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When teaching electronics to college students, we often use these large 1 W cylindrical resistors - not because we need the wattage rating, but because they are easy to handle and read the color code for students.

This got me thinking: almost all practical electronics these days uses small surface-mount resistors, and dissipating a whole watt of energy is wasteful. It is more efficient (and possibly cheaper) to use some small cheap silicon to rapidly switch it to limit current rather than dissipate heat.

Are there still any modern real-world practical applications that require something like a 1 W through-hole resistor?

If not, are they just made for hobbyists that repair old equipment?

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    \$\begingroup\$ Any time you need a power resistor, it's likely going to be either through-hole or chassis-mount. And yes, there are cases where you need to dissipate large amounts of power in a resistor; efficiency isn't always the primary concern. \$\endgroup\$
    – Hearth
    Commented Feb 22, 2023 at 16:10
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    \$\begingroup\$ and dissipating a whole watt of energy is wasteful - The 1W resistor will dissipate as much as 20mW resistor of the same resistance (given you are not exceeding 20mW there...) \$\endgroup\$
    – Eugene Sh.
    Commented Feb 22, 2023 at 16:11
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    \$\begingroup\$ @Hearth, I'm aware of a few applications of very high power resistors to dissipate a lot of energy (such as braking a train), but it's the "couple of Watts" middle ground that I'm asking about \$\endgroup\$
    – user1247
    Commented Feb 22, 2023 at 16:21
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    \$\begingroup\$ @user1247 I've used a handful of high-power resistors for discharging large capacitors, where basic safety principles dictate that they should be discharged as quickly as possible. I've also used power resistors for current shunts, when measuring high currents. I've also used power resistors as test loads for power supply designs. There are countless reasons you might need those medium-power resistors. \$\endgroup\$
    – Hearth
    Commented Feb 22, 2023 at 16:57
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    \$\begingroup\$ I understand that in real life not everything is possible, but I highly suggest you delve into some cheap modern lab equipment - at least electronic T&M stuff, like power supplies, soldering irons, etc. Some designs use through-hole assembly, and some devices need resistors with dissipation way above 1W, and there's overlap between the two. Most laboratory power amplifiers, 4-quadrant power supplies, and active loads use resistors rated way above 1W! Current production, expensive stuff does. \$\endgroup\$ Commented Feb 23, 2023 at 2:28

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You can get 1W (and more!) SMD resistors for situations where you need a lot of power dissipation. The reasons for needing that power dissipation are varied, but a common case is shunt resistors for current measurement.

However, in a case where you need that much power dissipation, but you want to limit the amount of thermal power that is transferred onto the board and surrounding components, a through-hole resistor part is actually very useful. Forced air cooling (e.g. from a fan) can blow across the larger surface area of the through-hole resistor package, and the thermal resistance of the leads will help reduce the amount of thermal energy that is transferred back onto the board, especially if the resistor leads are kept long so it stands off the board.

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  • \$\begingroup\$ Are shunt resistors really preferred? Seems wasteful when you can measure the magnetic field without much dissipation. Is it required for precision measurements in high power applications? \$\endgroup\$
    – user1247
    Commented Feb 22, 2023 at 16:24
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    \$\begingroup\$ Yes, shunt resistors are preferred. Hall effect sensors are far more expensive to build, are not as accurate or reliable, are physically larger, and (due to their inductance) find it difficult to measure high frequency changes in current. All modern computer motherboards and GPUs use shunt resistors for current measurement in their VRMs. \$\endgroup\$
    – Polynomial
    Commented Feb 22, 2023 at 16:27
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    \$\begingroup\$ Also magnetic parts like hall sensors can have hysteresis. I have seen 100A parts that have ±0.5A~1A hysteresis in measurement. Resistors don't have that. \$\endgroup\$
    – user4574
    Commented Feb 22, 2023 at 17:15
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    \$\begingroup\$ @user253751 I suspect they're not manufacturing them in anywhere near the level of bulk that they used to do, but I highly doubt that manufacturers are just sitting on new-old-stock. If the number of purchases was that low I don't think distributors would waste the space in their warehouses. Obviously they're not flying off the shelves like jellybean 0805/0603 reels, but they're still useful for plenty of things and are often preferred by hobbyists. \$\endgroup\$
    – Polynomial
    Commented Feb 22, 2023 at 22:26
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    \$\begingroup\$ When I worked for a modem manufacturer, we used 1W resistors between the PCB and the telephone line. They helped keep the PCB from getting burnt when lightning hit the telephone line (though not always, those were a pain to repair). \$\endgroup\$
    – bkb105
    Commented Feb 23, 2023 at 15:36
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Power resistors aren't going away, at least not any time soon. There are many places where you just can't return some power but must dissipate it in situ instead.

