# Fuses in parallel

Discussion in 'Electronic Repair' started by Lenny, Jun 17, 2004.

1. ### LennyGuest

This is probably going to sound like a stupid question but in a pinch
is it OK for example if you need to replace a .250A MDL ( or any type
fuse for that matter), to install two .125A MDL fuses in parallel?
Thanks, Lenny Stein, Barlen Electronics

Yep...OK

kip

3. ### Bill JeffreyGuest

Lenny -

You will still be protected, meaning that no more than 1/4 amp can flow,
but you will be subject to a lot of nuisance pops. Two fuses, even if
they are nominally identical, don't have the same resistance. And two
fuse holders, even if they are nominally identical, don't have the same
resistance. So the current through the two fuses won't be identical -
it will divide in (inverse) proportion to the resistances.

Say for example, that one fuse+holder combination has a resistance of
100 milliohms, and the other has a resistance of 50 milliohms. The
current will divide, 1/3 passing through the first one and 2/3 through
the second one. As a result, the second one will be overloaded while
the total current is still in bounds. In this example, when the total
current is 1/4 amp, the current in one fuse is 83 mA, and the current in
the other is 167 mA. Pow - the second one will blow, and then the first
will blow, even though the total current is OK.

Bill

4. ### Now and thenGuest

you are a funny guy Ha Ha Ha

5. ### AsimovGuest

"Lenny" bravely wrote to "All" (17 Jun 04 12:06:49)
--- on the heady topic of "Fuses in parallel"

Le> From: (Lenny)

Le> This is probably going to sound like a stupid question but in a pinch
Le> is it OK for example if you need to replace a .250A MDL ( or any type
Le> fuse for that matter), to install two .125A MDL fuses in parallel?

Be sure and put equal value swamping resistors in series with each to
make certain they share the current evenly... ;-)

Just kidding! Sure, parallel them... (seriously)

A*s*i*m*o*v

.... Bald spot? It's a solar panel for a sex machine.

6. ### johnGuest

Do you have a problem with my correct answer ?
kip

7. ### Robert StankowicGuest

Not necessarily - thre nominal current is usually much lower than the
current where the fuse blows, and the protection is OK
(But your explanation is of course correct)

8. ### Franc ZabkarGuest

parallel two fuses for the same reason that one should not parallel
two diodes or two transistors, this reason being that there is no
guarantee that current will be shared evenly.

See Bill Jeffrey's answer for a better explanation.

- Franc Zabkar

9. ### johnGuest

True! But close enough till the correct fuse can be purchased.

kip

10. ### LennyGuest

In my case the fuses were MDL slow blow type and selected. They
measured 21.05 and 21.12 ohms respectivly. I paralleled them by
soldering with half inch # 24 solid wire. One of them was then clipped
into the fuse holder. The extra length of wire connecting the other
fuse I suppose in theory may have increased that fuses' resistance but
if at all minutely I'm certain. I operated the equipment under worst
case conditions and everthing seems to be OK. It never occurred to me
(although admitedly it should have) that a rated fuse could be
something other its actual rating. I appreciate all the advice from
everyone particularly about matching resistances. Lenny Stein, Barlen
Electronics

11. ### Sam GoldwasserGuest

Also keep in mind that fuse ratings are not exactly very precise and also
vary based on type. The specifications for one type might say something
like: Time to blow at rated curret: 4 hours; time to blow at 1.5X rated
current: 15 seconds. It's not that a 1 A fuse will blow instanly if
1.000001 A is put through it!

So, worrying about matching fuses in this case may be obsessive compulsive
behavior. If the equipment was running so close to the fuse ratings that
it matters, the equipment was poorly designed in the first place. Parallel
the fuses for now but plan on installing a proper replacement.

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12. ### AsimovGuest

"Lenny" bravely wrote to "All" (19 Jun 04 06:13:28)
--- on the heady topic of "Re: Fuses in parallel"

Le> From: (Lenny)

Le> In my case the fuses were MDL slow blow type and selected. They
Le> measured 21.05 and 21.12 ohms respectivly. I paralleled them by

Wow, I didn't realize they had that much resistance! Well... I suppose
they must have *some* to act as fuses. This means I'd have to be extra
careful measuring the resistance of such low current fuses with an
analog ohmmeter for fear of blowing them with the internal current.

A*s*i*m*o*v

.... That was a fascinating period of time for electronics

13. ### Franc ZabkarGuest

I was taught that the current rating of a fuse specified the maximum
continuous current it could handle without rupturing. Theoretically, a
10A fuse protecting a 9.5A continuous load should never fail.

Have a look at the "Fuse Time-Current Characteristics" in Table 1 of
this interesting primer: