Multiple Power Loads and AC/DC adapter Question

Thread Starter

sonnyangell

Joined Mar 1, 2017
21
Hello!

I have a project that involves

2 - 3v Green Laser Diodes
1 - 4.8v DC Motor
1 - 12v Fan
1 - 110v / 400watt Smoke Machine (home depot)

I need to supply power to all of these un-interrupted. I also want one plug for this project.. could someone suggest a PSU for me? Seems like I can use a regular 400 watt Computer Power Supply Unit, but I would rather be cost and space effective (I need it to be as small as possible)

400W PSU

Since I want this to have ONE plug, How should I connect the Smoke Machine? If I hard wired it to the 110v input of the PSU (above) would that be super bad? and steal power from the other loads?

Last Question: I recently purchased a 12v 2A AC/DC power adapter to test my project. When I connect it to the 4.8v DC motor.. the motor only "pulses" in 2 second intervals.. Yet when I used a different and much older AC/DC adapter (12v - 700Ma) it works perfectly.
 

RPLaJeunesse

Joined Jul 29, 2018
86
+1 means "I agree with the above". The 4.8V motor will draw a lot of current at 12V, the newer supply senses this and shuts down, waits, and tries to restart. The old supply isn't so smart so it just sags to some compromise voltage between 4.8 and 12, and eats up the difference as heat. Leave it run long enough, and either the motor or old supply will burn up, perhaps literally.
 

Thread Starter

sonnyangell

Joined Mar 1, 2017
21
+1 means "I agree with the above". The 4.8V motor will draw a lot of current at 12V, the newer supply senses this and shuts down, waits, and tries to restart. The old supply isn't so smart so it just sags to some compromise voltage between 4.8 and 12, and eats up the difference as heat. Leave it run long enough, and either the motor or old supply will burn up, perhaps literally.
Thank you for the clarity... Damn..

Let me ask, If I had the same 12v 700ma adapter, and spliced the output to two step down buck converters (1) 12v>3 and (1) 12v>4.8v would that be un-wise?
 

RPLaJeunesse

Joined Jul 29, 2018
86
Yes, since the motor clearly needs more than 2A at start-up (as evidenced by the pow.er-supply shutting off).
Only if you try to feed it 12V. If you feed it 4.8V the current will be much less. I'd use a 12V to 5V buck step down for both the motor and LEDs. Remember, LEDs run off current, not voltage. So for each LED add a series resistor that sets the proper current given the 5V supply.
 

Thread Starter

sonnyangell

Joined Mar 1, 2017
21
Yes, since the motor clearly needs more than 2A at start-up (as evidenced by the pow.er-supply shutting off).
Only if you try to feed it 12V. If you feed it 4.8V the current will be much less. I'd use a 12V to 5V buck step down for both the motor and LEDs. Remember, LEDs run off current, not voltage. So for each LED add a series resistor that sets the proper current given the 5V supply.
Thank you so much, I feel silly now for the mistakes ive made =] I still have some to learn about electronics and power routing.

Since I want this project to be "portable" can you suggest some compact options I may not know about? IS it at all possible to use a computer PSU and use those outputs? and plug the smoke machine seperate? (2 plugs...ugh)
 

Alec_t

Joined Sep 17, 2013
10,907
Only if you try to feed it 12V. If you feed it 4.8V the current will be much less.
Good point.
If the fan is a typical computer fan or similar you should be able to run both the motor and fan from a computer PSU.
How much current do the laser diodes draw? A constant-current supply powered by the PSU 5V should hopefully cope. Be very careful with the laser diodes to avoid blinding anyone. :eek:
 
You can measure the winding resistance of the 4.8 V motor, but it's difficult. 5x the running current is an estimate of what your supply has to be able to provide. Usually you will see units of VA (Volt-Ampres) and not Watts.

There are plenty of step-down DC-DC converters available on ebay. I used something similar to https://www.ebay.com/itm/DC-Step-Down-Converter-2A-Constant-Voltage-Current-W-Voltmeter-Ammeter-Blue-led/122477727783 but mine has a voltage and current adjust. I used one to replace a 6V power brick from a 12 V DC input. In this case, the input has to be greater than the output by 2V and the currrent imit is fixed at 2 Amps. The unit I have has a variable current limit.

I;m not saying thus https://power.sager.com/rt-65b-2526486.html will work, but it's an open frame supply.

Voltage isn't enough to specify. 120 vac to 12 VDC is common. Checkout DIN rail mounted Meanwell or maybe an open frame power supply.

DIN rail is the erector set of control electronics. You start out with a T-shaped rail and you populate the rail. Here https://power.sager.com/ac-dc-power-supply/din-rail-mount/default.html are some power supplies.

Here https://www.winford.com/products/tta35.php is a place to explore.

This https://www.fluke.com/en-us/learn/blog/safety/why-you-care-mixed-cabinet-electronics-and-power is how complex you can get. This also uses wiring duct as a covered plastic channel for wiring.

This website https://www.asi-ez.com/

also gives you an idea.

You can buy small quantities of terminal blocks here: https://www.automation24.com

A terminal block is usually exposed on one side. You can put a bunch of them together and add just one cover, there are also partition plates which are higher. You then have end brackets that prevent side to side movement.

You can buy these that are multi-level. You can get relays and fuseholders etc. There are buss bars that can bus the terminals together.

process control stuff is usually 24 VDC.

One drawback is that your device may turn out bigger than you want, but it is a re-useable erector set. You can also get wire here by the foot.
 
I left out purposely the measurement of winding resistance, but the easiest way might be a resistor in series with a 1.5V battery.
You have to guess the resistance by knowing something about the motor. You have to measure the voltage ACROSS the motor and the current THROUGH the motor not using a meter in voltage mode and the same on in current mode.

Current is determined by the voltage drop across the resistor and then voltage across the motor is self explanitory. lead resistance effects go away and you should be able to measure a few ohms. Your DVM might have a lead resistance of 0.5-1 ohms. Another rule of thumb.

You then use the winding resistance with the planned motor voltage to get start current. Maybe add say 20%.`

An unloaded PM (permanent Magnet) DC motor acts as a tachometer. Voltage is proportional to RPM.

Amps are proportional to torque on a powered motor.

You can actually speed control based on V=Vm-I*Rm. V is the voltage applied to the motor. Vm is the voltage across the motor. 8-track tape decks did this. The speed control was in the motor.
 
Top