Electronic dimmer as Variac

[DaveC]

I want to turn up a switching power supply slowly after repairing it, so as
to see if anything else is damaged. I have no Variac, and I could use the
series light bulb trick, but I already have a light dimmer I'm using as a
variable-temperature control for my soldering iron that could do double-duty
in this application.

I know that the SMPS, when running correctly, will be a load pulse at 75 kHx.
Has anyone tried using a dimmer as an "electronic variac"? Is it feasible?

Thanks,

 

[John Popelish]

I want to turn up a switching power supply slowly after repairing it, so as
to see if anything else is damaged. I have no Variac, and I could use the
series light bulb trick, but I already have a light dimmer I'm using as a
variable-temperature control for my soldering iron that could do double-duty
in this application.

I know that the SMPS, when running correctly, will be a load pulse at 75 kHx.
Has anyone tried using a dimmer as an "electronic variac"? Is it feasible?

It is very doubtful. A dimmer needs a minimum current load to keep it
conducting the remainder of each half cycle after it is fired.
Resistive loads are ideal, and average the resultant waveform quite
well. SMPS usually rectify the line waveform and apply that directly
to an energy storage capacitor that charges up to the peak waveform
voltage each half cycle (and does that during a small part of the
cycle, near the peak).

When your dimmer fires, the rectifier and cap will look like a near
short circuit to a voltage that is something less than the line
voltage at that instant (whatever voltage remains on the cap from the
previous half cycle). And the moment the line voltage starts to go
down, the rectifier will turn off, isolating the cap from the dimmer,
causing the dimmer current to fall below its holding current, so it
will turn off. In other words, you will probably not get the smooth
control you are needing (any setting that includes the peak of each
half cycle will produce the same result) and and may damage the dimmer
and/or the rectifier and/or the capacitor with the big pulses of
current that occur as the dimmer switches on.

Sometimes you need the right tool.

 

[Ben Bradley]

sci.electronics.basics,sci.electronics.design,sci.electronics.misc,sci.electronics.repair,

I want to turn up a switching power supply slowly after repairing it, so as
to see if anything else is damaged. I have no Variac, and I could use the
series light bulb trick, but I already have a light dimmer I'm using as a
variable-temperature control for my soldering iron that could do double-duty
in this application.

I know that the SMPS, when running correctly, will be a load pulse at 75 kHx.
Has anyone tried using a dimmer as an "electronic variac"? Is it feasible?

My immediate response is don't do this. The SMPS will pull large
amounts of current as the voltage approaches peak (rectifier from
power line charging main capacitor), and it may exceed the peak
current rating of the dimmer, even accounting for the fact that light
filaments pull a large amount of current when cold (at least they heat
up rather quickly and pull less current).
My advice is (with all the caps dischharged) double-check all the
rectifiers and other 'main' semiconductors (such as the main flyback
transistor), then stand back and give it full power.
But then I'm posting from SED and don't do much repair. If you
don't want me steering you wrong, don't crosspost outside the *.repair
group.

 

[Rich Grise]

It is very doubtful. A dimmer needs a minimum current load to keep it
conducting the remainder of each half cycle after it is fired.
Resistive loads are ideal, and average the resultant waveform quite
well. SMPS usually rectify the line waveform and apply that directly
to an energy storage capacitor that charges up to the peak waveform
voltage each half cycle (and does that during a small part of the
cycle, near the peak).

When your dimmer fires, the rectifier and cap will look like a near
short circuit to a voltage that is something less than the line
voltage at that instant (whatever voltage remains on the cap from the
previous half cycle). And the moment the line voltage starts to go
down, the rectifier will turn off, isolating the cap from the dimmer,
causing the dimmer current to fall below its holding current, so it
will turn off. In other words, you will probably not get the smooth
control you are needing (any setting that includes the peak of each
half cycle will produce the same result) and and may damage the dimmer
and/or the rectifier and/or the capacitor with the big pulses of
current that occur as the dimmer switches on.

Sometimes you need the right tool.

I tried to use an SSR once to power a SMPS. I think I went through
about 3 SSRs before I figured out that you can't do that. And that
was just on/off!

 

[Sofie]

Dave C:
Cheap consumer grade quadrac, triac/diac and scr based light dimmers change
only the "duty cycle" of the power and NOT the voltage. Unless special
things are done they can not easily be used for induction, transformer and
ballast loads such as motors, fluorescent light fixtures and most devices
that use a power transformer which includes linear supplies and switching
supplies.....
Usually a mostly resistive load such as a soldering iron or incandescent
lamps will be the most appropriate application for cheap light dimmer
circuits. Special fan motor speed controls will cost more and must be
used with caution..... a setting that is too slow can stall the motor and
cause motor overheating and failure.
An auto-transformer based variac the one of the more useful pieces of
equipment on an electronics bench. EBAY regularly has fairly good deals.
In addition, a companion to the variac, an isolation transformer is a
"SAFETY must have" when working on "hot" chassis equipment.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

I want to turn up a switching power supply slowly after repairing it, so as
to see if anything else is damaged. I have no Variac, and I could use the
series light bulb trick, but I already have a light dimmer I'm using as a
variable-temperature control for my soldering iron that could do double-duty
in this application.

I know that the SMPS, when running correctly, will be a load pulse at 75 kHx.
Has anyone tried using a dimmer as an "electronic variac"? Is it

feasible?

