# Simple power supply question

#### Sailor K

Joined Jul 3, 2021
5
Guys, I'm out of my league with circuits, but I have a project and an issue someone may be able to help with. I have a rectifier circuit that I believe is taking 16V AC out of a transformer, going through a half wave rectifier and producing some ugly chopped 14V (MOL) DC. I'm just trying to close a relay with it and so I put it into a 12V DC relay and it chatters like the old time teletypes. To smooth out the output enough to keep the relay on, I was thinking of putting a 1000 micro farad capacitor across the input to the relay (e.g. parallel with the relay coil). Would that do it? A different value capacitor? Or am I better off with an AC relay? Or a different approach entirely? This is a simple question for someone knowledgable, but I'm rusty and never very good at this in the first place. Thanks in advance, Mark.

#### ScottWang

Joined Aug 23, 2012
7,117

If you want to make one by yourself then look at this:
Vout = 16Vac x 1.414 = 22.6Vdc
22.6Vdc is too high for a 12Vdc relay, so you need to add a 12V regulator like 78M12.
16Vac → Full rectifier → 1000uF → 22.6Vdc → 78M12 → 12Vdc

Good Luck.

#### MrChips

Joined Oct 2, 2009
25,011
No harm in trying the 1000μF capacitor.
It would be better to use a bridge rectifier which would give double the frequency.

#### Ramussons

Joined May 3, 2013
1,154
A Half Wave Retifier will cause a DC flow in the secondary winding.of the transformer. As suggested, use a Bridge Rectifier.

#### Martin_R

Joined Aug 28, 2019
103
A Half Wave Retifier will cause a DC flow in the secondary winding.of the transformer. As suggested, use a Bridge Rectifier
How? The transformer will still be AC. How can DC flow into the transformer secondary?
Back to the original question, depending on the relay coil current a capacitor will do the job fine. You may get away with a lower value cap.

#### Ian0

Joined Aug 7, 2020
4,102
How? The transformer will still be AC. How can DC flow into the transformer secondary?
Current flows in the secondary when the output is positive, because the diode is forward biassed. During the negative-going half-cycle, no current flows because the diode is reverse biassed. Therefore the net current flow is DC.
It doesn't do the transformer any harm, but it is quite a lot less efficient. Twice the current must flow during the positive half cycle, so the resistive losses are four times as much (I^2R), but as current flows half the time, the losses are only double.

#### crutschow

Joined Mar 14, 2008
28,532
No harm in trying the 1000μF capacitor.
I would think that 22.6Vdc into a 12Vdc relay coil would likely do it harm.

#### Sailor K

Joined Jul 3, 2021
5
Thanks for all the input guys. The rectifier is in the existing appliance. Its a pool salt system with a design flaw in the electronics. All I'm trying to do is keep power going to the electrodes that are in the water flow. The power that the existing system applies is the chopped DC. So, I was trying to power the plates with a relay that is closed by the water flow switch (closed when water is flowing). But the 12V power I have is the output of the rectifier that I was trying to smooth. Will probably try the 1000 mf capacitor when it gets here.

#### MrChips

Joined Oct 2, 2009
25,011
As noted above, when you rectify 16VAC you end up with about 22VDC which could be too high for your relay.

#### Martin_R

Joined Aug 28, 2019
103
It would be prudent to calculate the value for a resistor in series with the relay to give 12v to the relay.

Joined Jul 18, 2013
24,210
If you use a FW rectifier you don't need the capacitor, the result will be 16vdc, I doubt if it is going to harm the relay using it at that voltage, measure the resistance of the coil to confirm the current it would draw.
Relays have a fairly wide tolerance.

#### BobTPH

Joined Jun 5, 2013
4,267
If you put a 1000 uF capacitor across the relay you will also be altering the power to the rest of the system. If you do this, put another diode between the power supply and the relay + capacitor to prevent possible damage to other components.

Bob

#### Ian0

Joined Aug 7, 2020
4,102
Thanks for all the input guys. The rectifier is in the existing appliance. Its a pool salt system with a design flaw in the electronics. All I'm trying to do is keep power going to the electrodes that are in the water flow. The power that the existing system applies is the chopped DC. So, I was trying to power the plates with a relay that is closed by the water flow switch (closed when water is flowing). But the 12V power I have is the output of the rectifier that I was trying to smooth. Will probably try the 1000 mf capacitor when it gets here.
I'm not surprised that it uses chopped DC, if the electrodes are continuously powered then they will dissolve or become plated much more quickly. I'd be surprised if it samples more than once a second - after all, how fast can the salt concentration change?