A dramatic example is braking resistors: on a diesel-electric train for example, the engine can only generate power (there is no currently known reaction process to reverse fuel combustion, at least not in anything resembling an engine!), so the generator is no good to absorb the excess power that is available say while descending a hill, or when slowing down. However, if we simply toss a big fat resistor on the traction motor (or rather its controller), we gain the benefit of a wider-range and continuous control method (we can vary torque from positive (driving) to negative (braking)) while extending the longevity of the [mechanical] brakes on the train cars. That is, suppose you had a car that, instead of two pedals (throttle and brake), you had one lever that moved smoothly from "go" to "stop" (or to "reverse", even).

Another example is when it must be done very quickly. Snubbers are a typical application, where some unintended LC circuit elements become excited, say in a switching power supply. That excitation leads to increased peak voltage or current at the switch, and often increased EM emissions too. These can oscillate in the 10s of nanoseconds, just not enough time (among other constraints) to be able to transfer that energy somewhere useful. In that case, it's more effective to simply dissipate the energy as heat.

There are some ways around this -- SMPS can be built with topologies that recirculate more of this leftover energy, or indeed even harness it. But they come with some limitations of their own, too (e.g. the flyback topology places harder demands on its filter capacitors). Also, even more mundane: the cost of recirculating circuitry might simply not be warranted for a given product. It's all economics in the end!

Most generally, we have applications where we simply have a voltage or current, and want the other: resistors are used for current limiting (I = V/R), current sensing (V = IR), and measurement (R = V/I). Current sense (shunt) resistors can be made in quite small values (I have a few 100µΩ parts in stock, myself), but as currents go up and up, they can still end up dissipating quite a lot of power. And you can only reduce the sense voltage so far, before too many errors creep in -- even using good layout and a precision amplifier.

Losses can easily be improved, in some easy cases; when the sensed current is AC, it can be coupled through a transformer, and then a proportionally smaller "burden" resistor is needed. A transformer cannot convert current to voltage by itself, though: the resistor is always needed. (Well, you could use a transresistance amplifier to do the conversion instead, but that's most likely still constructed with a resistor!)

Similarly, resistor voltage dividers can be made with larger values, as long as this is suitable for the application. An oscilloscope probe could be made with 450Ω series into a terminated 50Ω cable (a "low Z" probe), but this draws quite a lot of power for many small-signal and DC biased applications. A probe might instead be made with 9M into 1M, where the 9M has some capacitance across it to compensate for the capacitance of the cable and scope input. The 10MΩ total draws very little power within ratings, so makes a good probe (as such).

As for the form of said resistors -- there are 1W SMTs too. A typical 2512 [imperial] chip size resistor is good for 0.5W or more. You need to dedicate quite a bit of board area to achieve this, though: the chip is heatsunk through its pads, so you preferably attach large copper pads on the PCB to it. If you're constrained on space... it might simply be better to stand that resistor up vertically.

THT resistors can also run much hotter. Constructed from ceramic materials, parts are available with temperature ratings that would literally desolder themselves -- if the hot part weren't kept responsibly away from the solder joints, that is. Higher surface temperature, means more heat into the surrounding volume of air, and faster rising convection currents. I have some 0.5W metal-oxide (and silicone enamel, I think?) resistors that are the same size as conventional (carbon film / epoxy enamel) 0.25W resistors, for instance. To avoid desoldering (not to mention PCB charring, etc.), it's a good plan to stand these resistors up off the board, perhaps using ceramic spacers, or by clinching the leads to stay up off the board during soldering.

