Author Topic: Converting 9V to 5V  (Read 10151 times)

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Offline dsesmgTopic starter

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Converting 9V to 5V
« on: August 23, 2009, 02:42:22 AM »
Hi, I was wondering whether this circuit would work and reliable to convert 9VDC to 5VDC using MC7805.
http://www.flickr.com/photos/34955071@N02/3847308673/sizes/o/
I have a 9VDC power supply and I need to convert 9V to drive a 5V photo detector.
http://www.eleparts.co.kr/epdf/SENSOR/ST-1CL3H.pdf
« Last Edit: August 23, 2009, 02:49:32 AM by dsesmg »

Offline SmAsH

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Re: Converting 9V to 5V
« Reply #1 on: August 23, 2009, 02:49:40 AM »
yes that will work fine.
are you sure the transistor needs 5v regulated?
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Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #2 on: August 23, 2009, 02:58:59 AM »
Hi, James.
No, I am not sure.  ;D
Isn't the reverse voltage in the maximum ratings is what I should look at?
I thought E-C voltage 5V is the one, since E(emitter)-C(collector) is reversed....
Please correct me if I am wrong.
« Last Edit: August 23, 2009, 03:05:03 AM by dsesmg »

Offline SmAsH

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Re: Converting 9V to 5V
« Reply #3 on: August 23, 2009, 03:01:48 AM »
can you take a screenshot of the datasheet page you are referring to... this would make things easier :P
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Offline dsesmgTopic starter

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Offline SmAsH

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Re: Converting 9V to 5V
« Reply #5 on: August 23, 2009, 03:26:14 AM »
yeah, the max is 20v i think...
where is says "C-E voltage".
this is when the voltage is flowing the right way. the transistor can only tolerate 5v going the wrong way...
i think it will be fine with just a 9v battery.
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Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #6 on: August 23, 2009, 03:40:44 AM »
I see, then I shall go with that circuit to convert 9v to 5v.
Do I need to use heatsink for MC7805 in this case?

Offline Truckstop

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Re: Converting 9V to 5V
« Reply #7 on: August 23, 2009, 03:42:33 AM »
smash is right the C-E voltage is max, it will work fine with a 9-volt battery. you shouldent need a heatsink, but it never hurts to add one

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Offline SmAsH

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Re: Converting 9V to 5V
« Reply #8 on: August 23, 2009, 03:43:26 AM »
what are you "switching" with the sensor?
chances are you wont need a heatsink on the regulator... not unless your pushing the current supply...
do you still need to have the regulator?
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Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #9 on: August 23, 2009, 04:03:34 AM »
oh.... so, I could just use 9V battery to power this detector without MC7805?
I am switching 9VDC to power the motor I mentioned in the other thread titled DC motor.
If I could use 9V battery to power the detector, what is the minimum operating voltage for the detector.
Please correct me if I am not getting your point.  :)


Offline SmAsH

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Re: Converting 9V to 5V
« Reply #10 on: August 23, 2009, 04:11:55 AM »
no, you don't need the regulator.
I don't know, but if i had to take a guess i would say somewhere around 3-5v.
i don't know if your motor will be able to take the 9v though :-\
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Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #11 on: August 23, 2009, 05:07:05 AM »
oh..Yes, it is rated 5V.
But, I will get the motor turned on only for a 2 or 3 seconds at 9V.
Will it descrease the life span of the motor even if the operation time is short?

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Re: Converting 9V to 5V
« Reply #12 on: August 23, 2009, 05:22:45 AM »
for only 2-3 seconds... most likely not...
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Offline Soeren

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Re: Converting 9V to 5V
« Reply #13 on: August 24, 2009, 04:08:43 PM »
Hi,

Never use a 7805 on a 9V-battery (since it's end-of-life voltage is only 5.4V and the 7805 needs at least 8V in.
Besides, you would not get a voltage regulator mounting just the 7805, since it would oscillate periodical or all the time - mount 100nF to 220nF on the input and 22F on the output - both as close to the pins of the 7805 as possible.

The optotransistor can be driven from 9V. If you're afraid to reverse the battery, just add a diode in series with the input.

You cannot mount the transistor through the motor you have. The optotransistor can handle a maximum of 20mA!
You need to use the optotranny in eg. a voltage divider and then amplify the signal to a useable current for the motor.

Your motor will have a much shorter lifespan if driven from nearly twice the rated voltage (the current will more than double).
However... If you just need it for very short time runs every now and then, it will still hold up for quite some time, as long as it isn't allowed to heat up too much (or the insulation of the windings may melt down and short the entire motor). A low value resistor could be used to drive the motor at ~5V from the 9V.
Regards,
Sren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #14 on: August 25, 2009, 05:21:55 AM »
Hi, Soeren. Thank you.
Ok, I will put the capacitors in case I use 7805 to convert 9V to 5V.

