[sdiy] Polyfuse Tripping Current values

[megaohm]

Great answers and info there, Harry.
Sheds a bit more light on the polyfuse for me. I always interpreted
the series resistance (while in normal operation) as a bad thing. I
never thought about it 'cushioning' the in-rush current at start up.
Thanks for taking the time.

p.



On Tue, Jan 11, 2011 at 10:49 AM, Harry Bissell <harrybissell at wowway.com> wrote:
> OK
>
> The polyswitch fuse is an odd duck.
>
> What are you protecting ?  The polyswitch can protect both the supply and the circuit, depending on the fault. If the current drawn by the
> fault is sufficient to make it high impedance, the power supply is mostly disconnected from the load. Enough current will still flow to keep the
> poly hot, therefore high impedance. You might expect a value of maybe 5K ohms when the poly is hot.
>
> Sizing ?   Size the poly for a little above the highest current you expect to have in normal operation, maybe 20%. If the ambient temperature is
> high, you need to make it even bigger as it will be closer to the trip point and higher in series resistance. (usually) Don't worry about inrush
> or very short transient currents, the poly takes quite a while to really get hot and switch. Its series resistance is actually a benefit as it will tend
> to limit inrush current as a side effect.
>
> The big problem with polyswitch fuses (indeed all fuses) is there can be a fine line between normal operation and the fault current that you want to stop.
> In many cases, a fault might not draw enough current to trip the thing anyway. It will usually stop a fire, but thats about it.
>
> Best use is a polyswitch with reverse shunt diodes, and maybe even a big zener diode. This could protect against over and reverse voltages, but size the
> Zener and Reverse diodes correct (they might need to dissipate watts of power. I'd suggest 5W parts, through hole, and stood off the board, they
> WILL get hot in a fault. The Zener should (of course) be higher than the supply voltage so it does not draw any (much) current in normal operation.
>
> A good power supply with current limiting might be a good, or better protection scheme...
>
> H^) harry
>
>
> ----- Original Message -----
> From: Justin Owen <juzowen at googlemail.com>
> Cc: SDIY List <synth-diy at dropmix.xs4all.nl>
> Sent: Tue, 11 Jan 2011 04:34:38 -0500 (EST)
> Subject: Re: [sdiy] Polyfuse Tripping Current values
>
> Not sure I explained myself that well - but the replies still make sense for the first question. Put the polys on the module/pcb/breadboard I'm testing - not the breakout module that will be powering it during testing. Fair enough.
>
> Second question still stands though - what should the value of the poly be in relation to the current draw of the module it's protecting? Next size up? +10%? Double?
>
> And I guess while I'm asking basic questions - what is the poly actually protecting against? Is it protecting the circuit from the power supply or the supply (and other modules?) from the circuit?
>
> I've changed a whole bunch of fuses - but I've never added one to a circuit, so apologies if this is on the verge of newb.
>
> Thanks,
>
> Justin
>
>
> -----Original Message-----
> From:  [blacet at blacet.com]
> Received: 10.01.2011 19:07:16
> To: Graham Atkins
> Cc: Justin Owen; SDIY List
> Subject: Re: [sdiy] Polyfuse Tripping Current values
>
> I use the polys on EACH module PCB. That way you can dial in the value for
> each module and any faults will cause that module to "blow".
>
> I can see problems trying to do a whole rack of modules at once. What
> happens if you want to rearrange your modules?
>
>> On 10 Jan 2011, at 16:12, Justin Owen wrote:
>>
>>> Hey all,
>>>
>>> Belated Christmas & HNY wishes to everyone - hope you all had good/
>>> fun/safe (or recoverable...) times.
>>>
>>> Based on the advice I've had here I'm going to add polyfuses to this
>>> power supply breakout module I'm working on.
>>>
>>> My Analogue Systems rack has each rail divided into 3 busses - each
>>> with a 1A fuse on, my Doepfer rack uses a single 400mA fuse on the
>>> mains transformer AFAIK.
>>>
>>> So, first up - considering a) that I'd like to build a breakout
>>> module for each rack and b) that I've no idea what current draw the
>>> prototype circuits that might be powered off it will have - what
>>> Tripping Current should I specify for the Polyfuse to be
>>> *reasonably* safe?
>>>
>>> ...and secondly, is there a rule of thumb for specifying a Tripping
>>> Current for a circuit that I *do* know the current draw of?
>>
>> You build the racks first thern measure the current draw, you can't
>> "specify" something unknown.
>>
>> Graham
>>
>>
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