Brake Transistor / Diode

Question to follow, having problems posting.

I have a 24vdc normally closed (voltage released) brake (draws 1.6A )that I have selected the following to interface to my dsPIC microcontroller:

SCHOTTKY RECT 20V 0.5A SOT23 NXP Semiconductors PMEG2005ET T/R

OPTO AC/DC 60V 2.5A 6-SMD Panasonic Electric Works AQV252GA


Does anyone have any better suggestions, I am trying to get used to the components needed to interface, optoisolate and protect my microcontroller (dsPIC) projects to higer voltage systems (24vdc mostly)

I also used a clare LA110 in place of where i should have used a relay, and I think my problems are due to the on resistance of the product. the system is closing a common on a outside relay, and i measure 35 v on the input to the LA110 (outside), but the microcontroller is correctly causing the relay to work because i un hooked it from the ouside relay and drove an led without problem...any ideas on a better product for this application?

If anyone is wondering, the reason i am not just using relays (they work very well) is that i am simply out of room.

The LA10 can only pass need 120mA and you need probably need more to switch your relay. The datasheet says the max resistance of the LA110 is 35 ohms which may require too high voltage to drive the required current to energize the relay coil (not including the resistance of the relay coil itself).

HOw about using the PIC to drive an opto-isolator which in turn drives a simple MOSFET which in turn energizes the relay coil? It's not too hard to get a MOSFET with a really low on resistance. YOu would also need a schotky diode in anti-parallel with the relay coil to protect the MOSFET from the inductive flyback of the magnetic coil when disconnecting current from it.

are you saying that the first part is (the brake app) and i could use it for the second too (the relay app)? Maybe i should have separated the two better.

I edited my first post though to answer only the second part of your question. I could tell you more if you could tell me what the coil resistance the relay is and what current (and/or voltage) is needed to activate it.

It would help if you could tell us what you actually did with the opto-isolator and the diode. I just don't see an opto-isolator providing enough current to drive a brake (unless the brake had some other power source driving it and the opto is just an activation signal)

Brake problem:

Yes, the brake is held in the released state by a solenoid.

I have tried to select components to do exactly what you are saying, the AQV252GA is listed on digikey as : RELAY OPTO AC/DC 60V 2.5A 6-SMD

and the data sheet shows it as an opto isolated pair of mosfet transistors: see info here

(http://pewa.panasonic.com/pcsd/product/pmos/pdf_cat/aqv252g.pdf)

and the second item is a schottky diode. what i don't know is if i sized them correctly for the first app (brake)

Relay problem:
First, the relay is inside a box that i can't open. The manufacturer of the product won't help me to fix the problem, they say get a relay.

So there is two wires coming out of a black box, and when you connect wire 1 to wire 3 the motor runs forward, and wire 2 to wire 3 and the wire runs in reverse. there is 2.7khm: of resistance between 1 and 2.

i just went and measured the resistance of the relay in the off state and it read an infinite resistance (Overload) but i could hear a clicking like the relay coil had energized, when i touched the leads, but the motor that the relay drives did not come on.

then i placed a 100 ohm resistor between 1 and 3 and it started to ran, same for 1 and 2. so i tried to measure the voltage drop from 1 to 3 and got 36 volts while not running and then repeated the same test and when my voltmeter leads touched the wires, it started to run.

then i put the resistor across from 1 to 3 and read the current across the resistor to be 33 mA.

Does that help at all????

Thanks for all your help.

If the solenoid requires less than 60V and 2.5A to run, then your opto relay should work. Your diode is also probably good enough to protect the opto relay from the motor's inductive flyback.


Are the 3 wires the relay wires? Motors wires? Or do you not know? THey just come out of a black box with a relay and motor inside? My guess is just that the LA110 simply cannot drive enough current to energize the relay coils to switch them- 120mA is pretty frugal. DO you know what voltage you are supposed to apply to the relay coils? You might need to apply the voltage to energize the relay coil and measure how much current it is drawing.

ok, good to go on the first.

relay.
According to the mfg. wire one and two are the low side of relay coils. wire three is gnd/common. so if wire one or two are connected to 3 then they are now at gnd and current flows through the solenoid, energizing it. I cannot measure the current draw across the solenoid b/c i cant access the positive side . that is why i put the resistor in parallel and measured current.

How low of an on resistance should i shoot for???

the bes thing i found so far is:this
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=FDS9958CT-ND

You don't need the positive end to measure current flow, just stick an ammeter between wire 1/2 and wire 3. Then you know how much current it takes to energize the relay coil (if I understand you properly).

Or just try this for now:
The 35 ohms of the solid state relay you are putting between wires 1/2 and wire 3 sounds like it's just limiting current too much. Just try a regular transistor (made easier since wire one and two are on the low side) and opto isolate to protect the PIC.

Thanks, I will order parts and give it a try. I appreciate your help.