2
\$\begingroup\$

I'm thinking about protecting a 12V/20W transformer with a polyfuse.

Quote from the Littlefuse datasheet:

I_Hold = Hold current: maximum current device will pass without tripping in 20°C still air.

I_Trip = Trip current: minimum current at which the device will trip in 20°C still air.

Let's take this example:

Part No. 30R185U, I_Hold 1.85 A, I_Trip 3.70 A

So this fuse will pass 1.85 A continuously without tripping, trip for any current greater than 3.70 A and yield unspecified behavior in between?

Now for the real question:

Is it feasible to use two fuses (for my example, two 30R090U with I_Hold = 0.90 A) in parallel for the benefit of the smaller time-to-trip?

Is the variance in resistance small enough that half of the current will pass through each fuse during normal operation?

\$\endgroup\$
3
  • \$\begingroup\$ Have you had a look at the resistance ranges? \$\endgroup\$
    – PlasmaHH
    Commented Jan 26, 2017 at 11:58
  • \$\begingroup\$ Yes, but what good are minimum resistance in un-soldered state and maximum resistance 1 h after tripping? \$\endgroup\$
    – Jasper
    Commented Jan 26, 2017 at 12:02
  • \$\begingroup\$ They denote the range of the resistance you can expect any fuse to have. \$\endgroup\$
    – PlasmaHH
    Commented Jan 26, 2017 at 12:11

2 Answers 2

Reset to default
2
\$\begingroup\$

It is generally recommended not to try using two poly fuse devices in parallel. Component variations in this device class are rather large enough that sharing of load currents is not going to be very good.

Also consider that when you decide to use a poly fuse type protection device you need to stop thinking about precise digital switching. This type of device needs to be considered as an On / Off device from a far out view of a low nominal load current range versus a serious overload current range. Do not expect any type of precision switching between these two extremes. Keep in mind that the device characteristics between these extremes also change considerably based upon environmental conditions and the number of times the device has tripped.

\$\endgroup\$
2
  • \$\begingroup\$ "the number of times the device has tripped." Do they degrade with every cycle? \$\endgroup\$
    – Jasper
    Commented Jan 26, 2017 at 13:47
  • 1
    \$\begingroup\$ Yes, they degrade every time they trip. A few cycles isn't going to matter too much but they should only be used in the same situations as a traditional fuse, they are for fault protection not switching things on and off. \$\endgroup\$
    – Andrew
    Commented Jan 26, 2017 at 13:59
1
\$\begingroup\$

I know from experience that putting two poly-fuses in parallel will result in a trip current that is not significantly higher than the current of a single fuse. Due to manufacturing differences they will have different resistances and so no load share well.

You can get around this by putting a small (in terms of ohms, physically fairly large) resistor in series with each fuse, the resistor needs to be larger than the fuse on resistance so that the unit variation becomes small in comparison to the total resistance. Obviously this has an efficiency hit.

Honestly the best bet is probably to prototype the circuit and verify it does what you want. Remember to take temperature and airflow into account since they are significant factors.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.