Control Transformer Help

[Yellowtail]

I have a small transformer I would like to use to control a 24VDC NC solenoid valve. I think I can just use a bridge rectifier to convert from AC to DC, but I have a few questions.

 

1. The transformer (below) has three leads on the secondary and is marked 24V 0 24V. Does this mean I can 24VAC using one red wire and the center black wire and 48VAV the two red wires, or does it mean I can get 12VAC (black and red) or 24VAC (two red)? 

 

2. Should the rectifier go before or after the transformer? 

 

3. Does the rectifier below look appropriate? 

 

 

 

Vishay Diode Bridge Rectifier 25A, 400V (GBPC2504A) | Lazada.co.th

 

[norbra]

You don't need a rectifier as your solenoid requires 24v AC

supply

[Yellowtail]

11 minutes ago, norbra said:

You don't need a rectifier as your solenoid requires 24v AC

supply

Thanks, I corrected my mistake in the post, the solenoid is marked DC24V

[norbra]

13 minutes ago, Yellowtail said:

Thanks, I corrected my mistake in the post, the solenoid is marked DC24V

Ok rectifier will be connected to the transformer output 24 V.

Terminals 24 - 0 

[Crossy]

You will probably need a smoothing capacitor on the output so the valve doesn't buzz, say 1000uF 50V.

 

Also, that bridge is waaaaaay over-sized, look at something rated around 50V 1A ???? 

[Yellowtail]

34 minutes ago, Crossy said:

You will probably need a smoothing capacitor on the output so the valve doesn't buzz, say 1000uF 50V.

Something like this?

34 minutes ago, Crossy said:

 

Also, that bridge is waaaaaay over-sized, look at something rated around 50V 1A ???? 

Will being too big hurt anything? 

 

 

 

[Crossy]

10 hours ago, Yellowtail said:

Will being too big hurt anything? 

 

 Weeeell ...

 

 

[Crossy]

Seriously, larger rectifiers (both V and A) tend to have larger volt drops.

 

Let's do this right.

 

What is the current drawn by your valve (should be on the valve)?

What is the current capability of your chosen transformer?

 

[bluejets]

Also be aware your 24VAC when rectified and smoothed will give you approx. 1.4 times the AC figure ........ so expect 33.6v DC on the output.

This may or may not be a problem as the details of the valve are not shown here.

 

Might pay to give some thought to a "pulsed" type valve which opens and closes with a voltage pulse (and usually lower voltage around 12v) rather than needing continual power to remain open.

Might also pay to look at plug pack type supply (smps) as they already have everything needed and are all enclosed for your safety.(and probably much cheaper alternative) 12v are more common type.

Each of the valve types have their advantages/ disadvantages.

[OneMoreFarang]

14 hours ago, Yellowtail said:

Should the rectifier go before or after the transformer? 

Anybody who asks that question should not play around with main voltage.

Don't do it! You will live longer. 

[OneMoreFarang]

And if you think that the output of a 24V transformer after the rectification is 24V then think again - or google it.

 

 

[Yellowtail]

4 hours ago, Crossy said:

Seriously, larger rectifiers (both V and A) tend to have larger volt drops.

If voltage drop is a concern, would it not make sense to put the bridge before the transformer? 

 

4 hours ago, Crossy said:

Let's do this right.

That's what I want to do. I don't mind buying what I need.

4 hours ago, Crossy said:

What is the current drawn by your valve (should be on the valve)?

I do not see it on the valve, photos below. It is a 1" water valve I want to open with a timer for an hour a day. 

4 hours ago, Crossy said:

What is the current capability of your chosen transformer?

0.5A 

 

[Crossy]

6 minutes ago, Yellowtail said:

If voltage drop is a concern, would it not make sense to put the bridge before the transformer?

 

If you want it to go into orbit!!

 

I strongly recommend you don't DIY this. Find a suitable 24V wall-wart and 24V timer or a mains plug-in timer.

 

Stay away from mains.

 

Your valve is 24V 2W so about 100mA.

 

[Yellowtail]

3 hours ago, OneMoreFarang said:

Anybody who asks that question should not play around with main voltage.

Don't do it! You will live longer. 

I've been playing with main voltage my whole life both at home and at work. I do not know much about electronics, which is why I am here for help. 

 

I assumed it would, but I do not understand why a rectifier rated 400V and 25A would necessarily go after the transformer, that's why I am asking. Do you know if it goes before or after, and if so, do you know why? 

[Crossy]

https://www.lazada.co.th/products/dc-24v1a-adapter-ac-100v-240v-to-dc-24v-converter-power-supply-adapter-55x25mm-i281783376-s458428237.html

 

Just buy one of these, it's far, far safer for you.

 

And a timer.

 

https://www.lazada.co.th/products/digital-timer-7-days-dual-outlet-i4442589628-s17823169857.html

 

[Yellowtail]

2 minutes ago, Crossy said:

 

If you want it to go into orbit!!

 

I strongly recommend you don't DIY this. Find a suitable 24V wall-wart and 24V timer or a mains plug-in timer.

 

Stay away from mains.

 

Your valve is 24V 2W so about 100mA.

 

Thanks, I already have the 24V timer, I'll buy a power supply. 

 

 

 

 

[Crossy]

2 minutes ago, Yellowtail said:

Thanks, I already have the 24V timer, I'll buy a power supply. 

