Which 240v Diodes Do I need Please? (1 Viewer)

Jul 18, 2009
11,313
18,353
Manchester UK + Javea/Xabia Spain + Abu Dhabi
Funster No
7,543
MH
HYMER B644
Exp
2004
Hi

I need to run two independent 240v Compressors with one 240v cooling fan.

The Compressors sometimes run independently, sometime together. So I need a feed off each compressor common (live) to feed to the fan.

Problem is, if just one compressor has a live feed to the fan, that in turn will power the other compressor when not required.

Ive looked on RS Components and was thinking of soldering in a 1-Way Diode to prevent voltage running back from compressors from the fan.

I just cannot seem to find the correct specification. Annoying because I had a pack of 50 I used to use twenty odd years ago for the same thing in my workshop. Threw them away two weeks ago as I was trying to clear space.

Any suggestions please?
 
Apr 27, 2016
6,949
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Since the 80s
Diodes won't work on 240V AC. The current is reversing backwards and forwards 50 times per second.

I'd suggest a couple of 240V relays, double pole single throw (DPST). It may be easier to find double pole double throw (DPDT) and ignore the extra pair of contacts. One can switch on when the first compressor switches on. The other can also switch on when the second compressor switches on. Because the relay power contacts are isolated from the coil contacts that trigger them, no current will back-flow to the compressors if the fan is on.

Because you are driving compressors and fans, ie devices with motors, make sure the relays are over-specified if anything, there will be a lot of surge currents.
 

Lenny HB

LIFE MEMBER
Oct 18, 2007
54,140
152,908
On the coast in West Sussex
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658
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Hymer B678 DL
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Since 2008 & many years tugging
The original fan was a capacitor fan, run from the main control board. The Capacitor fan is no longer available. When the replacement fan is connected to the output from the existing control board, it is pulsing.
The fan could be DC and the ac current was rectified & smoothed with a diode and capacitor.
The exact capacitor may not be available but there will be plenty of alternatives.

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OP
OP
EuroTrotters
Jul 18, 2009
11,313
18,353
Manchester UK + Javea/Xabia Spain + Abu Dhabi
Funster No
7,543
MH
HYMER B644
Exp
2004
The fan could be DC and the ac current was rectified & smoothed with a diode and capacitor.
The exact capacitor may not be available but there will be plenty of alternatives.
Thanks Lenny, I am away at the moment and the spec sheet is in my van. Just working whilst I am supposed to be on holiday !
 
Apr 27, 2016
6,949
8,119
Manchester
Funster No
42,762
MH
A class Hymer
Exp
Since the 80s
A capacitor fan, like a standard ceiling fan, has an AC motor that is not self-starting. If the power is turned on, and the fan is given a push, it will then rotate in the direction you pushed it. If a capacitor is wired in series with one of the windings, it will self-start in one direction. If wired to the other winding instead, it will start in the opposite direction.

Fan capacitors are standard components, designed to withstand normal 240V AC. It is important to avoid confusion between Alternating Current and Air Conditioning. Usually Alternating Current is 'AC' and Air Conditioning is 'A/C'.
 
Jan 19, 2014
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Derbyshire
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29,757
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Elddis Accordo 105
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since 2014
The problem with relays is the same, how to separate the relay coil, both compressor feeds need to feed it hence the same problem...
20220923_110610.jpg

Changing to DC will enable it to be blocked with a bridge rectifier (you can buy them as a single 4 legged component) a 240v dc relay will be uncommon so best step it down too.

This should work off the top of my head, there maybe a simpler way though. The fan relay contacts (not shown) will need a permanent live.

20220923_110634.jpg
 

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