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From: Rich Grise on 9 Dec 2009 17:04
On Wed, 09 Dec 2009 13:00:21 -0800, Matt wrote:
> On Dec 9, 1:41�pm, John Fields <jfie...(a)austininstruments.com> wrote:
>> On Wed, 9 Dec 2009 11:01:25 -0800 (PST), Matt
>> >I was hoping someone might be able to help with a problem I've been
>> >having with a TV lift I built. �I have an actuator that raises and
>> >lowers a TV for the bedroom. �It's wired with a DPDT switch. �I'm
>> >using a 12V 5A power supply. �When I first plug it in, it works fine.
>> >However, after a while, nothing happens when I flip the switch. Any
>> >ideas on if this is a power supply problem and what the remedy would
>> >be?
>>
>> If your DPDT switch doesn't have a center OFF position, then when the
>> lift gets to either the full up or full down position the motor's rotor
>> will be locked and the power supply will be driving a very low
>> resistance and outputting a lot of current.
>>
>> If you're lucky �and your power supply has a crowbar in it (or current
>> or thermal overload protection of some sort) then it'll just shut down
>> until things cool down or you unplug it and then plug it back in.
>>
>> If that's what's happening what you need is a circuit which senses when
>> the lifter gets to its limit(s) and then disconnects the AC side of the
>> supply from the mains or the DC side of the supply from the load until
>> the next time you throw the switch.
>>
>> Either that or replace the switch with one that has a spring loaded
>> toggle which goes to the center OFF position when you release it.
>
> Thanks. The switch does have an OFF position. The actuator has built- in
> limit switches at both ends, but I was operating it in the middle of its
> range when the problems occurred. I'll try a rocker switch that stays in
> the off position, but I'm not sure that was the problem since I could
> raise, lower and leave it in the OFF position. Thanks for the input.

Is the power supply properly sized for the motor? It could be going into
thermal shutdown, or the motor could have a thermal cutout if it's too
small for the job.

Is this a recent problem?

Good Luck!
Rich

From: Jamie on 9 Dec 2009 19:08
Matt wrote:

> Hello all,
> I was hoping someone might be able to help with a problem I've been
> having with a TV lift I built. I have an actuator that raises and
> lowers a TV for the bedroom. It's wired with a DPDT switch. I'm
> using a 12V 5A power supply. When I first plug it in, it works fine.
> However, after a while, nothing happens when I flip the switch.
> Any ideas on if this is a power supply problem and what the remedy
> would be?
> Thanks in advance.
> Matt
I don't know what type of power supply you're using how ever, I'll take
a guess and say it's a switch mode type that is shutting down from lack
of load after some time.
Since it appears that you don't seem to be super technically
inclined, I'll offer you a simple test solution. Place a 12 volt lamp
across the
output of the supply and let it burn.. If the unit remains working after
that, it confirms my theory how ever, if the lamp drops out on you when
not attempting to use the supply to move the unit, then you have a bad
supply..

Also, inductive loads on some supplies will cause them to shut down,
the lamp load should also cure this problem. If all works, then you can
use a resistor in the lamps place. You could also put a back EMF diode
on the output of the supply incase you do have a little kick back issue.

From: Jasen Betts on 10 Dec 2009 02:50
On 2009-12-09, Matt <matthew.n.johnson(a)gmail.com> wrote:
> Hello all,

> I was hoping someone might be able to help with a problem I've been
> having with a TV lift I built. I have an actuator that raises and
> lowers a TV for the bedroom. It's wired with a DPDT switch. I'm
> using a 12V 5A power supply.

That power supplky seems a little light, I guess it depends on how
hard the TV is to move. It could be the power supply overheating
and going into protective shutdown. if you haven't already try
adding some sort of counterweight like are used on lifts


From: Matt on 10 Dec 2009 15:41
On Dec 10, 1:50 am, Jasen Betts <ja...(a)xnet.co.nz> wrote:
> On 2009-12-09, Matt <matthew.n.john...(a)gmail.com> wrote:
>
> > Hello all,
> > I was hoping someone might be able to help with a problem I've been
> > having with a TV lift I built.  I have an actuator that raises and
> > lowers a TV for the bedroom.  It's wired with a DPDT switch.  I'm
> > using a 12V 5A power supply.
>
> That power supplky seems a little light, I guess it depends on how
> hard the TV is to move.  It could be the power supply overheating
> and going into protective shutdown. if you haven't already try
> adding some sort of counterweight like are used on lifts

The specs on the actuator I'm using say 12 VDC, 5Amp current draw at
full load. It's rated for 150 lbs and is lifting about 30 when the TV
is loaded. The power supply I'm using is one you'd use for an LCD
screen. It seems to lift things fine at first, but when it's been
inactive for a while, it doesn't start again.

I'll try place the lamp across the output and see what happens. (To
answer one poster's remark, Yes, I'm an electronics neophyte with no
formal training in the subject.)

Thanks again to all for the input.
From: default on 11 Dec 2009 10:29
On Wed, 9 Dec 2009 13:00:21 -0800 (PST), Matt
<matthew.n.johnson(a)gmail.com> wrote:

>On Dec 9, 1:41�pm, John Fields <jfie...(a)austininstruments.com> wrote:
>> On Wed, 9 Dec 2009 11:01:25 -0800 (PST), Matt
>>
>> <matthew.n.john...(a)gmail.com> wrote:
>> >Hello all,
>> >I was hoping someone might be able to help with a problem I've been
>> >having with a TV lift I built. �I have an actuator that raises and
>> >lowers a TV for the bedroom. �It's wired with a DPDT switch. �I'm
>> >using a 12V 5A power supply. �When I first plug it in, it works fine.
>> >However, after a while, nothing happens when I flip the switch.
>> >Any ideas on if this is a power supply problem and what the remedy
>> >would be?
>>
>> ---
>> If your DPDT switch doesn't have a center OFF position, then when the
>> lift gets to either the full up or full down position the motor's rotor
>> will be locked and the power supply will be driving a very low
>> resistance and outputting a lot of current.
>>
>> If you're lucky �and your power supply has a crowbar in it (or current
>> or thermal overload protection of some sort) then it'll just shut down
>> until things cool down or you unplug it and then plug it back in.
>>
>> If that's what's happening what you need is a circuit which senses when
>> the lifter gets to its limit(s) and then disconnects the AC side of the
>> supply from the mains or the DC side of the supply from the load until
>> the next time you throw the switch.
>>
>> Either that or replace the switch with one that has a spring loaded
>> toggle which goes to the center OFF position when you release it.
>>
>> JF
>
>Thanks. The switch does have an OFF position. The actuator has built-
>in limit switches at both ends, but I was operating it in the middle
>of its range when the problems occurred. I'll try a rocker switch
>that stays in the off position, but I'm not sure that was the problem
>since I could raise, lower and leave it in the OFF position.
>Thanks for the input.

When you say actuator I think of lead-screw type.

Can you check the PS voltage to see what it is doing?

An eccentric or cam (think piston crank) may require much more current
at the middle of the range than the ends.

Where did you find it and what was it's original purpose? It may be
binding or have some problem like rust.

I've had a satellite dish actuator on my bed for some time now. It
raises the headboard side to make reading easy. 24 VDC 6 amp supply.
One switch sends it up/down the other switches off the supply from the
mains. Immensely powerful - I mounted one toggle switch so it is off
in the up position in case my arm gets caught in it.
--
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