# Power supply help

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#### piotr1s

##### New Member
Hello! I build power supply for college project. It has 230VAC input and 24VAC, 24VDC output. I used multisim to simulate simple circuit with bridge rectifier, capacitor and resistor. I tried different values of capacitor and resistor, and i was able to get 24 VDC. At the same time ripple voltage is 4 VAC. Without the resistor the output is around 31-33VDC, but with small ripple voltage. Transformer im using has 24VAC output. How to get 24VDC and small ripple voltage at the same time. Thank You!

#### ericgibbs

##### Well-Known Member
Most Helpful Member
Hello! I build power supply for college project. It has 230VAC input and 24VAC, 24VDC output. I used multisim to simulate simple circuit with bridge rectifier, capacitor and resistor. I tried different values of capacitor and resistor, and i was able to get 24 VDC. At the same time ripple voltage is 4 VAC. Without the resistor the output is around 31-33VDC, but with small ripple voltage. Transformer im using has 24VAC output. How to get 24VDC and small ripple voltage at the same time. Thank You!

hi and welcome,
The problem is that the 24V is RMS, so when its rectified and smoothed with a capacitor the DC voltage will be approx 1.4 * 24Vrms = 33.6Vdc

To reduce it to 24Vdc with minimum ripple you need a 24Vdc regulator.
What current do want to draw from the 24Vdc...?

EDIT:
look at this pdf for a 7824 Vreg

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• ua7824.pdf
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#### piotr1s

##### New Member
Thank you for the answer. I will look up that voltage regulator. Im not sure what current i need. The power supply will power up PLCs or inverters in the workshop. Do i have to restrict the amperage with resistor to get required current?

#### ericgibbs

##### Well-Known Member
Most Helpful Member
Thank you for the answer. I will look up that voltage regulator. Im not sure what current i need. The power supply will power up PLCs or inverters in the workshop. Do i have to restrict the amperage with resistor to get required current?

hi,
No resistor.
The 7824 will shut down at a maximum of about 1Amp also if it gets too hot.
You MUST determine the maximum load current you expect the power supply to deliver...

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