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How much voltage could a voltage regulator regulate?

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MikeT1982

Thu May 17 2007, 08:47PM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

If a voltage regulator could regulate voltage? LOL alrite here is my question, If I have say a fixed 12v little 3 legged voltage regulator from Radio Shack that is rated at 12 volts fixed output at 1 amp max, just how high of a voltage source can it regulate? Like could I hook it to say a 100 volts DC power supply and it will regualte it to 12v as long as I don't pull over 1 amp?

Some articles I've been reading trying to figure this out say that the bigger the voltage I make it drop to its set voltage, the more heat it must dissapate....but this doesn't make sense to me as I thought that the heat was a product of the amperage it pulls through. So I'd think that the input voltage doesn't matter as the heat would be a product of the 12 volts out and the 1 ampout as a max of 12 watts of heat it can dissapate. I'm just specualting, can someone verify this for me?

Thanks a ton ahead of time,

Mike

Sulaiman

Thu May 17 2007, 09:07PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Typical (7812) voltage regulators are rated for up to 35V input
so for a safety margin say 30Vdc max.

The heat generated by the regulator is (Vin - Vout) x Iout (Watts)

e.g. for a 12V regulator with 20V input and 500 mA output, P=(20-12)x0.5 = 4W
IF Vin = 30V and Iout = 1A then P=(30-12) x 1 = 18W

Electroholic

Thu May 17 2007, 09:08PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

the 78xx series are rated for 37V in IIRC.
they are called linear regulator, and they drop the voltage for you buy turning the rest to heat.
so if you are using a 7812, 30V input and drawing 1A.
its gonna give you (30-12)*1 W.

GreySoul

Thu May 17 2007, 09:09PM

Registered Member #546 Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239

IIRC the little radio Shack voltage regulators have a maximum Vin is like 50v or so. You can over-voltage them a bit with proper (over) heatsinking. however there is an upper limit to the silicon used, and I doubt 100v could be dropped down to 12v safely with the little radio shack regulators. I've overloaded and burned out many over the years.

I am looking at the package for a LM317T variable regulator, 1.2 to 37V, and it's max in is 28v...

-Doug

Dr. Who

Thu May 17 2007, 09:11PM

Registered Member #326 Joined: Sat Mar 18 2006, 01:12PM
Location: Glasgow, Scotland
Posts: 66

Depends on the device. Find the device's data sheet (via Google) and it'll tell you the maximum input voltage. The power dissipated in the device will be the voltage drop between the input and output terminals, multiplied by the current. Keep this below the device's maximum dissipation (also in the data sheet), to avoid incinerating your regulator.

Edited to add: looks like we've cross-posted.

MikeT1982

Fri May 18 2007, 12:13AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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Posts: 119

You guys rock!! This makes life easier for me.

Well then I bet I could safely run a 24 VAC step down transformer through a full wave rectifer bridge, then to a smoothing bank of capacitors. From there I will hook one of the fixed 12 volt regs for my relays, lights and buzzers that want 12 volts. I will also hook to the smoothign cap bank 2 of Radio Shack's adjustable ones one set to 3 volts for my diode lasers, and one set to 24 volts for my Kilovac relay with a 24 volt coil. See thats my whole point of my project.

See since I want 3, 12, and 24 volts, at first I was looking into getting 3 separate step down transformers....which would mean I would need 3 separate rectifier bridges and 3 separate smoothing capacitor banks. When I discovered the V regulator, I thought I could simplify this....now that you've told me I can get away with all 3 voltages from a 24 volt source (I will heatsink all 3 VRegs adn have a small fan on them) this saves me sooo much space in my laser project! Thanks a ton!

Hazmatt_(The Underdog)

Fri May 18 2007, 04:31AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Another neat thing you can do with the regulators, is cheat a bit. If you ground-lift the ground tab of the regulator with a large resistor, you could regulate higher then rated voltages. So you could regulate a 150V source within 12V with a 7812, or within 37V with a LM317T, but you have to be careful about the ground lift resistor power handling, it has to be rated for the wattage and voltage you intend to drop during the ground lift.

MikeT1982

Fri May 18 2007, 07:06AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

Cool man thanks! I just thought of something. What does a voltage regulator do if your input voltage drops below what it is set to put out? Like suppose I had a 12 volt fixed 7812 hooked to 24 volts and doing its job outputting a nice 12 volts to my gadgets, then I dropped the input 24 volts to say...9 volts.... Would the output voltage be 9 volts or would the regulator be freaked out or raise the voltage?

Steve Ward

Fri May 18 2007, 07:38AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

These types of regulators need at least the output voltage + 2 volts to regulate properly. With 9V into a 12V regulator, you will see about 7V at the output.

Sulaiman

Fri May 18 2007, 10:13AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

MikeT1982,
A 24 Vac transformer will give 24 x SQRT(2) = 34 Vdc peak which is very close to regulator input voltage limit
but when there is no load on the transformer it will give 5% to 25% HIGHER output voltage
(depending on transformer regulation)
which will be above the maximum allowed input voltage.

If you feed a stepdown transformer with rectifiers and capacitors from a small Variac (and suitable fuse)
you will have an un-regulated but very robust variable dc power supply.
Incredibly useful for many many projects.
Only worthwhile if you can find a cheap Variac (eBay?)

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