Where to place a fuse in a circuit?

[bshi02]

I want to make a zener diode tester by following below schematic

There is no fuse in this schematic,But It use 3 Pin Plug which contains a fuse like below screenshot.

Unfortunately,There is no 3 Pin plug in the market of my nation ,Instead There is 2 Pin plug in the market which doesn't contains any type of fuse.

So I want to place a fuse in the schematic just in case,But Where to place a fuse in a circuit?
And what Ampere of fuse should I use in the schematic?

[S. Petrukhin]

In the primary coil of the transformer line.

[wasedadoc]

if the transformer were perfect then the primary current would be about 5% of the sum of the zener test current and the LED current.  You probably cannot obtain such a low current fuse.  I would use a 200mA one.  Allows for non perfect transformer and initial surge as capacitor on bridge output charges.

That may be an old circuit from a time when LEDs were not as efficient as modern ones.  You may find that you can increase the 1K resistor and still get acceptable brightness with a modern LED.

[Ian.M]

It is difficult to protect small line frequency transformers with just a primary side fuse, as the normal operating primary current is so small compared to typical readily available 250V fuse ratings.  It is often worth adding an extra fuse on the secondary side, between the secondary and the rectifier, chosen to have a rating less than or equal to the secondary's RMS current rating but greater than the expected RMS load current (which for a bridge rectifier directly feeding a reservoir capacitor will be approx 1.5 x the DC current).  Due to the reservoir capacitor inrush (surge) current, it will need to be a slow blow (T) fuse.   

[BILLPOD]

Good Morning Bshi02,   In similar situations, I have put an amp meter in the hot leg, feeding the xfmr., (a meter with mA capabilities).  My go-to meter in this application is one that has Min/Max capability, and then apply a maximum load on the circuit.

[bshi02]

It is difficult to protect small line frequency transformers with just a primary side fuse, as the normal operating primary current is so small compared to typical readily available 250V fuse ratings.  It is often worth adding an extra fuse on the secondary side, between the secondary and the rectifier, chosen to have a rating less than or equal to the secondary's RMS current rating but greater than the expected RMS load current (which for a bridge rectifier directly feeding a reservoir capacitor will be approx 1.5 x the DC current).  Due to the reservoir capacitor inrush (surge) current, it will need to be a slow blow (T) fuse.

If I place extra fuse(12v,1.5A) between "secondary coil of transformer" and one of "1n4007 diode" Is it enough for protecting transformer?

[S. Petrukhin]

If I place extra fuse(12v,1.5A) between "secondary coil of transformer" and one of "1n4007 diode" Is it enough for protecting transformer?

To calculate this, you need to know the parameters of the transformer.
But is the current in this device necessary to protect the output circuit?
Low-power transformers, which are quite enough here, easily tolerate secondary coil short circuits for a same time.

But! 1N4007 They have a maximum allowable current of 1A, if I'm not mistaken.

[S. Petrukhin]

It uses the LDO shift reception by the zener and current measurement on a 220Ohm load resistor.
Therefore, when the circuit is sjort, the maximum current will be about 50mA.
You will not find a suitable fuse. Unless they are resettable fuse, but I don't remember their lower limit.

[coromonadalix]

simply use poly fuses  simple efficient and resettable ....

[Ian.M]

The schematic says the transformer is rated at 1A.  The load current is only about 20mA,  about 5mA via the LM317 current source, the rest through the LED.  Therefore there is no difficulty choosing a secondary side fuse as the greater than 1.5 x DC load current and less than rated transformer current limitations are far apart.   Any time-lag fuse from a few hundred mA up to 1A will do.

However if the transformer is *NOT* as specified on the schematic, this choice should be revisited, with its actual secondary current rating.

I am curious why you want to build a mains powered Zener tester, especially one that is so limited in max. Zener voltage?   For most of us, 2x PP3 9V batteries in series + a resistor to limit the Zener current are sufficient, in combination with a DMM.

[bshi02]

simply use poly fuses  simple efficient and resettable ....

What type and specification of poly fuses Should I use for this schematic?

[bdunham7]

Your basic design is pretty bulletproof (self-protected against most user errors) so IMO you only need an input fuse to prevent fires and maybe protect your transformer from assembly errors or something like that.  I would use an MDL 1/10R or MDL 1/16R fuse on the primary side of the transformer and call it good.  The MDL fuses are expensive, but are good at both tolerating inrush surges while still blowing quickly and reliably when there is a fault.

I'd suggest two improvements to  your design.  First, you don't need a filter capacitor that large since your output current is only ~5.7mA.  50-100µF should be plenty.  Second, consider reconfiguring this as a full-wave voltage doubler.  You'll get about 30V compliance voltage and while you'll need two (small) capacitors you'll also need only two 1N4007 rectifiers.  Your LED resistor will have to be increased to 2.7k or so.

