Dimmer won't work when connected to power inverter

[Chuck]

Dimmer won't work when connected to power inverter

I successfully tested an 18ft long section of rope lights (Hunter brand) and a 3-stage touch dimmer in my house before taking out to install it in my RV. After plugging it into my 12VDC to 110VAC power inverter it just has one brightness level which is maximum brightness. Once its on, I can't turn it off again without unplugging it. I tried using a standard incandescent rotary dimmer with the exact same results. Why is this happening?

Why does it work properly on 110V 60Hz household current but not with the power inverter? Thanks.

 

[Chronic Philharmonic]

Dimmer won't work when connected to power inverter
I successfully tested an 18ft long section of rope lights (Hunter brand) and
a 3-stage touch dimmer in my house before taking out to install it in my RV.
After plugging it into my 12VDC to 110VAC power inverter it just has one
brightness level which is maximum brightness. Once its on, I can't turn it
off again without unplugging it. I tried using a standard incandescent
rotary dimmer with the exact same results. Why is this happening?

Why does it work properly on 110V 60Hz household current but not with the
power inverter? Thanks.

 

[Chuck]

Suggestion - try a cheaper dimmer! Most ordinary light dimmers,
designed for mounting in a wall switch are still the cap type here in
NZ - but they are probably electrically "noiser" than the more
expensive ones.

Eric Sears

OK... I will try various dimmers and try adding caps across the output to
see if I can get it to dim. Thanks.

 

[Chronic Philharmonic]

OK... I will try various dimmers and try adding caps across the output to
see if I can get it to dim. Thanks.

Mmm... not sure if adding caps will work. The type of dimmer that is most
prevalent in the US uses a TRIAC
(http://www.epanorama.net/documents/lights/lightdimmer.html).

These things simply trigger on the voltage level as the sine wave swings up
and down. This controls the "on" time for each cycle, and hence the
brightness of the (incandescent) lamp. If you feed in a square wave, there
is no variable voltage threshold to trigger on, because the square wave
turns on and off sharply. The dimmer will not dim.

You need a dimmer that is based on timing instead of threshold, but they are
not as common. The article I linked to should explain just about everything
you need to know about dimmers.

 

[Eric Sears]

Why does it work properly on 110V 60Hz household current but not with the
power inverter? Thanks.

-----------------------

That's because your inverter probably generates a square wave (with vertical
sides), but dimmers require a sine wave (with sloping sides). Without the
slope, it can't dim. It's either on or off.

I suspect that "Chronic P" is correct - but its a combination of TWO
factors - 1) the mod square wave of the inverter and
2) the way in which that particular dimmer works.
Not all dimmers are the same. Some use the slope of the sine wave, but
others use the charging of a capacitor to trigger them.

I use a mod square inverter ( probably like yours, only 230v), and it
works perfectly with a number of dimmers I have tried - including with
both incandescent lights and battery chargers (for varying the
charge). In fact I charge my electric car (a converted Daihatsu Mira)
with a battery charger that runs on my mod sq inverter. Charging is
controlled with a dimmer, and gives smooth charging from zero to 8
amps at 60 volts - through a transformer.
Suggestion - try a cheaper dimmer! Most ordinary light dimmers,
designed for mounting in a wall switch are still the cap type here in
NZ - but they are probably electrically "noiser" than the more
expensive ones.

Eric Sears.

 

[Chuck]

Mmm... not sure if adding caps will work. The type of dimmer that is most
prevalent in the US uses a TRIAC
(http://www.epanorama.net/documents/lights/lightdimmer.html).

These things simply trigger on the voltage level as the sine wave swings
up and down. This controls the "on" time for each cycle, and hence the
brightness of the (incandescent) lamp. If you feed in a square wave, there
is no variable voltage threshold to trigger on, because the square wave
turns on and off sharply. The dimmer will not dim.

You need a dimmer that is based on timing instead of threshold, but they
are not as common. The article I linked to should explain just about
everything you need to know about dimmers.

Thanks for the link. I have found a few "pure" sine wave inverters but the
cost is considerably higher.

 

[Eric Sears]

NOTE! I did not say anything about adding a cap to get it to dim. I
said they use a cap - its in the trigger circuit. You can't just add a
cap across the input or output.
Yes, they all use triacs - but there are various ways of firing or
triggering it.
If you use the straight sine wave, then when the voltage of the sine
wave reaches a certain level (set by the "dimmer" pot), the triac
turns on - and won't turn off again until the half-cycle finishes.

However, if you incorporate a cap with the dimmer pot, you have a time
constant, so that the cap begins charging, and reaches a point when it
triggers the triac "on". For this charging to occur, it doesn't matter
whether it is a square wave or sine wave (just a bit different
adjustment of the pot). The lower the resistive value that the pot is
set to, the quicker the cap charges on each half cycle, and the sooner
the triac turns on.

They are actually very simple devices and if you google "light dimmer
circuit" who will find many on the internet. Basically they have just
4 components - triac, pot, cap and a diac (some don't need the
latter).

Hope this helps.

Eric Sears.

Mmm... not sure if adding caps will work. The type of dimmer that is most
prevalent in the US uses a TRIAC
(http://www.epanorama.net/documents/lights/lightdimmer.html).

These things simply trigger on the voltage level as the sine wave swings up
and down. This controls the "on" time for each cycle, and hence the
brightness of the (incandescent) lamp.

Note - the problem with those that do NOT use a cap is that they must
trigger "on" during the first half of the "half-cycle". Since they
cannot turn off again until the zero-crossing point (for ordinary
triacs), full dimming is not possible. However, they are some triacs
which act more like a (bi-directional) transistor, and turn off with a
lower threshold of input - these will give full dimming.

 

[Chronic Philharmonic]

[snip]

Thanks for the link. I have found a few "pure" sine wave inverters but
the cost is considerably higher.

Yeah, that's the problem with the sinewave inverters. A compromise is a
stepwise inverter, which might give you a number of discrete levels of
dimming.