I built this circuit a little while ago and never got a chance to test it:
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i di dsome test and something i do not understand is happening. I can turn on some circuit with no problem but when it is time to turn a motor, its a whole different story. The motor would turn for a while then stop, no more voltage at the output. Or if i grab the shaft with my hand and make it stop, it would not start again until a few second. I do not know if its a bad power supply concept, or i was thinking that my wires are not big enough, but nothing is heating in the power supply. Any ideas?
Hi, Ken. I'd guess the answer is the LM317, and it's not a bug, it's a feature.
The LM317 (one of the all-time great ICs) has thermal overload protection as well as overcurrent limiting. When the IC die temperature exceeds 150C, the IC has a self-shutdown feature to help prevent it from letting the magic smoke out. When overtemp protection kicks in, you have to let it cool down a bit before it will turn on again.
I'll bet that when you run the motor load, the current rises and the heat's ramping up until it shuts down. There are a couple of things you can try. First, use the pinky test to feel the LM317 (moisten your finger a little first to avoid blisters!!). If the IC is painfully hot, you're probably running it at too high a temp. Slap on a big heat sink and be sure to use a small amount of white zinc-oxide thermal grease to ensure good thermal contact between the IC and the heat sink. If you've already got a heat sink there and it's painfully hot, get a bigger one.
If you've got very brief overload situations, you can help yourself by putting a big cap (say, 1000uF) at the output of the LM317. But if you're gonna drive the motor, the best thing to do might be to go with a voltage regulator that can handle higher currents. You might try just dropping in an LM338 -- it's pin-for-pin compatible and can handle
5A peak loads. But remember the heat sink. And also remember that you'll need to upgrade your transformer, too. The transformer current should be rated for 1.8 times the DC current you need. So, if you need
2A DC, your transformer should be rated for 24VAC @ 3.6A or more.
that sounds good, I do not have a heat sink an it is getting hot. Although I thought it was a transformer problem so I did change to one that can supply more current, but that did not fix the problem. I just tried a heat sink and it works better. still getting hot but much better. the current is about 1.2 amp, I thought the chip could handle 1.5 amp without heating too much. thanks ken
Read the LM317 data sheet carefully. The 1.5A rating is at a junction temperature of 25C but you know your junction is higher than that since the package tab is. In any case, the LM317 protects itself by shutting down when the unction temperature gets to 150C. Assuming you are using the TO-220 package, the LM317 can only disipate 2.5 Watts at a 25C ambient and no heatsink. With your 1.2A current, if the Vin - Vout of your LM317 is greater than 2V then it will shutdown shortly.
over heating and the LM317 is shutting down? or, the brushes in the motor are causing noise? in which, put a load Resistor with a small disc cap on the load.. etc..
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"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Over-Current Shut-Down circuit is being tripped. Too much current is going through the LM317. You need to drive the motor using an external Pass Transistor to shunt the Motor Current around the Regulator circuit. This is NOT an overheating problem. Heat-sinking will not help !
Well I only had some LM350 around, I added a heat sink to that and put it in the circuit The motor is running good, I stalled it and it starts again by its own, something it did not do before.
Look at page 13 for the circuit labelled "High Current Voltage Regulator". You need to add only 1 resistor and 1 transistor to your existing LM317 circuit. Use an MJ2955 transistor on a nice heatsink. It can handle up to 15 amps or up to 115 watts.
This is a really nice, simple circuit, and you have the perfect opportunity to use it now.
Having this circuit in your arsenal will serve you well, certainly for your present project, and in the future.
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