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Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
I have a question here. Here is a random picture I found of a resistor chart on an LM317T V-Reg information site. But it doesn't specify if it is Radio Shack's LM317T or not. The chart shows the resistor values needed when choosing to set the desired V-Reg's output. Will the Radio Shack LM317T's package actually have a chart like this on the back (or the same exact chart?). I can't find one in a local store to look at so I have to order, and Radio Shack's site does not have a picture of the package, or tell if it has a little insert, or even what values it wants! arghh. I simply want to order my resistors at the same time as I order my Radio Shack V-Regs. Can I follow the chart to heart and just get like 1/2 watt resistors or something and that will be good for up to the max 1 amp I may pull?
(I need one for 3 volts, one for 12, and one for 24) while my rectified AC to smoothing Cap bank of my power supply floats at about 25 volts DC. (FYI, the one set to 3 volts will only be supplying like 90mA so it will have it easy)
And more importantly, my big question is, will these resistor values keep the voltage steady at all voltage ranges within the V-reg's limit? (of course observing the rule that the Vin should be a few volts greater than the desired Vout). In other words what I mean is say I choose the resistor values from the chart for a 12volt output. Must their values be altered if my input voltage changes or could Vin range from like 15-30VDC and still I will get 12V out? (all with the chart's set resistor values).
Registered Member #69
Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116
One LM317T will behave the same as any other (in theory) regardless of where you buy it from. You are correct the output voltage is independent of the input, as long as the input is high enough.
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
An LM317 is an LM317, the chart should work just fine. The only thing the suffix indicates is, in this case, the fact that its a TO-220 package. Some of the other suffixes are for lower current models. This particular one is 1.5A
It will indeed keep voltage output the same no matter what the input. Thats what regulators do. The greater the difference between input and output voltage, the greater the heat it will produce though.
Never anticipate ratshack providing any documentation for their parts. Also, expect to pay 100-300% more for each part than you will anywhere else.
In this case, you could sample two or three from
Or you could buy them for about $0.50 each from mouser.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
You will need a heatsink of some sort. If your input voltage is 30V and your ouptut voltage is 12V then you have a drop of 18V. At 1A current that is 18W of heat and the regulator would overheat in an instant without a heatsink.
Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
Ok cool thanks a million guys! Yeah I was thinking of bolting each one to its own Rat Shack aluminum finned sink they sell for transistors, and using the white electrolytic grease too. Plus I will have a small centrifigul blower nearby the sink fins!
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Question on Radio Shack LM317T Adjustable V-Regulator
