# Convoy SST40 drivers now coming with R020 sense resistors?

I’ve just received some Convoy 6A drivers (Biscotti-clone firmware) that have R020 sense resistors. The image on AliExpress shows a R010 sense resistor.

With my bench power supply at 4.2V the driver only pulls 3.5A, but at 5.2V or higher the current tops out at 6.5A.

Based on my very limited understanding of this thread it seems like on the driver rather than doubling the sense voltage the output has been (approximately) halved.

Does this make sense? It seems surprising that Simon would be selling a 3.5A driver in pace of a 6A driver.

Can you confirm that once it’s installed into a flashlight?

When you set your power supply at 4.2V, there are 4.2V between the power supply output connectors. In fact, and to be precise, these 4.2V are “where the measurement is taken”, this is somewhere inside the power supply close to the output connectors.

Once the circuit is closed and current starts to flow every resistance in the circuit causes a voltage drop. This voltage drop is V = I × R.

Physical connections themselves have some small resistance, this is why battery cells are soldered or spot welded in battery assemblies, and not set inside battery holders. You could for example assemble a big battery out of battery cells using battery holders, but each resistance in the contacts between each battery holder and its own cell would severely limit the effective current output of the cell. Resistance in the contacts versus cells' own internal resistance is significative.

Then, PSU output probes have resistance; the alligator clips in them have resistance too. And the contacts you make between alligator clips and the driver… have resistance.

Once all of the above is considered, you understand that in a closed circuit the actual input voltage at the driver when fed from the power supply is lower than what the PSU's voltmeter is saying. For you to know what is the actual input voltage at the driver when this is done, you could hire some assistant for him/her to peek right at the driver with the probes of a multimeter.

The purpose of these regulated variable load drivers is to take the input voltage and regulate or limit it in a way that the current for the different modes matches what is claimed, this is done by “sensing” the current flow as the current goes through the driver's sense resistor, something which causes a subtle voltage drop at the sense resistor which, after magnification (amplification), is seen by the driver microcontroller, and the driver microcontroller uses this information to “regulate the gas valve” at the MOSFET (it adjusts the MOSFET's VGS), causing the output voltage to match what is needed for the exact mode current to flow. With the recent changes which resulted in a more than doubling of the sense voltage, the voltage drop in the sense resistor stage is higher. The problem with this is that the sense voltage is an “inevitable loss” which mathematically speaking is V = I × R. This loss is more noticeable in the higher modes, and despite not being very noticeable, is comparable to replacing a high current battery with a not so high current battery.

The actual problem (for sense resistor modders) is that we made our math presuming the older sense resistor and voltage value, which is very important to precisely limit the current for smaller emitters like SST-20's and Osram white flat 1mm² units.

With all of the above which has been said, I hope for you to understand that the sense voltage has been actually more than doubled in fact, and that the actual maximum output (when input voltage is enough) is now higher than what is claimed. The wrong part in all of this is that maximum output current is a tad higher than what is claimed (this matters a lot for those of us who do sense resistor modifications), but also that to attain this current at the emitter the driver needs more voltage than the older versions of the driver with lower sense resistors (they cheaped out on the driver).

The reasons why you could only measure 3.5A with the power supply are explained above. The maximum driver output has not been halved, it has been raised a tiny bit instead, but at the price of a slightly higher internal resistance now versus the old version (higher sense resistance), resistance whose effective recovery is unavoidable without laborious modifications. The sense resistance is important because the current regulation of the different modes is proportional to the inverse of its value (in other words, current regulation of the different modes is proportional to the sense resistor's conductance), and thus if you bridge the sense resistor you lose regulation, this means the driver will let all the voltage and current to go through because it will be unable or barely able to sense the sense voltage, i.e. all modes will be maxed out without driver limit except maybe the lowests (0.1% - 1%) modes which will just look very bright or nearly as bright.

So, are you measuring correctly if you set the power supply output at 4.2V and stick the wires at the driver? No. In practice the PSU output wires and contacts have a lot more resistance than the flashlight internals (springs, switch, and wires), this is the reason you must raise the output voltage of the power supply until it hits a current wall; this current wall is the point at which the driver regulates or limits the current. See what you said (5.2V), and also what I said (6V). Since each time you move the output wires and connectors you also vary their resistance, this is the reason I used a high figure: to ensure that these movements would have no chance of reducing the input voltage at the driver too much while I was switching modes and etc.

