Looking for simple power relay
JBWolf
Posts: 405
Hello,
I am looking for a small (and preferably cost effective) power relay to use with the prop.
I want to make a 'sleep mode' for my camera project. The camera uses exactly 7.5vdc @ 2a.
I have tried a mosfet, transistor and also a ULN2803, but these do something funny to the 7.5v power, the sony camera detects this and says 'not an infolithium battery'.
I can get the camera to run fine off the 7.5vdc power supply when run direct. so I'm guessing I need to completely isolate the power instead of using a common ground for everything.
Just looked through digikey and believe something like this might work:
http://www.digikey.ca/product-detail/en/IM01TS/PB1092-ND/1633978
Any ideas?
I am looking for a small (and preferably cost effective) power relay to use with the prop.
I want to make a 'sleep mode' for my camera project. The camera uses exactly 7.5vdc @ 2a.
I have tried a mosfet, transistor and also a ULN2803, but these do something funny to the 7.5v power, the sony camera detects this and says 'not an infolithium battery'.
I can get the camera to run fine off the 7.5vdc power supply when run direct. so I'm guessing I need to completely isolate the power instead of using a common ground for everything.
Just looked through digikey and believe something like this might work:
http://www.digikey.ca/product-detail/en/IM01TS/PB1092-ND/1633978
Any ideas?
Comments
I suspect any 2 amp or more rated relay will work, if you use a 2N2222 transistor to drive the relay. I guess you want to power the relay at 7.5 volts, which is a bit odd ball as relays are usually rated for 5 volt, 6 volt, or 12 volt. But I suspect a 6 volt relay would work well since the insertion of the 2n2222 is going to cause some voltage drop in the circuit - about 0.7 to 1.0 volts.
You will see a lot of relays contacts rated for so many amps at 120VAC and similarly at 24VDC. DC switching puts more wear on the contacts than AC switching. So you only get a fraction of the voltage that a relay is capable of at 120VAC.
I suspect your problems are caused by internal voltage drops in your choices of switching devices. In all cases, you are getting something less than a true 7.5VDC. The UNL2803 is a darlington pair add drops at least 1.4 volts, Mosfets vary in Rds, and so on. You might be able to use what you have if you can adjust the supply to a higher voltage and verify you are getting a good clean 7.5 VDC output.
I have several Hasco relays that fire directly off a Propeller pin: http://forums.parallax.com/showthread.php/140126-Nice-Relays-for-1.10-ea-Prop-Potential
The relay's contact specs are: 2.5A max. carry 0.5A max. switching
If you want a couple, PM me. I just happen to like the simplicity of low (coil) current relays that can be driven directly by a uC.
Per Loopy, there are plenty of larger relays that can be driven through a simple switching transistor.
2.5A is more than enough, I wont be exceeding 1.5A for this project.
I am not familiar with the term "uC", is that just a prop pin in high state?
@Loopy, I'm pretty sure the 7.5V is the load not the coil voltage.
@JBWolf, You can use 5V (coil) relays with the Prop if you use a transistor such as Loopy suggested. You also need to use a diode across the coil to keep the Prop safe from a voltage spike when the relay is turned off. Let us know if you need help with the circuit.
This leads to problems with solid-state switching as people persistently seem to ignore or not know that there is a voltage drop involved with solid-state switching in all cases. Mosfet is the least voltage drop, but the most difficult to drive directly from 3.3v logic.
A relay is the right solution if the voltage is going to drive the coil and the output load. And a 2n2222 transistor will handle 500ma of current coil load or less. Most relays seem to demand 100ma or less.
In other words, by working with an adjustable bench supply before you finalized the supply voltage, you can actually get exactly what you want in output via a complete solidstate solution. Of course, the draw back is creating a supply to an odd ball voltage requires some device that wastes more power.
And I try to NOT drive the Propeller i/o at 25-30ma, just because it is marginal. I always use a transistor to handle the coil load and to isolate the Propeller from coil transients. but there are a lot of people that think driving a coil directly from a microprocessor is just fine.
If you do drive a 5 volt coil from a 2n2222 with a 5 volt supply, the coil is going to get 4.3volts or less. Therein lies the problem again, voltage drops in solid-state switching. Coils tend to be tolerant of a bit of over-voltage as the insulation and cooling are pretty good. So I suggested an over-voltage to the coil to assure it fully pulls in.
BTW, a 2.5 amp rated relay with 0.5 amp switching is unwise. You need a full 2.0 amp or more switching as the wear is most in the switching.
The relays work perfectly with the propeller.
The threshold is perfect for high/low state on a prop pin.
I am directly driving it with a capacitor to buffer any fluctuations.
I have tested it out with led's, input buttons, motors driven from a uln2803 and even an xbee running off of other prop pins and none have caused the relay to hiccup even once.
Best of all it fits nicely on a breadboard for prototyping.