Power supply- MOVs, isolated, etc.

Thanks to a great post by MikeyBoo on isolated power supplies (http://www.picaxeforum.co.uk/showthread.php?30096-Isolated-power-supplies&highlight=isolated+power) , I have decided to upgrade my power supplies for picaxe projects. Seeing as most of my projects use motors and relays which have significant back EMF and really noisy supply lines, it is probably worth putting in filters and all sorts of other goodies.

Question 1: if I use an isolated power supply, is it worth putting in the usual precautions as I would for regulators (LM2937 regulator) like:
- fuse, well upstream, then within your box
- forward biased diode in supply line
- MOV in supply line
- low value resistor in supply line
- TVS/varistor/zener across supply line
- inductor in supply line immediately before regulator
- large and small caps upstream of regulator
- reverse biased diode across regulator output and input
- large and small caps downstream of regulator
- 5v1 zener on output of regulator

Question 2: if an MOV is placed accross the supply lines, then as I see it there are 2 modes:
1 - high resistance when the MOV is below is threshhold voltage
2 - low resistance mode when the MOV is above the threshhold voltage.
but surely in mode 2 this means that the supply lines are short circuited by the MOV. How is that a good idea? It would save the circuit but short circuit the power supply until the power supply is switched off?

Thank you in advance for any advise.

toxicmouse said:

Question 2: if an MOV is placed accross the supply lines, then as I see it there are 2 modes:
1 - high resistance when the MOV is below is threshhold voltage
2 - low resistance mode when the MOV is above the threshhold voltage.
but surely in mode 2 this means that the supply lines are short circuited by the MOV. How is that a good idea? It would save the circuit but short circuit the power supply until the power supply is switched off?

Click to expand...

I would imagine that MOV induced short is intended to cause a large enough current to flow that it is guaranteed to blow the fuse, which disconnects the equipment from the mains and prevents the power supply getting more voltage than the MOV allows.

Even if it doesn't blow the fuse it prevents more voltage than intended from reaching the power supply.

Thanks hippy. That was my understanding as well- to ensure that the fuse would blow but this arrangement only seems useful if there is a polyfuse installed.

Repairs that I have run across where I have visibly seen the results of high voltage (instead of 120 someone wired to 240), the MOV always had one of the leads blown off. Therefore, once it disintegrated the full voltage that caused the problem was applied downstream.

The fault was that there was no fast-blow fuse in close enough proximity to save the day. Even if there was, the power rating on those MOVs wasn't sufficient to hold the current long enough to blow a fuse.

I suppose it depends on what one is intending to achieve; what "useful" would be. In most cases the intent would be to not cause damage, not expose things to damage.

In some cases it may be appropriate to limit then recover, small transients for example, but for other cases it may be best to completely disconnect until the cause is investigated, the fuse replaced.

I would also suppose that a MOV implicitly does that, absorbs smaller transients without taking out the fuse, more inclined to take out the fuse when something more than a small transient.

As well as asking "why is it a good idea?" one can ask "why it would be a bad idea to do otherwise?".

Between the two one can probably decide what is best for a particular situation.