 

Linked one above.

 

I don't bother building power supplies these days, far simpler and cheaper to just buy one.

[Yellowtail]

2 minutes ago, Crossy said:

https://www.lazada.co.th/products/dc-24v1a-adapter-ac-100v-240v-to-dc-24v-converter-power-supply-adapter-55x25mm-i281783376-s458428237.html

 

Just buy one of these, it's far, far safer for you.

 

And a timer.

 

https://www.lazada.co.th/products/digital-timer-7-days-dual-outlet-i4442589628-s17823169857.html

 

Perfect, thanks for the help!

[OneMoreFarang]

1 hour ago, Yellowtail said:

I've been playing with main voltage my whole life both at home and at work. I do not know much about electronics, which is why I am here for help. 

 

I assumed it would, but I do not understand why a rectifier rated 400V and 25A would necessarily go after the transformer, that's why I am asking. Do you know if it goes before or after, and if so, do you know why? 

A transformer works with AC.

When you connect AC to a transformer on one side, and nothing on the other side, then almost no current flows.

If you connect DC to a transformer, then a lot of current flows. If you would connect mains to the rectifier and the output to that transformer then when you are lucky the breaker disconnects or the transformer burns very fast a "hole" in the wire and that disconnects it. Worst case it makes boom and you have a big problem.

 

You need a rectifier for after the transformer and that needs specs like maybe 50V and maybe a few Amps.

 

I wrote above because if you do it wrong, then that is dangerous. Life threatening dangerous. Like little kids playing with fire. Don't do it! 

[Yellowtail]

7 hours ago, OneMoreFarang said:

A transformer works with AC.

When you connect AC to a transformer on one side, and nothing on the other side, then almost no current flows.

If you connect DC to a transformer, then a lot of current flows. If you would connect mains to the rectifier and the output to that transformer then when you are lucky the breaker disconnects or the transformer burns very fast a "hole" in the wire and that disconnects it. Worst case it makes boom and you have a big problem.

 

You need a rectifier for after the transformer and that needs specs like maybe 50V and maybe a few Amps.

 

I wrote above because if you do it wrong, then that is dangerous. Life threatening dangerous. Like little kids playing with fire. Don't do it! 

Thanks, I did not know a transformer did not work with DC. 

[Yellowtail]

8 hours ago, Crossy said:

 

Linked one above.

 

I don't bother building power supplies these days, far simpler and cheaper to just buy one.

I'm pretty sure I ordered a 24VAC valve, but I've had it over a year, so I just want to make it work. 

 

Thanks again...

 

 

 

[bluejets]

23 hours ago, OneMoreFarang said:

And if you think that the output of a 24V transformer after the rectification is 24V then think again - or google it.

Correct but....the output shown is only half wave rectification so the output value shown is incorrect.

It's actually closer to 1/2 that shown.

[Fruit Trader]

On 10/11/2023 at 6:36 AM, OneMoreFarang said:

And if you think that the output of a 24V transformer after the rectification is 24V then think again - or google it.

 

 

The output voltage of 33V given in your diagram is correct (approx) for half wave rectification with filter and no load. 

 

 

 

[bluejets]

16 hours ago, Fruit Trader said:

The output voltage of 33V given in your diagram is correct (approx) for half wave rectification with filter and no load. 

You forgot to knock off the bottom half of the sine wave, which, when in use, loaded filtered and all the jelly beans removed, will be 1/2 of that.(approx)

Known as half wave rectification...don't believe it...?? look it up...

educate yourself.

[Fruit Trader]

5 hours ago, bluejets said:

You forgot to knock off the bottom half of the sine wave, which, when in use, loaded filtered and all the jelly beans removed, will be 1/2 of that.(approx)

Known as half wave rectification...don't believe it...?? look it up...

educate yourself.

I forgot to knock off nothing.

Does not matter how much jelly bean nonsense you apply, my answer and the diagrams result are correct.

There is a reason why the diagram shows 33V at the output and its not a mistake. Educate yourself and all will be revealed.

 

PS

Its possible the widely used diagram was taken from here

https://rayshobby.net/wordpress/24vac-to-5vdc-conversion

 

A little more help understanding half wave rectification with and without filter.

https://www.youtube.com/watch?v=N7EUf1GAQuI

[Crossy]

Gentlemen.

 

That capacitor (C1) makes all the difference, with no load it will charge up to the peak voltage of 33V which is what you will measure.

 

Once you add a load it will discharge during the negative half-cycle, the rate of discharge determined by the load resistance.

 

[Crossy]

More reading here https://www.homemade-circuits.com/how-to-design-power-supply-simplest-to/

 

[Fruit Trader]

33 minutes ago, Crossy said:

Gentlemen.

 

That capacitor (C1) makes all the difference, with no load it will charge up to the peak voltage of 33V which is what you will measure.

 

Once you add a load it will discharge during the negative half-cycle, the rate of discharge determined by the load resistance.

 

Which I would expect our so called DIY & Electrical Forum Expert to fully understand and not belittle people with jelly bean nonsense.

[Crossy]

2 minutes ago, Fruit Trader said:

Which I would expect our so called DIY & Electrical Forum Expert to fully understand and not belittle people with jelly bean nonsense.

 

In fairness, this is really delving into the electronics world.

 

At this point to prevent further conflict this topic is closed.