SST-90 current draw - confused??

[plasmaman]

OK - so I finished it!
2D body, FM 2.5" deep throw head, solid custom ally finned heatsink with pedestal machined to fit the centre part of the SST-90, D2flex v2 board, momentary modded mag switch and 2 x Li 7.5ah cells built into a parallel 3.7v pack.
Very bright - 2.5 times the output of a P7 running the same cell.
Need to measure the current draw on full power.
Great tint, good throw/spread with the 2.5" reflector - a standard size MOP reflector didn't give as much focus to the beam.
Tricky build, but worth the effort I think.


I tried this current draw measurement method on my new SST-90 build (because I can't get directly to the power lead between the driver board and the LED):

Battery pack (2 x 7.5ah Li D cell in parallel) removed from tube and held in clamp
Bat -ve attached to tube body
Bat +ve attached to DMM
Switch +ve attached to DMM
DMM set to 10A
Switched on power.

On full power current shows 2.55A draw!

I tried the same method on a single Li D cell with a P7 - that showed 2.75A

Both lights use a dFlex board. Both use 20awg teflon between the switch/board and LED, and both have a +ve bypass wire direct from the switch +ve contact to the board (so the switch is only acting as a momentary switch to turn on the board)

The P7 is probably about right.

The SST-90 is definitely not - although it is much brighter than the P7.

I'm confused! Any ideas?

 

[calebra]

i wonder what the draw would be if u went direct drive?

 

[plasmaman]

i wonder what the draw would be if u went direct drive?

mmmm - but the d2flex does not limit the current passing through it.....

 

[calebra]

mmmm - but the d2flex does not limit the current passing through it.....

are u sure? a quick direct drive test may shine some light on that...

 

[plasmaman]

are u sure? a quick direct drive test may shine some light on that...

No current limiting function in the D2flex according to George (taskLED).
But problem is I can't easily get to the LED side of the board now its all put together, without pulling it all apart - which I really do not want to do!

 

[calebra]

hmm, on checking draw on a light i have it gave 3 different readings as per the 3 different modes, then i noticed it was a maxflex driver...so i am not sure about the D2flex...

 

[HKJ]

The meter and probes adds some resistance to the path and this reduces the current.

 

[calebra]

in fact i would be surprised if the D2flex driver didnt control current, given the different modes, just read the manual. i am no boff on the subject, perhaps the driver needs to be configured correctly, good luck, its not easy.

 

[pyro]

Datasheet says:

The D2Flex driver is a PWM direct drive controller.

Do you know the forward voltage for the SSC?

 

[plasmaman]

Datasheet says:


Do you know the forward voltage for the SSC?

The Luminus spec sheet says vf of the SST-90 is 3.87v at 9A

I have measured the voltage on the battery pack at full power at 3.88v......

so - I'm still confused!

 

[smootik]

Apologies if you already knew what's below.

Checklist:

- what type of DMM you use? I had a supermarket-variety with a large internal resistance, it "measured" about 1.5A when real current should be almost twice that. Better DMM showed correct value.

- is the current constant, or alternating? DMM might have two measurement settings, one for DC, one for AC. If the driver uses PWM, I would expect the current not to be constant.

- even when current is alternating and DMM is set to AC, be aware that DMM will "average" current reading, so what it shows is not the peak current value. Search internet for "True RMS" to see how different meters calculate this "average" value. It is not as simple as it seems, especially for square waveforms.

 

[AEHaas]

You need a True RMS reading meter for any signal that is not a true sine wave on AC or a straight line on DC. Many signals even contain both together. For todays electronic devices a meter that cannot measure True RMS is not going to be a much use except for a pure, constant, resistive load. And these are uncommon.

aehaas

 

[plasmaman]

Thanks to everyone so far....its quite interesting!
My DMM is a middle of the road $50 jobbie, so not high end by any means.

I decided to try a different method - I removed the cell pack and rigged up the light to run from my DC bench supply.
Set V to 3.8 and switched on to low and then stepped up to high.
Current draw on high started at 5.5A and worked up to just over 6A steadily in a minute or so, and then slowed down, increasing by .01A roughly every 10 seconds or so. I switched off at 6.1A

So that's quite a different result from the DMM test, but still not the 9A or more suggested by the spec sheet or some other users who have posted.

I could have stepped up the voltage but decided that was unwise on a $60 emitter!

I'm less confused - clearly there is a whole range of issues linked to DMM abilities/accuracy, but still missing 3A or so of draw. Does that all get eaten up in the D2flex I wonder?

A direct drive test is obviously the answer to that question, but I don't have a second SST-90 to play with just now.

 

[HKJ]

So that's quite a different result from the DMM test, but still not the 9A or more suggested by the spec sheet or some other users who have posted.

Try measuring the voltage directly at the led, i.e. use one set of wires to supply the current to the led and another set of wires to measure the voltage at the led.
You will probably see a lower voltage than you expected, due to voltage drop in your wires and connections.

It might be a good idea to buy one or two extra cheap DMM's, making it possible to do multiple measurements at a time. Use one meter to measure current draw and another meter to measure voltage directly at the led will give much more reliable results. Cheap DMM's usual has a good precision for low voltage DC and DC current.

 

[JohnF]

I'm less confused - clearly there is a whole range of issues linked to DMM abilities/accuracy, but still missing 3A or so of draw. Does that all get eaten up in the D2flex I wonder?

A direct drive test is obviously the answer to that question, but I don't have a second SST-90 to play with just now.

I just finished a mag build using the d2flex and SST-50, using a homemade parallel pack of 2 IMR 26650's. Fresh off the charger, the pack measures 4.19v. I tested the emitter / heat sink wired directly to the pack first, measured 4.6a. After wiring in the d2flex, (and putting it to max mode) I still measured 4.6a.

The d2flex IS direct drive. I believe your problem is related to the fact that your battery voltage is so close (essentially equal) to your emitter vf. There are small resistance losses in wiring and meter to take into account, so you may be under-driving the emitter using a supply voltage equal to emitter vf.

I avoid max mode until my pack voltage sags just a little (a few minutes on level 4), but using a high bin SST-50 with a measured vf of 3.6v, it is perfectly happy with a supply voltage of up to 4.1. Beyond that, it starts to turn angry blue...
I consider the pack depleted when its voltage is about 3.6. At that level, I can tell a difference in brightness, but current draw is WAY down at that point.

With your supply voltage = emitter vf, you are STARTING at that low point.

Again, I've tested with and without the d2Flex in the mix, and not an iota of change in measurements.

I'm no expert, and am sure I'll be repremanded if obviously wrong, which is great.

John F