[bshi02]

The schematic says the transformer is rated at 1A.  The load current is only about 20mA,  about 5mA via the LM317 current source, the rest through the LED.  Therefore there is no difficulty choosing a secondary side fuse as the greater than load current and less than rated transformer current limitations are far apart.   Any time-lag fuse from a few hundred mA up to 1A will do.

However if the transformer is *NOT* as specified on the schematic, this choice should be revisited, with its actual secondary current rating.

I am curious why you want to build a mains powered Zener tester, especially one that is so limited in max. Zener voltage?   For most of us, 2x PP3 9V batteries in series + a resistor to limit the Zener current are sufficient, in combination with a DMM.

I got this schematic from a old ebook which was written by one of top-notch in Electronics Repairing field.So I think that this schematic is reliable without any defective.

[bshi02]

Your basic design is pretty bulletproof (self-protected against most user errors) so IMO you only need an input fuse to prevent fires and maybe protect your transformer from assembly errors or something like that.  I would use an MDL 1/10R or MDL 1/16R fuse on the primary side of the transformer and call it good.  The MDL fuses are expensive, but are good at both tolerating inrush surges while still blowing quickly and reliably when there is a fault.

I'd suggest two improvements to  your design.  First, you don't need a filter capacitor that large since your output current is only ~5.7mA.  50-100µF should be plenty.  Second, consider reconfiguring this as a full-wave voltage doubler.  You'll get about 30V compliance voltage and while you'll need two (small) capacitors you'll also need only two 1N4007 rectifiers.  Your LED resistor will have to be increased to 2.7k or so.

Just as you pointed out,MDL 1/10R or MDL 1/16R fuse is quite expensive in the market of my nation.
By the way, I stumbled on "LITTELFUSE - 0218.050HXP" which seems to be cheaper than MDL 1/10R Fuse.
https://www.devicemart.co.kr/goods/view?no=15448385
Is it possible to make use of "LITTELFUSE - 0218.050HXP" on the primary side of the transformer in the schematic?

[soldar]

It is difficult to protect small line frequency transformers with just a primary side fuse, as the normal operating primary current is so small compared to typical readily available 250V fuse ratings.  It is often worth adding an extra fuse on the secondary side, between the secondary and the rectifier, chosen to have a rating less than or equal to the secondary's RMS current rating but greater than the expected RMS load current (which for a bridge rectifier directly feeding a reservoir capacitor will be approx 1.5 x the DC current).  Due to the reservoir capacitor inrush (surge) current, it will need to be a slow blow (T) fuse.

This is a very pertinent way of looking at the problem.

I just finished making a (drill) battery charger and was pondering the same question. A fuse on the primary side is not really very effective for the reason you gave. I opted for a fuse on the secondary AND a thermal fuse in series with and enclosed with the winding. I think this is a good balance.

[bdunham7]

Is it possible to make use of "LITTELFUSE - 0218.050HXP" on the primary side of the transformer in the schematic?

Yes, I think that would be OK for your circuit.  It's a 50mA fuse, so it can't tolerate too much inrush even though it is time-delay (slow blow). That makes it even more important to not use excessively large filter capacitors.  I would note that your transformer is much, much larger than it needs to be and the circuit might make more sense with a 2.5 to 5.0VA transformer if you can find one.

[Zero999]

The LM317 is not an LDO, but an ordinary linear regulator. It's configured as a current source, with a 220R resistor between the adjust pin and output, to give a current of only 5.7mA. This will probably work, but it's a little below the recommended minimum operating current of 10mA.

I would put a fuse both on the primary and secondary sides of the transformer, to provide protection against fire. The values aren't critical, as it's not worth attempting to protect the electronics. 100mA will be fine. Going for PTC on the secondary is a good idea because they're very slow blow, so the inrush current due to the capacitor won't cause nuisance tripping.

[bdunham7]

It's configured as a current source, with a 220R resistor between the adjust pin and output, to give a current of only 5.7mA. This will probably work, but it's a little below the recommended minimum operating current of 10mA.

Good point, I guess his regulator is too large just like the transformer.  A small-die variant like the LM317L-N in a TO-92 case would be more appropriate. 

[SteveThackery]

I wonder why the voltmeter is switched.

[bshi02]

Is it possible to make use of "LITTELFUSE - 0218.050HXP" on the primary side of the transformer in the schematic?

Yes, I think that would be OK for your circuit.  It's a 50mA fuse, so it can't tolerate too much inrush even though it is time-delay (slow blow). That makes it even more important to not use excessively large filter capacitors.  I would note that your transformer is much, much larger than it needs to be and the circuit might make more sense with a 2.5 to 5.0VA transformer if you can find one.

