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Voltage Failsafe?

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modularama

New Member
Hi, I have limited experience with electrical components (and theory), but I'm working on a "battery eliminator" for a portable audio device in my car (which will replace a 1.5v AAA battery with 1.5 volts drawn from the car battery).

Basically...

[12v to 1.5v converter/power adapter]--[failsafe?]--[1.5v portable audio]--[auxiliary input adapter]--[factory radio]

The question is, what I can put between the converter and the player to prevent too many volts from going to the player (if the converter should fail, or is on the wrong voltage setting)?
 
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modularama

New Member
Thank you. I'm looking into those now.

Thinking out loud...

So I have two wires from the converter and two wires from the player. In between those I need a clamp diode and fuse...

The converter already has a fuse, so where does the other one need to go?

Are there inline clamp diodes, kind of like inline fuses to which the wires can just plug into, or is more assembly required here? What sizes do I need, and where can I buy them?

Hmm...
 

ke5frf

New Member
There are different ways of doing this. One way might be a zener diode rated for just above your operating voltage. Perhaps 1.8 to 2.4 volts? Depends on the tolerance of your load. The zener would be reverse biased to ground from your converter positive output. If the zener voltage is reached the zener will avalanche and recover when the overvoltage goes away. This is better for transient spikes.

Other options exist with fast switching diodes. In this case your diode may become a sacrificial lamb so to speak and destroy itself. And in this case you probably need a fuse. When the reverse breakdown voltage is exceeded the diode will momentarily short, drawing supply power to ground. The diode will overheat and open up, and the overvoltage condition will re-establish. This is why you need a fuse. Don't put it in series with the zener, because when the fuse blows it will be, in effect, the same as the diode blowing, so you'll have two blown components and no protection. The overvoltage will find its way back to the load.

Put the fuse in series with the POWER LEAD, BEFORE the diode. This way, when the diode is shorted it will overcurrent the fuse and blow it, opening the circuit so that power no longer can be drawn by the load. Of coarse the diode will likely be shot too. So, both of these components will have to be replaced. Like I said, this is a sacrificial lamb method but it does protect your load, and hopefully it will protect your power supply too, but then again the power supply is probably already faulty when this circuit becomes needed.
 

modularama

New Member
That all sounds interesting. I'll probably have to get into this at a lower level to really understand the hook ups. I may try a D battery in the mean time. I've also found a DC Car Adapter (AH55), which is said to feature "IC regulated overload and short circuit protection to protect it from damage", though I'm not sure if "it" refers only to the adapter.
 

audioguru

Well-Known Member
Most Helpful Member
You could use an LM317 voltage regulator IC adjusted to 1.5V.
 

modularama

New Member
Thanks; I guess that means I could use a standard 12v power adapter. So far, I'm finding more basic information on using the LM317T (1, 2, **broken link removed**) than the diodes and fuses. If someone would like to detail how they might configure things between a power adapter and portable audio player, I'm wondering about various specifics, because I don't yet know enough to generalize with the components. Examples are welcome.
 

smanches

New Member
I thought most diodes short closed when they fry? At least that's the consensus I've seen on using 15V zeners on mosfet gates. The diode will short closed and cause something else to fry instead of the spike traveling all the way back to your control circuitry.

Oh, and what ke5frf was describing is called a crowbar circuit I think. Purposely failing shorted to cause a fuse to blow.
 
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audioguru

Well-Known Member
Most Helpful Member
I have used many LM317 voltage regulators and never had one fail.
I use one to charge lithium batteries and if it fails then my home will be on fire. It charged my lithium batteries safely many times.

I drive my car every day. There are many stupid people who cause accidents but I avoid them so I never had an accident.

I had a heart attack. I got to the hospital quickly so they fixed me before my heart was damaged. Now I am better than before.
 
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