# How to calculate capacitor value after making a bridge rectifier ?

Discussion in 'General Electronics Chat' started by LETITROLL, Oct 13, 2013.

1. ### LETITROLL Thread Starter Member

Oct 9, 2013
218
2
Hello everyone .

I have a 220V/400V/415V/440V AC to 24V AC transformer , and i have power bridge rectifier .
the specs of the transformer are :

100 VA
50/60 Hz frequency

i don't know if this is the right value for the current supported by this transformer :
220V/100VA =2.2A

How can i calculate the capacitor's value in order to make the 24 VAC a DC voltage ??

2. ### MrChips Moderator

Oct 2, 2009
14,242
4,174
Depends on how much ripple you are willing to tolerate.

The ripple voltage can be calculated using the formula:

Peak-to-peak ripple voltage = I/2fC

where,

f = line frequency (Hz)

3. ### LETITROLL Thread Starter Member

Oct 9, 2013
218
2
The load is a simple power relay
The coil consumes 0.93 W for a 34 mA current

Last edited: Oct 13, 2013

Jul 18, 2013
12,884
3,520
For simple inductive devices like relays, contactors, brakes, clutches etc, it is common to use just the output from the bridge, No Cap, and the transformer secondary voltage can be the same as the device rated voltage.
If using a capacitor then the secondary would be .7 x desired DC.
That is assuming a simple linear transformer originated supply.
In the case of the transformer you are using, the resultant DC would be 34vdc if using a capacitor.
Max.

5. ### LETITROLL Thread Starter Member

Oct 9, 2013
218
2
I only need 24 vdc at output , so would it be safe to use that rectified output without capacitor ??

Nov 23, 2012
7,108
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Yep.

Just fine.

7. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,309
Maybe not. Some small relays will buzz if driven from full wave rectified "DC".

Your relay seems to be a 24v 34mA DC coil relay, I would use a cap and series resistor. So if the DC at the cap is 32v (etc) then the series resistor needs to drop approx 8v at 34mA.

8. ### LETITROLL Thread Starter Member

Oct 9, 2013
218
2

Jul 18, 2013
12,884
3,520
It is very rare to see 'buzz' on most inductive devices, if fed from 100/120hz ripple, especially if using a BEMF diode across the coil, this also aids in retaining the device through the zero transitions due to the increased time taken for the current to collapse through the coil.
If you have scope you could measure the amount of ripple starting with a 100μf capacitor at maximum current and you would not need anything less than 5% ripple.
Go too high and it increases the Va requirements of the supply transformer.
Max.

Last edited: Oct 14, 2013

Jul 18, 2013
12,884
3,520
Another solution, if using simple inductive device switching and you already have the 24vac if you prefer to use capacitor smoothed DC, to replace the need for a regulator, is to see if it is practical to take a few turns off the secondary.
The secondary is wound on last as a rule, so although it is initially tricky to remove the insulation layer, once removed the turns can usually be removed simply.
A rough guide is 2t/volt, but it could be measured a few turns at a time to get the number of T/V.
Max.

11. ### LETITROLL Thread Starter Member

Oct 9, 2013
218
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Well in that case i prefer using a 24 v regulator .

I have a question max : i have an AD736 RMS to DC converter , i want to check if the input gives accurate output results , but i don't know what is the max input AC voltage i can put .

Jul 18, 2013
12,884
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I have never used that particular IC but looking through the data sheet it does not appear to be ideal for this application?
Also in going through this I would have thaought it more expedient to pick up a transformer with correct secondary.
Hammond have the ideal secondary with 16v to 18vac.
There are also simple bell transformers in this range.
http://www.hammondmfg.com/5cpwr.htm
As long as the mean voltage is sufficient, Relays do not require a regulated supply.
Max.

Oct 9, 2013
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Jul 18, 2013
12,884
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Max.

15. ### LETITROLL Thread Starter Member

Oct 9, 2013
218
2
Yes .
Can you take a look at my DMM repair thread , i have done the test you asked me to do BTW .

Jul 18, 2013
12,884
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That is a totally different situation, the other thread is on repairing the VOM portion of a clamp on meter and this one is on powering two 24vdc relays?
Max.

17. ### LETITROLL Thread Starter Member

Oct 9, 2013
218
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LOL no they are not , i just asked you a favor sir .

Jul 18, 2013
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Well it would be nice to know 'A How to' on powering up two relays from 24vdc is somehow connected to repairing a clamp-on meter??
Max.

Oct 9, 2013
218
2