Fuse/Diode sizing for rev. polarity protection 1

[techlist]

I have a small circuit, powered by a 24VDC 1.5 amp regulated AC/DC "wall wart".

I need to protect my circuit against a user wiring it backwards. A blowable fuse is fine.

I planned to put a fuse and a diode across teh input terminal, but I'm not sure what size of each to place.

Seemed simple enougn at the start, but after reading the datasheets on the fuses, I'm not so sure. The circuit will pull a max of 1.2A, it's a uP and a step motor driver.

Can someone help me choose the correct fuse size and a diode to go with it?

Thanks, Scott

 

[richs]

Hiya-

Can you put a germanium (low forward drop) diode in
series with the supply at the board and still have the
circuit function within specs? This way, you don't even
need the fuse. If reverse connected, there would be only
leakage current flow, it just wouldn't work.

This is assuming that you aren't using the fuse as an
indicator that the thing was wired up wrong.....

Cheers,

Rich S.

 

[IRstuff]

If you provide the connector, how can the user reverse the connection without voiding the warranty?

TTFN

 

[ScottyUK]

Fuses have an I[²]t characterisitic in which I represents current and t represents time. The I[²]t essentially represents the amount of energy which the fise can withstand without blowing. The diode has a similar characteristic. Choose the diode I[²]t to be significantly higher than that of the fuse. This method is pretty common for selecting fuses to protect big semiconductors.

Rather than the germanium diode suggested above you could also consider a Schottky diode at the low voltages you are contemplating.

And finally there's the brute force solution - at low power levels it's possible to just oversize the protection diode without picking up much extra cost. Something like a 1N5401 3A rectifier would almost certainly survive blowing a 1.5A fast-blow fuse.


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Sometimes I only open my mouth to swap feet...

 

[waross]

How about a bridge rectifier so that the polarity will be automatically corrected? 24 volts should have enough headroom.
respectfully

 

[techlist]

Thanks guys.

1. The board is done so I'd prefer to stay with a Diode and Fuse, but ti's not mfgr's yet so a minor change wouldn't be a big deal.

2. I don't have to have an indicator that the user wired it wrong, but it woudl be good. Of course having it so they can't screw it up would be good, too.

3. Some user may try to "extend" or shorten his PS wiring for cosmetic reasons. Our use their own PS. Thus the risk. It is a minimal risk.

4. Rich: I need most of the 24v but might be able to spare a little drop for a Ger. diode. I'll look some up.

5. Scotty: I'm happy to put in a larger diode that can handle enough currnet long enough to blow a fuse. So if a 1N5401 and a fast blow 1.5A would work, that woudl be fine.

I'm more worried about preventing the thing from catching fire than protecting the electronics <smile>. And I thought a fuse would have the added benefit that if some catastrphic component failure casued a short it would have the fringe benefit of protecting (agains fire) for that, too.

??

Thanks, Scott

 

[BrianG]

For my two pence (cents) worth:
When fuses blow the temptation from non-technical users is to put in a bigger one! If they try really hard they will use aluminium foil or even pieces of metal.

Use a bridge rectifier and they can't get it the wrong way round. Protect against faults from your equipment with a non-accessible wired-in fuse on the circuit board. If it blows then you've got problems and the user should not be encouraged to mess with it further.

 

[techlist]

Thanks. I'm kinda in the middle. It's a replaceable fuse, but not generally accessable.

What about sizing same fuse. The device uses 1.5A (well really about 1.1), shoudl I just use a 1.6A fast blow? I don't know anything about sizing them and teh charts on the datasheets made me worry if I sized it wrong it woudl be useless.

I don't want it to blow under normal operations but I don't want to let it have more than necessary in a fault/component failure condition.

 

[Skogsgurra]

In applications where two diode drops was considered too much, I have used a relay with a diode in series with the coil. Relay pulls in only when polarity is correct and stays off (no power to circuit) when polarity is wrong. An LED may be used for polarity indication so that user knows what is wrong - but probably not necessary.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[logbook]

If you can still buy germanium rectifiers at 2A ratings from distributers I would be very surprised. A schottky drop of 0.4V doesn't seem like much on a 24V supply and is a lot easier and cheaper than blowing a fuse.

Watch your fuse ratings. IEC and UL fuse ratings are different by about 20%. An IEC specified 1A fuse will carry 1A. A UL specified 1A fuse will break at 1A.

Your circuit make take 1.1A steady state, but it will surge much higher than that (electrolytic capacitors across the power rails). Fuses are inaccurate components. I would be looking at more like a 2A to 2.5A fuse. The shunt diode is cheap anyway needing only 50V reverse rating. Use at least a 3A or 5A device.

 

[jimkirk]

One difficulty is that if your supply is truly limited at 1.5 amps, finding a fuse that reliably powers your 1.2 amp load but clears at 1.5 amps is impossible.

