Power supply help

[moody07747]

I just built this circuit to drive a small 25V 1A motor

https://www.aaroncake.net/circuits/supply.htm

it normally puts out 0-30V at 5 amps

I only needed amp so I modified the circuit

here is what I did...

different voltage regulator
**broken link removed**

smaller filter cap. (C1)
**broken link removed**

smaller transformer
changed from: 24 V 5 A
changed to: 24 V 2 A

everything else is the same

now for the problem...

this circuit keeps burning up POTs
i installed the 5K Ohm POT as in the wiring sheet but i guess it was no good for this type of power.

why is this circuit burning up POTs?

 

[Roboticinfo]

reply

His circuit on the powersupply did not work if you check the message boards on his website you will here all the people complain that it did not work and they had to modify it. I myself thought it was a bad circuit although he has some circuits that work.

:lol:

 

[moody07747]

hmm

are there any power supply circuts here that will put out 1 amp and 30 volts?

(the volts must be variable)

 

[k7elp60]

The only thing I see wrong with the original powersupply circuit is possibly the value of R2. It is specified as 240 ohms. The National Semiconductor data book that I have specifies for the LM338 to use 120 ohms.
I have built numerous powersupplies with the LM338K and used 120 ohms instead of the 240 ohms. Instead of the ceramic capacitors on the output I generally use a 1 to 2.2 uf tantalum capacitor.

Replacing the LM338 with a LM317 should work fine with a 240 ohm resistor and a 4700 uf filter. However with no load the voltage on the input(across the filter capacitor will be very close to 33 volts. (secondary rms x 1.414). When you put a 1A load the voltage across the filter capacitor may be very close to the 24 volt rms secondary voltage.

I can see no reason why the 5K pot is burning up. Have you monitored the output voltage when you first connect the motor. The motor may have a high surge current. But if the regulator is on a adaquate heatsink it should be okey as the LM317 has built in current limit and power dissipation shut down.

DC motors tend to create a lot of noise on the DC line so a bigger capacitor on the output terminals, may be necessary to suppresss which could be causing problems for the regulator. With a larger capacitor on the output it is necessary to connect a diode from the output terminal back to the input terminal of the regulator. The anode goes on the output and the cathode on the input. A one amp diode should be okey.

 

[Roboticinfo]

reply

That is extremly easy, goto these website

http://www.hobbyprojects.com/

http://www.discovercircuits.com/list.htm

 

[moody07747]

Have you monitored the output voltage when you first connect the motor.

what i did for the first test was i just pluged in the circuit with nothing attached to the output lugs and tested it to see if any would blow. nothing blew up so i decided to put a 120v 20W light bulb on the output lugs. the bulb lit up very little but i knew the circuit worked.

after that i brought it upstairs and pluged my spool gun motor into it and thats when i had trouble.

first the motor didnt turn. i thought i blew somthing up but after about 5 seconds, the motor started to turn when i hit the trigger. it was turning at a good speed but it was changing speed quite a bit. when i touched the POT to change the speed I noticed a strange noise and then saw the smoke come out of the vent in the project box. i opened the box then pluged the circuit back in and saw the POT light up like a light bulb.

I have my multi meter at work so i could not use it today.

i dont really want to plug the thing in a 3rd time because i dont want to blow up any of the parts in there but if it will help me fix it, ill plug it in to test some valtage and what-not.

 

[moody07747]

i may even have the wrong POT on there

heres the rateing on the package

5k - Ohm
Audio-Taper
Potentiometer
250VDC * 0.25W

i was told on another site to take the pot off and test the amperage comming to it to see what wattage POT was needed.

do i have the correct wattage POT on there now?

 

[moody07747]

OOPS :x

i powered up the circuit and touched the leads where the POT was with my dads tester and the LM317 VR blew into 2 parts

maby it would have been better to have the teater leads on the POTs wires before plugging it in :roll:

ill stop my radio shack and pick up anther LM317 tomarrow after work then try that again...maby i should get 2 chips while im there :roll: :roll: :roll:

 

[audioguru]

Hi Moody,
As explained on the other site your LM317 is wired backwards and you should have looked at the clear pin layout on its datasheet.

Also explained is the reason to use a linear pot not an audio-taper one.

I agree with K7 and always use a 120 ohm resistor for R2 on LM317 and LM338 circuits and therefore use a pot that is half the value of one needed with a 240 ohm resistor for R2. A 240 ohm resistor will cause the output voltage to rise without a load for some IC's. It can be used with the higher-priced LM117 and LM138. To drive your motor it doesn't matter. :lol:

 

[moody07747]

WOW

i had that all wires quite wrong :roll:

thanks for all that info

 

[moody07747]

radio shack does not carry a 2.5k-Ohm linear POT


are there any substitutions i can put in place of that?

if not, wheres the best place to get one 2.5k-Ohm linear POT?

 

[Roboticinfo]

reply

Order it one the internet

 

[moody07747]

whats a good site to order one part?

digikey charged $5 for handling and they almost never sell just one item.

 

[audioguru]

Hi Moody,
I just finished answering this question on the other site. :shock:
A 5k linear pot will work fine in an LM317 regulator circuit with a 240 ohm or 220 ohm matching resistor since the regulator will always have a load.
If the load is disconnected then its output voltage will rise with some low-spec IC's. :lol:

 

[moody07747]

thanks for all the help everyone

i think i got this under control now

if i have any problems ill be back here

ill also tell you guys if it works

 

[moody07747]

I just put together the new circuit and it works great

from 1 - 30V at 0.66 amps

thanks for all the help guys

 

[audioguru]

Hi Moody,
Congratulations! :lol:

Ah, no more smoke? Your new expertise has spoiled our fun. :lol:

 

[Roboticinfo]

reply

You should make one with more amps


That is extremly low

 

[moody07747]

it does not have to be high at all

its only driving a motor thats not under any stress

the motor is only pushing a small wire through a 3" tube

 

[Roboticinfo]

reply

Ohh