# Thread: Fuse calculation for power supply

1. Junior Member
Join Date
Nov 2007
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8

## Fuse calculation for power supply

Anyone know what is the formula for sizing the line side of a power supply?

Thanks,

2. Unless I misread your post..........WOW, now that is a open question.

3. Multiply by Pi.

4. Cow
Senior Member
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Originally Posted by mdshunk
Multiply by Cow Pi.
Yep.:wink:

5. Originally Posted by mdshunk
Multiply by Pi.
Originally Posted by Cow
Originally Posted by mdshunk
Multiply by Cow Pi.
What's wrong with this picture? :-?

6. Originally Posted by LarryFine
Larry Fine for President!
I'm not sure.

7. Junior Member
Join Date
Nov 2007
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8
e.g. power supply is 115vac to 24vdc and it is a 10amp power supply.

What size fuse is the line side (115vac)?

8. gar
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CrazyCowPie:

There are many factors.

What do you want to protect? The load, the rectifiers, or the transformer?

Inherently you have an inrush problem even with no load on the power supply. This might be the primary criteria because it may size the fuse larger than needed for the load. If you use a series input thermistor you can greatly reduce the peak inrush current and therefore more closely size the fuse to the load. Still you may want to use a slow-blow fuse.

You need to take measurements. If the power supply will be fully loaded, then you certainly need more than a 2 A fuse just for the load and losses.

.

9. Senior Member
Join Date
Jan 2008
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## Have you checked with the MFR?

Most low voltage power supplies have a MFR, check the internet for a specifications sheet. Note: most 10A 24 VDC PS's have nearly the same fusing requirements. Example....
http://web1.automationdirect.com/sta...s/rhinopsm.pdf

If you can't find your MFR's spec sheet, cheat, reference 2 or 3 other MFR's spec sheets, the input fusing requirements should be pretty close for a given size ( amp load ). Don't forget to include your loads inrush current... Oh if your the MFR its back to measuring for a given load.. ( ie bench testing )

10. gar
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CrazyCowPie:

Another question:

Is this a switching power supply or an ordinary transformer-rectifier supply?

A switching supply is primarily a bridge rectifier feeding a capacitor input filter with some current limiting impedance for control of inrush current. Whereas a transformer-rectifier-capacitor supply has the added inrush from the transformer.

To determine transformer inrush current you need to create special test circuitry, or experimentally run many trials to determine the peak value.

To answer your first post I would suggest that there is not a simple formula to determine the sizing of the fuse.

.

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