Read the instructions that came with the dimmer. It probably says that
you're only supposed to use it with a resistive load such as a light
bulb. It'll probably burnout if used on a PS.

 

[Frithiof Andreas Jensen]

Is it feasible?

Likely not, use the light bulb trick instead - I prefer a Toaster in series
mysel just to save the fuses.

A SMPS will, if it is a decent design, have a threshold where the supply
will start - so the first 1/3 of the input voltage range it will be off
anyway and ramping up the voltage does not buy you anything other than
saving a fuse: It will start suddently by itself.

A SMPS is usually a capacitive load, the dimmer is designed for an Ohmic
load i.e. a light bulb, and the TRIAC in the dimmer will not appreciate
piring directly into a capacitor that will look like a dead short. They you
can stick an inductor in series - and you might get at resonant overvoltage
blowing the supply/dimmer away.

IOW: It creates more trouble than the problem you started with.

 

[El Meda]

"Frithiof Andreas Jensen"

Likely not, use the light bulb trick instead - I prefer a Toaster in series
mysel just to save the fuses.

A SMPS will, if it is a decent design, have a threshold where the supply
will start - so the first 1/3 of the input voltage range it will be off
anyway and ramping up the voltage does not buy you anything other than
saving a fuse: It will start suddently by itself.

A SMPS is usually a capacitive load, the dimmer is designed for an Ohmic
load i.e. a light bulb, and the TRIAC in the dimmer will not appreciate
piring directly into a capacitor that will look like a dead short. They you
can stick an inductor in series - and you might get at resonant overvoltage
blowing the supply/dimmer away.

IOW: It creates more trouble than the problem you started with.

And if the OP uses the dimmer to control a light bulb, and connect the
SMPS in parallel with the bulb? Will it work?

 

[DaveC]

OK, OK, I think I get the idea. I had no plans to try it unless others here
thought it might work. Guess not...

So it's a light bulb (or toaster) in series.

Thanks for lending me your minds,

 

[John Popelish]

OK, OK, I think I get the idea. I had no plans to try it unless others here
thought it might work. Guess not...

So it's a light bulb (or toaster) in series.

Thanks for lending me your minds,

A divider made of two light sockets wired in series may be helpful,
also. Connect the supply across the bulb on the neutral side of the
divider. If the two bulbs are the same wattage, the voltage is
limited to no more than half the line even if the supply does not draw
current. It takes little time to screw in various bulb combinations.

 

[Ian Stirling]

It is very doubtful. A dimmer needs a minimum current load to keep it
conducting the remainder of each half cycle after it is fired.
Resistive loads are ideal, and average the resultant waveform quite

A lot of the nastiness can be reduced by putting an appropriate resistor
in series with the dimmer.
It's got to be high peak wattage so that it can take series connection
between the (effectively) peak mains voltage, and zero of an uncharged cap.

If you pick the R so that the RC filter formed is some 5ms or so, then
that'll probably be around the right number.

 

[John Popelish]

A lot of the nastiness can be reduced by putting an appropriate resistor
in series with the dimmer.
It's got to be high peak wattage so that it can take series connection
between the (effectively) peak mains voltage, and zero of an uncharged cap.

If you pick the R so that the RC filter formed is some 5ms or so, then
that'll probably be around the right number.

It might just work if you use one resistive load (Say, a 25 to 40 watt
light bulb) as a minimum load and put a second resistor (say, a 40 to
100 watt light bulb) in series with the input of the supply as a
second load. That one would function as part of the low pass filter
you describe.

 

[Ian Stirling]

It might just work if you use one resistive load (Say, a 25 to 40 watt
light bulb) as a minimum load and put a second resistor (say, a 40 to
100 watt light bulb) in series with the input of the supply as a
second load. That one would function as part of the low pass filter
you describe.

I assume that the first one is to keep the triac conducting.
Why?
If it's going to a rectifier/cap, then how does it matter?
When the current goes to zero, the rectifier turns off anywya.

 

[John Popelish]

I assume that the first one is to keep the triac conducting.
Why?
If it's going to a rectifier/cap, then how does it matter?
When the current goes to zero, the rectifier turns off anywya.

The phase timing circuit in many dimmers only functions properly if
there is a resistive load on the dimmer. The timing current passes
through the load.

 

[Sam Goldwasser]

The simple answer is that if the dimmer sees mostly a resistive load, it
will work reasonably well and survive. This *can* include things like
shaded pole motor-based fans (but probably not other types of induction
motors or universal motors). However, if it's driving a lot of capacitance or
inducatance, at the very least it will behave strangely and more likely
will self destruct or damage the equipment. There's also usually a minimum
load below which it won't do anything predictable. In short, get a proper
Variac. You know my motto: "You can never have too many Variacs!".

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header is ignored.
To contact me, please use the feedback form on the S.E.R FAQ Web sites.

 

[James Sweet]

I want to turn up a switching power supply slowly after repairing it, so as
to see if anything else is damaged. I have no Variac, and I could use the
series light bulb trick, but I already have a light dimmer I'm using as a
variable-temperature control for my soldering iron that could do double-duty
in this application.

I know that the SMPS, when running correctly, will be a load pulse at 75 kHx.
Has anyone tried using a dimmer as an "electronic variac"? Is it feasible?

Thanks,
--
DaveC
[email protected]
This is an invalid return address
Please reply in the news group

This won't work, a light dimmer doesn't vary the voltage, it varies the duty
cycle, so if you plug a switching supply into one you'll fry it.