If the stall current of the motor is 46A, that is what it will try to draw at start-up. Whatever transistor ot FET you use will need to be able to take that kind current repeatedly for a second or two or will fry eventually. Plus, the power supply would need to be able sustain high current until it was able to get the motor up to sufficient speed that it will draw less current.

A 2n3055 will only handle 15 amps and you will need to drive it with about 400 ma to ensure it saturates.

Can you get by with a smaller motor?

 

I think someone mentioned a MOSFET earlier, they are generally better for these kinds of use.

 

BillO,

Thanks. I don't think I have anything else lying around the house that I cam pull a motor from, but I will think about it.

 

A 1N4004 diode is made to rectify 60Hz or 120Hz from a mains transformer and is not a high speed diode.
Don't buy garbage parts from RadioShack because they know nothing about electronics and know nothing about the garbage that they sell. No spec's whatsoever for the 3A "barrel" diode. Usually they say only the weight or the color of an electronic part.

 

Audioguru,

Thanks for the advice. If I can avoid it, I will try to order online instead of Radio Shack.

 

Audioguru,

Can you recommend a diode that might be appropriate for my circuit? The power (signal) from the control board to the base is 5V.

 

Audioguru,

Can you recommend a diode that might be appropriate for my circuit? The power (signal) from the control board to the base is 5V.

I don't know a very high current fast diode. I am a low current dude.
The output of your control board is 5V (it is probably Cmos) when it has no load. You need to find out its output voltage when it has the load that it will drive.

 

You mentioned this is a project. Does it have to be a hand vac motor? If not and the goal is just to turn a motor on and off to demonstrate the process, it seems that you could purchase a small electric motor from radio shack that would not draw the current that the hand vac is trying to draw. Outside of that you can probably find motors all over your house. Old cell phones will have a small motor that vibrates the phone when a call comes in, toys that have any kind of movement will have motors, old cd and dvd drives will have motors that eject the disc when you hit the eject button (I pulled four of them out of 2 cd drives just the other day as well as the brushless motor that spins the disk although you won't be able to drive that one with a simple on and off process).

 

My theory about what is going on (this is a learning experience for me reading this; This post is more for me than OP, so please correct me if I'm wrong):

#1. the reason why your motor runs in "pulses" when connected directly to the p/s is that that when a DC motor is initially started up, in the absence of back EMF, it's basically almost a direct short; Amp draw could be the mentioned 46A or even higher. This immediately discharges the capacitors in the power supply, so the output stops until the caps can charge back up (which apparently takes about 1/2 sec).

#2 the reason why it doesn't "pulse" when connected to the circuit is bacause your transistor is not fully saturated due to the 4700Ω resistor (or because the transistor is undersized for the job) so there is some current limiting going on there. Because of this current limiting, the motor is allowed to run for a little while until the power supply eventually shuts down due to overheating.

Basically a recap of what everybody else said; is it correct?

 

jerseyguy1996,

Yes, it is a project, but the main use of this project is that I need it to vacuum some objects. So, yes, I am using the motor from the hand vacuum, but also am using the parts connected to it that do the actual vacuuming.

 

strantor,

I will let others (who have a better understanding of things than I do) comment as to whether or not your summary/analysis is correct. Thanks.

 

what if instead of using the transistor to drive the motor, you used it to drive a relay that would switch the motor. & instead of using the laptop p/s you used a battery or whatever power supply came with the hand vac?

 

strantor,

Yes, I have thought about using a relay instead - and I might just do that. Without going into too much detail, this project is part of a larger project and this vacuum will be turned on and off many many times. I kind of did not want to use a relay because it is a mechanical switch so it only has a lifetime of a few hundred thousand switching cycles which may last me only a few months of use. I thought a transistor would be better long term solution. But, since I have been having so many problems using a transistor, I might just use a relay as well as connect the original rechargeable battery pack as the power source - and always have the battery pack connected to the recharging cradle (and have the recharging cradle plugged into the wall). That should work I think, but it's just a little bit bulky.