#### ElectricSpidey

Joined Dec 2, 2017
2,018
I second what Bob said.

Joined Jul 18, 2013
24,210
A lot of over-thinking going on, I side with @crutschow, simple bridge, no capacitor, 16vdc - bridge drop = 15vdc ,nothing else needed.
Post #11

Last edited:

#### BobTPH

Joined Jun 5, 2013
4,267
A lot of over-thinking going on, I side with @crutschow, simple bridge, no capacitor, 16vdc - bridge drop = 15vdc ,nothing else needed.
Post #11
He is using power off an existing board. Changing that to a full wave rectifier would change its operation.

Bob

Joined Jul 18, 2013
24,210
He is using power off an existing board. Changing that to a full wave rectifier would change its operation.

Bob
Come off the same transformer with a separate bridge?
An insight into the present circuit setup would help!
.

#### Sailor K

Joined Jul 3, 2021
5
Guys, I'm confused.
I'm not surprised that it uses chopped DC, if the electrodes are continuously powered then they will dissolve or become plated much more quickly. I'd be surprised if it samples more than once a second - after all, how fast can the salt concentration change?
Good insight. I'm confused. I appreciate all the comments, but don't know how to proceed. I will attach a drawing of my construction. The application is the following: I have a pool salt generator that has a design flaw in the circuitry--its sampling of salt concentration in the water is bad--hundreds have complained about it. No one has a circuit diagram of the system. I am trying to bypass the offending circuitry and avoid spending \$1000 for a new salt generator. The existing salt generator has two sets of plates, in the upper left of my drawing. The system rectifier is in the middle of my drawing. The rectifier output records as 14V on a dc multimeter, and has power whenever the system is plugged in. It is clearly chopped DC and not smoothed at all. I believe Ian0 hit the nail when he said it helps the plates not get plated with calcium. If I can get the chopped "14V" to the plates, I am good. I'd like the "flow switch" to shut off the current to the plates when water is not flowing (switch open) and apply current to the plates when water is flowing (switch closed). The flow switch is bottom middle of my drawing. I used a simple 12V automotive relay, not realizing that the chopped 14V would cause it to chatter like an old teletype. So I was hoping a 1000 mf capacitor across 85 and 86 (relay coil) would smooth the input enough to let the relay work normally. I'm not trying to condition the power to the plates--only condition the power to the relay so it will operate normally. Thinking the capacitor or perhaps use an ac relay? I was trying to avoid writing a book, but I think the additional detail might be helpful.

#### Attachments

• 336.9 KB Views: 4

Joined Jul 18, 2013
24,210
Simple, treat the flow switch and the relay coil as a separate circuit, and carry out the suggestion of bridge rectifier on the transformer 16v secondary.
I can post a diagram if needed.
From your diagram, the relay and flow switch are the only devices involved?
If an automotive relay has been used, I assume this was a DIY put-together project?
Doesn't sound very professional.?.
Incidentally the automotive supply goes up to 14,6vdc on the car.

#### Sailor K

Joined Jul 3, 2021
5
Simple, treat the flow switch and the relay coil as a separate circuit, and carry out the suggestion of bridge rectifier on the transformer 16v secondary.
I can post a diagram if needed.
From your diagram, the relay and flow switch are the only devices involved?
If an automotive relay has been used, I assume this was a DIY put-together project?
Doesn't sound very professional.?.
Incidentally the automotive supply goes up to 14,6vdc on the car.
Truly a non-professional effort. I'm a retired software engineer. NO circuits background. I will have to try to tap into the 16V transformer output. The other connections are push or screw terminals. If I tap into the 16V ac transformer output, why not just an ac relay? Just asking.... Isn't an ac relay using mechanical means to "smooth" the input? In the meantime, to get the salt system cleaning our pool, I've just hard wired the rectifier output to the plates and put the power cord on a hockey puck timer, with android application to time it and turn it off.