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Choosing the correct size wires

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waterbug

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I've posted questions before about a 1.5-30V 5A power supply I am building. My latest question is how to choose the the correct wires for connecting the various resistors, capacitors etc on the board and if that would differ from the output and variable resitsor wire sizes.
Thanks for all your help
 

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Willbe

New Member
More important is that the regulator sense circuitry be at a single point ground (at the large cap negative terminal).
5A will almost melt #30 and the resistance of wire halves for each 3 AWG numbers down from there. Teflon insulation can withstand very high conductor temps, other insulations not so high.
It's on the Web. Look up CM (circular mils) and ampacity.
 

Sceadwian

Banned
According to this AWG chart
American Wire Gauge table and AWG Electrical Current Load Limits
You should be using approximately 22 gauge wire. Too big is okay, too small is not. PCB traces are different I don't have any good charts for ampacity vs trace width/thickness because the thickness of a PC board can be pretty variable.
 

Sceadwian

Banned
Right willbe, which is why you use 22 AWG or equivalent traces.
 

crutschow

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Most Helpful Member
More important is that the regulator sense circuitry be at a single point ground (at the large cap negative terminal).
For best voltage regulation you want the regulator sense circuitry lines to be connected to the output terminals. That way any resistance in the wire carrying the load current from the regulator to the output terminals is canceled. Some lab power supplies bring the sense lines out to separate terminals so you can connect them to a remote load and cancel any resistance of the wires going to the load.

There should also be a decoupling cap connected across the output terminals so the power supply has a low output impedance to high frequencies.
 

audioguru

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your circuit will not work. Look at the datasheet:
1) The LM338 guarantees a 5A output only if the input to output voltage is 10V or less. So if the input voltage is 35V then you will have 5A with an output from 32V to 25V. At a 5V output the current might be only 1A.
2) The more expensive LM138 works with 240 ohms from the output to the adj. pin. But the LM338 needs 120 ohms. Also then you must use a pot with half the value you have.

Calculate the power dissipation to see if the thing gets too hot.
 

waterbug

New Member
Your feed back is great

The power supply is using parts from a former McIntosh 0-13.5v 0-6A programable power supply. T1 is a Toroid #617.152 117vac in 2x15V/5.9A out. It originally had two regulators that I can not ID. The are the same construction as the LM338K and are marked 132189 over C9044. the original pot was a wire wound 1K. The amp and volt meters are Modutec. All enclosed in a really nice case. I'll do what ever you all suggest to get a working power supply. I just read some theory on decoupling caps that say the .1uF and 1uF caps in the output are decoupling caps. Am I reading this right?
 
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crutschow

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I just read some theory on decoupling caps that say the .1uF and 1uF caps in the output are decoupling caps. Am I reading this right?
Any cap that is connected from a dc voltage to ground is considered a decoupling cap. If it's connected in an ac signal path (as a dc block or filter component for example) then it's not.
 
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