 

strantor,

Yes, I have thought about using a relay instead - and I might just do that. Without going into too much detail, this project is part of a larger project and this vacuum will be turned on and off many many times. I kind of did not want to use a relay because it is a mechanical switch so it only has a lifetime of a few hundred thousand switching cycles which may last me only a few months of use. I thought a transistor would be better long term solution. But, since I have been having so many problems using a transistor, I might just use a relay as well as connect the original rechargeable battery pack as the power source - and always have the battery pack connected to the recharging cradle (and have the recharging cradle plugged into the wall). That should work I think, but it's just a little bit bulky.

Unless you are controlling the motor with PWM, the relay life shouldn't really be an issue, just get one with a high amp rating on the contacts and use a supressor across it. If you are using PWM then throw the relay idea right out

 

The question of using the relay is not relevant, since the motor still does not run properly (right?) when directly connected to the under-powered AC adapter. Fix that, and then the switching options are all in play again.

Between the 3 options of MOSFET, relay or BJT, I'd rank them by suitability in the order I've listed them. The MOSFET is more versatile and can enable PWM, whereas the relay is not up to that but would be just fine if PWM is not planned. You may already have scrap relays laying around, but maybe not the right FET.

 

I don't think I need the motor to run at maximum power/strength because it is pretty powerful even at lower "speeds" so I think I can accept running it at less than maximum power. I also would like to avoid using a power source that can supply a lot of amps because I have this feeling that it would be a bit dangerous if someone in my house accidentally touched some exposed wire or connection. Also, I think I can accept using a lower voltage power source because I have a few other AC adapters in my house I can use (e.g. 12V 2A and a few others similar and lower).

With that in mind, can I still use my "underpowered" power source if I somehow limit the current draw from the motor?

Here are some articles I found that I think discuss some of these ideas. Do you think they are possible solutions?

http://forum.allaboutcircuits.com/showthread.php?t=15411

http://www.electro-tech-online.com/.../20636-dc-current-limit-circuit-low-cost.html

http://www.electronics-lab.com/articles/LM317/

Thanks again for everyone's help so far on this matter.

 

With that in mind, can I still use my "underpowered" power source if I somehow limit the current draw from the motor?

I don't understand this thinking. If the motor fails to run properly at the full power available, how is reducing that power going to make it run? I guess if the motor will run at a lower current than it is drawing from your supply, and that lower current daw would allow the power supply to keep running, maybe it could all work. Is this what you're thinking?

The simple way to reduce the current would be to put something in series with the motor, perhaps a 12v light bulb. If you have power resistors that can handle the current, you could try those.

A more complex approach - one more likely to work - would be to use PWM to reduce the duty cycle to the motor. This would still give the motor the volts it needs for good torque but would reduce the average draw on the power supply. PWM is the more-or-less standard way these days to control a DC motor.

I'd steer away from "linear" current limiters (other than the really simple approach above), since they are inefficient, and I think you need every drop of power you can squeeze out.

 

The LM317 article at electronics Lab is wrong. It copied a circuit from the datasheet designed for the more expensive LM117.

If you use an adjustable power supply voltage to reduce the voltage to the DC mortor then its speed drops but also its torque drops. Then the torque might be so low that the motor does not start running and it might stop when loaded.
But if you use PWM to reduce the speed of the motor then the motor still has plenty of torque and will barely slow when it is loaded with some work.

 

I'm coming into this thread late at #39. After reading through all of it I can't believe that the OP is still trying to power a vac motor like that with a PC wallwart.... Really!

PWM, relay switching, current limiters, etc are irrelevant at this point. The OP needs an adequate power supply first.

 

wayneh,

Actually, I think I need to mention something more so that there is no confusion. The motor ran fine with the AC adapter for like a minute. Then, I "think" I had blocked the vacuum nozzle for a few seconds so the motor had to work even harder - and then - that is when the whole thing shut down. I am not sure if that was definitely the cause of the shutdown or not because I have not had a chance yet to rewire everything to test it again.

Let's assume in my project I will not be blocking the nozzle again (or at worst simply a partial blockage).

I mention this because I think you might have come away from my description thinking that the motor never ran at all with this ac adapter.