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If I had to have a single test tool for electrical maintenance work it would be a voltage tester (test lamp) as they are the best way to check for the presence of voltage reliably by applying a load to the circuit being tested.
Normally I'd suggest a Fluke tester for professional work, but for home use by DIYers or trades that might not use them so much, I'd suggest a cheap and simple set like these.
The cheap sets actually have a big advantage over the higher profile brands in that they often power themselves directly from the circuit, and in doing so load it down enough to shunt stray capacitively coupled leakage current that can cause false readings on some more sophisticated and expensive testers. It also means you don't have to worry about batteries going flat or leaking and destroying the tester.
That does also mean that these pass enough current to give a strong shock if you hold the end of a probe while sticking the other into a live connection. Use them with suitable caution.
The circuitry in this unit is quite sophisticated, but does lack the reassurance of a fuse. As such I'd only recommend its use on lower energy circuits like home circuits beyond the distribution board, and not in high current industrial equipment. Use a Fluke for the industrial stuff to "tick the box".
If one of the four rectifier diodes failed in this tester it could cause a high current fault with nothing but the tracks as fuses, and that might not deal with the fault as well as a proper HRC fuse.
The diodes could also have been spaced a little further apart, as the DC side does have full mains potential across it.
The tester does come with tip covers (which you will lose) but does not have shrouds around the probes so only the tips are exposed. I'd suggest adding a bit of sleeving if using in areas where you could bridge onto adjacent grounded metal or other connections.
I'd guess this same tester is available in other countries under different branding, as Duratool is a fairly generic brand. I got this one from CPC in the UK for the surprisingly low price of £2.78 including VAT. I couldn't even buy the components for that cost.
Note that the initial peak of current can trip an RCD/GFCI if used between live and earth. To avoid that, test between live and neutral first so the thermistors are pre-heated and then immediately test between live and earth, as there won't be the same current peak. This applies to some of the professional testers too, with one manufacturer implying that it's a "feature".
It's always recommended to check your tester on a known live circuit before use. Also check the leads of test equipment for signs of damage when stored in amongst other tools.
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I've always recommended, if you're doing electrical work, you should have a set of test lamps and for most people. The name that comes to mind is fluke and they're kind of expensive and it kind of takes out the scope of uh people doing diy stuff. But there are cheaper test lamps available. I've looked at this very shady chinese, one which turned out not to be too bad a while back, and it's super simple.

It gives a no-go indication. It does load the circuit down with about 11 milliamps in this case, and it doesn't just show you the presence of voltage. It could be any voltage from 6 to 380 volts, it says, but it shows the polarity as well, and the structure in this one is super simple. I shall show you the circuitry, even though i've covered this in a video.

This one is more sophisticated uh. It shows the voltage and the polarity, but it has it uh in steps. So it's got six twelve, twenty four, fifty hundred and twenty two thirty and four hundred volts. So it's more useful uh.

Let me just bring up the hopping, we'll stuff it into it, and we'll see what sort of current it draws. So here's the hoppy. So i should just stick these connections randomly into uh here. So it's showing 230 volts.

The current it's drawing is 147 milliamps. Initially, that's quite high uh dropping down. Did it really do that? I'm just going to test that again one moment, please i'm going to let this cool down because they usually have a little temperature, sensitive resistor in them one moment per years. Okay, let's try this again! So i'm stuffing it and now it takes a wee while for that to respond 134 milliamps, but then it drops rapidly down to about 20 milliamps.

It's currently 18 milliamps uh, maybe if held on longer, it would go lower, but it does suggest there is a temperature sensitive device in here and although it's not designed for continuous operation, these things, it does look like it has that self-adjustment, but it is currently dissipating About four watts in this plastic case, which is quite high, but that's interesting right, tell you what i'm going to open this one i'll get the whole pie out of the way? Actually, no we'll get the hoppy out of the way i'll stuff this one in watch. The current at the top uh 11 milliamps and after a while, when it's been sitting 11 milliamps, a while, which is about 2.7 watts, apparently 2.8 watts after a while, it will suddenly start regulating down uh once its little temperature. Sensitive resistor is heated up and it will just limit the dissipation in the unit, so it's still showing about 2.0. It's cutting down now: 2.6 watts, 2.5, 10 milliamps, and it's going down nine eight.

Oh, it's really flying down now and it will finally settle out around about uh. Well, it's actually gone way off the scale. Here. It's gone so low, it's not picking up anymore.

Okay right, we will take a look at the structure in this one. It's super simple, we'll tell you what let's uh just pop the lid off. This is not what this video is about. That's the circuitry.

It's got two leds a resistor, and then it's got the temperature sensitive resistor here. That is it. What is in this one, though? It's warm, or is that just me holding it with excitement? No, it is warm, i'm kind of expecting a resistive divider of some sort, oh price, wise uh. This one comes in at a staggering three pounds or something and that's my reputable supplier.

