ok, had momentary brain fart... was thinking the pnp base resistor in series between two transistors.
Like this (R3 is opto current limit resistor):
the engineers that manufactured and implemented these high current optos surely also designed a non discreet component to drive them at a reasonable current level? ....... The PIC is happy at 25ma but what about 45ma, Is there an IC that can operate multiple I/O's at the increase in current using non discreet electronics?
Sorry, but that's complete nonsense
Check my PIC 'extras' for an example of high side switching using NPN/PNP.
But the simple emitter follower suggested earlier (post #5) would do all that's required, as you're only powering a grounded LED.
You can always parallel two pins together to give double capacity
But as for 'design', if you design things correctly then such problems don't occur - as for a suitable IC, the much mentioned ULN2003 would do it if you didn't ground the opto-couplers (and any sensible design wouldn't cripple itself by not offering the option).
And yet again, simple emitter followers would do the existing job anyway, and you can (or certainly could) get multiple transistors in IC type packaging.
Sorry, but that's complete nonsense
Check my PIC 'extras' for an example of high side switching using NPN/PNP.
Aren't the transistors in the wrong places? Doesn't the Vb of the PNP have to be at least .7v less than the emitter?
Actually, your post is utter nonsense. We were talking about a way to give a positive output without the .7V loss. We both understand you don't have to worry about the .7V drop using an NPN to turn on a PNP because you are driving an LED that's grounded. That's what my post #6 commented on that using the NPN to drive the PNP would drop .7V, and Crutchow mentioned an NPN follower and PNP follower wouldn't.
He's trying to interface to an existing board, so the optos are already grounded.
And then you suggested the dual follower from post #5 would do all that's required. So show me the schematic of this follower pair, if mine isn't correct... and by the way, mine would work nicely, giving a hi out for a hi in, positive logic like he wanted. What is your better suggestion (for the follower pair)?
Please read the entire thread before picking apart a post 'out of context'...
Do you have a schematic of the cheep chinese motor driver board?Hey guys, don't get your shorts in a bunch....... This was a very good learning thing for me..... You know whats a POOR design..... This crappy Chinese controller...... I have decided to modify whatever is wrong with it or throw it away...... because It is flawed does not work with higher current either!!!! going remove the improperly connected optos. and go right to the chip, which is pulled low internally, and accepts 5v inputs...... all common grounding anyways, and somewhere along the line they have the inputs pulled high......Its a simple design.....I don't know how they could have screwed it up that badly...... thought it was lack of current, but looking much, much more messed up than that!!! arg...
Hi Mike,ok, had momentary brain fart... was thinking the pnp base resistor in series between two transistors.
Like this (R3 is opto current limit resistor):
Active low is used a lot because nothing bad happens when a board is disconnected. Take a motor controller with a DIRECTION and NOT ENABLE input.
Low causes the motor to move. Disconnect the signal and the motor doesn't move.
Now say your driving the chip with a CPU. The pins are normally tri-stated or configured as inputs. So, what happens, again nothing.
Some problems do arise if a standard TTL gate is used. When disconnected it may be the same as floating high. This can cause problems with some circuits.
With the ULN2003 or a simple transistor, you get DISCONNECTED=OFF, LOW=OFF and HIGH = ON.
motor moves, with NOTHING connected!!!! then power opto, it stops. I would be happy to send this ILL designed controller to someone if they feel the need to be irritated. p.s my controller is active High then (-) low or visa versa +5v - low (-)Active low is used a lot because nothing bad happens when a board is disconnected. Take a motor controller with a DIRECTION and NOT ENABLE input.
Low causes the motor to move. Disconnect the signal and the motor doesn't move.
Now say your driving the chip with a CPU. The pins are normally tri-stated or configured as inputs. So, what happens, again nothing.
Some problems do arise if a standard TTL gate is used. When disconnected it may be the same as floating high. This can cause problems with some circuits.
With the ULN2003 or a simple transistor, you get DISCONNECTED=OFF, LOW=OFF and HIGH = ON.
I do not have a schematicDo you have a schematic of the cheep chinese motor driver board?
looks as low current?Use a high side driver: https://www.st.com/web/catalog/sense_power/FM1965/SC1037/PF63388 This one is active low too, so you would need an inverter as well.
I smell smoke........once you let it out you cannot get it back in there......3 channel; +-5 to +-30 uA is what I get. There are lots of other "high side drivers" out there.
Hi Mike,
What program do you use to draw your schematics?
My name is Robert.
Thanks Mike.Robert, Hi,
I use PADS Logic for schematic entry and PowerPCB for pcb layout.
The suggestion from post #5 is a simple emitter follower, and is all that's required, the 0.7V loss is meaningless for driving an LED - I've no idea what you mean by 'dual follower'?
A simple solution would be to use a small NPN transistor (such as a 2N2222) as a emitter follower buffer but that reduces the output voltage by about 0.7V. If that's a problem then you could use an NPN driving a PNP.
The example high side drivers in my tutorial would do it without the loss, as the link already posted.
I never took it out of context, it was an exceptionally poor 'solution'.
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