12v to 5v reducer

[Takid455]

Need assistance PROPERLY doing a voltage reduction for a sensor (VSS)

I have read that a LM7805 in a T0-220 package along with a in4002 diode will do the trick. yay or nay? Do I need a step-down reducer? DC/DC converter?

Is it worth 40 bucks to buy one? I would like it is compact as possible if I did buy one.

I dont mind building what I need, just want to make sure I am doing ot correctly as to not damage the sensor or the circuit. the 12V (+) runs off a 5amp fuse. I doubt the VSS uses anywhere near that.

do I need this?
https://docs.google.com/viewer?url=http://www.cui.com/pdffiles/VCDE6%2520series.pdf

[jtm311]

I would think you can use a resistors depending on the wattage you need.

Here is a good page . http://hobby_elec.piclist.com/e_resistor.htm

John

[shep]

I would use a 5V zener diode in series with a 1/2 watt resistor it will clamp the voltage at 5V. what kind of load is this using you said VSS. Any idea what the current draw is? http://www.kpsec.freeuk.com/powersup.htm Zener circuit towards the bottom of the page right below the regulator.

[jtm311]

Shep would you use the Diode so you do not get a bounce back of voltage?

John

[Samckitt]

I don't remember the part number, but there is a transistor that you can use that has 3 legs, 12V in on two of the legs & 2 legs out gives you 5V. The negative from 12V is the negative for the 5V, they share one leg. Let me look for it.

[Samckitt]

Found it, at the top center of this image with 7805 on it:
https://static1.pt-content.com/images/pt/2010/06/aldlif-1.jpg

[esenior]

Be careful when using these...they can produce a substantial amount of heat.

I needed a 5V DC supply to power a digital photo frame from a car battery (for my display board). I ended up buying an Audiovox power supply for one of their hands free units. Can get these for around $10 on ebay (Example (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250636588011)).

Good luck! ---Eric

[Rhino]

I could be wrong about this, but wouldn't the heat output increase with current draw? If it's just a sensor, I wouldn't imagine it to draw nearly the amps the photo frame does.

[CarlC]

Way back I used one (6V if memory serves) to power an external CD player. It's a standard Radio Shack type part. I screwed it to the firewall to ground it and to dissipate heat.

[shep]

Shep would you use the Diode so you do not get a bounce back of voltage?

John

A zener circuit is very simple and the heat is disapated in the resistor instead of in the device. Typicaly a half or 1 watt resistor will work if the input voltage and current draw is with in the circuit spec. I used to know the formula to figure all this but haven't done it in about 7 years. You can get resistors that are built in a heatsink if needed. The link I posted has a circuit in it designed for 60 mili amps. A little googleing will come up with a correct circuit design if you can figure the current draw of the vss. Since the heat is disapated in the resistor the circuit should last virtualy for ever provided the resistor is sized appropriately

[jtm311]

A zener circuit is very simple and the heat is disapated in the resistor instead of in the device. Typicaly a half or 1 watt resistor will work if the input voltage and current draw is with in the circuit spec. I used to know the formula to figure all this but haven't done it in about 7 years. You can get resistors that are built in a heatsink if needed. The link I posted has a circuit in it designed for 60 mili amps. A little googleing will come up with a correct circuit design if you can figure the current draw of the vss. Since the heat is disapated in the resistor the circuit should last virtualy for ever provided the resistor is sized appropriately

Thank You I'm in the same boat I knew this stuff many years ago..
John

[David Pozzi]

I bought a little 5v regulated power supply on a circuit board from an Ebay vendor, very cheap. I wanted to power a Map sensor for my Innovate wideband.
This is not it, but similar: http://www.superdroidrobots.com/shop/item.aspx?itemid=508

[DCx]

i run 12V to my VSS on my carbed ls1/t56 swap and it communicates with my autometer electric speedo just fine.

fine tuning the autometer speedo is another story..

[Takid455]

I'll look into some of the ideas. no clue the current demand on teh vss which would be very helpfull.

DCx - you have a 2 wire unit (GM) the 3 wire viper unit uses 5 v.

[79PonchoUK]

I'd just use an LM7805...it just wants a couple of capacitors (one each side) as defined by the calculations of the datasheet. It's good for 1000mA, which is plenty for a speed sensor...You won't have heat troubles and it will cost about $1 in total. It's just in a standard 3 pin package....tiny.

$0.65 for the regulator
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=LM7805CT-ND

Everything you need to know is on here:
http://www.fairchildsemi.com/ds/LM/LM7805.pdf

[TD509EFI]

Sorry for the dumb question, but why would you need 5v for the VSS? I thought most VSS were small PM AC generators and shift voltage and frequency acoording to speed?

John

[79PonchoUK]

Sorry for the dumb question, but why would you need 5v for the VSS? I thought most VSS were small PM AC generators and shift voltage and frequency acoording to speed?

John
It sounds like it will be a hall effect sensor....generally you need a voltage across the sensor to get the hall effect.

I would imagine a standard 2 wire pickup wouldn't work or would be inaccurate for low speed....whereas a hall effect works at any speed.