# Problem with stepper motor generator circuit

#### ecircuit1

Joined Jul 17, 2014
8
Ok, my work asked me to design a charger for a sensor that is driven by a stepper motor. I accepted...but I think I'm a little over my head now.

I rectified the output of the stepper and fed it to a LM317. My goal was to maintain 5v and supply about 120 mA of current to charge a battery. The battery supplies a sensor that pulls about 50mA, which is why my current target was so small. I have several physical constraints limiting the speed that I can spin the stepper as well.

Before I had the battery selected, and the regulator in place, I ran some tests by putting resistors across the voltage to act as a load. I found that at 50 Ohms, I had about 6-7V and 150mA. Here are the issues:

Issues:
*I didn't account for drop out voltage. I am on the low end (mid 4's now) for voltage now that I have the LM317 added. Is there anything that can be done to minimize the drop out voltage?

*When the battery was selected, and finally attached it to the circuit, the voltage made large swings (-1V) which causes problems with the USB charger. Once the Voltage is out of range, the charger turns off. The only two things I can think of are: 1. Some sort of feedback issue due to the wrong capacitor ( I only had a 1000Uf sitting around) on the LM317. 2. The charger is drawing too much current, and without the available charge, the voltage is dropping. If it is number 2, how can I limit the battery so that it only tries to draw around 100mA? I need the battery to only draw what the 50 Ohm resistor was doing

#### Alberto

Joined Nov 7, 2008
169
Just an idea: why not connecting the lm317 as a current limiter!

Cheers

Alberto

#### THE_RB

Joined Feb 11, 2008
5,438
...
I rectified the output of the stepper and fed it to a LM317. My goal was to maintain 5v and supply about 120 mA of current to charge a battery. ...
...
Use a low dropout 5v regulator, there are many common types good for 120mA output. That fixes your dropout (headroom) problem.

Regarding your instability, I would suggest using some very large caps before the regulator. The stepper motor might be making quite a low frequency so your caps need to be large enough tomaintain a stable DC voltage before the regulator.

A couple of 2200uF caps in parallel should do it?

It will help if you can supply a scope shot showing the waveform into the regulator.

Also, how did you design the rectifier bridge? Generally a stepper motor needs dual bridges, 8 diodes total, to rectify both phases.

#### ecircuit1

Joined Jul 17, 2014
8
Thank you very much for the suggestions. I'll start looking for a new regulator with a lower voltage drop. I'll also try adding the capacitors before the regulator. Right now I have them going from Vout to ground. Would I just add several capacitors to Vin to ground? Sorry if thats a dumb question.

I used bridge rectifiers from radio shack (GBU4G), hopefully those are ok.

#### ecircuit1

Joined Jul 17, 2014
8
I just ordered this: http://www.newark.com/texas-instrum...-reg-5v-1a-to-220/dp/41K4636?CMP=TREML008-005

That should fix the voltage drop problem. I also went ahead and added 4 1000uf capacitors in parallel on the input voltage.

Someone suggested using a regulator to regulate current. Would I regulate the voltage first, then set up a current regulator? I'm a little confused how to go about this. I need to make sure I have 5v, and that the battery cannot draw more than 100mA. Thanks for all of the help

#### THE_RB

Joined Feb 11, 2008
5,438
You should not need a current regulator because you are using a USB charger. It needs a reasonably stable +5v DC input, and it will take care of the rest.

Your GBU4G rectifier bridges are 4A bridges (I think) and should be fine.

If you originally had no caps at all on the Vin pin, that would explaint he instability! And 4 times 1000uF on Vin should be plenty to stabilise it now, with a 100mA output.

#### ecircuit1

Joined Jul 17, 2014
8
Thanks for the help so far.

Maybe for the current part I'm not explaining enough though. Once I get the constant 5v at the USB and I attach a battery, I cannot have the battery draw more than 100mA. If it does, the load on the stepper motor will be too large, and it will not be able to continue to rotate.

I ran a few tests, and with 50ohms between the output of the circuit and ground, I get the right amount of current without requiring too much torque input to the stepper motor.

How can I make sure that the battery is not allowed to draw more than 100mA??

Thanks again

#### THE_RB

Joined Feb 11, 2008
5,438
Proper current limiting costs you headroom, so will be inefficient and require the stepper motor to spin faster.

I think choosing a battery charger IC that runs from 5v and has a 100mA limit will be a better idea. Most of these smart Li charger ICs have a current limit, either fixed or settable.

#### ecircuit1

Joined Jul 17, 2014
8
Proper current limiting costs you headroom, so will be inefficient and require the stepper motor to spin faster.

I think choosing a battery charger IC that runs from 5v and has a 100mA limit will be a better idea. Most of these smart Li charger ICs have a current limit, either fixed or settable.
Thanks! I post the final results when it is finished.

#### ecircuit1

Joined Jul 17, 2014
8
The new voltage regulator worked great, but the IC I got to regulate current isn't working. I just doesn't turn on for some reason.

I got this one http://datasheets.maximintegrated.com/en/ds/MAX1811.pdf

Then I tried a few different current limiting IC's, but they aren't working.

Still struggling with this. I"ve ordered a few IC's but I must not be buying the right things.

I have a LDO voltage regulator in place now and a constant 5v. When I attach the battery, it draws 200ma or so, and it requires too much input torque from the motor.

I just need to add something to the circuit so that I have 5v at the battery, but the current is limited to 100ma. Can someone help me pick the right components?

#### alfacliff

Joined Dec 13, 2013
2,458
have you tested the stepper motor to see how much voltage and current are available at the speed you are trying?