Where to place a fuse in a circuit? - Page 1
Forum section: electronics / beginners

author: bshi02
author_rank: Regular Contributor
timestamp: May 17, 2024, 11:55:04 am
content: I want to make a zener diode testerIt use 3 Pin Plug which contains a fuse .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?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: May 17, 2024, 12:04:27 pm
content: In the primary coil of the transformer line.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 17, 2024, 12:11:25 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: May 17, 2024, 12:16:14 pm
content: 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.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: May 17, 2024, 12:35:31 pm
content: 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.

author: bshi02
author_rank: Regular Contributor
timestamp: May 17, 2024, 01:38:56 pm
content: Quote from: Ian.M on May 17, 2024, 12:16:14 pmIf I place extra fuse(12v,1.5A) between "secondary coil of transformer" and one of "diode" Is it enough for protecting transformer?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: May 17, 2024, 01:46:40 pm
content: Quote from: bshi02 on May 17, 2024, 01:38:56 pmTo 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.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: May 17, 2024, 01:53:33 pm
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 17, 2024, 02:13:00 pm
content: simply use poly fuses simple efficient and resettable ....

author: Ian.M
author_rank: Super Contributor
timestamp: May 17, 2024, 03:00:32 pm
content: 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.

author: bshi02
author_rank: Regular Contributor
timestamp: May 17, 2024, 03:07:37 pm
content: Quote from: coromonadalix on May 17, 2024, 02:13:00 pmWhat type and specification of poly fuses Should I use for this schematic?

author: bdunham7
author_rank: Super Contributor
timestamp: May 17, 2024, 03:24:08 pm
content: 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.

author: bshi02
author_rank: Regular Contributor
timestamp: May 17, 2024, 03:44:45 pm
content: Quote from: Ian.M on May 17, 2024, 03:00:32 pmI think that this schematic is reliable without any defective.

author: bshi02
author_rank: Regular Contributor
timestamp: May 17, 2024, 04:08:54 pm
content: Quote from: bdunham7 on May 17, 2024, 03:24:08 pmJust 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=15448385Is it possible to make use of "LITTELFUSE - 0218.050HXP" on the primary side of the transformer in the schematic?

author: soldar
author_rank: Super Contributor
timestamp: May 17, 2024, 05:04:30 pm
content: Quote from: Ian.M on May 17, 2024, 12:16:14 pmThis 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.

author: bdunham7
author_rank: Super Contributor
timestamp: May 17, 2024, 06:02:52 pm
content: Quote from: bshi02 on May 17, 2024, 04:08:54 pmYes, 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.

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2024, 07:45:11 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: May 17, 2024, 07:57:09 pm
content: Quote from: Zero999 on May 17, 2024, 07:45:11 pmGood 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.

author: SteveThackery
author_rank: Super Contributor
timestamp: May 17, 2024, 09:40:49 pm
content: I wonder why the voltmeter is switched.

author: bshi02
author_rank: Regular Contributor
timestamp: May 18, 2024, 12:15:11 pm
content: Quote from: bdunham7 on May 17, 2024, 06:02:52 pmIt make use of transformer.Is it also possible to replace with 250v 100uf rubycon YK capacitor.

author: Ian.M
author_rank: Super Contributor
timestamp: May 18, 2024, 02:00:49 pm
content: 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 aLM317HV, which is good up to 60V (abs.Max.)Use a JFET current source, good up to VDS(max)of the JFET (subject to power & SOA limitations)Use a BJT current source, similar to a JFET, but needs a stabilised base voltageDon'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

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2024, 04:24:22 pm
content: A couple of transistors and a zener diode can be added to the LM317 to reduce the voltage across it

author: wasedadoc
author_rank: Super Contributor
timestamp: May 18, 2024, 04:56:53 pm
content: Quote from: bdunham7 on May 17, 2024, 07:57:09 pmI suspect the original choice of the large 317 was at a time in the distant past when the 317L did not exist.

author: bshi02
author_rank: Regular Contributor
timestamp: May 19, 2024, 12:20:06 pm
content: Quote from: Zero999 on May 18, 2024, 04:24:22 pmIs 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?

author: Zero999
author_rank: Super Contributor
timestamp: May 19, 2024, 09:57:22 pm
content: Quote from: bshi02 on May 19, 2024, 12:20:06 pmYes.QuoteIt'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.QuoteYou mean theschematic attached to the original post? The represent the test probes going to the zener diode.

author: bshi02
author_rank: Regular Contributor
timestamp: May 22, 2024, 08:10:56 am
content: 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

author: Ian.M
author_rank: Super Contributor
timestamp: May 22, 2024, 09:38:09 pm
content: 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.

author: bshi02
author_rank: Regular Contributor
timestamp: May 23, 2024, 05:43:03 am
content: Quote from: Ian.M on May 22, 2024, 09:38:09 pmThank 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=94And 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/___6eOr 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

author: Ian.M
author_rank: Super Contributor
timestamp: May 23, 2024, 04:44:37 pm
content: pkle.cp.kr seems to be blocking access from outside Korea, so I cant comment on that transformer or its wiring.Quote from: bshi02 on May 23, 2024, 05:43:03 amAlmost 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.QuoteAny 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.

author: bshi02
author_rank: Regular Contributor
timestamp: May 23, 2024, 09:14:00 pm
content: 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=1Quote from: Ian.M on May 23, 2024, 04:44:37 pmIt 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

author: Ian.M
author_rank: Super Contributor
timestamp: May 23, 2024, 10:33:47 pm
content: 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=1A 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!

author: bshi02
author_rank: Regular Contributor
timestamp: May 24, 2024, 08:38:29 am
content: Quote from: Ian.M on May 23, 2024, 10:33:47 pmThank 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.

author: Ian.M
author_rank: Super Contributor
timestamp: May 24, 2024, 01:09:17 pm
content: 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.

author: bshi02
author_rank: Regular Contributor
timestamp: May 25, 2024, 08:11:59 am
content: 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.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: May 25, 2024, 09:36:45 am
content: 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

author: bshi02
author_rank: Regular Contributor
timestamp: June 18, 2024, 02:49:56 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: June 18, 2024, 04:10:17 pm
content: 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.

author: bshi02
author_rank: Regular Contributor
timestamp: June 19, 2024, 03:20:16 am
content: 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.