Where to place a fuse in a circuit?

[bshi02]

By the way Is it possible to use "10W AC Input 220V to Output 12V-0-12V" of below link?
https://www.aliexpress.com/item/4000075844762.html

[Ian.M]

Yes, but the no load voltage of the smaller transformer is likely to be higher, increasing the voltage stress on the LM317.  See my reply #20 and Zero999's reply #21 above for ideas to fix that.

[bshi02]

Yes, but the no load voltage of the smaller transformer is likely to be higher, increasing the voltage stress on the LM317.  See my reply #20 and Zero999's reply #21 above for ideas to fix that.

Thank you very much for all of your reply!

I finally found below POWER TRANSFORMER from online Electrical store of my nation which has specification of 110V, 220V input voltage and 12V 0V 12V output voltage and 0.8Ampere and Its price is less than $7 including shipping fee.
https://www.pkele.co.kr:50010/main_kr/main.php?ctt=../sub/view_product&Code=HT801-12V&CatNo=94

And taking your advice, I plan to use LM317HV in order to cope with secondary voltage excess just in case.

But Is it okay to connect two Ground in the schematic(below "LED" and "Black probe to annode") to  secondary terminal of 0V on the transformer in order to avoid risk of electrocution?

And Is it possible to use below glass tube fuse of Time-lag type(160ma Ampere rate,250v voltage rate,Melting Integral 0.075)on the primary side and secondary of the transformer?
https://www.changdaefuse.co.kr/post/___6e

Or It it possible to use one of below Radial lead PPTC fuse(FRV or FRHV series) on the primary side and secondary of the transformer?
https://www.changdaefuse.co.kr/blank-17

[Ian.M]

pkle.cp.kr seems to be blocking access from outside Korea, so I cant comment on that transformer or its wiring.

But Is it okay to connect two Ground in the schematic(below "LED" and "Black probe to annode") to  secondary terminal of 0V on the transformer in order to avoid risk of electrocution?

Almost certainly not.   The general rule of thumb when bridge rectifiers are involved, is: You can ground the AC side or the DC side but *NEVER* both.

And Is it possible to use below glass tube fuse of Time-lag type(160ma Ampere rate,250v voltage rate,Melting Integral 0.075)on the primary side and secondary of the transformer?
https://www.changdaefuse.co.kr/post/___6e

Or It it possible to use one of below Radial lead PPTC fuse(FRV or FRHV series) on the primary side and secondary of the transformer?
https://www.changdaefuse.co.kr/blank-17

Any 250V rated glass tube fuse of appropriate characteristic and current will be suitable.  I would regard any self-resetting PTC fuse as a safety hazard on the primary side of a small transformer - if anything happens serious enough to trip it, you really don't want power to be reapplied until a technician has looked at the cause of the fault.

[bshi02]

Thank you very much for reply!
Below link is the same power transformer with power transformer of previous post.

https://parts-parts.co.kr/product/detail.html?product_no=2011&cate_no=398&display_group=1

But Is it okay to connect two Ground in the schematic(below "LED" and "Black probe to annode") to  secondary terminal of 0V on the transformer in order to avoid risk of electrocution?

Almost certainly not.   The general rule of thumb when bridge rectifiers are involved, is: You can ground the AC side or the DC side but *NEVER* both.

It seems that the only way to ground all of two Ground in the schematic(below "LED" and below "Black probe to annode") is to make use of AC inlet socket of below link(connecting all of two Ground in the schematic to ground pin of AC inlet socket).
https://elecloud.co.kr/product/detail.html?product_no=1422&cate_no=60&display_group=1

[Ian.M]

Even smaller and cheaper is this 300mA 12V-0-12V transformer: https://parts-parts.co.kr/product/detail.html?product_no=2005&cate_no=398&display_group=1
A secondary rated 300mA RMS AC gives you max. 200mA DC after the bridge rectifier, plenty for any sort of Zener tester.

The schematic grounds the negative terminal of the bridge rectifier.  Do *NOT* ground any of the transformer secondary terminals!

[bshi02]

The schematic grounds the negative terminal of the bridge rectifier.  Do *NOT* ground any of the transformer secondary terminals!

