1.2v to 21.2v variable power supply with meter

[delphijustin]

1.2v to 21v variable power supply with meter

hi, hope you like this variable psu, the original circuit come from **broken link removed** with help from ETO I have been able to make modifications to it so it can run a built-in meter. Thanks to 3v0 and Hayato. PLEASE NOTICE THIS CIRCUIT DOES NOT MAKE UP TO 21.2V SINCE THE XFORMER IS NOT THAT HIGH, IT WAS GOING TO UNTIL AUDIOGURU SAID I THAT I HAVE TO USE A LOWER XFORMER. IT NOW ONLY PRODUCES ACCORDING TO ELECTRONICS WORKBENCH 1.2V TO 21V
Here's the parts list with AllElectronics.com catalog numbers:

• 240Ω resistor CAT# 291-240
• 2 1N4XXX Series diodes
• Heatsink Compound for Voltage Regulators CAT# TG-200
• 5KΩ Pot. CAT#LPP-5K
• 100nF or .1µF capacitor CAT#104D50
• 18VAC 1.4A xformer CAT# TX-1814
• ½ amp fuse fast-acting 120V or higher
• 7808 Positive Voltage Regulator(for built-in volt meter) CAT# 7808T
• Bridge Rectifier 8Amp 100V CAT# FWB-802
• LM317 Voltage Regulator CAT# LM317T
• Knob CAT#KNB-125
• 2 of the 1µF Electrolytic Capacitor CAT# 1R50
• 2 of 2200µF capacitor CAT# 2200R35
• Power cord CAT# LCAC-407
• 2 heatsinks (for lm317 and 7808) radioshack cat# 276-1368
• A case.
• PCB
• Wire
• Fuse holder for the ½ amp fuse.
• Digital Volt Panel Meter CAT# PM-22

The schematic is attached to this post, note the xformer symbol has 3 leads on 1 side 2 on the other, please ignore that its just that PCB Wizard does not have the 2-leads only xformer symbol. I have tested it in Electronic Workbench and it has proven there to work. I will upload photos of the project when i have it fully built. If parts are not on the list that are in the circuit please reply to this post letting me know them.

 

[audioguru]

Your 24VAC transformer produces a rectified and filtered 32.4VDC which is too high and makes a lot of heat. Instead use a 20VAC transformer that makes 26.8VDC.
You are using a 240 ohm resistor that is used with the more expensive LM117. An LM317 uses 120 ohms then the variable resistor needs to be 2.5k.
The 1000uF main filter capacitor is too small for 1.5A output. Use 2200uf or 3300uf for less ripple voltage.
The 3A fuse on the input might blow when the output current is 27.5A so you got the arithmatic backwards.
The datasheet for the LM317 shows that it reduces the output current when its input to output voltage exceeds 15V so if you use a 20VAC transformer the max output current might be only 650mA at 1.25V.

 

[delphijustin]

the xformer is 24v, if you read the page correctly you would see the capacitor C1 is 2200uF. What I see your doing is looking at the original schematic. The original schematic has been changed to this other one. I don't see why there's a problem with it works in ewb, 3v0 and hayato helped me designed it.

 

[audioguru]

I don't see why there's a problem with it.

1) your transformer voltage is too high which makes extra unwanted heat in the regulator.

2) The datasheet for the LM117 and LM317 shows that 240 ohms is used with the more expensive LM117 and 120 ohms is used with the less expensive and lower spec's LM317. So your circuit is WRONG and the output voltage will probably rise without a load. Most websites make that error (because they simply copy each other?). When the output voltage rises and you are testing a new Cmos logic circuit then the CMOS circuit will blow up when the voltage gets too high.

3) The datasheet also shows that the LM317 has a minimum of 1.5A output only when there is 15V or less from its input to its output so the output voltage of your circuit must be 17.4V or more for 1.5A output.

4) Your fuse is very wrong and will set your house on fire if something goes wrong.

You should read the datasheet and design a proper power supply circuit yourself so it does not have these errors.

 

[delphijustin]

I just fixed it. I'm using a 18v 1.4a xformer and a 1K ohm pot and a 120 ohm resistor. If this is still wrong please reply back soon as possible.

 

[audioguru]

I just fixed it. I'm using a 18v 1.4a xformer and a 1K ohm pot and a 120 ohm resistor. If this is still wrong please reply back soon as possible.

An 18VAC transformer produces rectified and filtered 23.6V. The LM317 with 120 ohms and a 1k pot has a max output voltage of only 11.7V so 11.9VDC is wasted and makes a lot of heat in the regulator if the load current is high.

 

[delphijustin]

me, hayato and 3v0 made it clear that this new adjustment is correct.