# Resistor Help

I’m trying to create a board that uses PWM signal to control laser duration (simulating ttl) I found some schematics, but they are for at best 12v input. I am using a 24v system. Using the attached diagram, can someone tell me the correct resistor values for R1,R2, R3, R4.

I believe R1 should be 1125 Ohm (1.5v .2ma for optocoupler), but have limited knowledge of the other side of the equation.

I just need the R values, but really would like to know what each resistor is actually doing and how to figure out the needed Resistor. Thank you for your help.

Pretty crappy schematic. The logic flow is almost impossible to follow, no labels. It is more of a block-diagram than a schematic.

R1 is a current limiter to protect the LED in the optocoupler, but the “schematic” doesn’t show the voltage source for the LED side of the optocoupler, so how do you calculate a current limiter? What is the block under the IRFZ44N? Another IRFZ44N? What does the block labeled “PWM 24V+” contain? What does any of this have to do with Arduino?

There are a number of problems with that circuit.

If we assume that both "blocks" represent IRFZ44s, then the one that actually switches the laser has no pull-down on its gate and will not turn off at all. Actually, as shown it will never turn on. And using one to switch another is entirely unnecessary in any case.

R2 and R3 are in series, so one of them is redundant. You require 10 V to positively switch the FET, so just delete R2 and the second FET.

You need the optocoupler to switch 2.4 mA to the two 4k7 resistors, so you need to drive he LED with at least 5 mA. At 24 V, that would make R1 (24 / .005) another 4k7.

I appreciate your help. the block at the bottom is a on-off-on switch. When in position 1 power comes directly from the buck converter (full power) and in position 2, it is simulated ttl.

The voltage source for everything is a 24volt power supply. PWM is from the board, the block that states 24v power supply is the source.

I have a pass through to the buck converter.

When i remove R4, the test light stays on at full. However, when i leave R4 as the 4.7k, it does not come on at all.

My understanding is R2 drops the voltage from the power supply through the optocoupler, but still dont understand what R3 or R4 is doing?

R4 has several functions.

1. the opto transistor 'leaks' a bit when 'off'. R4 drains that to ground (source), so the fet can fully turn off.
2. gate discharges faster, so less time spent in the 'dangerous transition zone.
3. voltage divider with R2,R3, so not the full 24volt on the gate (Vgs(max) = 20volt).

R2,R3 have the same function and are in the same series circuit, so R2 can basically be omited.

Transistor current is 24/(R3+R4) = ~2.5mA.
The C-version of the opto has a minimum CTR of 200%, so 1.25mA opto LED current would be enough.
But you charge the gate through R3 only, so give it a bit more. About 2.5mA LED current is probably fine.
Opto LED Vf is ~1.2volt at that current.
If you know PWM voltage for the LED, then you can calculate the LED CL resistor.
Leo..

Thank you, that helps me understand it

Wawa:
R4 has several functions.

1. the opto transistor 'leaks' a bit when 'off'. R4 drains that to ground (source), so the fet can fully turn off.
2. gate discharges faster, so less time spent in the 'dangerous transition zone.
3. voltage divider with R2,R3, so not the full 24volt on the gate (Vgs(max) = 20volt).

R2,R3 have the same function and are in the same series circuit, so R2 can basically be omited.

Transistor current is 24/(R3+R4) = ~2.5mA.
The C-version of the opto has a minimum CTR of 200%, so 1.25mA opto LED current would be enough.
But you charge the gate through R3 only, so give it a bit more. About 2.5mA LED current is probably fine.
Opto LED Vf is ~1.2volt at that current.
If you know PWM voltage for the LED, then you can calculate the LED CL resistor.
Leo..

So if I'm understanding you correctly,

Its (24-1.2)/.0025 or 9.1k ohm resistor for R1 to set the forward voltage (PWM is 24volt)
R2- Omit
R3- 4.7k resistor
R4- 4.7k resistor

What does any of this have to do with Arduino?

When searching for the answer, I found this, Computing resistor value for LED - General Electronics - Arduino Forum Figured, it would be a good place to start.

Wawa:
the opto transistor ‘leaks’ a bit when ‘off’. R4 drains that to ground (source), so the FET can fully turn off.

100 nA - nanoAmps maximum.

More to the point, since the FET gate is an insulator, it would never turn off even with no leakage from the phototransistor.