In the ebook, It make use of TELETRON T1201 transformer in order to make zener diode tester which is able to test zener diode voltage rating from 2.4 Volt to 30 Volt.
https://we.tl/t-WD0h8MZQE7

Is it possible to replace 1A TELETRON T1201 transformer with 3.6VA transformer of below link in the schematic?
http://hanamelec.com/main/view.php?kod=03010102&idx=466
and Is it also possible to replace 50v 1000uf capacitor with 250v 100uf rubycon YK capacitor in the schematic?

[Ian.M]

Although doubling the transformer secondary voltage makes for a more useful Zener tester, capable of testing >15V Zeners, depending on the transformer's regulation factor, you are likely to be pushing the abs. max voltage limits of a LM317 regulator.  A mains transformer's output voltage is normally specified at its full rated load current (or VA), and rises when unloaded or minimally loaded.  For very small transformers the rise may be extreme, maybe even to double the nominal voltage.  A LM317 is normally never operated with more than 35V across it from In to Out, and has an abs. max. limit of 40V.   Allowing for the drop across the current set resistor, diode drops in the bridge rectifier etc.  if the unloaded secondary voltage exceeds 29V, there is a significant risk of LM317 failure.   There are a few simple alternatives here:

• Use a LM317HV, which is good up to 60V (abs.Max.)
• Use a JFET current source, good up to V_DS(max) of the JFET (subject to power & SOA limitations)
• Use a BJT current source, similar to a JFET, but needs a stabilised base voltage
• Don't try to maintain a constant current, just use a simple dropper resistor!

The LM317HV offers best current regulation in a simple circuit.  The  JFET and BJT options are less precise, and tend to drift with temperature, + require the current set resistor to be initially adjusted to get the desired current due to device to device parameter variations. 

The simple resistor doesn't make any pretence at current regulation thus the Zener current will decrease with increasing Zener voltage.  This isn't necessarily a bad thing unless you need to test if a particular Zener meets specification (for which you would need an adjustable current source anyway so it could be set to the specified Zener current) as higher voltage Zeners are typically specified at a lower current than the 5mA typical of <1W low voltage ones.  If you go down this route, I'd choose a resistor that gave about 6mA with the probes shorted, E24 6K2 1/2W should do nicely.   That will drop the current to 5mA at around 6.2V Zener voltage, and to 2mA at around 24V to suit lower current high voltage Zeners.

Personally, I'd probably go 'whole hog' and use that transformer to build an 'octopus' curve tracer, or even an Arduino based digital curve tracer rather than a simple Zener tester, but if you've got thousands of unmarked Zeners to test and sort, YMMV.

A 100uF capacitor is suitable if you reduce the power on LED current considerably by increasing its series resistor  - the ripple at the expected Zener current will only be about half a volt pk-pk.  Modern high efficiency red LEDs are plenty bright at only a couple of mA If.  However if you keep the high LED current, you'll need at least 220uF to keep the ripple under 1V

[Zero999]

A couple of transistors and a zener diode can be added to the LM317 to reduce the voltage across it

[wasedadoc]

.. I guess his regulator is too large just like the transformer.  A small-die variant like the LM317L-N in a TO-92 case would be more appropriate.

I suspect the original choice of the large 317 was at a time in the distant past when the 317L did not exist.

[bshi02]

A couple of transistors and a zener diode can be added to the LM317 to reduce the voltage across it
(Attachment Link)

Is it possible to use 1N4744-15V for D1 symbol(Zener15v) in your schematic?
and What voltage Should I use for V1 symbol in your schematic? I think that V1 symbol must be series of several DC battery.
Does Load symbol contains parts of "Red probe to Cathode","Black probe to Anode","Ground","30 VDC analogue panel meter","switch" in the schematic which I asked?

[Zero999]

A couple of transistors and a zener diode can be added to the LM317 to reduce the voltage across it
(Attachment Link)

Is it possible to use 1N4744-15V for D1 symbol(Zener15v) in your schematic?

Yes.

and What voltage Should I use for V1 symbol in your schematic? I think that V1 symbol must be series of several DC battery.

It's supposed to be a simulation, not a practical circuit. Anyway, V1 represents the power supply voltage. It's swept from 5V to 60V.

The pink trace shows the voltage differential between the LM317's input and output pins.

The green trace is the current through RL, which represents the load. In your case the zener diode.

To answer the question, in reality, V1 is just the power supply. It can be the rectified voltage from your transformer + voltage doubler circuit.

Does Load symbol contains parts of "Red probe to Cathode","Black probe to Anode","Ground","30 VDC analogue panel meter","switch" in the schematic which I asked?

You mean the schematic attached to the original post? The represent the test probes going to the zener diode.