Okay, oh there's the resistors, oh there's a little uh current, limiting thermal resistors, oh and there's another one and that's kind of interesting. Let's get this out, there is a screw here. There is a little rudimentary strain relief. Let's pop this out, there's no screw! At this end, i shall prise it out.

Oh, that is kind of wedged over that pillar. I think oh there, it goes. We've got some surface mount components in the back uh, notably the leds there's what looks like some sort of rudimentary resistive divider a couple of diodes. Well, four diodes, not sure what they are right.

Tell you what tell you what i'm going to reverse engineer this, but i think i'm going to have to actually remove some of the components off this side, or am i yes? I think i am to be able to actually see where a lot of these tracks are going at the back of this radio. I shall explore one moment. Please, reverse engineering is complete. Let's explore that was quite complex to reverse engineer.

I did have to remove all the resistors off it to actually make it easy to follow out, so this circuit board has seven of these high power resistors. It's got. Four 5.1 k resistors, which it turns out, are all in series for part of the circuitry, and then it's got three separate 3.9 k resistors, because the circuitry divides into four distinct sections things worthy of note. There is the six volt pair of leds that indicate polarity and they're on the ac side of the circuit, because it does have a bridge rectifier based on these two diodes and these two diodes connected to each probe and the reason they've got them mounted separately.

As out just because of the fact that the supply the probe is at one end and the flying probe is the other, i didn't see a fuse um i'll mention that later and i'll show you the schematic, but these bridge rectifiers diodes, and so you get the Inverse parallel leds they've got their own resistor and then there's a ptc thermistor. It's just like these little dinky uh symbol. Testers then they've got another set of circuitry with a couple of zener diodes uh one for the 12 volt led one for the 24 volt led. I guess they're just going for precision now and then the other ones at 50, volt, 120, 230 and 400.

Volt are just part of a resistive divider, which is ensures that big cluster of resistors - that's it described, let's bring in the schematic here, is the schematic. I shall zoom down that just a little tiny bit more. So, let's start off with the very simple one: it's got: a 3k ptc thermistor and sears, the 20k resistor and then two red leds in inverse parallel and if you put it across dc enough, current will flow even at low voltage to make one of these leds Glow, if you put it across ac, it will actually make them alternate backwards and forward really quickly so they'll both look lit, and that indicates that they see no specific polarity. The 20k resistor will initially limit the current to a fairly low level, but this 3k positive temperature coefficient thermistor will rapidly heat up and it will then actually reduce the current flowing through the circuit.

Further, that's why it started off about 11 milliamps and then it trailed off very quickly to just about one or two milliamps. So now we've seen how that bit works. It's repeated another one. This potential ptc should say positive.

Temperature coefficient thermistor increases in resistance as it gets hotter and in this case it's used almost like an electronic fuse itself heating, so it does increase in resistance. It had a rough value of 3000 ohms. The other ones did not. Let me flip this over and show you the schematic of this.

So here are the two probes, and we've got a repeat of that circuitry here. So that's. The first section of circuitry is the ac circuitry, which has a 1k ptc thermistor a much much beefier. 3.9 thousand ohm resistor and the two leds, the i've, marked it flying probe and case probe, because the case probe, as you probe it, that it will indicate the polarity.

So, for instance, if this is connected positive, the current will flow through this led and then the resistors. Then there's a bridge rectifier based on those discrete diodes. That's where i would not categorize this for industrial use, because i don't think, there's a. I don't think there's a fuse in the probe as such.

There's no obvious sign of that. So you've got the two probes here with this bridge rectifier and the only thing that's likely to blow as a fuse is that the tracks there are some thin tracks. I suppose they would works a fuse, but it's not really what you want. An industrial environment as such i'd only rate this for uh home use after the distribution board i'll just put in that dot, because it's missing next section, the 12 volt section has its own 1000 ohm positive temperature coefficient thermistor another of those 3.9 k resistors and then A 12 volt an indicator led and then a zener diode to actually match that.

So this will be around about, say a 10 volts. You know or something like that. Likewise, that will be about 20 volts. You know just to give it uh, because you have to allow for the two volt drop cross led as well.

The 24 volt section has the same again: the ptc thermistor, the resistor and then the leds. When you're testing low voltage like a car battery um or a truck battery, these won't be dissipating a lot of power and the leds won't be super bright it'll just be basically a 5k in sears, with an led which will be okay. The leds are fairly efficient. So they'll light up visibly and when it's used across higher voltages, the ptc thermistors will kick in so they'll, initially start quite bright, those leds and then they'll tame down rapidly.

As those heat up. The fourth section circuitry is the bit that measures for 50 volt 120 volt, 230 volt and 400 volt. Let's get those for 5.1 000 ohm resistors rated about 2 watts, each i'd, say and then a divider and at low voltage the because of the amount of current flowing through that it will create effectively a voltage across the resistor and initially at low voltage. The only led that can light is the 50 volt led because the these resistors are so low that the voltage across them will be um lower than the threshold of the led, as the voltage increases to say, 230 volts, the voltage across these rises and then effectively That 234 one can land at the full voltage or above um, it's high enough to actually make all these leds, like they're, being used, basically as two volt threshold led indicators, um initially, if you're, to connect this to 400 volts.

