hello
am using relays to control high power motors but need to make very fast on off pulses (20ms bursts). How long do they last?...it's meant to be installed for 6 months, so will be used everyday
cheers
K
am using relays to control high power motors but need to make very fast on off pulses (20ms bursts).
I don't think that mechanical relays that are able to switch that fast at all. The usual contactor for a 4KW motor (which would I regard as the smallest size for high power) needs about 100ms to switch the motor current on and off.
What you should be looking for is a SolidState Relay. They have no mechanical parts.
How long do they last?...it's meant to be installed for 6 months, so will be used everyday
I would place my bet at 2 weeks 
BTW:
What the purpose of switching the motor current that fast ? There might be better solutions around ...
Eberhard
eek. Install on Saturday.
Ok, so we are using relays to switch 2 things (not high power):
Screwdriver motor 2amp 12V (fast switching 20ms burst).
Mains power to answer machine (250ms switching, once every 3 seconds (for 5 min periods on then 30mins off)).
For other elements of our installation (with lower power) we have been using tip120's & H-bridge circuts (L292D), which drive the relays.
We're looking at ssr's. Will these one's be good to use in this instance....
4-32dc control voltage
12 mA input current
4kV isolation voltage
24-265 load voltage
1.6 max on state
25A rms max load current
350 or 150 rms min load current
1.2dc release voltage
cheers for any advice anyone can lend. I think I have travelled down a cul-de-sac
K
P.S we are switching the motor that fast to create ratlting patterns - the motor drives a crank which moves some files around in a filing cab.
As I understand your last post, the 20ms switching applies to the 12/2A thing?
Is that motor running on DC or AC?
I think the ssr you pointed out, will not work for 12V (24-265 load voltage).
At http://www.conrad-direct.co.uk I found
this thing :sharp S202T01 in the solid state relay section.
But I must admit i don't have that much experience with ssr's, so I'm not sure if this one will do the trick.
If you have to stay with relays I would at least use a type that comes with a socket and have someone change them whenever they die.
Here in germany I would go for relays made by "Finder". That company operates worldwide and is not too expensive!
If your application really runs 24-7 with the relay switching on-off every 20 milliseconds that adds up in half a year to:
25 Switches p. sec * 3600 secs in an hour * 24 hours * 180 days = 622.080.000 switches
Oh poor relay
Eberhard
hi
solid state relay's won't work unless they are the Mosfet version... make sure to get one that is specifically deigned to switch DC loads. The ones that use SCR's inside of them will just latch in the "on" state.
Crydom makes a DC output solid-state relay that switches 20A, with on time of 100 Us and off time of 1Ms:
Crydom part D1D20. Digikey has it.
You might want to check out the electronic speed controllers at the hobby stores, the ones used for radio-controlled cars. they take a servo signal input, and the output can control the speed of a high-current motor. About $50.
You could also use a big transistor, but you'd have to have really good noise clamping. Actually, you have to have really good noise clamping for any switching method! Big, fast diodes (Schottky, 10A, 200V), a few bypass capacitors (1000UF, 35V & .1UF, 35V) and maybe even a choke in line with the power supply to the motor.
D