Thank you very much for reply!
I just have understand that It is very dangerous to connect negative terminal of bridged rectified to 0V of the transformer secondary terminals or ground pin of AC inlet socket because any of transformer secondary terminals or ground pin of AC inlet socket is not able to ground DC voltage.

 

[Ian.M]

You misunderstand me.  See my comments about a similar situation (except with ground via an oscilloscope probe) here: https://www.eevblog.com/forum/beginners/safe-to-hook-osciloscope-probe-directly-to-this-bridge-rectifier/
and the LTspice simulator schematic showing the problem (reply #5) here: https://www.eevblog.com/forum/beginners/full-wave-rectifier-measured-on-one-oscope-channel/

Provided the bridge rectifier is isolated from the mains by a transformer, you can safely ground any single node of the power supply circuit, *EITHER* one terminal of the transformer secondary (e.g at one of the AC terminals of the bridge rectifier), *OR* one node of the DC part of the PSU, (e.g. the bridge rectifier -DC terminal) but *NEVER* both.  Attempting to ground both will at best blow a fuse, but can let the 'magic smoke' out, destroying the transformer and/or bridge rectifer.

For safety, unless the transformer has a split bobbin (primary and secondary windings fully separated by a plastic barrier that's part of the former the coils are wound on) or has an inter-winding screen you can ground,  you should probably ground *somewhere* in the DC/secondary side of the PSU to mains supply Ground (PE), unless the typical mains installation wiring in your country is so unreliable that supply 'ground' may be not connected to real earth ground, or may even be at a dangerous voltage.

[bshi02]

Thank very much for your reply!
It seems that It is safe to ground earth symbol of below "LED" and earth symbol of below "Black probe to annode" to ground(earth) pin of AC inlet socket in the schematic.Because negative leg of "LED" and "Black probe to annode" are directly connected to negative terminal(negative DCV) of bridged rectified.So below earth symbol in orange circle is actually the same type of negative DCV terminal.

But, It is not possible and rather dangerous to ground together any terminal,which is directly connected to negative terminal(negative DCV) of bridged rectified, and any terminal, which is directly connected to terminal of the transformer secondary such as AC terminal of rectifier diode, to ground(earth) pin of AC inlet socket.because they aren't the same voltage type.

[Solder_Junkie]

That circuit is excessive to just test a zener diode. I use a variable Voltage supply set to 24V and a series resistor and a multi-meter. No need to make a special tester just to measure the Voltage across a zener.

Note, mains rated fuses are needed in the primary side of the transformer, ideally your transformer should also include an over temperature trip/fuse integral within the winding (part of the transformer and built in by the manufacturer).

Polyfuses are excellent, but I do not think they are rated for mains electricity, you want a fuse that will safely blow and the Polyfuse works by going high resistance when drawing excessive current through it.

SJ

[bshi02]

I finally made diode tester using strip board.

I doubt that transformer don't work correctly,But I dont know how to measure voltage of transformer with switching on.

[Ian.M]

You've wired the transformer wrong.  That particular transformer has two completely separate 12V secondaries (a very common arrangement).  You can either connect them in parallel (link '0'-'0' and '12'-'12') for more current  or in series (link middle '0'-'12') for double the voltage or to use as 12-0-12 center tapped, or use them independently to power separate circuits.

As is you have only one terminal of each secondary connected so you get nothing.

You say you are using LM317HV, so wire the windings in series: move the wire from the middle '12'  to the outside '0' and link middle '0' to middle '12', and it should work.

[bshi02]

Thank you very much for reply!
I saw green led lit as soon as I connect AC power cord,But When I switched on ,It was measured more than 30v in analog panel meter.I measured DC voltage between negative and positive pin of 200v 100uf capacitor.It was measured 33v.

and I also tested LM317HV, it displays 4v in analog meter when I placed red probe to pin1 of LM317HV and black probe to pin2.
and It displays 10v in analog meter when I placed black probe to pin1 of LM317HV and red probe to pin2.
I think that this diode tester works correctly.
Thank you once again for all of your reply which is always very helpful for solving the problems afflicting me.