[sdiy] Polyfuse Tripping Current values

[Justin Owen]

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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