Power resistors can also handle higher voltages. A typical 1W THT resistor is rated 300 or 400V, and HV types are available up to a couple kV. (A nontypical 10W HV resistor might be rated 10kV or more; but it will be much longer to avoid arcing along the body length itself, and the power rating is just kind of a formality at the megohms many such resistors have.)

Mind, it is usually worthwhile to sense modestly high voltages with SMTs. It's a common sight in off-line SMPS to have a chain of three or more 0805 chips in series, say across-the-line (to discharge the line filter capacitor), or to sense supply voltage (dividing it down to a comfortable say ~5V range that the controller IC is comfortable with).

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Airplanes still use them because it takes so much money to recertify so things are used for decades.

In Avionics we had to learn to rework boards where the boards were two boards sandwiching a forest of through-hole components standing upright connecting the two boards together like a sandwich. Really old.

@Ken Shirriff mentions that this type of construction is called "cordwood". So now I can search for pictures.

enter image description here https://hackaday.com/2017/03/24/retrotechtacular-tinkertoy-and-cordwood-in-the-pre-ic-era/

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    \$\begingroup\$ By the way, that construction technique is called "cordwood". \$\endgroup\$ Commented Feb 23, 2023 at 21:56
  • \$\begingroup\$ @KenShirriff Thanks. \$\endgroup\$
    – DKNguyen
    Commented Feb 23, 2023 at 21:56
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I still use THT resistors for three purposes:

  1. When a circuit is used in multiple products but requires slightly different resistances, e.g. a circuit that is designed to work in a product that runs on 12 V, a second product that runs on 24 V, and a third product that runs on 48 V. Rather than maintain three separate inventory parts that only differ by a single resistor value, the assembler can simply solder in the correct THT resistor with minimal difficulty. SMD resistors can be hand-soldered, but if space isn't at a premium, THT is faster and more reliable.

  2. When a resistor needs to go inside a cable or in series with a wire. Wires can connect directly to THT leads, no PCB required. I'll only do this if the wire is internal to the device, and never on a cable that actually flexes in use.

  3. When the power the resistor needs to dissipate is just more than an SMD part can handle. You write

dissipating a whole watt of energy is wasteful

which, I think, has it backwards. A resistor will only dissipate \$P = \frac{V^2}{R}\$, but some resistors are capable of dissipating more heat. A high-power THT part in a circuit that only burns milliwatts will still burn milliwatts even if it's rated to handle multiple watts. Conversely, if the \$V\$ and the \$R\$ that you need to use mean your \$P\$ is above the rating for any available SMD part, you either need to use multiple SMD parts to share the load, or simply use a beefier THT part.

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    \$\begingroup\$ Not worth it when your annual sales are in the mid hundreds. \$\endgroup\$
    – Matt S
    Commented Feb 22, 2023 at 16:31
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    \$\begingroup\$ "THT is faster and more reliable" ... For a single part on an already-populated board? Placement is always perfect and clumsy hands can hit a THT much easier than a pad. We also have space-constrained parts where the specific value is selected during testing, and even the 1206s I put down on that board with oversize pads are a struggle for our assembly team and a soldering iron. Better equipment would surely help, but again -- 100's of sales a year. \$\endgroup\$
    – Matt S
    Commented Feb 22, 2023 at 16:37
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    \$\begingroup\$ @Polynomial -- why a jumper and not just a THT resistor? Multiple parallel SMDs would still need to be added or removed by assemblers. Removal is far worse than addition of course. I might see the value in a set of parallel resistors with scratch traces, but I think people fundamentally understand "add the right resistor" a lot more clearly than "cut this exact spot with a knife". But anything else still involves breaking out the soldering iron. \$\endgroup\$
    – Matt S
    Commented Feb 22, 2023 at 16:41
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    \$\begingroup\$ @MattS With the jumper you wouldn't remove the parts. The PCBA would fit all the resistors on every board, and you'd just short the necessary jumper pads with a solder blob to configure the specific value you need. No soldering components at all. And if your shunt resistances work out to be even multiples (e.g. 0.2/0.1/0.05Ω) you can fit the same maximum shunt resistance value (e.g. 0.2Ω) multiple times and bridge 1/2/4 jumpers to put them in parallel to get the final value required, which means instead of buying N of each value, you buy 2N or 4N of one value, which works out cheaper. \$\endgroup\$
    – Polynomial
    Commented Feb 22, 2023 at 16:46
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    \$\begingroup\$ FWIW, I always spec THT parts if they're meant to be hand assembled, unless space constraints dictate otherwise. I agree with your comment but would counter that in 2023, all uses of THT resistors are "a bit of a niche case." \$\endgroup\$
    – Matt S
    Commented Feb 22, 2023 at 16:46
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When teaching electronics to college students, we often use these large 1 W cylindrical resistors - not because we need the wattage rating, but because they are easy to handle and read the color code for students.