Let's just ignore the leds because they would add up to a grand total of 8 volts, so not really significant um, so that would be four times five thousand one hundred ohms equals twenty thousand four hundred ohms uh 400 volts divided by twenty thousand four hundred ohms. Would equal approximately 19 milliamps flowing through these so a decent amount of current, but multiply that by the 400 volts and the dissipation of these resistors here would be about almost 8 watts. But that's what they're rated i mean i wouldn't leave it on all the time, because the case would get very hot. It probably is only rated for a short duty cycle, but they are, you know it's a fairly generous rating of resistor.

It looks fairly decent and the way they're spaced from the circuit board. It was all right, it was all kept separate. So it's very simple when it comes down to the crunch that simple ac section, the two more precise uh 12 and 24 volt sections, the simple resistive divider for the 50 122 3400 volts, and that is it other things worthy of note. The cable, where we'll put the cable there's a cable, has fairly thick insulation.

It's fairly flexible. It's got a thin core going through it because there's not much current involved. It doesn't have the two color insulation, which is a shame, that's quite useful, for where you want to actually see if the cable's been damaged uh. I just don't think this is fused.

It says category 3, that kind of suggests industrial issues - 1 000 volt max, which will be determined largely by the diodes um yeah to push i'd, use it in industrial equipment, but not. I certainly wouldn't definitely help cat for a bullet doesn't say cat4, which is substation grade stuff, because i, as i say this bridge rectifier if the diodes fail, the only thing that really is there to blow. Is these tracks they're not very thick tracks, but you can get that effect of flashing over having said that, the two ends of the circuitry are quite well separated in the case. I'm not really sure what would happen there.

Don't fancy trying it not an industrial supply. Other than that, it's simple it's functional and extraordinarily cheap. That came from cpc in the uk who are part of element14 group they're. They usually supply stuff that is meeting reasonable standards.

So i would actually rate that as acceptable for general maintenance use on appliances and stuff like that, it's not a bad little tester.

16 thoughts on “Inside a cheap multi-voltage tester with schematic”
  1. Avataaar/Circle Created with python_avatars Chloe Hennessey says:

    Hi Clive!
    Got my first Fluke tester. It’s a 115!

  2. Avataaar/Circle Created with python_avatars manitoba2445 says:

    radioshack used to sell ones of these with neons to measure plate voltages between 80 and 300v in 60v increments for tube gear. i have quite a few laying around xd

  3. Avataaar/Circle Created with python_avatars Jason McKain says:

    Thanks from Texas Clive

  4. Avataaar/Circle Created with python_avatars Matty England says:

    Wake and bake special.

  5. Avataaar/Circle Created with python_avatars Blown Capacitor says:

    I'm so Hopi to see you Clive!!!

  6. Avataaar/Circle Created with python_avatars adebabay tech says:

    Let's check my experiments

  7. Avataaar/Circle Created with python_avatars Stephen Eyles says:

    I wouldn't trust those probes on 1,000V!! At work we have to use GS38 rated probes; they have shrouded tips and finger protection so there's less chance of zapping yourself while testing! Also have proving units for our voltage detectors which we test before and after testing plant circuits.

  8. Avataaar/Circle Created with python_avatars houseatwrongtown says:

    I guess it just shows how naive I am that I'm surprised at the absence of a fuse. Really good point you make about testing on a known-live circuit before anything else!

  9. Avataaar/Circle Created with python_avatars Johan S says:

    With a start current of 100 mA you will trigger the RCD if you plan to check if there is a voltage with respect to ground.

  10. Avataaar/Circle Created with python_avatars Todd Sharp says:

    I was really hoping you'd run 1k volts though it purely for entertainment purposes.

  11. Avataaar/Circle Created with python_avatars Tim Gooding says:

    To be honest I would never use one of these on mains power. There is no way it's Cat III. But it's better to have than nothing. 🙂

  12. Avataaar/Circle Created with python_avatars DC Allan says:

    A nice cheap bit of tat from China. Really makes a nice change when things like this are not designed to kill you. great video 2x👍

  13. Avataaar/Circle Created with python_avatars Barrie Shepherd says:

    Remarkably low cost from a reputable supplier – thanks for pulling it apart. Would be interesting to see what the innards of a equivalent Fluke are like.

  14. Avataaar/Circle Created with python_avatars Harris Chalk says:

    Great video Clive! That's another great excuse to spend more money on electrical stuff , thanks Clive 🙂

  15. Avataaar/Circle Created with python_avatars Paul Smith says:

    Another tool I'll have to add to my collection, along side my isesamo prying tool, my cat3 meter etc etc 👍 Might even have to buy a cliff quick test one day. The Hopi might be a bit far though

  16. Avataaar/Circle Created with python_avatars Gregory Thomas says:

    Very nice for such a small amount of money 🙂

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