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Resistance needed to drop from 13VDC to 9VDC

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#1 steveeb

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Posted 26 November 2008 - 03:12 AM

I just bought a Vixen cigarette lighter adapter to power my SD-1 single axis controller. The controller comes with a battery pack for x6 1.5V C sized batteries for a total of 9VDC. Well the cigarette lighter adapter is measuring over 13V and although Vixen says it's for use with the SD-1 controller, I would rather not burn it out or shorten it's lifespan.

I would like to install a resistor to drop the voltage to 9V. Can someone help me calculate what resistance I need to make that drop? TIA.

#2 ccs_hello

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Posted 26 November 2008 - 07:33 AM

Steve,

Using resistor to drop voltage won't work in your application due to various load characteristics. A simple "variable voltage output cigarette plug DC-DC adapter" will be the solution.

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#3 Arctic_Eddie

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Posted 26 November 2008 - 07:37 AM

A resistor won't work in this case. It would if the current were constant but motor drive current is intermittent. You'll need something with a 9V regulated output. I would look for another auto adapter with the correct output voltage and enough current to supply the max amount needed. Be aware of some adapters as the regulator chip often has a 1A current limit.

#4 steveeb

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Posted 26 November 2008 - 08:44 AM

So where would I find such a thing? I just spent $ on a Vixen product they said would work, but would probably just burn out the controller in no time. When plugged in to my power source, it produces 13+V and the controller gets quite hot to the touch. I thought there would have been some sort of circuitry between the lighter plug and the other end, but it's a straight thru hookup.

#5 Skywatchr

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Posted 26 November 2008 - 09:00 AM

Radio Shack, or other electronics store. Maybe the electronics dept. in Walmart or something. Look for the "Universal" power adapters. Just be sure the polarity is right, if it's center positive, or center negative.

Jeff

#6 Midnight Dan

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Posted 26 November 2008 - 09:31 AM

Usually, devices are designed with a range of acceptable voltages. My guess is, since the adapter you bought is made by Vixen, for a Vixen product, that the voltage range for the controller is something like 9v-15v. It is well known that a 12v lead acid battery can produce up to 14volts, so I'm sure they designed it with that in mind.

The heat you are feeling is probably a regulator circuit that is built into the controller. It is doing it's job by reducing the incoming voltage to the 9 volts required to actually run the device. Will the added heat reduce the life of the controller? Perhaps. If you're concerned, I would call them and see what they say.

-Dan

#7 hudson_yak

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Posted 26 November 2008 - 10:56 AM

A good possibility, yes, that there's a regulator already built in.

If not, you can accomplish this with silicon diodes (the most-common type) wired in series, each drops voltage around .6-.7 over a wide range of current flow. Get some rated for several amps. They'll get warm while in operation, depending on the amount of current.

Mike

#8 N6VMO

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Posted 26 November 2008 - 12:55 PM

For about $1 and a little bit of time you can use a 7809 9 volt regulator. It will handle one amp of current and that should be plenty enough for your application. These are available at Radio Shack, Fry's, etc.

Here is a simple circuit using the 7809:
http://www.n6vmo.com\Telescope\9 Volt Regulator.jpg

#9 Skywatchr

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Posted 26 November 2008 - 03:32 PM

True, but if it's getting "hot" on the outside, the 7809 regulator inside is getting quite hot. Apparently the 9 volt battery pack supplied is the "normal" voltage to run it at. 12 or 13 volts may be pushing it on the internal regulator. A heat sink would help, but it's enclosed and won't dissipate much heat after it's saturated. The good news is you'll have a nice and toasty hand in cold weather. :lol:

Jeff

Usually, devices are designed with a range of acceptable voltages. My guess is, since the adapter you bought is made by Vixen, for a Vixen product, that the voltage range for the controller is something like 9v-15v. It is well known that a 12v lead acid battery can produce up to 14volts, so I'm sure they designed it with that in mind.

The heat you are feeling is probably a regulator circuit that is built into the controller. It is doing it's job by reducing the incoming voltage to the 9 volts required to actually run the device. Will the added heat reduce the life of the controller? Perhaps. If you're concerned, I would call them and see what they say.

-Dan



#10 steveeb

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Posted 26 November 2008 - 04:18 PM

Thanks for the info. I went to Radio Shack to look for the 7809 and they told me that it is not carried company wide. So I ordered it for $1 online. That should do the trick. I will try to attach it with a heat sink on the outside so that air can circulate but I will not get burned. I'll let you know how it goes. Thanks again for all the good info.

#11 ccs_hello

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Posted 26 November 2008 - 06:30 PM

Steve,

Just FYI.
Many 780x ICs require a 0.1uF capacitor placed in between pin 1 and 2, as well as another 0.1uF in between pin 3 and pin 2 to operate properly.

Also don't forget the heatsink. A small but thick Aluminum plate will do.

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#12 Arctic_Eddie

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Posted 27 November 2008 - 09:05 AM

I would make the capacitor from output to ground at least 22uF. This will prevent oscillation within the regulators feedback circuit and provide a stable noise free output.

#13 steveeb

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Posted 27 November 2008 - 11:08 AM

Ok, now we're getting a bit complicated.

I'm relying completely on your expertise. I took some electronic's classes about 25 years ago, but have forgotten 90% of what I learned.

