Which 240v Diodes Do I need Please?

[EuroTrotters]

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?

 

[Lenny HB]

A diode will only work if the compressors are D.C. as they are 240 v they will be A. C.
Should be easy enough to rig up a couple of relays to do the job.

 

[meanders]

Jaycar Electronics do 400V diodes in 1amp and larger

 

[EuroTrotters]

A diode will only work if the compressors are D.C. as they are 240 v they will be A. C.
Should be easy enough to rig up a couple of relays to do the job.

Ive used them on A/C before. They were specified by the Manufacturer of the machines too.

The compressors are A/C

 

[Odd job]

A diode will give pulsed DC if used on AC

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[autorouter]

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.

 

[tonyidle]

Diodes will appear to work in that the compressor unit won't start. But it will be fed with rough DC through the diode that will do nothing for longevity or power consumption. A relay or relays is the correct method. How much current does the fan draw?

 

[EuroTrotters]

Diodes will appear to work in that the compressor unit won't start. But it will be fed with rough DC through the diode that will do nothing for longevity or power consumption. A relay or relays is the correct method. How much current does the fan draw?

< 0.5 amp

 

[EuroTrotters]

A diode will give pulsed DC if used on AC

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.

 

[Lenny HB]

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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[EuroTrotters]

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 !

 

[Richard n Ann]

It's usually just a little 4uf run capacitor on fans.

 

[autorouter]

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'.

 

[Richard n Ann]

The problem with relays is the same, how to separate the relay coil, both compressor feeds need to feed it hence the same problem...


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.

 

[Tiny]

Use a DIAC, which is a diode but for AC volts

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[Gromett]

I agree with tonyidle and autorouter that the correct answer here is relays not diodes.

 

[Richard n Ann]

Or use a thermostat on each compressor discharge set to about 40°c

 

[tonyidle]

Use a DIAC, which is a diode but for AC volts

A diac has an entirely different purpose. Its a bit like a pair of back-to-back Zeners.

 

[Richard n Ann]

Here's a 2 relay diagram, I prefer the thermostats though I think