# regulating amperage

Discussion in 'General Electronics Discussion' started by Dude22, Jun 24, 2012.

1. ### Dude22

66
1
Mar 30, 2012
Hi everyone,

I have recently finished a CNC build and am working on a better power supply... I would like to use an old computer power supply, but the problem is the amperage. I need the voltage (12) to be unchanged but the amperage must be limited to 1 amp. How would I go about doing this???

NOTE: Please don't answer by saying that my driver will only draw the amperage that it needs... because that is not true with my DIY driver It will pull AS MUCH AMPERAGE AS IT CAN EVEN IF THAT MEANS IT BLOWS... sorry for ranting but I have had one to many people tell be that my amperage does not need to be reduced.

2. ### Raven Luni

798
8
Oct 15, 2011
You can have a current limiting circuit using a couple of transistors. This page gives a good explanation of how to calculate the component values, but the circuit itself as they show it is flawed. The load should be placed on the collector side of Q1 instead of where they have it. That way the current levels off with very little voltage drop and little power in the limiting circuit. With their version, the voltage drops proportionally to the current drawn and far too much power is wasted in the limiter circuit.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Your driver will only draw the amperage that it needs.

If it is so designed that it applied a short circuit across the supply, then that is a load you're going to have problems with.

I can't see any other load that will not limit the current to some value (even if it is a high value)

You can't limit the current to a device without changing the voltage applied to it.

4. ### CocaCola

3,635
5
Apr 7, 2012
In addition to what Steve said, my questions are... What Blows? Maybe a faulty circuit design that can't handle the current it's supposed to? What 'mystery' device in your circuit is doing this claimed infinite current drain?

Because whether you like to hear it or not, the circuit will only draw the current it needs/wants depending on the load, not some infinite amount beyond that...

5. ### Dude22

66
1
Mar 30, 2012
Ok, people want to know more about my driver so here it is I am using uln2003's here is a link to an image showing how it is wired: http://www.eleccircuit.com/wp-conte...03-control-stepper-motor-by-parallel-port.jpg The data sheet for the chip can be found here: http://www.datasheetcatalog.org/datasheets/120/489337_DS.pdf Notice how right on the front page it says: 500mA rated collector current(Single output) I have exparamented with this chip a lot and whenever my amperage goes above 500ma it heats up and after a few minutes glows red and smokes. I am using simple dc wall wart's.

NOTE: the motor is powered directly from the chip so as the motor can use up to a huge amount of amperage it try's to pull to much through the chip

Last edited: Jun 25, 2012
6. ### CocaCola

3,635
5
Apr 7, 2012
So basically your circuit is grossly underrated for the task you are trying to do... As suspected it's only drawing the current it needs but the circuit isn't designed to handle that current...

This is not an issue with the motor, it's the fact your circuit isn't up to the task... It's not time to limit the current it's time to redesign a proper circuit for the job at hand...

You have two primary choices, read the next line of the datasheet "The darlington pairs may be parrlleled {sic} for higher current capability. "

So you can parallel a few extra ULN2003s and up the current capacity that way... Example using 2 ULN2003 in parallel will give you 1A (1000mA) using three will give you 1.5A or 1500mA and so on...

or

use the Darlington to switch something else (FET or RELAY) that actually switches the motor and can handle the motors current requirements...

7. ### gorgon

603
24
Jun 6, 2011

Note that one ULN2003 chip can only handle 500mA in total.

So it doesn't matter how many of the separate drivers you parallel, you will not get more than 500mA safely from one chip. You need to find the needed current for the motor and use a driver big enough for that, or reduce the voltage to adapt it to the ULN2003. This will of course reduce the stepper's parameters accordingly.

TOK

8. ### CocaCola

3,635
5
Apr 7, 2012
Pretty much what I said, but yes for clarification you need to parallel multiple chips not simply the individual darlington pairs...

9. ### Dude22

66
1
Mar 30, 2012
That is what I have done already, I am currently paralleling 3 chips which gives me a max of 1.5amps. I want to use an old computer power supply for the driver and motors but the problem is that all my 20 or so old power supply's rate at around 10amps sometimes up to 20amps on the 12 volt power. Instead of paralleling 20 or 30 chips for each motor; I want a circuit to reduce the amperage without changing the voltage so that I will only need to parallel a manageable amount of chips.

I like Raven Luni's idea, but if someone thinks they have a better solution I would love to hear it.

10. ### CocaCola

3,635
5
Apr 7, 2012
This is the wrong approach... You need to determine what the motors want and use divers that will accommodate the motors required draw... I doubt you need anywhere near 20 or 30 chips only what the motors will draw not what the supply is able to accommodate... If you are currently paralleling 3 chips and it's working then it should work with the new supply...

What are the specifications for your stepper motors?

It's the wrong approach in this circumstance...

I have given you several viable solutions, please review them...

11. ### gorgon

603
24
Jun 6, 2011
The only way to reduce the current is to reduce the voltage. If the motor is rated for 12V, you'll also reduce the motors power range when reducing the voltage.

You'll need to get a rated driver for the rated current.

TOK

Last edited: Jun 25, 2012
12. ### MrEE

84
0
Apr 13, 2012
Each of the ULN2003 7 drivers has about 1V drop from and will therefore consume about 500mW at 500mA. I would definitely replace the ULN2003 with a set of 4 power FETs (enhancement mode logic level gate fets) that can handle about 5A to 10A each. Your stepper motor will then work nice even when powered from a 12V truck battery with 500A current capability. Yes, you should limit the current with a standard fuse against component failure, (or use a PTC resettable fuse).
As an example,here are 2 mosfets I found after a quick search, but there are many more.
NTD4960N-1G or a IRLZ14PBF