#0010: Device analysis of a USB resistive load

Preamble

This will (hopefully) be a start to a series where I take a look into various devices and analyse them. My intention is to begin with very simple devices and steadily ramp up to more complicated ones as time goes on. The idea is to give the reader enough information about the device (design, function, components, etc) to the point that they can conceivably create their own. As well as in some cases recommending certain modifications, tools, adapters, or companion devices for optimal practical use.

Device information

This is a resistive load with a male USB plug interface. It’s intended function is for testing the current output of USB power-supplies and power banks. It has 2 current draw settings, these include: a 1 ampere and a 2 ampere mode. This device is operated using a two state slide-toggle switch, which allows alternating between the two current draw modes. Additionally it uses a common anode bi-colour LED as an indicator for these modes. Green for the 1 amp and red for the 2 amp mode.

The general layout and configuration of the 5 ohm wire-wound power resistors is what makes this device function. One resistor (R1) is always in circuit, in both the 1A and 2A mode. Within the 2A mode, R2 is added (in parallel to R1) to the circuit. Adding R2 in parallel to R1 reduces the circuit resistance to 2.5 ohms, which in turn draws 2 amps from the power supply.

1A mode: R1 in circuit (5V/5R=1A)
2A mode: R1 and R2 in parallel (5V/2.5R=2A)

Schematic

Key

R1: 5 ohm wire-wound power resistor
R2: 5 ohm wire-wound power resistor
R3: 560 ohm resistor
L1: light emitting diode (green)
L2: light emitting diode (red)
S1: toggle slide switch

Recommended modifications

• 3-state switch or additional switch (for device OFF state)
• Heatsink / fan (to efficiently dissipate generated heat)
• PTC resistor (as a safety temperature cut off)
• male USB to female USB extension cable (for allowing easier multimeter access)

Additional switch

Due to both states of the USB resistive load’s two-state slide-toggle switch being used during the operation of the device, the device has no plugged-in OFF state. This means that it should not be left unsupervised whilst plugged into a power-supply as it would be active at either switch state. Either adding an additional two-state switch or replacing the current one with a three state-switch, will allow the device to have an OFF state. Whether or not this is considered valuable, is largely subjective. However personally, I like the option of turning a load off without necessarily having to unplug it.

Heatsink and fan

The reason why leaving this resistive load active and unsupervised is a concern; is predominantly due to the functioning of the two large 5 ohm power resistors. These resistors dissipate around 5 watts each (5V*1A=5W) and generate considerable heat as a result. Because of this, I recommend the addition of an appropriate heatsink to be attached to these resistors in order to dissipate this resultant heat. As it is now, during continuous operation the resistors heat up to the point that they can not be handled with a naked hand. This level of heat could pose a possible burn risk, or fire hazard.

The addition of a heatsink will allow the device to run continuously without reaching these same high temperatures, it does this by dissipating the heat generated within the resistors in a more effective manner. I.e. moving it into the environment quicker, so that it doesn’t concentrate within the device. An addition of a mounted 5 volt mini fan will enable the cooler ambient air to run through the fins of the heatsink and further improve it’s ability to move heat out of the device. I specify a 5 volts fan because it can be powered from the device itself.

In addition to allowing the device to be handled after extended periods of operation, a good heatsink will in all likelihood also extend the lifespan of the two 5 ohm power resistors. It does this by dissipating any generated heat before it reaches levels that may damage either the components themselves or their neighbours. Generally speaking: devices that run cooler, live for longer.

PTC thermistor

As an alternative to the above two fairly common sense modifications; one could also choose to incorporate a Positive Temperature Coefficient (PTC) thermistor as some kind of safety shut off in the cases where the device reaches any critically high temperatures during operation; essentially as a reusable temperature fuse. The reason why I am dubious in recommending this is that: although it will make the device safer to use in a continuous application; attaching a PTC thermistor in series with the two power resistors may actually affect the resistive load’s performance if not it’s ability to function entirely. I am not sure as I am inexperienced with the application of thermistors in general, and have yet to try this particular use-case out.

My working theory is that since a PTC thermistor increases resistance as it heats up, and with this device’s current draws; it will quickly lead to a positive feedback loop where it’s resistance generates heat, which will generate more resistance, and so on until no current can pass through it – essentially becoming open circuit. I believe that the heat generated from the low resistance power resistors will kick start that sequence; and that the increasing circuit resistance from the PTC thermistor will quickly negate the the power resistors’ ability to draw current. Even if the thermistor’s resistance doesn’t increase to the point of open circuit. It only needs to be consistently higher than the power resistors’ 5 ohms in order to hamper device functionality.

Still, perhaps there is a configuration where the PTC resistor will not negatively affect the power resistors, while still being functional as a thermal fuse. Perhaps if the trip temperature of the thermistor was high enough, or it had a position on the device where it only heated up in cases of catastrophic environmental temperatures. Basically a setup that allowed for the added safety of having a thermistor in circuit without it negatively affecting the functionality of the device.

USB extension cable

I decided to create my own USB extension cable rather than use a prebuilt one. The main reason for this is that I fear that a random off the shelf unit my not be designed to handle current draws of 2 amperes (even if it’s vendor says that it is). This would mean that the cable itself would provide a level of resistance that would become a limiter to the amount of current that the resistive load can draw.

Generally if a USB cable is offering resistance, it will be in it’s cable and not in it’s plug and socket. Either the cable itself is too thin of a gauge, made of inferior materials (aluminium instead of copper), or is long enough to cause voltage drops at higher currents. With this in mind, I used thick gauge copper power cables (taken from an extension cord), and a salvaged USB plug and socket. I also made sure to keep the total length of the cable short; just being long enough to allow comfortable use of a clamp style multimeter.

Which actually brings me to the purpose of this little extension. The pictured USB extension cable is designed to allow the use of a clamp multimeter (one that can measure DC current). Alternatively, if I didn’t have a clamp multimeter, I would cut the 5 volt line in half and terminate it’s two ends with banana plugs. That way I can insert a regular multimeter in series within the circuit; and measure the actual current draws.

As an alternative to using a DIY USB extension and a multimeter, one could just buy a USB voltmeter/ammeter. Such as the one pictured. Although these things, in my opinion can skew the results: due to them (in my experience) imparting some level of resistance on the circuit; they are certainly more convenient to use. Which one a person prioritises: accuracy, or speed (neatness, etc.) will largely depend on their preferences and use-cases.

Closing statements

Although the build quality of this device is more than acceptable (i.e. it doesn’t feel like a shoddy product), it is clearly built to a price point. Only having near enough the absolute necessary components to function. I believe this to be the case, not just because of the miniscule bill of materials (BOM), or the use of inexpensive components in that list. But rather because of the in my opinion necessary things that were left out; namely a basic heatsink.

Although it actually functions fine as is; at least for short operations. If you wish to use this device safely continuously for longer periods of time than a couple of minutes, modifications will need to be made.

Sources, references, further reading

https://en.wikipedia.org/wiki/Thermistor#PTC

https://sciencing.com/difference-between-resistive-inductive-loads-12181159.html