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Author Topic: How to measure current draw in a multiplexed display?  (Read 889 times)
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I have a 24x16 LED matrix - the 16 rows are connected by transistors driven by a pair of 74ac138s and the 24 rows by 3 TLC5916s. Refresh rate is 10K Hz (0.1us).

I want the lights to be as bright as possible so I'm overdriving them with 100 mA pulses...at least that's what things should be if I did the calculations right.

When I measure a row's draw I know I won't see 2400 mA (24 LEDs @ 100 mA ea) since it's only being powered 1/16th of the time, but my meter says ~38mA instead of the 150 I'd expect (2400 / 16).

On each column the meter says ~63mA. I know it won't say 100mA because the display is off for 3-4ms each update, but 63mA seems low...

Maybe my meter is so off because it doesn't average pulses very well?

My meter only goes up to 500mA so I can't do the easy thing and just measure total circuit draw.

My question is should I just get a new meter with a higher current capacity, or is there something like an oscilloscope etc. to more accurately measure pulsed current? If so are there any models you recommend?

Thanks in advance.

Chris
« Last Edit: October 16, 2011, 01:12:00 am by Chris Magagna » Logged

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I don't think any normal meter can read this, maybe if it has a peak hold facility.

I'd use a scope to measure the voltage across the current-limiting resistor then consult Mr Ohm about his law.

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If so are there any models you recommend?
Anything will be able to do it, I can't recommend a modern one though.

EDIT: Just noticed you're using a constant-current driver chip, can't you just trust the 5916 to do as advertised?

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Thanks for the scope info. I'll start researching.

BTW I do trust the TLC5916s, they're working great.

I'm trying to figure out if the row transistors are supplying enough current. 74ac138 outputs go to DPLS350Y-13 transistors via 43 ohm resistors. If I did the math right then 24mA from the outputs and a gain of 100 from the transistors will give me 2.4A on each row (for 24 LEDs @ 100mA ea).

The problem is with the setup I have right now is the lights aren't as bright as when I drive them directly with 20mA. I realize I won't be able to get exactly as bright, but I was hoping for more than what I have.

Here's the transistor data sheet:
http://www.diodes.com/datasheets/ds31149.pdf

« Last Edit: October 16, 2011, 02:39:22 am by Chris Magagna » Logged

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So worst case is that a transistor has to drive 2.4A, they have a max pulse rate of 5A so I guess that's OK (Transistors aren't my strong point).

They have an Hfe of 80 at high currents, so the base current has to be 2400/80 or 30mA, can the ac138 provide that much on a single pin?

One way to measure the current would be to break out the TLC5916 GND pin and measure the current flowing out that. I assume that with all LEDs on there will always be 16 LEDs driven at a given time. Different LED but still 16 so you could use that measurement /16 minus the chips quiescent current to see what each LED gets.

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43 ohm resistors. If I did the math right then 24mA
43 x 24 = 1.032 V.   Where it came from?
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@Graynomad - Hfe is 130 @ 2A and 80 @ 3A, so I'm estimating I should be able to get at least 100 @ 2.4A. The data sheet on the 74ac138 (http://www.fairchildsemi.com/ds/74/74AC138.pdf) says it can provide up to 50mA; in another thread (http://arduino.cc/forum/index.php/topic,72866.msg547333.html) Crossroads said it should be able to do 24mA.

I think you're right, if I measure current from a TLC5916's ground pin I'd hope to see about 800mA (8 outputs @100mA ea) but my current meter only goes up to 500mA. I know I need a new meter but am trying to figure out if I should get a new basic multimeter, or if there's something fancier that would let me measure these pulses.

@Magician - sorry if I was unclear. The output pins from the 74ac138s are connected to 43 ohm resistors, which are connected to the bases of the DPLS350Y transistors. I'm using the formula from "Choosing a suitable transistor" at http://www.kpsec.freeuk.com/trancirc.htm ; Rb = (Vc x hFE) / (5 x Ic), so (5 x 100) / (5 x 2.4) = 500 / 12 = 41.67.

Thanks again.
« Last Edit: October 16, 2011, 02:27:30 pm by Chris Magagna » Logged

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Your formula isn't correct for pnp transistor.
Rb = Vr / Ib; Ohm law
Vr = 5V - Vbe - Voic; 5V - power line, Vbe = 0.7 base-emitter drop of transistor, Voic - Low voltage IC = 0.8.
Vr = 5 - 0.7 - 0.8 = 3.5 V
Ib = Ic / hfe;
Rb = 3.5 / Ic / hfe = 3.5 x hfe / Ic = 3.5 x 100 / 2.4 = 145.8 Ohm.
For R = 43 Ohm , output current of IC is : Io = 3.5 / 43 = 81.4 mA
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Thank you for that information. Where do Vbe and Voic come from? Are they standard values or from the specs of the transistors or the 74ac138?

Thanks again!
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Vbe from Fig. 5 datasheet of the transistor you provided above, Voic is standard, Fairchild gives 0.44V / 24 mA  (74AC138SC)
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Thank you again for your help!
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