12V to 5V

[shaiqbashir]

HI Guys!

well! my dear friends!

Im making a robot. In which i will be requiring 5V for a motor. I have a 12VDC, 5AH battery.

Now how i will supply 5V to a motor from a 12V battery. Should i use a regulator 7805. Will it be able to handle an input voltage of 12V??????

and one thing that i was also thinking is this that first i use 7809 regulator to regulate the battery voltage to 9V and then use 7805 to further regulate that 9V to 5V.

And what about the curent ?

i mean my stepper motor requires 1.4A per phase of current. so It will be around 5.6A total requriement. Will the regulator be able to supply such a high current.

If not what should be the best method to do that.

I shall be thankful to u if u will make an eary reply

thanks in advance

[audioguru]

Now how i will supply 5V to a motor from a 12V battery. Should i use a regulator 7805. Will it be able to handle an input voltage of 12V?

Look at the datasheet for a 7805 regulator. It shows that its max input voltage is 35V and that it has a well-regulated output up to 1.0A, and up to 1.5A with a small loss of regulation. More than half the power of the battery (7V) will be wasted as heat in the regulator so maybe you should use a 12V motor.

i was also thinking is this that first i use 7809 regulator to regulate the battery voltage to 9V and then use 7805 to further regulate that 9V to 5V.

A single regulator will work the same as two that are in series.

And what about the current ?

i mean my stepper motor requires 1.4A per phase of current. so It will be around 5.6A total requriement. Will the regulator be able to supply such a high current.

If not what should be the best method to do that.

Some datasheets show how a current-booster transistor can be added to the regulator.

[ante]

Hi AG, shaiqbashir,

The L4970A can supply 10A and will do the job without wasting 60% of the energy from the battery; it will waste only about 12 – 15%! An alternative is the MAX724 with a boost power transistor which also have the same high efficiency!

[shaiqbashir]

Hi guys!

thanks for ur tremendous help.

Well! as far as Ante advice is concerned, i have checked the datasheet of Max724/726 and found it really good. I have attached below an application diagram of it i found in that datasheet. What i want to ask here is this, that you can see in the application diagram that it is providing 5V at 5A. If i have a requirement of around 5.2V at 6A, will this diagram work????


Secondly, i have found myself a high current voltage regulator named as MSK5012. I have also attached an application diagram of it below. This was also found in the datasheet. Now considering the same requirements as above i.e. 5.2V at 6A, what will be the value of Cin and Cout? How current can be adjusted in this IC.

[img width=680 height=485]http://www.geocities.com/shaiqbashir/MSK5012.JPG

Please keep this thing in minds that i have a 12V 5Ah DC battery as a source.

Your early reply shall be highly appreciated.

Thanks in advance!

Regards.

[audioguru]

The MAX724 is rated for a max current of 5A. More current might break it.

The MSK5012 is a 10A linear regulator that will do the job if it has a huge heatsink to dissipate the 7V x 6A= 42W.

[shaiqbashir]

thanks a lot Audioguru!


but one question of mine still remains intact. What should be the values of Cin and Cout in order to make it suitable for 5.2V @ 6A.

should i go with 10uf because datasheet says that for Cout, 10uf is a good starting point.

but i didnt find any clue regarding Cin.
can u tell me what function this Cin is performing?
Thanks in advance


take carez

Regards,

[windoze killa]

A good rule of thumb concerning power supply caps is 10 uF per amp. Seeing as you require 6A then 60uF would be a good choice. As 60uF isn't a standard value the 47uf or 68uF would do the job. Capacitance is really only critical in switch mode PSUs. If you decide to go with a SMPS then I would think 100uF or higher maybe needed. Sometimes it is required to use a few caps in parallel to lower the ESR.

[ante]

Hi AG,

I don’t know why you think a MAX724 Circuit as I described it would brake!?!?  “MAX724 with a boost power transistor”!! I attach a circuit for 12A!

[ante]

Hi shaiqbashir,

Have you looked at the other alternative the L4970A? Any of these alternatives should work but as I mentioned before; in the case of the MAX724 it needs a boost power transistor since you are going above 5A in your application. 8)

[indulis]

There is plenty of information out there on buck converters. Check out http://www.linear.com/index.jsp they even have a cad program you can download to help you with your design.

As for cap's on the output of a SMPS… you have to be careful! First, if the ESR is to low, the zero in the transfer function caused by the output cap and it's ESR can cause stability problems. Sometimes, you have to put a "crappy" cap on the output in order to stabilize it, or really slow down the feedback loop which would make transient response worse (not good with motors because of start-up currents). Second, the value of the cap has a LOT to do with the switching frequency. The higher the frequency, the smaller the cap. There is plenty of information out there on buck converters. Check out  http://www.linear.com/index.jsp  they even have a cad program you can download to help you with your design.

As for cap's on the output of a SMPS… you have to be careful! First, if the ESR is two low, the zero in the transfer function due to the output cap and it's ESR can cause stability problems. Sometimes, you have to put a "crappy" cap on the output in order to stabilize it. Second, the value of the cap has a LOT to do with the switching frequency. The higher the frequency, the smaller the cap.