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How to select resettable fuses

I have a question related to resettable fuses. These devices have a “holding current” which is a safe current through the device and also a “trip current”, the point at which the device interrupts the current flow.

For example, the MF-R300 has a holding current of 3A and a trip current of 6A. What I’m confused about is what happens when the current is, say, 4.5A? I am guessing the fuse wouldn’t trip at 4.5A but would it eventually trip if that current is maintained?

Furthermore, the data sheet at, under the “typical time to trip” section, states that the MF-R300 will trip after a second at 20A. This seems very high seeing that earlier in the data sheet it says the trip current is 6A.

Can you inform me on the correct way to select a resettable fuse? Let’s say my device under normal conditions can draw anywhere from 0-3A. Any more than about 3A is not normal and a fault may be present.

How would I go about determining what resettable fuse I’d need? (B. W., via email).

• Fuse rating for holding and trip current are based on time. The MF-R300 fuse is designed to stay closed at the holding current for at least 30 minutes (1800s). It will trip before five times that period (ie, 2.5 hours). In other words, the MF-R300 will trip at 3A between 30 minutes and 2.5 hours.

According to the graph in the data, the fuse will trip at 6A after about 1000s. At higher currents it will trip more quickly. The data table shows that it will trip at 15A within 10.8 seconds.

So the holding current is the current that the fuse will trip (eventually) but at higher currents it will trip faster. So if your device does not draw a continuous 3A, then the MF-R300 will be suitable. If it does draw 3A on a continuous basis, then the MF-R400 (4A) might be better to prevent nuisance tripping.

Setting the delay on a VOX circuit

I have been looking for a VOX relay that could switch on a very small amplifier when it receives an audio signal (for example, from an Apple Airport Express) and would subsequently turn off a couple of minutes after the signal disappears, ie, so it doesn’t turn off between tracks.

Through the wonders of Google, I stumbled on the VOX circuit published in July 2011. Will this project be suitable? Where can I buy a kit or the components? (M. P., via email).

• The VOX from July 2011 should be suitable for your application. Time-out is a maximum of 10 seconds but this could be increased to a longer period using a 470µF capacitor at the cathode of diode D4 instead of the 100µF value.

There is no kit available but the PCB, code 01207111, can be obtained from SILICON CHIP for $25. Contact

The components are available from Jaycar and Altronics: and

Using the VOX for PA work

The VOX article in the July 2011 issue of SILICON CHIP is stated to be suitable for PA systems or amateur radio use. However, I believe that the circuit has an omission which precludes its use for these purposes. It does not have any means of extracting the audio from the microphone to then use it for the PA or amateur transceiver.

A small addition to this circuit would enable this to be done. Any ideas for a modified VOX to be able to control the relay and allow the audio to be used as well? (C. M., via email).

• Typically, a signal is taken or tapped off from the transceiver or public address (PA) system to obtain a signal to trigger the VOX. Since we are not breaking the signal to the transceiver or PA but just tapping it off, the transceiver or PA unit will still have an audio signal. There is no need to tap the signal from the VOX itself for the PA or transceiver.

An electret microphone is usually connected when the VOX is used for a stand-alone sound activated switch rather than when used with a transceiver or PA.

However, tapping the signal from the VOX is possible using either the 3.5mm jack socket or the screw terminals. These essentially provide a signal in and loop out (as a parallel connection). If you require an amplified signal, the pin 1 output of op amp IC1a can be used provided it is fed via a capacitor to block the DC level.

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