Using a Chassis Transformer for a power supply. Help please?

[dasatcopthorne]

Hi all.

 

Can anyone help with this please.

 

I want to use a chassis style transformer to build a PSU for testing various 12v low power things such as DC controllers (need 12v ac supply) LED signals (12v dc supply) other LEDs and things.

 

I have removed a transformer form a Gaugemaster M1 cased unit. Originally it gave two 12v dc supplies at 1amp.

I have removed one Bridge Rectifier from the PCB and inserted a wire link so that the outputs are 12v dc and 16v ac (no load).

I feel confident on wiring this unit up inside a new project case with all the normal precautions except one that I am not sure of.

 

I want to have some auto-reset short circuit protection inside the case but I am not sure of the best way to provide this and at what amperage.

 

The writing on the labels is my own.

 

Can anyone help with this please.

 

Dave.

 

 

 

 

[Crosland]

Those orange square components look suspiciously like Polyfuses to me. If they are, then you already have overload protection.

 

Normally they are very low resistance. When you try to draw too much current through them they start to heat up and eventually they break the circuit (to all intents) acting like a fuse. Once you fix the overload they will cool down and "reset".

[dasatcopthorne]

Thanks for that.

 

Very helpful as I've never heard of them before.

 

Cheers.

 

Dave.

 

[Phil S]

Type RX110 Polyfuse. Into google and you will find both round and square versions. Depending on manufacturer.  Or RX110 PTC (positive temperature coefficient .... When they heat up do to the current they increase their resistance.  .. A lot.

Various different current ratings and voltage !units. .... Browse a few of the suppliers and look at the data sheets to get a feel for them.

 

 

[dasatcopthorne]

Thanks for both replies.

 

Have Googled and read up on Polyfuses/Polyswitches.

 

Also managed to decipher code numbers etc.

 

One more question please. The RX110 type seem to be rated at 60/72v with trip current at 3.75v. 

 

Is it acceptable to have the trip current so far above the transformer rating of 1amp and the voltage so much higher than the 12/14v of the transformer?

 

Dave

Edited November 3, 2019 by dasatcopthorne
spelling

[melmerby]

The RX110 datasheet:

https://www.bourns.com/docs/technical-documents/technical-library/multifuse-pptc-resettable-fuses/publications/bourns_mf_ptc_short_form.pdf?sfvrsn=b72982f1_20

 

Where did you see 3.75A trip current?

It shows trip @2.2A and hold @1.1A which seems reasonable for a 1A supply

 

The voltage will be the breakdown voltage, i.e. that at which it will be destroyed so it should be rated well above a 12/15v supply for safety.

Edited November 3, 2019 by melmerby

[dasatcopthorne]

12 hours ago, melmerby said:

The RX110 datasheet:

https://www.bourns.com/docs/technical-documents/technical-library/multifuse-pptc-resettable-fuses/publications/bourns_mf_ptc_short_form.pdf?sfvrsn=b72982f1_20

 

Where did you see 3.75A trip current?

It shows trip @2.2A and hold @1.1A which seems reasonable for a 1A supply

 

The voltage will be the breakdown voltage, i.e. that at which it will be destroyed so it should be rated well above a 12/15v supply for safety.

 

Thanks for that.

 

I don't know where I goty the Trip current from that I quoted but I did realise it was as you have said.

 

Thanks for the clarification of voltage part of the spec. I understand that now.

 

Many thanks.

 

Dave.

 

 

[dasatcopthorne]

Thanks all for the replies and the clarifications.

 

Much appreciated.

 

Dave.

Edited November 4, 2019 by dasatcopthorne
spelling

[sharris]

Just a FYI - The technical  datasheet on the MF-RX range (including the RX-110):

 

Link to Farnell...

 

suggests that the MF-RX100 60V range is no longer recommended for new designs and for future designs the 72V MF-RX/72 series is used instead - in this case the MF-RX110/72 would be an electrically similar device, it just takes a few more volts to kill it! 

 

The MF-RX/72 range also has more trip-current options down to 0.2A should you need to protect low-current devices. 

[melmerby]

2 hours ago, sharris said:

Just a FYI - The technical  datasheet on the MF-RX range (including the RX-110):

 

Link to Farnell...

 

 

Just as I posted a few back! (Actually the Manufacturer's Bourns original)

[sharris]

36 minutes ago, melmerby said:

Just as I posted a few back! (Actually the Manufacturer's Bourns original)

 

When I saw that the brochure you linked to was the 12 page short-form it fell into my TL;DR category compared to the 4 page datasheet (also Bourns') that Farnell use with the useful info on pages 1 and 2.