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Might anyone recommend an opamp?

by mishima
Tags: opamp, recommend
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mishima
#1
Dec20-13, 08:33 PM
P: 303
I have a 30V, 500mA power adapter that I would like to step down to 15 V with a voltage divider and run through a voltage follower (opamp) for connection to an experimental circuit.

I have really only ever used 741s, and honestly have no idea how to shop for an appropriate opamp here.
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berkeman
#2
Dec20-13, 09:29 PM
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Quote Quote by mishima View Post
I have a 30V, 500mA power adapter that I would like to step down to 15 V with a voltage divider and run through a voltage follower (opamp) for connection to an experimental circuit.

I have really only ever used 741s, and honestly have no idea how to shop for an appropriate opamp here.
What does this voltage input do for your experimental circuit? If you are meaning to power the circuit, an opamp is not the right device (you should use a buck power supply). If you are meaning to monitor the voltage of the input supply, you can step it down further and use a single-supply opamp like the LM324 jellybean opamp to handle that chore.
mishima
#3
Dec20-13, 09:38 PM
P: 303
Right, it was supposed to power it. For reasons I don't understand, my circuit has an equivalent resistance of about 10 ohms (might be shorted somewhere). It wasn't getting enough when just connected to the output of a 1k/1k voltage divider (~0.29V). I thought a voltage follower would solve this, bringing the voltage output of the divider back up to 15 and the current to the most my load could handle. Seems I need to hit the books again.

meBigGuy
#4
Dec20-13, 10:51 PM
P: 1,074
Might anyone recommend an opamp?

The proper component is a voltage regulator, like the LM317. http://www.ti.com/lit/ds/symlink/lm317-n.pdf But, you need to calculate the power it dissipates to determine whether you need a heatsink. The TO-220 package can dissipate about a watt without a heatsink, which is 1watt/15V = 66ma. That is equivelent to a 15/.066 = 225 ohm resistor.

Or, you can look for a 15V fixed output regulator.

Again, regardless of the regulator you choose, you need to consider the power dissipated in the regulator (voltage drop across regulator times current drawn) and its thermal conductivity (junction to case, and case to ambient) to determine the proper heat sinking.

There are several LM317 tutorials out there. But feel free to ask more questions.
mishima
#5
Dec20-13, 11:19 PM
P: 303
It seems to be drawing almost an amp. I will take a look, I've used 7805s before for logic stuff.
meBigGuy
#6
Dec20-13, 11:48 PM
P: 1,074
If it is drawing an amp, a 30V 500ma supply won't survive.

To supply 15V at 1 amp with a passive regulator you need a supply that can supply 1 amp. If you choose 30V 1 amp (bad choice) your regulator will have to dissipate 15 watts. Some LM317's can handle 1.5A, 40V, so all you need is to get a heat sink that will keep the case below 120 degrees. 15W and 100 degrees (some margin) is a heatsink with a 100/15 = 20 C/watt rating. Here is a cheap one that is 10 C/W.

http://www.mouser.com/ProductDetail/...a2raVWidJys%3d
mishima
#7
Dec21-13, 12:07 AM
P: 303
Thank you, that's quite helpful. I'm sure there is a better power supply out there, but the 30V .5A is what I have at the moment for free (until it dies, apparently).

The project is a commercial satellite finder which is intended to run off of a coaxial power supply running from something like a directTV box (13-18V). The guide I am following suggested batteries but they were draining after less than one hour (obviously due to the 1A current) so I am trying to slap together a wall wart kind of setup.

http://www.gb.nrao.edu/epo/ambassado...anualshort.pdf

Since that guide (and others on the net and SARA publications) never mention this problem it is likely I have done something wrong. It is probably an impedance mismatch between the twin leads I am currently using from the wall wart and the stripline on the circuit board of the signal meter.
meBigGuy
#8
Dec21-13, 01:25 AM
P: 1,074
I'm not sure what you understand about power supplies. Power supplies have to be low-impedance.

The 30V supply WILL fry the receiver (if it hasn't already). You can't use a series resistor or a resistor divider. You need to buy a cheap supply or use a regulator. There are 18V wall supplies on ebay.
mishima
#9
Dec21-13, 01:48 AM
P: 303
I understand little about anything electronics. :) I've probably fried it then, given the odd behavior. I'll try again with a new finder (they are only $7) and an actual 18V power supply. Thanks again.
mishima
#10
Dec24-13, 10:29 PM
P: 303
I built a variable power supply with a LM317 as suggested, the current when measured at the same location is a much more reasonable 75 mA with this setup. Thanks for the tip.


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