This would replace all the power switches and reversing mechanism you have now.

https://uk.farnell.com/stmicroelect...al-board-high-bridge-motor-driver/dp/2797977

Hmm. That looks like the numbers were picked by the marketing department. 35A, with a 0.04 ohm driver impedance, and about 40C/W thermal resistance die to ambient. Lost power at 35A is 35*35*0.04 = 49W in that little PCB. I don't think so. Working back from the 150C max temperature, 9A would be more honest. Maybe 35A for a very short time. It is worth being wary of pokey motors, the winding resistance can be very low. When they are spinning, the back EMF opposes the current flow - when stalled, it can be HUGE.

Perhaps making a nice cabinet that you can actually put something in, and just placing the TV on top, might not be such a bad option after all :lol: (Runs for cover ...)

 

[Mike G]

I appreciate all the suggestions here. I really do. And I wish I understood them, but I don't. I really don't.

I can say this with absolute certainty: the only way I going to build this thing is with stuff I can understand, so that limits me to controls which are switches, and only switches. That means that this will be electro-mechanical in nature, not electronic or computer controlled. I just don't have a scooby do about that stuff. Logic circuits and programming would mean getting one of you guys to do it for me, because there is nothing more certain than I can't do that sort of thing for myself.

 

[AndyP]

In the best interests of men in sheds and good ol' British ingenuity I do so hope that you can pull this off Mike.

Plonking TV's on top of cabinets is what mere mortals like me do. Where is the fun in that?

 

[stevegruk]

Sheffield Tony,

Agreed - without extra cooing it can’t provide 35A continually. I was trying to keep it simple and the eval board would most likely be adequate.

35A is the current limitation. When running the motor will not draw this much and it wouldn’t be stalled for very long at all with limit switches and logic. The transient thermal impedance is much lower –see datasheet and it ought to easily start the motor and load moving. The IC has thermal protection as well.

Anyway, none of this is a help to Mike.