This explanation also reveals why people who measure current by dismantling a flashlight's tailcap and sticking multimeter probes attached to a multimeter between the cell cathode and the flashlight body, do it very wrong. Among contacts, probes and the multimeter itself have a lot of resistance, way more than that of the flashlight's tailcap by itself. No wonder why they measure low values.

To measure the current in a flashlight you must use a clamp meter around the flashlight body, this works because the flashlight body is used as a current path. This measures the current which goes from the battery to the driver, and this current value is also equal to the current which hits the emitter in flashlights without switching drivers (boost, boost-buck, or buck drivers).

I actually took my measurements with the driver installed in the head of a Convoy L2 with a white flat. I don’t have a clamp meter so I connected the power supply to the to the driver spring and flashlight head using alligator clips.

As Barkuti pointed out this is not an optimal setup as there is the potential for a lot of voltage drop due to the test leads and poor contact. However, I have checked the voltage between the ground ring of the driver and the use of the spring and it’s the same as the power supply voltage so I don’t think that’s the issue. I have also tested a different driver (led4power A4) using the same method and it’s pulling the expected current.

I’m going to set up another one of the Convoy drivers outside a flashlight for another test.

Edit: After writing this post I realised that I wan’t measuring the voltage drop properly. This could have been avoided if I had used a clamp meter, or perhaps just thought about what I was doing a bit more carefully. Thanks Fantastic and Barkuti for helping me understand.

Thanks Barkuti!

I was in the middle of typing a reply that said your explanation made sense but it didn’t match my measurements when I realised that I hadn’t been measuring properly.

I was checking the voltage at the driver when I first turned the power supply on and the driver was in the lowest mode. When the driver was in higher modes the voltage drop due to the test leads was higher, but I wasn’t re-testing the voltage at the driver (not enough hands).

So why was the led4power driver pulling the expected current? I expect it’s the lower internal resistance in the driver, but I’m also no longer sure what current I had it configured to - I’ll have to look at the manual and check.

So now my issue is that the high mode on this driver is actually >6A which isn’t great for a white flat, but I can focus on understanding sense resistors so I can adjust it.

What flat white do you have in there?

Thanks Funtastic, I think you’ve solved my problem.

I had a CSLNM1.TG in the L2 and for some reason I got it into my head that ~6A was the sweet spot and decided to swap the driver. Of course I should have been aiming for 5A maximum - think I got confused because I’m using a 6A led4power driver with a CSLNM1.TG in another light, but it’s set to 5A maximum output.

Although this driver swap was a fail I’ve learned at least two valuable lessons - remember to account for voltage drop, and don’t do an impromptu driver swap late at night. If it wasn’t for the wonderful people at BLF I’d probably still be wondering why my driver was underperforming while simultaneously burning up my emitter,

In djozz’s test of the CSLNM1.TG the output at 6.5A was ~250 lumens lower than the peak at 5A. That might explain why felt my L2 was underperforming compared to my C8 with the same emitter driven at 5A, which was the reason I attempted to measure the current in the first place.

First of all, I am going to say something which is common among humans, and this is to imprint some sort of fearful, bad energy around something in their minds. This bias someone's perception around it, particularly when strong emotions are involved. This is something which must be avoided at all costs… for the light to flow correctly, i.e. to understand and etc.

What I mean with this is that if you are only accustomed to fairly simple unregulated “MOSFET” drivers, it may be harder for you to see this clearly, but it certainly is possible.

These regulated drivers employ a MOSFET to regulate or limit the current, using a sense stage to do this. They are fairly simple too, the only technical difference between a blind MOSFET driver and these regulated ones is the presence of a sense stage. Thanks to this sense stage they can precisely limit the current, and they can do it without PWM (pulse-width modulation).

I somehow sense that you probably feel that limiting the current is bad, but this is not. Without current limiting you could find yourselves with lower than expected output or damaged emitters due to overcurrent. Drivers which properly sense the current can take care of this. The problem I came here to discuss is that these “revised” SST40 drivers are using a coarser sense stage, with more resistance (the R020 versus the older R010), which is undesirable as it unnecessarily drops input voltage.

How did you checked that? :D The voltage difference or voltage drop between the power supply output's voltmeter and the driver's input terminals only appears when the current is flowing. The driver's spring also drops some voltage as it also has some resistance, by the way.