The same is true in a factory. Having a simple hand assembly process for your products can sometimes make sense. Especially if you don't have all the expensive SMD machines (possibly costing hundreds of thousands - millions of dollars) and don't want to pay for outside assembly services.

Also, one has to be using a lot of SMD parts (probably millions) before you can justify the cost of SMD assembly machines. And at least a moderate amount (hundreds ~ thousands) before you can recover the cost of NRE and stencils for outside assembly. So, for low volume production through hole can make sense.

This got me thinking: almost all practical electronics these days uses small surface-mount resistors, and dissipating a whole watt of energy is wasteful. It is more efficient (and possibly cheaper) to use some small cheap silicon to rapidly switch it to limit current rather than dissipate heat.

Efficiency is only one part (Power) of the overall SWAP-C (Size, Weight, Power, and Cost) that go into the design of a product. Sometimes using a small $0.05 resistor is better than designing a switching circuit costing several dollars that takes several cm^2 of board area. Especially if the power dissipation is only occurring occasionally.

Also, sometimes power dissipation is the point. For example...

  • In the heating element of a temperature controlled inclosure.
  • Discharging stored energy for safety or other purposes.
  • Dumping excess energy being generated that can't otherwise be stored.

Are there still any modern real-world practical applications that require something like a 1 W through-hole resistor?

Generally speaking, a through hole resistor might be used whenever you need voltage and wattage ratings that exceed what might be possible for an SMD resistor, and you don't want to use a chassis mount resistor.

I use power resistors to protect digital and analog IO pins all the time in safety critical stuff. Normally, almost no current flows into/out of the pin (so very little power loss in the resistor). But in the event of a fault the pin may connect to high voltage.

In order for the IO pin to work properly the resistor often needs to be kept in the low k-ohm range, which means that it may need to be designed to dissipate >1W during the fault. A resistor can be a good fit for that type of application. If high wattage and voltage ratings are needed, it may need to be through hole.

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    \$\begingroup\$ In my previous workplace, where the circuits we did just required SMT, the owners decided to buy a small PnP machine and an oven. Iirc the total was on the order of 50k USD, but they simply had to do it - our usual external prototype assembly was getting longer and longer lead times, and eventually upscaled and just refused to take our small prototype runs. \$\endgroup\$
    – jaskij
    Commented Feb 23, 2023 at 4:49
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Snubbers, Q control in matching networks, sometimes you REALLY don't want a switcher hashing things up, but a modest power resistor and a Zener diode will work just fine, current sensing, current sharing, there are all kinds of applications for the 1-10W power resistor.

Bipolar power transistors typically need a low value emitter resistor to parallel reasonably safely, guess what power range that usually falls into, yep, 1 - 5W more often then not.

I have a modern design burning 140W in bias power in an output stage, you think I care about a watt? Efficiency is contextual.

There is a whole world of parts optimised to very specific jobs, and as long as for those specific jobs the parts remain sufficiently superior to justify their existence, they will not be going anywhere soon. While I CAN find a 1W SMT resistor, they are a pain in the arse and do your board life no favours with the heat.

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Another use is resistive fusible links, especially where line voltages are present. The physical size of a large resistor allows higher voltage separation. The resistor limits initial inrush when power is applied. While short in duration, inrush can be 100s of amps on startup for DC-DC supplies with large filter caps.

If there is a sustained short downstream, it fuses open. Plus on devices designed to be repaired, a fused thru-hole doesn't trash the PCB.

While it can be done with large SMDs, regulatory approval is easier with a large thru-hole device.

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