# Center tap microwave transformer.

#### dl324

Joined Mar 30, 2015
11,485
Would you agree that the word “cartoons” in reference to my cartoonish schematic was a clever jab.
What you posted wouldn't be considered a cartoon.

This would:

The original diagram:

Note that this version has unnecessary wire crossings and scenic routes. I modified this diagram to get the first one to show how it could have been drawn better.

#### dl324

Joined Mar 30, 2015
11,485
The last photograph clearly shows a KBPC 5010, which is a bridge rectifier!
I prefer schematics and didn't look at any of the pictures.

#### Hutch2793

Joined Jul 7, 2020
133
What you posted wouldn't be considered a cartoon.

This would:
View attachment 212944

The original diagram:
View attachment 212945
Note that this version has unnecessary wire crossings and scenic routes. I modified this diagram to get the first one to show how it could have been drawn better.

Wow.... so there is an actually term for it. I apologize then. I was completely wrong about everything I just said. It’s funny how we can interpret text the wrong way. Must be my own self consciousness about my lack of understanding the subject at hand.

Back to business! Could you help me calculate capacitor and resistor values to achieve 6-6.5 Vdc? I’m a little confused with the other poster agreeing that it is a bridge rectifier.I would draw a new schematic, but you both disagree about what it is.

#### dl324

Joined Mar 30, 2015
11,485
The intended output is 14 Vac total on the secondary with the center tap being 7. It is right where I want it. The dc output is supposed to be 6-6.5.
7VAC will give you 8.5V pulsating DC after removing 2 diode drops. If you add a filter cap, you'd be able to measure the voltage with a meter.
Could you help me calculate capacitor and resistor values to achieve 6-6.5 Vdc?
What is the load and current it requires?
I’m a little confused with the other poster agreeing that it is a bridge rectifier
It is a bridge rectifier. The confusion comes from calling it a full wave rectifier.

When you have a center tapped transformer, you can accomplish full wave rectification with 2 diodes. Without a center tap, you need a bridge rectifier.

#### Hutch2793

Joined Jul 7, 2020
133
7VAC will give you 8.5V pulsating DC after removing 2 diode drops. If you add a filter cap, you'd be able to measure the voltage with a meter.
What is the load and current it requires?
It is a bridge rectifier. The confusion comes from calling it a full wave rectifier.

When you have a center tapped transformer, you can accomplish full wave rectification with 2 diodes. Without a center tap, you need a bridge rectifier.
That is the load and current required this isn’t an exact thing I’m working on here. 5-7 volts and 3+ amps.
I have a bridge rectifier attached to my center tap should I have just used two diodes and the 14 Vac would have been split down the middle to yield 7? I have a filter cap, but I’m scared to hook capacitors up without calculating the minimum capacitance for the current I am producing at the moment. I want to learn the math involved for those calculations. I know I need a capacitor and a resistor to smooth the Vdc.

#### dl324

Joined Mar 30, 2015
11,485
That is the load and current required this isn’t an exact thing I’m working on here. 5-7 volts and 3+ amps.
To regulate the voltage, you need a voltage regulator. With a 7V output, you don't have much headroom and will be more susceptible to ripple on the filter cap.

What current is the secondary capable of?
I have a bridge rectifier attached to my center tap should I have just used two diodes and the 14 Vac would have been split down the middle to yield 7?
No. The benefit of using a full wave rectifier instead of a bridge rectifier is that you only have 1 diode drop from the rectifier.
I have a filter cap, but I’m scared to hook capacitors up without calculating the minimum capacitance for the current I am producing at the moment.
The important thing is to connect them correctly and use an appropriate voltage rating. For power supplies, you also need to consider ripple current.
I want to learn the math involved for those calculations. I know I need a capacitor and a resistor to smooth the Vdc.
You don't use a resistor by itself for voltage regulation. That would only work if the load current was constant.

From a Nat Semi Voltage Regulator Handbook:

With a 3A current and ripple voltage of 1V (peak to peak), you'd need 18,000uF; that's a big cap.

If you had a higher secondary voltage, you could use a smaller cap. For instance, if you used the full winding, you could tolerate a ripple voltage of 5V and the capacitor could be reduced to 3600uF. But now you have more power dissipation in the voltage regulator.

