next topic »

[on:]

Hi friends,

Newbie here for electronics,

I need your help in estimating power supply requirements for my project.
Quick Info:

Project consist of
1 DC motor 30 RPM 12 volts
1 DC motor 60 RPM 12 volts
1 proximity sensor 5 volts
3 standard servos 5 volts

Now what happens is my present power supply 1 amp is not able to fulfill power requirements for above said components(lets say when 2 dc motors starts simultaneously, servo starts flickering)

What I want is even though both motors are moving none of servos should start flickering or loose their arrow.  The following is my power supply..


In this image I am only having single transformer, due to above said problem I added one more transformer for both different extensions.  But it doesn't improve the problem.

So can you give my your advice how to go further?  I am thinking of buying new power supply with 12 volts and (1+1+1+3)X0.5 = 3 amperes is it correct?

[Reply #1 on:]

I have a motor 12-24 volts, 2amps. My power suplys could only give about an amp, so I used two power suplies.

[Reply #2 on:]

It's impossible to say what your power requirements are without knowing what current the motors take. However, an ATX computer power supply will probably be sufficient, and gives you both +12V and +5V outputs (and -12V and +3.3V as well). You may need to place a dummy load on the 5V output to get thr voltage about right.

[Reply #3 on:]

To find out the peak current drawn by a motor you can measure it thus:

disconnect motor from the circuit.

Use multimeter on resistance range across the terminals.  Take a few readings, rotating the shaft a little between
each reading.  Discard all but the lowest reading.

Measure the resistance of the multimeter leads by shorting them together, subtract this from the motor reading
to give a best estimate of the winding resistance.

Say this gives 1.6 ohms (for instance).  At 12V the motor can pull 12 / 1.6 = 7.5A.  [ insert your own figures here of course! ]

If your supply cannot supply this current its voltage will dip momentarily as the motors start moving, or if they stall.

[ Adding a large electrolytic cap can reduce the dipping associated with starting ]

If the supply dips then you can't share it with the servos and other parts of the system - it may still be
perfectly adequate for powering the motors under normal load.

[Reply #4 on:]

Those little TO-220 linear regulators aren't good for much current.  Even trying to get a full amp out of them requires more effort in effective heat-sinking than it would be to move to switching regulators.  But, the real question is this:  Do your engines even need regulated power?

What's the voltage rating on the transformer?  Maybe you can power the engines straight from the DC rectifier?  (BTW, you do have a rectifier, right?  I don't see any diodes...)  Speaking of the transformer, what's its VA rating?  You may need a bigger one to supply enough current to everything.

Then, the servos... What are their current demands?  You might be able to use one 5v regulator for the servos, and power your control circuitry from another.

[Reply #5 on:]

Hi friends,

Thanks all for reply,

dc42:
I am looking forward as per your recommendation, I do have one ATX PSU, but have few queries regarding it. 
Have a look at its specs

1) It is having much more ampere capacity then my requirement, won't it blow up my sevos,dc motor and stepper drivers (In my present PS I have connected ground to arduino ground, I think ground connectivity wont blowup my arduino board)
2)How much (exact) load requirement will be there and one which line(5v or 12v), I tested that ATX without any load with multimeter and it gave right readings.(12.03 v and 5.12v).

[Reply #6 on:]

I won't estimate the motor current in the way explained above. Motor's currents are not the simple Ohm's law calculation from the windings resistance. The current varies depending on the mechanical load applied to the motor, that is, how much force they are doing to move the load. Thats' why they get burnt when mechanically overloaded. Better to place an ammeter (your multimeter measuring current) in series with the motor and measure the real current with the motor working with its normal mechanical load. That's the normal current it will take from the power supply. Remember if the mechanical load increases it will take more current. I don't think the relationship between force and current is even linear by the way (I have to check). Therefore, careful with the calculations and estimates...

[Reply #7 on:]

I won't estimate the motor current in the way explained above. Motor's currents are not the simple Ohm's law calculation from the windings resistance. The current varies depending on the mechanical load applied to the motor, that is, how much force they are doing to move the load. Thats' why they get burnt when mechanically overloaded. Better to place an ammeter (your multimeter measuring current) in series with the motor and measure the real current with the motor working with its normal mechanical load. That's the normal current it will take from the power supply. Remember if the mechanical load increases it will take more current. I don't think the relationship between force and current is even linear by the way (I have to check). Therefore, careful with the calculations and estimates...

The stall current of a DC motor at a known voltage can be calculated from the voltage and the DC winding resistance. Since the stall current is then worst case value (unless the motor is forced to rotate backwards while power is applied), it is the stall current that you need to know when choosing the peak current rating of an H-bridge and power supply used to drive the motor.

[Reply #8 on:]

dc42, I was also going to suggest using an old PSU from a puter. There are some very good instructions on the web for converting them to standalone PSU's - Lots of grunt and pretty stable!

e.g. c/w 3.3V http://www.instructables.com/id/Convert-an-ATX-Power-Supply-Into-a-Regular-DC-Powe/

Lots of other similar pages.

Beware of mains when the unit is open!

[Reply #9 on:]

Hey guys thanks again for reply,

But for using ATXPSU only my concerns are as below.
1. This is having much more ampere capacity then my requirement as you can see in above posted image, won't it blow up my sevos,dc motor and stepper drivers.
2. How much (exact) load requirement will be there and one which line(5v or 12v), I tested that ATX without any load with multimeter and it gave right readings.(12.03 v and 5.12v), so is there requirement for dummy load resistor or dummy light bulb...


Thanks,

[Reply #10 on:]

Just because an ATX power supply can supply more current than you need doesn't mean it will. The whole point of a regulated power supply (such as an ATX power supply) is that it supplies a more-or-less constant voltage whatever the current demand from it, within limits.

Many ATX power supplies do give better regulation when they have a minimum load applied. So you might want to put a load on the +5V line (a 12V brake light bulb will probably do nicely).

[Reply #11 on:]

dc42, thanks for your clarification. Now I understand it better.
Thanks.

[Reply #12 on:]

Many ATX power supplies do give better regulation when they have a minimum load applied. So you might want to put a load on the +5V line (a 12V brake light bulb will probably do nicely).

Indeed, most if not all will not run without a load and the sense line properly connected.

This is a nicer site, note the resistor in the parts list. (like the idea of a 21W bulb BTW)
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

[Reply #13 on:]

Thanks for suggestions guys,

dc42, I have tried using ATX PSU but the scenario doesn't improve.  Whenever my 100 RPM DC motor starts, all servos tries to go to zero position and that too flickering.  But when I stop my DC motor all servo are back in their position.

Here is my ATX


and this is my extention all 12vs and 5vs


I can't understand this behavior.  Any help appreciated.  Let me know if you want further information

[Reply #14 on:]

Can you show your circuits (not a photo)?
You may just need to suppress your motor.
1 x 10nf cap across contacts and 2 x 10nf from each contact to (metal) chassis.
The commutator will/can produce huge electrical noise.