Diodes in the Flex Cable

Hi. My first post on this forum so go gentle with me. I am not an electrician just a DIYer.

I recently moved in to my new house and needed to get the boiler serviced. The house is a 70s build and has warm air heating from a Johnson & Starley Modairflow boiler. I managed to get hold of a service engineer who came and did the service but also replaced the flex cable to the boiler (as he thought the lump on the cable was a join). Afterwards he took the heat shrink off to reveal two 1N4007 diodes in-line with the line & neutral. Ever since he removed it the timer makes a buzzing noise whenever we switch it on.

All I want to know is what the diodes are doing?

why not get him to put it back to stop the hum?

are you sure he has worked on this type of heater before? removing cables he's not sure what they do!!

A very crude rectifier, why it isn’t built in to the boiler is beyond my ken.

A lash up like that can't possibly be original to manufacture, but may well be a way of replacing a failed internal rectifier. 

If it worked OK before I would say removing it without understanding why it was there is, at least, unwise and will probably lead to more problems.

It has to be a rogue alteration I'd imagine .     Does the timer actually work now ? .  Is it a DC timer ? 

Sorry I haven't gotten back to the forum. I had email notifications set but I haven't received any so thought there weren't any replies. I should have known the diodes were to rectify the current as I've been teaching myself electronics the last few years. Doh!

Anyway, having investigated further, this flex is not the main power supply to the boiler. It runs from the timer control to the hot water solenoid valve.

I think I'm going to put it back on myself. I've left messages with the service engineer but he hasn't replied.

Looking at what I said above I thought 'solenoid', surely that needs a DC supply? It does seem to be chattering.

Solenoids can be rated for AC. 

Should say on the side of the solenoid coil. The AC/DC rating along with current and voltage. 

OK, thanks Rob. I'll have a look & report back.

Its sounding like the solenoid coil is DC  but thats probably still AC .  My knowledge of electronics is not great but I'd say the configuration doesn't give DC  .  Another two diodes and a condenser  thrown in , possibly .  

half wave, Deke

Deke......halfvwave is smaller than a Microwave and not as big as a Mexican wave, but nowhere near as big as a tidal,wave

i knew my C&G in Electronics would come in one day

I'd defininatly say thats to feed a DC coil, you have one diode in series with the supply for half wave recification, then the other one seems to be in reverse parallel with the load, which is generally done to deal with the back EMF when the coil is switched off

Phoenix said:

I'd defininatly say thats to feed a DC coil, you have one diode in series with the supply for half wave recification, then the other one seems to be in reverse parallel with the load, which is generally done to deal with the back EMF when the coil is switched off

Click to expand...

Yes, I've been trying to get my head around what's happening here, and I suspect that the second diode is not actually doing much at all. 

Like you, my electronics theory has suffered from a lack of practical application for too many years. 

I had to draw the “circuit” (I use the term loosely) to make some sort of sense of it. The lower diode would seem to be reverse biased.

OK the solenoid may be DC, half wave is likely to cause the coil to buzz.