Remember what I said before, by Ohm's law V = I × R. If you use a multimeter and measure the voltage at the driver's input or at the end of the test leads without load, you'll of course get the same value. In these conditions the only current flowing is a veeeery small tiny bit of current which the multimeter needs to get its measurement done, but since it is so tiny the voltage drop in the power supply leads and connectors is also insignificant. However, let the current flow and you'll see. This is the reason I said “hire some assistant for him/her to peek right at the driver with the probes of a multimeter”, this is because when you are connecting the PSU probes to the driver with your hands and you “close the circuit” so current is flowing, you may need someone to put the multimeter probes above the driver ring (negative) and somewhere in the spring. You will then notice a difference: the larger the current flow the larger it will be.

Hmmm, I think the change came when Simon’s supplier fixed the mode reset bug, maybe a component was clashing with the firmware? The drivers had the R010 with the bug and after it was fixed was changed to R020.

At the beginning it was running biscotti and all was perfect until Simon announced his own firmware “12 groups”

The first fixed batch was running at 5.4A and I have 15pcs . I also have them in 10pcs S2+ Osram’s Simon sent and the LED turns a slight blue. I’m just going to sell them as is under my 12 month warranty, too much messing around otherwise and it’s difficult to discuss with Simon since 6A is still okay by his view.

Well, the driver I was above speaking about (see #39) has a working memory, but uses the old sense voltage and resistor. I measured ≈1.3A for an R050, but please consider I don't know how good or precise is the sense resistor I used and my reflowing job (and so I can't say 100% sure that the sense voltage in such driver is 65mV). Furthermore, when later I installed an R020 in parallel with the R050 I was expecting to see ≈4.55A maximum (presuming 65mV sense), but it reached ≈4.7A. No worries because an SST-20 can take that, but if you set your emitter theoretically to the brim and actually you get even more current… :facepalm:

Mind you, considering 60mV sense with an R020 and an R050 in parallel the maximum driving current should have been 4.2A.

Tue, 08/24/2021 - 14:46

I received a L21B CSLNM1.TG with what appears to be a 12 group 20mm 6A driver.

Measured 6.5A at the tailcap on 100% with a clamp meter.

Is there an easy way to mod the 6A driver to be limited to 5A so that it doesn’t burn out the CSLNM1 or would be best to install a 12 group 17mm 5A driver?

Is the driver coming with an R020 sense resistor, TimMc?

If so, this means that the driver's sense voltage is V = I × R = 130mV. With this in mind, replacing the onboard R020 resistor neatly with an R025 will limit the current to 5.2A maximum; with an R027 the limit would be 4.815A (4.8̅1̅4̅A), and with an R030, 4.33A (4.3̅A).

Sense resistor standard values extracted from this document at venkel.com.

Yeah, it’s an R020:

Would WFMA2010R0250FEA work?

If the old resistor is a size “2010” (5mmLx2.5mmW), then it would work; that WFM is a really high quality part with high thermal rating, much better than the original.

Guess that's an R025, doesn't it?

What kennybobby said, plus pretty high or highest resistor accuracy. 5.2A is on the limit for a CSLNM1.TG, for my liking. Remember to do a neat reflow job, because if the resistance between the sense resistor pads ends up being a tiny bit lower the current will go up.

I just searched for R025 on mouser and digikey to get that part number. Haven’t ordered from them before.

Is there a more appropriate part?

You should first use some calipers or a fine-scaled ruler to measure the Length and Width of the part on the board to determine its dimensional size, then you can find the correct part to order. The resistor value is a separate parameter, but the size must match or it won’t fit to the solder pads.

Then with the size and value known, it will be possible to find a part with a desired tolerance and power rating, etc. Plenty of folks here that will give you some assistance.

I measured the R020 as 5mm by 2.5mm with some plastic callipers.

Yes you are good to go Tim, that is a good part number that you found and will fit and work.

WFM A 2010 R0250 FEA

“WFM A” is the model and construction material code;

the resistor size is “2010” (5mmL x 2.5mmW);

the resistance value is “R0250”, which is 0.0250 Ohms, or 25.0 mOhms;

“FEA” is the tolerance and packaging code.

More than likely I'd personally choose a higher value sense resistor, since I care less for virtually indiscernible gains and more with reliability. O:)

By the way, there are these P5000 and H17A drivers at Kaidomain, regulated variable load drivers too. Asked about them in the Kaidomain thread many days ago. Did someone around here gave 'em ago? They seem to come with R010 sense :THUMBS-UP: resistors.