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By Athul
#189982
Hi,

I've been using a bidirectional logic level converter in my CO2 laser cutter to convert 3.3V signals in to 5V signal, but I'm having some problems with it.

For emitthing laser following condition needs to satisfied
TL<0.3V
WP<0.3V
IN= 0-5 PWM

I'm using smoothie board to control the machine,http://smoothieware.org/, which produces pwm signal of 3.3V,, and my laser PUS requires 5V pwm http://www.laserpsu.com/laser-power-supply-p-767.html

But 3.3V pwm from smoothie only giving me 4.04V at the output when it's connected to IN terminal
1.jpg
and when it is disconnected it gives me 4.98V.
2.jpg
Anyone have any idea about that???


I've tried my best to explain with some pictures, please take a look\



Thanks
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By DanV
#189984
It's possible that the "5V Output Power Output 5V, the maximum output current is 20mA. " spec from your laser power supply is not enough current.

That 5V has to not only power the level converter, but also provide the current to drive the TTL level input.

But 'TTL' level says more about the voltage than the current draw, so you could put the meter (on mA setting and probably need to move one lead on your meter) in the "5V to HV" circuit path and see what the current is.
By Athul
#190001
@DanV
Thanks for the reply.

My first doubt was that, so I tried to power the HV side of LLC from an external DC power supply capable of supplying current upto 5A.
Untitled.jpg
See the connection diagram and please let me know if there is any problem with it. Shouln't I connect all the GNDs together?

But the result was same, without connecting to IN, I got 4.95V and when connected to IN , voltage reduced to 4.04V.

Also I tried one more thing, 5V is given to HV (no LV, LV1, LV2, LV3, LV4) and measured voltage across all the HV cahnnels(HV1, HV2, HV3, HV4) with respect to GND, I got 4.95V in two channels and 4.8 and 4.35 in remaining channels(without connecting to IN). But when I made connection to HV , it reduced to 4.04V.(see image)
4.jpg
Also what will be the voltage across LV and GND when 5V is applied between HV and GND(no LV1, LV2, LV3, LV4, HV1 ,HV2, HV3, HV4)
5.jpg
I'm not an electronic guy, but from schematics and from what I know , I think it'll be 0.
http://cdn.sparkfun.com/datasheets/Brea ... tional.pdf

Since, without any gate signal, source and drain won't conduct.
That 5V has to not only power the level converter, but also provide the current to drive the TTL level input.
5V has to power the LLC? How?

When LV1=0 gate and source become forward bias, OV will be available at HV1, and when LV1=3.3V, MOSFET won't conduct and 10K pull up resistor will give 5V to HV1. So how come 5V used for powering LLC.

May be 10K resistor will be reducing 20mA current given by the 5V. Is there any chance for that?
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By Athul
#190063
I figured out what's the problem.

10K pull up resistor on the HV side of logic level converter was too large in my case. I figured this out by doing,

1. connected 5V to IN terminal by a wire , measured voltage across it. Got 4.95 on DMM.
2. Connected 10K between IN and 5V got 3.7V if TL is not grounded and 4.04 when grounded. What I need is 5V when TL is LOW.
3. Connected 1K between in 5V and IN , got 4.8V at both TL disconnected from GND and at TL is LOW

I think this is due to the high input impedance of IN terminal

So replaced 10K with 1k in logic level converter and I'm getting around 4.8V for S1(100% duty cycle)

I'll check with CD4040 and https://www.sparkfun.com/products/11771 and see what'll happen in case of these