[sdiy] Polyfuse Tripping Current values

[Harry Bissell]

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
>
>
> _______________________________________________
> Synth-diy mailing list
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>




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-- 
Harry Bissell & Nora Abdullah 4eva