PTC resettable fuse questions

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Hi,

I am trying to choose a good (as in proper values) PTC for my USB power supply project for micro-controller. I am looking to make a simple power supply for breadboarding purposes. The little gadget will provide USB 5V at 500mA or 3.3V/3.6V at 300mA via an LDO. So, my questions are:

From DigiKey's list of thousands of avail PTCs ( http://search.digikey.com/script... ), I am a bit confused of the right values to choose for time to trip, current trip, and current hold.

I would choose 10V as the max. voltage for both the 500mA and 300mA case, since the max voltage is 5V. So, that should be safe enough

1. For the current trip value, should I go about 5% less than the max current output? That is, for example, for the 500mA max output at 5V, is 475mA a good value? Or ok to have 500mA and still safe?

2. current hold. 5% less than the max current output? A good figure?

3. Time to trip. What would be a good value?
Thanks

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A slow blow fuse will hold 100% overload for 10 seconds I think.

Imagecraft compiler user

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The perfect one will be 475mA with a lower time trip possible at 500mA. But I don't think has something like this, so you need to play with numbers.
Lower the trip current, it'll lower the time trip at 500mA.

Choose some ptcs and put it here...

Regards,

Bruno Muswieck

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Hi:

There are PTC's specially targeted for USB apps. Look at the Littelfuse Tyco and Bourns catalogs (they are the most comprehensive) and don't forget AN's!!!

But bear in mind that PTC's are complimentary of "crowbar" devices although they can be used alone.

It's difficult to find a device suitable for protection at 500mA that could trip in a reasonable amount of time. For example, the 1206L050YR PTC will hold 500mA of continous current, but at a current of 1A will trip in > 10S (enough to set a flame!) You need more than 1A to trip the PTC in a reasonable time, and for that purpose is that you need a crowbar.

On the other hand, USB has a current limitation by protocol. I think that such a current limitation will shutdown the bus on an overcurrent event more quickly than a snail PTC alone. Why not use it?

Nachus

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Hi,

nachus001 wrote:
But bear in mind that PTC's are complimentary of "crowbar" devices although they can be used alone.

Can you elaborate on this a bit? What is a crowbar device?

Thanks,

Alan

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Think of a Crowbar Circuit as a three terminal circuit, like a linear voltage regulator, with Vin, Vout, and Gnd.

If the Vin > Vsetpoint, the crowbar "shorts" to ground. This protects Vout, so that there is no output voltage even briefly exceeding the set voltage.

The "excess" input (current) is typically shorted to ground, which then trips a fuse/breaker on the input line.

As an analog circuit, it is very fast. (Digital ones exist...). It can have a very specific set point. A normal fuse won't do this. A normal fuse on the input won't trip quickly, or at a precise current, and is a current device, not a specific voltage monitoring and protection device.

Crowbar circuits have been around forever, the first ones I saw were on tube curcuits to protect radar tubes.

A voltage regulator has many of the same charasteristics of a crowbar circuit, but it doesn't shut down the input, (by popping the input fuse/breaker), when the Vout is exceeded.

JC

edit: Not the paper I was looking for, but This AN/SPQ9A Radar Transmitter Upgrade is an example of a digital, (and hence also auto-resettable) Crowbar.

JC

Last Edited: Tue. Dec 7, 2010 - 04:06 AM
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Hi,

Thanks the explanation JC!

Alan

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One more example, if one were to feed Vin through a fuse, then into a voltage regulator and then into their circuit, there could be a problem if the Vin (e.g. a battery) were connected backwards.

A in-line series diode protects the circuit, but has a 0.3-0.7 V drop across it.

Instead of an in-line series diode, one could put a reverse biased diode across the Vin terminals, after the fuse.

If the Vin, (battery), is connected backwards, the fuse conducts heavily, and the fuse pops. The diode also keeps the circuit from seeing more than 0.3-0.7 reversed Vin. (Diode and fuse must be selected carefully!, This minimal circuit isn't optimal!).

This is a form of reverse Vin protection "crowbar". Note that a typical crowbar protects against excess Vin in the Vforward direction, and does so very rapidly.

JC

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DocJC wrote:
One more example, if one were to feed Vin through a fuse, then into a voltage regulator and then into their circuit, there could be a problem if the Vin (e.g. a battery) were connected backwards.

A in-line series diode protects the circuit, but has a 0.3-0.7 V drop across it.

Instead of an in-line series diode, one could put a reverse biased diode across the Vin terminals, after the fuse.

If the Vin, (battery), is connected backwards, the fuse conducts heavily, and the fuse pops. The diode also keeps the circuit from seeing more than 0.3-0.7 reversed Vin. (Diode and fuse must be selected carefully!, This minimal circuit isn't optimal!).

This is a form of reverse Vin protection "crowbar". Note that a typical crowbar protects against excess Vin in the Vforward direction, and does so very rapidly.

JC

IMO that if you don't use "crowbaring" in your circuit, then you face the (certain and real) possibility that your circuitry make the fuse pop as it dies :(

Nachus

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Hi,

My main project has two PTC's and two Schottky diodes because it supports two power sources 7.5-20VDC.

The diode is rated for 20V.

Would a crowbar device be something like a TVS with the breakdown voltage at 20V?

Thanks,

Alan

So what would/could be the crowbar device if I added one?

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alank2 wrote:
Hi,

My main project has two PTC's and two Schottky diodes because it supports two power sources 7.5-20VDC.

The diode is rated for 20V.

Would a crowbar device be something like a TVS with the breakdown voltage at 20V?

Thanks,

Alan

So what would/could be the crowbar device if I added one?

It depends on how you connect the schottky diodes. If connected reversed (as in polarity protect) they will clamp at polarity reversal (0,4 V +- for schottkys) and trip the PTC. But you can't count on the shottky breaking down at 20V (for overvoltage prot.). Usually it will suck hard current when in breakdown but in a voltage greater than 20V. Check the mfr. datasheet.

A TVS on the other hand, will clamp the input voltage at the rated conduction voltage (let's call it "referenced crowbaring"), but, here arises a second problem: Supply impedance. If your Vin source has a very low impedance, you can count with the TVS clamping will generate enough current to trip the PTC, but if the Vin has high impedance, the TVS will clamp, but will act as "regulator" and the PTC will sit there, happy of life and won't trip.

So, the ideal device for crowbar is a thyristor. The thyristor once ignited will short the power bus until it gets destroyed OR there's something (the PTC) that disconnect the power bus.

There are much literature on this over the net and the variations of crowbar protection circuits. Overcurrent, overvoltage, undercurrent, undervoltage, differential... Look for AN's on circuit protection from the speciallists in circuit protection. They want to sell, so they offers many many notes on their devices and techniques to use them.

On the other hand, drop an schematic here to see about what are we talking about.

Nachus

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A crowbar circuit could be a SCR connected across the supply after the fuse with a zener or other circuit setting the voltage trip value.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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With respect to the opinions mentioned above about crowbar,i think it acts as overvoltage protection circuit not current.Instead of fuse or ptc breaker a constant current source maybe could be used.