Getting more current, sacraficing voltage 1

[leonar40]

I have a system that runs on 28V and can provide 1A. I would like to run a motor using this power. However, the motor runs on 5V and between 3 and 4 A to get the torque that I need. It seems like I should be able to sacrafice some voltage and increase my available current, but I don't have any experience designing a circuit for this. What do you guys think would be the best way to go about it?

 

[buzzp]

Just some food for thought.

This must be a DC supply or is it AC? Just going by the wattage, it looks like your supply can source this power. I would see what the supply is capable of sourcing (transformer size, regulator size, or whatever the source originates from). You should also consider the conductor size (I dont believe this will be an issue but would definately check size). Also the overcurrent protection should be reviewed.
If its AC, you could use a transformer (might be hard to find one with 28V pri and 5V sec) or build a switching supply.
If its DC, you could use a regulator but it would probably be best to build a switching supply to reduce heating.

 

[leonar40]

Thanks for your response. The system is DC. As far as seeing what the supply is capable of sourcing, I really don't have access to that part of the design. I am working on a small part of a large project, and was told that I have a power line that I can access that has 28V and 1A DC. So I need to do something after the fact of the original source to convert down to 5V and 3A. I had also though of using a 5V regulator previously, but would that automatically give you the increase in current? Luckily for me, heating is not going to be a problem because the motor only has to run for 2-3 seconds, and only once.

 

[analogkid2digitalman]

Use an off the shelf DC to DC converter, 25-30W range. For example, a Cosel ZUS252405 (25W unit) takes 18-36 VDC input and produces 5a VDC at up to 4A output. Under $100 in a couple square inch package.

A bare 5V regulator will not increase the current as you asked.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[Comcokid]

What you need is a switch-mode buck regulator circuit. You may have to design one to meet your exact needs given the input voltage. These will be ~85% to ~92% efficient which will meet your maximum 5V 4A needs ( 28V at 1A source can supply 28 watts of power, and 5V at 4A is a 20 watt load. 28 watts times 85% efficiency gives 23.8 watts which is more than your max need).

For circuits, check

http://www.National.com,

http://www.micrel.com,

http://www.ti.com.

Ther are other companies as well. You might just check the TI site first as they acquired a company several years ago (Power Trends) which made modules that might meet this input/output requirement.

 

[buzzp]

Better check more into the 1A from the supply. How is this limited to 1A? By the power rating? OCP device? Regulator? This has to be known before you can draw more than 1A from it. Chances are it is not an issue but the switcher will still draw more than 1A on the input and if the supply is limited by wattage AND current then you may run into problems with a switcher. Here is the link to the switcher analog referred you too.

http://www.cosel.co.jp/en/products/parts_products/se_zus25.pdf

It sounds to me like the supply is from a 3 pin regulator and if this is the case, you better test well. Might be better to find a 28V motor. Also, depending on the switcher, some will not start when they have a nonlinear load connected to it. I am not well versed with DC motors but I believe they are nonlinear loads just as an AC motor is (might be wrong on this, sure someone will correct me though).
Since it only has to run once and for only 2-3 seconds, this will probably mean your fine with a switcher provided you dont take out the supply you have in the meantime. I just wanted you to be aware of the fine line you may be walking with the supply they have given you.

 

[VEBill]

"...motor only has to run for 2-3 seconds..."

Use the 28v at 1A max to slowly charge a supercap (perhaps 10 or 20F) to 5 volts. The supercap can easily supply 4A for several seconds. This assumes that you have time (30-60 seconds) to charge the supercap.

"...and only once."

Or use a separate battery.

 

[leonar40]

Thanks a lot for all the responses! I am going to look into the DC to DC converters. I have already looked at using a capacitor to supply the power that I need, but unfortunately they tend to be pretty large when you get up in the farad range. That is still a possiblity though.

 

[VEBill]

Make sure that you're looking at the latest 'Supercaps' - not the much larger, good old fashioned capacitor.

 

[logbook]

Note that 4A for 3 seconds giving a 1V droop requires a 12 Farad capacitor.

 

[VEBill]

With the latest Supercaps, probably just a few cc in volume (!!!!) and perhaps about $10 or so.

The tricky bit will be the 5 volts versus the common 2.3 volt rating of many supercaps. Even two in series is only 4.6v, so you'll need three in series and that means you have to buy at least 36F at 2.3v to reach 12F at 6.9v. It would be very much worth searching for one with a 5v rating.

 

[felixc]

Are these new supercaps made to stand discharge rates of 3amps, or will the internal resistance spoils it?

As Comcokid says, have a look at the TI modules the former Power Trends ones. They're using TI's buck converters, of course, only leaving the input and output caps to add, and maybe some ferrites or chokes for the noise.

 

[Myndex]

Another good source for DC-DC converters is

http://www.maxim-ic.com/PowerSupplies.cfm

And supercaps can discharge at incredibly high currents. They really are super.....


A

 

[VEBill]

One application for supercap technology is to improve on the peak current capacity for electric cars.

Yes - they're good for high currents.