Do I need ONE or TWO capacitors? The simpiler the better. Clean power is a plus, but I believe the hand controller's circuitry also has it's own electronics/reducers to do some of that.

Artic_Eddie so if you look at the schematic above from N6VMO, could you please tell me where the capacitor would be placed. I'm thinking between pin 2 and 3. Is this correct?

And trust me, being such a novice, I will be testing this extensively before touching the the power supply to my hand controller.

Thanks for all the help.

#14 rmollise

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Posted 27 November 2008 - 11:30 AM

I just bought a Vixen cigarette lighter adapter to power my SD-1 single axis controller. The controller comes with a battery pack for x6 1.5V C sized batteries for a total of 9VDC. Well the cigarette lighter adapter is measuring over 13V and although Vixen says it's for use with the SD-1 controller, I would rather not burn it out or shorten it's lifespan.

I would like to install a resistor to drop the voltage to 9V. Can someone help me calculate what resistance I need to make that drop? TIA.


If Vixen supplies the 12vdc cord for this unit, the logical assumption is it will work with this unit.

Before you start worrying about voltage regulator circuits, why not check with some Vixen users (maybe a Vixen Yahoogroup) and see what they are using?

:cool:

#15 steveeb

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Posted 27 November 2008 - 01:08 PM

I've actually called VixenOptics a number of times and never really got a good answer. It seems that they import the products but don't really know all the in's and out's of them. At first I was going to purchase their $99 AC converter but again was concerned because of the 12v output. I called them and they said it was OK and now that I have the cigarette lighter thing, it probably would have regulated the power better, but it was 100 bucks!

The cigarette lighter has no regulation and will deliver whatever voltage comes off the battery which can be as much as 14V. I've hunted down a spec sheet on the SD-1 and DD-1 and the SD-1 was rated for 6.5v to 12v. My battery is delivering nearly 14v to the hand controller and that's just plain too much. I figure 9v is a happy medium and is in fact equivalent to the 6 C batteries of the battery pack.

I didn't know there was a Vixen user group, I will go check that out to see if someone has simalar experiences. Thanks for the tip.

#16 Arctic_Eddie

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Posted 27 November 2008 - 03:44 PM

The 22uF capacitor would go between pin 2(ground) and pin 3(output). I would also add 10uF between pin 2(ground) and pin 1(input). These are the only two capacitors needed. They would be the electrolytic type with the negative terminal of both connected to pin 2(ground). Be sure to mount the chip on a good heatsink. Your electronic supplier should have all the parts.

Before building this circuit I would look for an auto adapter with a 9V regulated output. Just be sure it's regulated with a 1A current capacity.

#17 Arctic_Eddie

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Posted 27 November 2008 - 04:33 PM

Here's one from Radio Shack.

http://www.radioshac...oductId=2102592

It seems overpriced to me. You'll also need an adapter plug to fit your unit. Most are 5.5mm x 2.5mm.

#18 ccs_hello

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Posted 27 November 2008 - 09:13 PM

This DC-DC step-down adapter
might be cheaper.

BTW, as I stated, a 78xx series regulator needs small capacitors on input and output sides for stability (as seen in IC datasheet). Can add additional large cap in parallel for better ripple rejection but is optional.

Clear Skies!

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#19 Arctic_Eddie

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Posted 28 November 2008 - 06:52 AM

Excellent choice, higher max current, correct voltage, plug adapters included, and much less cost than RS.

I'll bookmark the site for future use.

#20 Skywatchr

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Posted 28 November 2008 - 10:11 AM

What circuit diagram? I don't see one.

Jeff

For about $1 and a little bit of time you can use a 7809 9 volt regulator. It will handle one amp of current and that should be plenty enough for your application. These are available at Radio Shack, Fry's, etc.

Here is a simple circuit using the 7809:
http://www.n6vmo.com\Telescope\9 Volt Regulator.jpg



#21 ccs_hello

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Posted 28 November 2008 - 02:12 PM

Jeff,

Example: LM78xx series
7805 IC datasheet, see page 21.
(P.S., although it uses .33uF, in my experience, .1uF is good enough.)

Clear Skies!

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#22 Bill Weir

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Posted 28 November 2008 - 03:22 PM

So let me see if I have this story straight. First of all you have Product A. Then you bought Product B, that the company of Product A says is for using with Product A. Then you say you want to modify Product B (can you say "Void Warranty?") and then plug it into Product A. (also pronounced "Void Warranty")

Am I reading this correctly?

Bill

#23 Skywatchr

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Posted 28 November 2008 - 05:22 PM

Thanks! That will come in handy.

Jeff

#24 Skywatchr

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Posted 28 November 2008 - 05:24 PM

What I'm gathering is to make the voltage regulator so it will just "plug in" with no mods done to anything.

Jeff

So let me see if I have this story straight. First of all you have Product A. Then you bought Product B, that the company of Product A says is for using with Product A. Then you say you want to modify Product B (can you say "Void Warranty?") and then plug it into Product A. (also pronounced "Void Warranty")

Am I reading this correctly?

Bill



#25 steveeb

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Posted 29 November 2008 - 12:55 AM

What I'm saying is:

I purchased Product B to power Product A both made by the same Company C. Product B, although made for Product A does not regulate the power to fall within the specs of Product A given by Company C.






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