Quote
A low value resistor could be used to drive the motor at ~5V from the 9V.
The motor current is 1A(when it starts) and 0.6A at 5V. But, I need to use 9V at the moment. Because, I need a certain speed and the motor spins at that certain speed when it is powered by 9V. so...
 In this case, how much resistance does the motor need?
Do I need to use around like 4ohm?
« Last Edit: August 25, 2009, 05:24:51 AM by dsesmg »

Offline SmAsH

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Re: Converting 9V to 5V
« Reply #15 on: August 25, 2009, 05:23:57 AM »
yes, 4ohm but choose your wattage carefully...
too low and your room will smell like bad...
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Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #16 on: August 25, 2009, 05:49:29 AM »
Hi, James, how are you?
then, If I choose a resistor that would be used in a circuit which is operated at 1A, 4ohm, do I need to use wattage more then 4W?
I have 1/2W, 5%, 4.7ohm available right now.
« Last Edit: August 25, 2009, 08:14:28 AM by dsesmg »

Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #17 on: August 25, 2009, 09:19:51 AM »
Quote
Never use a 7805 on a 9V-battery (since it's end-of-life voltage is only 5.4V and the 7805 needs at least 8V in.
Besides, you would not get a voltage regulator mounting just the 7805, since it would oscillate periodical or all the time - mount 100nF to 220nF on the input and 22F on the output - both as close to the pins of the 7805 as possible.


Are these capacitors ok to use?
http://www.eleparts.co.kr/epdf/rlc/SHL.pdf
I am still searching for 100nF~220nF.

Offline SmAsH

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Re: Converting 9V to 5V
« Reply #18 on: August 25, 2009, 03:53:50 PM »
yes, they're fine...
where are you looking for them?
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Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #19 on: August 25, 2009, 04:40:50 PM »
Hi, James.
I am looking for them in a web shopping site.
I am in Korea so it is a Korean site.
www.eleparts.co.kr

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Re: Converting 9V to 5V
« Reply #20 on: August 26, 2009, 12:16:58 AM »
oh my god, i cant read korean! lol
can you link me to the capacitor page?
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Re: Converting 9V to 5V
« Reply #21 on: August 26, 2009, 12:23:39 AM »
Hi, James,
oh.. then, could you write "AL-CAP" in the search engine of this website? http://www.eleparts.co.kr/

Offline Soeren

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Re: Converting 9V to 5V
« Reply #22 on: August 26, 2009, 01:46:44 AM »
Hi,

I am still searching for 100nF~220nF.
You do know that it means "from 100nF to 220nF" (0.1F to 0.22F), right?
Exact value isn't very critical, 100nF will do, 150nF will do and so will 180nF etc. (hey, 82nF or 330nF would still keep it in check).

You don't need to use 100V caps, 16V or 25V will be quite OK.

Don't you have a defect old VHS-player or similar that you can strip for parts?
Regards,
Sren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #23 on: August 26, 2009, 02:01:15 AM »
Hi, Soeren.
Unfortunately, I don't have a defect VHS player.
So, I need to buy them.  :)

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Offline Truckstop

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Re: Converting 9V to 5V
« Reply #25 on: August 26, 2009, 03:18:17 AM »
you could try other defective electronic like computer tower power supply's, bad battery chargers, stereo amps etc... every piece of "garbage electronics" are gold mines of components, is dumpster diving allowed in your country? its legal here in the us altho frowned upon, i get load upon loads of free parts out of the dumpster behind the radio shack up the street. any applience store should have a simular gold mine waiting for you to claim it..

Here is some good reading for those of you unfamiliar with the art... And remember. Don't leave a mess...

http://www.emoware.org/dumpster-diving/dumpster-diving-laws.asp
http://www.wikihow.com/Dumpster-Dive

Truckstop
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The Hyena: Nature's A$$hole
And.....
The Lemming: Natures Retard

Offline dsesmgTopic starter

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Re: Converting 9V to 5V
« Reply #26 on: August 26, 2009, 04:35:21 AM »
Quote
heres both of the caps
http://www.eleparts.co.kr/front/productdetail.php?productcode=002002023001000623
http://www.eleparts.co.kr/front/productdetail.php?productcode=002002023001000624

That's very nice of you. Thank you, James.

Quote
http://www.emoware.org/dumpster-diving/dumpster-diving-laws.asp
http://www.wikihow.com/Dumpster-Dive

That's very interesting articles. Thank you Truckstop.
i shall find out whether it is legal here. I think I shall post a quiry to the city hall, then.



Offline Soeren

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Re: Converting 9V to 5V
« Reply #27 on: August 26, 2009, 09:07:42 PM »
Hi,

Spread the word among friends and relatives, that you'd like to get whatever piece of electronics they might throw away when it's dead.

If you use a resistor in front of the motor on the 9V supply, the motor won't be getting the full 9V and if you need the speed at 9V, you don't really have other options than hoping it won't die or get a better suited motor.
Unfortunately, you cannot just calculate such a resistor with Ohms Law, since the motor is an inductive component. For that you need to know the inductance (and the resistance) of the motor windings and use Imaginary Numbers to calculate the load (or do as most engineers with the problem of an unknown motor at hand - test it empirically, starting with an educated guess and working it out from there).
Most important part is keeping an eye at the temperature... Or rather a finger - when you find it too hot to touch reasonably comfortable, cut power immediately and let it cool completely.
Regards,
Sren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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