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Old 21st Jan 2019, 10:29 am   #1
Don_Zalmrol
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Default Revision of a full wave rectifier with centretap questions

Hi all,

I'm currently doing minor repairs on my AMI K-200 jukebox and the power supply unit is in need of some repairs.

The capacitor that is used is unknown and only mentioned as capacitor in my service manual. On another forum (link) I have started a topic, but comments are a bit slow and I wish to continue with my repair as half my living room is now a mess.

The transformer has 3 windings where the center winding is the centertap. The voltage after rectification should be between 25 and 29VDC (Ideally 28VDC).

As you can see from the pictures the current connections are in place.
I would replace the diodes as described in the original post on the different forum (no more 2 diodes in parallel, only 1 on each winding).

But I will also replace the resistor with this one (link) and the capacitor.

In the other forum they suggest a 1000 uF 63VDC capacitor. But when I look at the specs of most of the 1000 uF caps I see that the ripple current is max 1.2A.

The fuse used in the power supply is rated 2A. So the question is shouldn't I order a larger capacitor that could work under max load with 2 amps ripple current?

So would a 2200 uF capacitor (link) not be better then the 1000 uF capacitor?

Thanks!
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Old 21st Jan 2019, 1:11 pm   #2
G8HQP Dave
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Default Re: Revision of a full wave rectifier with centretap questions

The ripple current is largely set by the DC current drawn by the circuit, although increasing the capacitor value may increase the ripple current too. Be aware that increasing the ripple current is a bad idea, as it will cause the capacitor and transformer to get hotter.

Unless you have a hum problem there is no reason to use a bigger capacitor value.

A 2A fuse does not mean that it is good to draw up to 2A ripple current; it means that you can draw up to 2A ripple current without risk to the fuse.
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Old 21st Jan 2019, 1:13 pm   #3
Don_Zalmrol
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Default Re: Revision of a full wave rectifier with centretap questions

Thanks Dave, then it's safe to say to stay with the 1000 uF cap.
I thought that both needed to be similar of value.

Edit: There is a hum, but it's minor and more coming due to worn out caps.
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Old 21st Jan 2019, 6:31 pm   #4
G8HQP Dave
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Default Re: Revision of a full wave rectifier with centretap questions

Both what? The fuse and the cap ripple current rating?

In both cases the current rating is a rating (i.e. what the component can cope with) not a value (what the component will do). You will get more reliable operation by staying below the rating, not trying to achieve it!
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Old 21st Jan 2019, 8:40 pm   #5
Don_Zalmrol
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Default Re: Revision of a full wave rectifier with centretap questions

Ah ok, I see.
Thanks, I will stick with the 1000 uF capacitor then.

I thought that the transformer outputted around 56W as the fuse was rated 2A. And as the original capacitor could provide a max ripple current 1,2A it would strain the cap too much when on full load.
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Old 21st Jan 2019, 10:25 pm   #6
Argus25
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Default Re: Revision of a full wave rectifier with centretap questions

Quote:
Originally Posted by G8HQP Dave View Post
The ripple current is largely set by the DC current drawn by the circuit, although increasing the capacitor value may increase the ripple current too.
The capacitor value and load current are only 1/2 the story, the other two factors are the internal resistance of the transformer and rectifiers.

Since the main issue for the capacitor is generation of heat in its core due to the square of the rms ripple current and the capacitor's ESR, one thing that can help is a small resistor in series with the rectifiers which reduces the peak ripple current, lowers the capacitor core temperature and shifts heat to the resistor instead, which is a bit more tolerant of long term heating than the electrolytic capacitor, where the heating always shortens its life. Of course doing this does degrade the overall supply regulation, but only a little.
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Old 22nd Jan 2019, 1:17 pm   #7
G8HQP Dave
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Default Re: Revision of a full wave rectifier with centretap questions

Quote:
I thought that the transformer outputted around 56W as the fuse was rated 2A.
No, a 2A fuse means that the transformer is protected from having to supply more than 2A. It does not mean that the transformer will necessarily try to supply 2A. The transformer will supply around 28V, but the current depends on what is connected to it.
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Old 22nd Jan 2019, 2:54 pm   #8
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Default Re: Revision of a full wave rectifier with centretap questions

Quote:
Originally Posted by Don_Zalmrol View Post
... The capacitor that is used is unknown and only mentioned as capacitor in my service manual.
In the other forum they suggest a 1000 uF 63VDC capacitor. But when I look at the specs of most of the 1000 uF caps I see that the ripple current is max 1.2A.

So would a 2200 uF capacitor (link) not be better then the 1000 uF capacitor?
Hi,
I bet the capacitor has information printed on the metal can. Peel back the black insulation and have a look. (I guess it is a Mallory from the shape and end tag.)

You do not need a 63 volt replacement. 40 or 50 volts is plenty for a 28 volt supply line.
It will not hurt to fit a 2200uF replacement either once you replace the diodes. Like this for example with 1.9 amp ripple rating? https://uk.farnell.com/united-chemi-...-in/dp/2842059

Any replacement will be much smaller in size than the original.

The resistor will be more than good enough at 50 Watts...
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Old 23rd Jan 2019, 10:36 am   #9
Don_Zalmrol
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Default Re: Revision of a full wave rectifier with centretap questions

Thank you all for the clarification.
I've ordered the replacement parts for the rebuild.

New diodes, new resistor and cap (though stuck with 1000uF 63V to be safe).
About the resistor, it's rated for 50W so in any case it can handle anything thrown at it.
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