Ready Steady Eddy

[cncJim]

Yes that's what the Charge Pump relay is doing. The BOB is turning a 12khz signal from Mach3 into a 0 or 5vdc output, on my particular BOB the output of ENA is 0v when Mach3 is healthy and +5v dc when it stops.

Of course, what a spanner! sorry that makes perfect sense now.

[EddyCurrent]

There are also 4 contactors which I haven't taken out yet. Are there any practical/safety differences between a relay and a contactor?

With electrical systems there are generally two main circuits, the 'control circuit' and the 'power circuit'. Contactors are regarded as components of the power circuit whereas relays belong to the control circuit, it's mostly to do with the current carrying capacity and the ability to break that circuit without arcing or at least controlling the arcing. There are relays with positive guided contacts and these are for use in safety circuits, I have not come across equivalent contactors but that's not to say they don't exist. I'm not 100% on this but I think relays originated in communications areas such as telephone exchanges. Contactors can have auxilliary contacts fitted which in essence are relay equivalents and are used in the control circuits for those contactors.

[EddyCurrent]

I'm thinking this is going to be my Charge Pump relay circuit. My BOB ENA signal is active low so the relay will be in the 'healthy' state when de-energised, this is not fail safe and I could have inverted the signal but it's going to do. The BOB is supplied by 24vdc and so GND is common to the 5v and the 24V.

[irving2008]

I'm thinking this is going to be my Charge Pump relay circuit. My BOB ENA signal is active low so the relay will be in the 'healthy' state when de-energised, this is not fail safe and I could have inverted the signal but it's going to do. The BOB is supplied by 24vdc and so GND is common to the 5v and the 24V.

You have C and E swapped over. Its an NPN so E is always to -ve rail.

with only 3 or 4 more components you could make it failsafe and i would...

[EddyCurrent]

Irving, you are right, I just threw the drawing together on Paint and I have indeed got e and c crossed over. What would your plan be to make it failsafe ?

Maybe this ? (sorry for poor drawing, no tools at hand on this pc)

[irving2008]

Irving, you are right, I just threw the drawing together on Paint and I have indeed got e and c crossed over. What would your plan be to make it failsafe ?

Maybe this ? (sorry for poor drawing, no tools at hand on this pc)

that'll do it. i'd put a series resistor between BOB and base of transistor, value depends on pull-up on BOB. If there is no pull-up (i.e. its an o/c output) use 10k to 5v rail.

R needs to be something like 2k7 0.5W rating if connected to 24v, or 470R 0.125W if connected to 5v rail.

BD679 is fine as output transistor if a bit of an overkill. Any small signal NPN with a gain >50, Vce >40v and Ic >200mA will do for the input transistor e.g 2N3904, BC337 or similar

[Jonathan]

If the breakout board output is open collector, why not reduce the component count by using a PNP transistor to invert the signal?

e.g:


Edit: Also don't forget the back-emf protection diode!

[EddyCurrent]

If the breakout board output is open collector, why not reduce the component count by using a PNP transistor to invert the signal?

e.g:


Edit: Also don't forget the back-emf protection diode!

That would be a sensible solution Jonathan and one that I shall test, thank you

[irving2008]

That would be a sensible solution Jonathan and one that I shall test, thank you

Be very, very sure the BOB output is truely open collector if Vs = 24v or at best it won't work (I'll leave it to the reader to explain why :) ) and at worst you could fry the BOB!

Alternatively make Vs = 5v

Rb for Vs = 5v is 1k
Rb for Vs = 24v is 4k7

1/4w resistors will be fine

[cncJim]

I'm thinking this is going to be my Charge Pump relay circuit. My BOB ENA signal is active low so the relay will be in the 'healthy' state when de-energised, this is not fail safe and I could have inverted the signal but it's going to do. The BOB is supplied by 24vdc and so GND is common to the 5v and the 24V.

Irving, you are right, I just threw the drawing together on Paint and I have indeed got e and c crossed over. What would your plan be to make it failsafe ?

Maybe this ? (sorry for poor drawing, no tools at hand on this pc)

that'll do it. i'd put a series resistor between BOB and base of transistor, value depends on pull-up on BOB. If there is no pull-up (i.e. its an o/c output) use 10k to 5v rail.

R needs to be something like 2k7 0.5W rating if connected to 24v, or 470R 0.125W if connected to 5v rail.

BD679 is fine as output transistor if a bit of an overkill. Any small signal NPN with a gain >50, Vce >40v and Ic >200mA will do for the input transistor e.g 2N3904, BC337 or similar

R needse 2k7 0.5W rating if connected to 24v, or 470R 0.125W if connected to 5v rail.

BD679 is fine as output transistor if a bit of an overkill. Any small signal NPN with a gain >50, Vce >40v and Ic >200mA will do for the input transistor e.g 2N3904, BC337 or similar

Please forgive my ignorance but could someone explain how this will work in basic terms? Why is Eddy's first circuit not fail safe and how does the next circuit achieve that?

Sorry, I've not had a lot of sleep over the past few nights (babies all have colds....) and I think my brain may be turning to mush.