Fuses blowing in pcb's

[Agile]

If you measure voltages applied to components with that component out of circuit then you are asking to become confused.

A digital voltmeter has an input resistance of perhaps 10 MOhms and will read a voltage which would not exist if the supply was terminated with the normal load. Rub a baloon with a velvet cloth and you can meaasure 1000 volts !

Always measure the voltage across the supply to a fan, pump etc with the component connected. First however measure the resistance of the component to ensure its not short or open circuit. Fans 40-80 Ohms, pumps 150-280 Ohms and gas valves 0.8 -3-2 KOhms.

Tony

 

[newcomers]

Always measure the voltage across the supply to a fan, pump etc with the component connected. First however measure the resistance of the component to ensure its not short or open circuit. Fans 40-80 Ohms, pumps 150-280 Ohms and gas valves 0.8 -3-2 KOhms.

Tony

And what about if one of those components is blowing the actual pcb, not the fuse? Surely you wouldnt leave them connected would you?

 

[Agile]

There are some PCBs ( particularly Potty ) where they have resistors which are designed as "fusible resistors" in series with gas valves, fans etc and are intentionally designed to blow if the part protected becomes short circuit or too low resistance.

If you were well experienced you could read from the PCB which part has caused it to blow.

Until then you will have to reply on ALWAYS measuring the pump, fan and gas valve resistances BEFORE replacing the PCB. It may take five minutes but thats always far quicker than blowing a second new PCB.

If you were really experienced then you would use an LCR meter but probably only Chris, Raden, myself, Dave Hickey and perhaps Paul Barker use them on boilers.

Tony

 

[geoff@cetltd]

Like somebody else has said, being competant is about knowing whats safe and what isnt, and working safley, not about knowing everything about everything

No, being competent (in this sense) means being able to work safely AND knowing what you are doing

In my opinion, for someone who is working in a professional capacity, this would include (in this case) being able to think to test the voltage to the solenoid ti identify whether the problem was with the pcb or gas valve

and if you read the post youre referring too, you will see that I did that. You would have also have read that with the leads unplugged from the solonoid, the voltage on the leads stayed at 240V. You would have also read that when the leads were put back onto the solonoid, the voltage kept jumping from 0v to 240v..................you do remeber that dont you????

With the load removed, you could be picking up any crap a DVM has a very high input impedence

So, with the PCB connected to the solenoid, if it's oscillating between 0 and 240 volts, it HAS to be the pcb (or a short - which would blow a fuse)

 

[ChrisR]

It's often worth seeing if your fan / gv / pump is getting supplied by a relay. They'll usually provide a solid voltage, usually 240vac, rather than some jumpy intermediate value, unless, and it's very common, the solder joint on the relay pins are cracked, in which case you're likely to see the the intermittent contact.
Digital meters can be a pain in the proverbial when the voltage isn't constant. A cheapo analogue one (swinging needle) can be much better for finding intermittent contacts.

If the solenoid has an intermittent short L-N or L-E, that will blow a fuse of course.