EDIT: Here's an illustration of ripple voltage from Stack Exchange:

The red line represents the ripple voltage on the filter cap.

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#### Hutch2793

Joined Jul 7, 2020
133
To regulate the voltage, you need a voltage regulator. With a 7V output, you don't have much headroom and will be more susceptible to ripple on the filter cap.

What current is the secondary capable of?
No. The benefit of using a full wave rectifier instead of a bridge rectifier is that you only have 1 diode drop from the rectifier.
The important thing is to connect them correctly and use an appropriate voltage rating. For power supplies, you also need to consider ripple current.
You don't use a resistor by itself for voltage regulation. That would only work if the load current was constant.

From a Nat Semi Voltage Regulator Handbook:
View attachment 212947
With a 3A current and ripple voltage of 1V (peak to peak), you'd need 18,000uF; that's a big cap.

If you had a higher secondary voltage, you could use a smaller cap. For instance, if you used the full winding, you could tolerate a ripple voltage of 5V and the capacitor could be reduced to 3600uF. But now you have more power dissipation in the voltage regulator.

EDIT: Here's an illustration of ripple voltage from Stack Exchange:
View attachment 212952
The red line represents the ripple voltage on the filter cap.
I have seen many of these setups done without a voltage regulator. I thought I could just use the rectifier, capacitor, and a resistor to supply 6-7Vdc.
I have a transformer with a rewound secondary that can supply 14+- .02 Vac. I made a center tap on the secondary that supplies 7+- .02 Vac. I thought that after rectification of the entire secondary it would provide 7 volts, Instead it supplied 12.8 Vac. I then hooked up the start of my secondary to the rectifier component and the center tap to the rectifier component. This gave me 6 volts exactly. My schematic was correct. I was under the impression that I should have been able to wire the rectifier to the first turn and the last turn of my secondary and that it would split the 14 volts down the middle as long I used the right rectification method?

#### dl324

Joined Mar 30, 2015
11,485
I have seen many of these setups done without a voltage regulator.
There's all kinds of crap on the internet. No reputable person would claim to regulate voltage with a resistor. A lot of newbies think they can do this, but they can't. They also try to use voltage dividers which just reinforces how little they know.
This gave me 6 volts exactly.
You can't measure unfiltered pulsating DC with a voltmeter. With a filter cap, you don't know what the meter is measuring because the DC range can't measure quickly varying voltages.

How many times have I said that in this thread? How many more will it take before you get it?

#### Hutch2793

Joined Jul 7, 2020
133
There's all kinds of crap on the internet. No reputable person would claim to regulate voltage with a resistor. A lot of newbies think they can do this, but they can't. They also try to use voltage dividers which just reinforces how little they know.
You can't measure unfiltered pulsating DC with a voltmeter. With a filter cap, you don't know what the meter is measuring because the DC range can't measure quickly varying voltages.

How many times have I said that in this thread? How many more will it take before you get it?
We are talking about many different things. I’m trying to keep it all straight, I apologize. You say that it can’t be measure without one and you say it can’t be measured with one? “You can't measure unfiltered pulsating DC with a voltmeter. With a filter cap, you don't know what the meter is measuring because the DC range can't measure quickly varying voltages.” How do I measure then? I need a 7 Vdc voltage regulator and a capacitor, correct? I know I need a fuse, but is there anything else I need?

#### dl324

Joined Mar 30, 2015
11,485
How do I measure then?
Referring to the image in post #26.

The gray dashed line represents the full wave rectified secondary output with no filter cap. That voltage can't be measured with a DVM on the DC or AC settings. The DC range can't measure voltages that are changing rapidly. AC is for measuring RMS, which the rectified output isn't, so it can't measure that waveform accurately either.

The red waveform represents the full wave rectified secondary output with a filter cap and some load. A DVM can't measure that voltage either because it's changing too rapidly.

For both waveforms, an oscilloscope is the appropriate tool to use.

I need a 7 Vdc voltage regulator and a capacitor, correct?
You may require multiple capacitors. Voltage regulators sometimes require some ceramic or tantalum capacitors. This is an excerpt from a Nat Semi LM317 datasheet:

#### Hutch2793

Joined Jul 7, 2020
133
Referring to the image in post #26.

