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 ??

 

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,

I = load current (amps)
f = line frequency (Hz)
C = capacitance (farads)

 

The load is a simple power relay
The coil consumes 0.93 W for a 34 mA current

 

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.

 

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.

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

 

Yep.

Just fine.

 

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.

 

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.[/QUOT ]

I want to make this rectified 24 ac a dc voltage and then i will use a voltage regulator to make it 24 dc exactly .
i just don't know how to choose the right capacitance value for it .
The current i need shouldn't be less than 1 A

 

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.

 

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.

 

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.

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 .

Datasheet link :

http://pdf1.alldatasheet.com/datasheet-pdf/view/48197/AD/AD736.html

 

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.

 

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.

Actually the AD 736 test is related to my other thread about DMM repair

 

What about a simple 7824?
Max.

 

What about a simple 7824?
Max.

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

 

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?
Are these linked some how??
Max.

 

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?
Are these linked some how??
Max.

LOL no they are not , i just asked you a favor sir .

 

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.

 

you sure have a max head room for bad sarcasm bro

 

Simple question?
You have two different threads/subjects going without any reference to the other.??
Max.