Starting over with an abandoned project ( replacing AC transformer PS with a switching PS)

Thread Starter

rolo95

Joined Dec 25, 2017
18
Guys,
i started a project around 2017 to modify some Club DJ scanner lights
removing the heavy iron 120V ac step down transformer and replacing it with a switching power supply

Wife got very sick and i had to abandon the project, she is stable now
so i want to continue on my quest

Special Thanks to dendad
who give me invaluable input on the project

Original thread is here for your reference
https://forum.allaboutcircuits.com/threads/help-to-convert-a-ac-stepping-down-transformer-to-switching-supply.143687/?cb=1583170936

do you guys think it can be done ?
 

MaxHeadRoom

Joined Jul 18, 2013
21,451
If this involves stepper motors, many manuf do not recommend that SMPS supplies be used.
What is the present problem with using the Linear supplies?
You already have them!
Max..
 

Thread Starter

rolo95

Joined Dec 25, 2017
18
This are the fixtures
https://prnt.sc/rai0uz
https://prnt.sc/rai1hh

they weight around 24lbs each
i already removed the transformer on one to measure the weight and
it weights 15lbs, so i wanted to remove linear supply and add switching power supply

like these new designs
https://prnt.sc/rai4jd
that still use step motors but the light engine is LED
and there are no linear PS inside
for reference look at the size of the mirror
and the weight is only 4.6LBS

i know i can better just buy the new ones, but you know
when you like to DIY
 

MaxHeadRoom

Joined Jul 18, 2013
21,451
Personally I much prefer linear over SMPS for this type of application, not only sturdier but easier to fix Should something go awry!
Max.
 

Analog Ground

Joined Apr 24, 2019
416
Converting to a switching power supply does not get rid of the transformer. If you go to a "direct from line power" type of power supply, I believe these are not allowed to be discussed on this forum. They are fundamentally too dangerous because of the possibility of electric shock. You really want to stay with a transformer for isolation from line power and, perhaps, a switching type of voltage regulation on the output of the transformer. Switching to LED lights will reduce the power required and the weight. Using switching regulation often makes the power supply more efficient and reduces the size and weight of the transformer. But, you need a transformer.

Forum members: Am I right about the line direct power supplies not being allowed for discussion?
 
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MaxHeadRoom

Joined Jul 18, 2013
21,451
Most SMPS supplies are isolated from the AC line source via HF transformers etc..
At least all the ones I have had contact with, including Wall-Warts!
Max.
 

Thread Starter

rolo95

Joined Dec 25, 2017
18
just trying to save weight, if that cant be done, i will need to resign myself and start to looking for new lights
i cant use my light duty truss with those 25lbs lights
 

MisterBill2

Joined Jan 23, 2018
7,048
Getting back to the original thread topic of switch mode power supplies and ignoring the noise, I can provide questions that need answers. First, what is being powered? I am guessing lights and motors, but what else?. Next, do you know the voltage and current ratings of the supplies removed? And what are those ratings? Then there is the question about the stepper motor drive packages, and how fast the motors need to go. Step motor performance requirements have a large effect on power supply demands, and it is a complicated relationship.
So really there is a whole lot about the operation of the package that needs to be fully understood before starting. And that information is rather important from the beginning.
Converting to a switching power supply does not get rid of the transformer. If you go to a "direct from line power" type of power supply, I believe these are not allowed to be discussed on this forum. They are fundamentally too dangerous because of the possibility of electric shock. You really want to stay with a transformer for isolation from line power and, perhaps, a switching type of voltage regulation on the output of the transformer. Switching to LED lights will reduce the power required and the weight. Using switching regulation often makes the power supply more efficient and reduces the size and weight of the transformer. But, you need a transformer.

Forum members: Am I right about the line direct power supplies not being allowed for discussion?
All of the AC mains powered switching mode power supplies that I have used in equipment are adequately isolated from the mains supplies. So leave off from the panic, if you please. Unsafe ones may be available in some parts of the world, but we will avoid stuff from those places.
The transformers in the switchers run at MUCH HIGHER frequencies and so they can be much smaller and lighter. The advances in power supply technology over the past 45 years are very impressive.
 

Thread Starter

rolo95

Joined Dec 25, 2017
18
Before the lamp turn on i measure 0.40 Amps of current flowing through the power cable

when the lamp turned on i measured 2.08 amps, with the lamp on and all 4 stepper motors operating i measured 2.15 amps
that mean that the lamp itself draws around 1.68 amps
 

Thread Starter

rolo95

Joined Dec 25, 2017
18
So, going back to the topic
does the current consumption figures help?

Idle current 0.40 Amps
with Lamp on 2.08 amps
Lamp on + 4 servos (X and Y mirror servos + 2 wheels ( color and gobo ) 2.15 amps

so lamp is around 1.68 amps
 

MisterBill2

Joined Jan 23, 2018
7,048
So, going back to the topic
does the current consumption figures help?

Idle current 0.40 Amps
with Lamp on 2.08 amps
Lamp on + 4 servos (X and Y mirror servos + 2 wheels ( color and gobo ) 2.15 amps

so lamp is around 1.68 amps
This tells us that the steppers are not working very hard, probably nowhere near their max abilities. If you can try tempoarily running the steppers on a switch-mode supply that will tell you if they will perform as needed. LED lights should be happy with a switch supply.
 

MaxHeadRoom

Joined Jul 18, 2013
21,451
There is a little more to controlling steppers than the type of supply, the standard recommended method now is to use a power supply with a much higher voltage rating than the plate voltage rating of the motor.
This is in order for the DRIVE to control the current throughout the RPM range of the stepper in order that the current remains equal to the rated value.
This is due to the inductive reactance increasing with rpm, subsequently reducing current.
Sites such as Gecko Stepper drives have useful tech sheets on sizing etc.
Also explanation on selecting a drive and P.S. Both equally important! ;)
Max.
 

MisterBill2

Joined Jan 23, 2018
7,048
There is a little more to controlling steppers than the type of supply, the standard recommended method now is to use a power supply with a much higher voltage rating than the plate voltage rating of the motor.
This is in order for the DRIVE to control the current throughout the RPM range of the stepper in order that the current remains equal to the rated value.
This is due to the inductive reactance increasing with rpm, subsequently reducing current.
Sites such as Gecko Stepper drives have useful tech sheets on sizing etc.
Also explanation on selecting a drive and P.S. Both equally important! ;)
Max.
Max is correct about steppers running fast enough for current rise time to be an issue. BUT based on the reported current draw and the speeds that steppers run in those applications it is not likely that these steppers need anything close to maximum performance. AND, in addition, there has been a huge amount of advances made in stepper drives in the past 20 years. It is not likely that these are the classic Superior Electric stepper motors that were used in the 1975-1985 era. In 1987 I designed in newer Superior electric steppers that did not use the high voltage and resistors scheme any more. They did it quite differently.
 

MaxHeadRoom

Joined Jul 18, 2013
21,451
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