The gray dashed line represents the full wave rectified secondary output with no filter cap. That voltage can't be measured with a DVM on the DC or AC settings. The DC range can't measure voltages that are changing rapidly. AC is for measuring RMS, which the rectified output isn't, so it can't measure that waveform accurately either.

The red waveform represents the full wave rectified secondary output with a filter cap and some load. A DVM can't measure that voltage either because it's changing too rapidly.

For both waveforms, an oscilloscope is the appropriate tool to use.

You may require multiple capacitors. Voltage regulators sometimes require some ceramic or tantalum capacitors. This is an excerpt from a Nat Semi LM317 datasheet:
View attachment 212961
Ok so do I have any other options, like the cheap build your own scope kits? School is draining the bank lol. Technically if I measure the ac output before rectification it should be 1.4v drop factoring in the diode voltage loss, correct? Also can I use a zener diode for regulation? Lastly, help me understand capacitor voltage ratings, for example if my calculated Vdc is now 5-6 what voltage rated capacitor would be best? One that is rated for more than that, as well as my capacitance being what you said (18,000 uf)

#### dl324

Joined Mar 30, 2015
11,485
do I have any other options, like the cheap build your own scope kits?
You could go that route; I have one of the cheap digital scopes in the mail from Ali Express ($20 assembled). You could also use try one of those sound card scopes. can I use a zener diode for regulation? Not for 3A. help me understand capacitor voltage ratings, for example if my calculated Vdc is now 5-6 what voltage rated capacitor would be best? One that is rated for more than that, as well as my capacitance being what you said (18,000 uf) The filter cap needs to be rated for the peak secondary voltage, minus the 2 diode drops for the rectifier. You should derate the capacitor voltage for safety; 20% or so is fine, but many go crazy with derating. Thread Starter #### Hutch2793 Joined Jul 7, 2020 133 You could go that route; I have one of the cheap digital scopes in the mail from Ali Express ($20 assembled). You could also use try one of those sound card scopes.
Not for 3A.
The filter cap needs to be rated for the peak secondary voltage, minus the 2 diode drops for the rectifier. You should derate the capacitor voltage for safety; 20% or so is fine, but many go crazy with derating.
Even if their are zener’s rated For over 3 amps? If not what method would you implement for voltage regulation? Do you like your scope?

Btw thank you very much for the help!

#### vu2nan

Joined Sep 11, 2014
162

#### vu2nan

Joined Sep 11, 2014
162
I used a 50 amp Rectifier because of the initial voltage rush when turning power on/off. My transformer settles around 4.20 amps no load.
What do you mean by saying, 'My transformer settles around 4.20 amps no load.' ?

#### Hutch2793

Joined Jul 7, 2020
133
What do you mean by saying, 'My transformer settles around 4.20 amps no load.' ?
After rectification, measuring with a multimeter it was 4.20 amps. I was told this wasn’t accurate though. Before rectification it was at 4.39, if I remember correctly.

#### vu2nan

Joined Sep 11, 2014
162
After rectification, measuring with a multimeter it was 4.20 amps. I was told this wasn’t accurate though. Before rectification it was at 4.39, if I remember correctly.
I understand you have tried to measure the short-circuit current of this power supply (there is no such thing as 'no-load current' !).

What you need to know is what current the transformer can deliver with an acceptable voltage drop.

What is the intended application of this power supply?

- Nandu.

#### Hutch2793

Joined Jul 7, 2020
133
I understand you have tried to measure the short-circuit current of this power supply (there is no such thing as 'no-load current' !).

What you need to know is what current the transformer can deliver with an acceptable voltage drop.

What is the intended application of this power supply?

- Nandu.
Electrolysis, so I need a minimum of 3 amps and 5-7 volts dc. My secondary provides 14 Vac and the center tap provides 7vac.

#### dl324

Joined Mar 30, 2015
11,485
Even if their are zener’s rated For over 3 amps?
This is what the circuit would look like:

The zener doesn't need to pass the load current, but the load current would have to go through R1.
EDIT: If the voltage divider between R1 and R2 is sufficiently "stiff", the zener would stop regulating voltage.
what method would you implement for voltage regulation?
Are you even certain that you need voltage regulation?
Ordered it from Ali Express a couple months ago and it's still in transit. For me, it's just another toy. I already have a dozen oscilloscopes; mostly vintage analog, but a few digital (also vintage).

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