Does a series diode cause so much of a drop that your circuit can't handle it? I'd guess your wallwart has a +/- 5% tolerance anyway, which is about a volt. Can't operate with another half volt drop or so?

Another possibility replaces Skog's relay with an FET (at 24 volts, you'll need a resistor divider to drive the gate safely) to minimize diode drops if that's a concern. You can even build a full wave rectifier with 2 P channel and 2 N channel FETs, but you'll need to be careful driving the gates. This, like a full wave bridge, will power your circuit regardless of the applied polarity, without the two diode drops.

Using a DPDT unit for Skog's relay you could do a similar thing, powering it with either polarity connected, but you'd have to be sure that nothing bad happens during the short time it takes for the relay to pull in.

 

[jimkirk]

Um, no, I don't think that circuit will do what you want. What are the gates connected to? Looks like ground. Let's say, for symmetry it's half way between the + and - indicated on the input. Just looking at the top two FETs, with + on the drain of the left hand device. The substrate diode will be on, so the source will be pretty near your 24 volts. So for the right hand FET the source is +24 and the gate is +12, turning on the device, drawing as much current as it can.

There's a similar circuit in thread240-149960 with two P channel and 2 N channel FETs. You'll need to do something to limit the gate-source voltages. This should select the polarity safely. Note that you can't put AC on the input and a big filter cap on the output and expect a near-ideal fullwave bridge.

 

[vitormt]

The fuse has to have a current rating beetwen 1.2 and 1.5A. It can be a one-time-use fuse or a resettable fuse. The blow time should be as fast as possible, so that you do not damage the AC/DC wall wart.
For the diode, I sugest that you use a unidirectional tranzorb,
brecause it protects your circuit agains transient voltages. The working voltage of this diode should be equal or greater than (24 + output voltage tolerance of the wall wart).
If you need more details, ask me.

 

[Skogsgurra]

Welcome vitormt,

It is not quite as simple as that.

Fuses typically have a broad tolerance between "must not operate" and "must operate" so selecting a fuse "between 1.2 A and 1.5 A" is not possible. I don't even think you can find one.

Also, the wall wart usually can take a lot of abuse. Many of them are actually short-circuit proof. The problem is mostly about protecting the attached circuit from wrong polarity.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[vitormt]

What about a 1.25A fuse in series with the positive supply terminal? You can use the Belfuse 5MF1.25.
After the fuse, in parallel with the supply terminals, I would use a 1N6283A tranzorb.

 

[Skogsgurra]

As I said, it will not work. The power supply is current limited to 1.5 A and a 1.25 A Belfuse (5x20 mm) is not guaranteed to break until current is 135 % of rated current - or around 1.7 A. And even if you can get that current out of the power supply, it will need around one hour to break. It will take even more time at 1.5 A.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[richs]

logbook-

Yep. My bad. Your comment:
"If you can still buy germanium rectifiers at 2A ratings from distributers I would be very surprised." is true.

It was in the late 70's that I really had to worry about
a power supply with that little overhead. Nowadays,
I just throw in a voltage regulator as a matter of course.
Don't really worry about what the wall wart voltage is
too much. Sorry, yes a Shottky is the new equivalent.

Next thing that you'll tell me is that I can't get a
distributor in the US for a 12AT7 anymore either! ;-)

Thinking about the OP problem of using a "wall wart" and
being that sensitive to voltage drops, one wonders what
will happen with voltage drops along the supplying cable.
Worse yet, what will happen when the power connector
between the wall wart and his circuit gets
corroded or dirty..........

Cheers,

Rich S.

 

[Skogsgurra]

Perhaps not a distributor - but this guy has some 12AT7;

http://www.vacuumtubes.com/12at7.html

- and I have got some ECC81 for European applications ;-)

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[techlist]

Hi guys:

The Wall wart is regulated pretty well, and it does have short protection. What Gunnar described is exactly what I gathered as teh problem when I started looking at teh datasheets for the fuses. If the PS can't give more than 1.5, then a dead short in the circuit wont' pull enough to burn the fuse. On the other hand a smaller fuse might pop too easily if it was rated near the operating current. It appears one needs a very large difference in available current vs needed current to make a fuse useful at all. Obviously one would not want to oversize a PS just so it could blow a fuse in case of component failure.

I may be worrying about nothing. I just want to be sure the device can't catch on fire in the event some component fails and shorts out. Maybe 24v at 1.5 A won't be able to get it too hot anyway. I haven't done any destructive testing .

I can live with the drop from a diode in series to protect against the unlikely reverse polarity issue. But I worry I'm not providing adequate electrical failure protection. Wonder what would UL/CS require?

 

[Skogsgurra]

There is a simple question to be asked: Are the 1.5 amperes the rated current or the current limit?

If rated current is 1.5 A, then short-circuit current can easily be 4 or 5 A if there is not a current limit function. But, if the current limit is 1.5 A - then you will have a problem with a fuse.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...