# Issue with a PS2501-4, I cant seem to get it to work :(

#### pythonholum

Joined Jun 9, 2014
3
Hello, I am trying to use a PS2501 to help make an H bridge ( using http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html as a base design )

I am attempting to get one leg of this working first before going on to the rest,
Here is a hand drawing of how I have everything hooked up

If I connect point a to point b my motor turns on, but even though there is power to the diode side of the PS2501-4 it does not turn on through it

Any help would be greatly appriciated, Thank you

Oh one last thing here is a picture of the bread board how It is set up right now
Thank You

Color key
Brown 5v+
Brown/White ground
Orange 12v+
Orange white ground
blue and blue white go to the 2 terminals of the motor

There are 2 green white cords, 1 goes from pin 9 of the PS2501 to ground and the other goes from +12 to the firthest pin on the tip107 ( I beleave it is pin 3 )

https://www.sparkfun.com/datasheets/Components/ps2501.pdf

#### praondevou

Joined Jul 9, 2011
2,935
Did you try any of the other optocouplers inside the 2501?
Is it really a 470R resistor and not something else? (can't be seen on the picture)

What is the voltage drop from A to B when the 5V is connected? What's the voltage drop from emitter to base of the TIP?

#### djsfantasi

Joined Apr 11, 2010
5,620
In reading the data sheet, it appears that 470Ω is too large. To calculate the resistor value, you need to know the signal voltage (V=5V), the forward voltage of the diode (from the data sheet, Vf=1.14V), and the current required by the diode (from the data sheet, If=80mA). Then calculate R.
Rich (BB code):
R=(V-Vf)/If
Plugging in the above values, R=48.25.

Either a 47Ω or 51Ω resistor is close. Also, it is interesting (perhaps only to me) that this is one tenth of the value in your sketch. Also, note that on the data sheet the resistor used for the test circuit is 50Ω.

Last edited:

#### pythonholum

Joined Jun 9, 2014
3
I will have to try a different resistor, I was going off of a scimatic I found online for an H bridge, and didnt double check anything ( and honistly I dont rilly know how to read a spec sheet other then getting just the basic info )

http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html

I did figure out thought how to build my H bridge without the opical issolator and that is working now I basicly took the point A of this one and hooked it up to the emitor of my tip102 so that it is brought to ground when the tip102 is turned on and it seems to work well for the basic test's I ran, I need to get some heat sinks and transfer it to some perf board

#### pythonholum

Joined Jun 9, 2014
3
Also i did try removing the resistor completly, That worked for a 10 seconds ( Then I am guessing I blew it up becoust it never worked again ) But on my next shipment from digikey I will get a few more to play with allong with some 50 ohm resistors

Thank you both for your responses and help

#### AnalogKid

Joined Aug 1, 2013
8,109
The max diode current is 80 mA, but the test current is 5 mA. It looks like OP is running 6-7mA. CTR is 80 min., 300 typ. I know this part, and my experience is 200 min. with diode currents of 10-20 mA.

ak

#### THE_RB

Joined Feb 11, 2008
5,430
TIP107 is a NPN darlington, pinout BCE looking from the front.

All you needed to do was swap the GND and 12v wires to the TIP107, you had them reversed.

#### JohnInTX

Joined Jun 26, 2012
3,810
The max diode current is 80 mA, but the test current is 5 mA. It looks like OP is running 6-7mA. CTR is 80 min., 300 typ. I know this part, and my experience is 200 min. with diode currents of 10-20 mA.
ak
+1.
Also, change the 10K to something big, all its there for is to swamp any leakage current from the phototransistor. For a Darlington, 68K-100K is fine. Also, move the resistor to the other side of the 1K.

It should work OK:
TIP107 has a high gain so you don't need much current from the base. With the 1K you can pull 12Vsupply - 2.8Vbe(on)-.3Vce(sat) / 1K = 8.9ma (ignoring the small current through the new, relocated 68K-100K resistor).

8.9ma * 200 hFE(min) = 1.78 amps(minimum) to the load. IF that's OK,

8.9ma(Ic-opto) / .8(min coupled CTR) gives 11.125ma If(LED).

(+5V - 1.4Vf(max)) /11.125ma ~= 323ohms for the LED resistor. Call it 300.

This is rough but conservative. If you need more load current, you'll have to pull more from the base of the TIP (reduce the 1K) then adjust the LED resistor accordingly. Since you are way below Iforward(max) for the LED, you could settle on something like 180-200 ohms to ensure enough Ic in the PS2501 photo transistor.

Good luck.
EDIT: if you are driving a coil (relay etc) be sure to install a diode across the coil - banded end to +12V to protect the transistor.