Will a .125A inline fuse protect reverse voltage to mega48

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I messed up on a design and I have the boards made now. It is a solar panel and 4 volt re chargeable battery configuration. I neglected to put a diode inline to the chip. If the battery were to be connected in reverse the surge through the solar panel would I think would blow the fuse before the chip had any damage.

The board draws a maximum of 36 mA when running. The 16 LEDS, only one is on at a time, one fades up as the next one fades down alternating on and off. The two center LEDS run at 50% duty cycle. The pins sink current to turn leds on.

The solar panel maximum current when charging the battery is 60mA

One thing I see is a .125A fuse has a 7 ohm cold resistance, I don't know much about fuses but that might be a problem. If I can go to a more standard fuse like .5A I think it would be better.

The other problem is the solar panel would fry if the battery was connected in reverse. I was going to protect with a diode but don't see how to do it as current has to go both ways charging and powering the circuit.

Finally got the vodeo working, this is what it does. A simulated rotating light for a solar lighthouse.
When I disconnect the clip I am just simulating darkness on the solar panel because the panel is outside my window. I use the ADC and the 1V on chip reference to shut it down when there is enough voltage on the panel. Then I put the chip to sleep and it draws about .04A

video
http://www.metron9.com/lighthous...

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metron9 wrote:
e: Will a .125A inline fuse protect reverse voltage to megaR

Solder a through hole 1N4001 across the power supply. Connect the bar (cathode) to VCC and the other side to ground so when the battery is reversed the diode will short the supply, thus protecting the uP. Any inline fuse will blow as well.

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You circuit seems to have a few problems:

1/ There's no current limiting resistors for the leds.

2/ Using a jfet to turn leds on/off?

3/ No voltage protection for the AVR

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Kartman wrote:
You circuit seems to have a few problems:

1/ There's no current limiting resistors for the leds.

2/ Using a jfet to turn leds on/off?

3/ No voltage protection for the AVR

1/ Yes there is, one common 50 ohm for leds on Vcc?

2/ How is this a problem? I don't know about J-FETs, I'd use a transistor or enhanced N-channel MOSFET.

3/ Does it need it? What solar/battery voltages are?

- Jani

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Jepael wrote:
Kartman wrote:
You circuit seems to have a few problems:

1/ There's no current limiting resistors for the leds.

2/ Using a jfet to turn leds on/off?

3/ No voltage protection for the AVR

1/ Yes there is, one common 50 ohm for leds on Vcc?

2/ How is this a problem? I don't know about J-FETs, I'd use a transistor or enhanced N-channel MOSFET.

3/ Does it need it? What solar/battery voltages are?

- Jani

1. Only one LED is on at a time of the 16 that connect common to the one 50 ohm resistor. I use an interrupt to sequence through them.

2. The mofet I am using is a 2n7000 I see I used the 2n3819 symbol but the footprint is the same.

3. The solar panel maximum is about 4.6 volts.

"Solder a through hole 1N4001 across the power supply. Connect the bar (cathode) to VCC and the other side to ground so when the battery is reversed the diode will short the supply, thus protecting the uP. Any inline fuse will blow as well."

I see, with this could I then use a resettable fuse in series with the diode like the:
RESETTABLE CIRCUIT PROTECTOR 0.65 - 1.3 AMP
CAT# RXE-065

from
http://www.allelectronics.com/cg...

Like this:

POSterm-----[|doide]---(neg return from circuit here) --------NEGterm

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Yes - go for a polyfuse. They have a very low resistance until triggered. You may need to experiment with a couple of types to get the right trip rating. You need to power the circuit via the polyfuse, with the diode connected between the output side of the polyfuse and ground. That way the polyfuse will trigger when the diode conducts.

I hate non resettable fuses. I used to repair a particular type of (Cleartone) PMR transceivers which had a pcb fuse on the logic control board. It's all that ever used to go wrong with them but was a mare to replace. Judicious use of a polyfuse stopped that.

Your customers will thank you for it.

Kind Regards,
Robin

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I see what you mean, put a new hole in the board and connect the battery + wire to the fuse and the diode across the existing battery connection holes. So a short circuit is created is the battery wires are corssed. So simple but I have never looked at fusing things before.

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That's it.

Watch out for the capacity of the battery though as it may be able to produce a large instantaneous current which could result in the diode going short circuit.

It would have done it's job and protected the Mega48 of course but once it's gone s/c then it's back to you for repair or it goes in the bin. Careful selection of the polyfuse will reduce the chance of this happening.

I've also seen a design which used a Transient protection diode (similar to a zener diode) across the polyfuse output to ground - iirc it was a 1N6277, but this was for a device using a 13.8v supply so you'd need another device from the same 1N62xx family selected to suit your supply voltage.

The other way you might be able to do it is to put a Schottky diode in series with the battery +ve lead. Schottky diodes have a lower forward voltage so won't cause as big a volts drop as say a 1N4001. No need for the fuse then.[Edit] Ignore last paragraph - just realised the need to charge too... [/Edit]

Regards,
Robin

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just realised the need to charge too... [

Yes that was my problem, I had that diode on the board it was one of those last minute oops I better protect reverse voltage with a diode....

I see what you mean, if the diode goes open circuit (fries) then the circuit is no longer protected from reverse voltage. I better nake sure the diode won't fry.

We are using a cyclon battery dual cell pack. Yes it can supply quite a few amps in a short circuit condition. I will have to give this some deeper thought.

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Gibbon's crowbar idea is a good one. But the fuse by itself will not work. It is too slow to protect semiconductors. The polyfuses are even slower.

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Doing some research I found this PDF file about how to use the polyfuse devices. Now I have to read about TV S diodes as well, perhaps not needed for this circuit but I think over voltage and spikes apply more to a TV S diode function.

What I plan to do is solder the diode and poly switch inline with the battery wire going to the board and heat shrink the two to the wire. I see typical voltage drop is only .1V. This could be a good choice for thermal runaway as well for the solar charging part of the circuit if charging that type of battery.

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I'd be putting a TVS diode in there! If there's a solar panel involved, one would think this device is mounted outdoors. What about lightning? A neaby hit will induce some voltage into your wires - you'd want some protection methinks. As for the solar cell - what happens if there's no battery connected? The 12V solar cells I use hsve a 21V open circuit voltage.

The gate voltage (Vgs) of the 2n7000 has quite a spread at a drain current of 1mA - 0.8V -> 3V. So you'd want to be running at a voltage rail of over 4V, probably more depending on what the gate threshold voltage is at a drain current of 16mA and taking into account voltage drops over diodes.

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Could you suggest some digikey part numbers for TVS Diodes that are through hole parts that should work for this application, I don't really know what I am looking for. I should buy a few different sizes I would think.

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goto digikey, enter tvs into search box. Viola! Unidirectional and 5.0V.....

http://www.digikey.com/scripts/D...

Be sure to protect the TVS diode - like with a polyswitch. Otherwise the TVS diode will most likely go up in smoke if activated.