Hi, everyone.

I’m having issues with a relay (HE721A0500 - see datasheet attached) for a simple test…

The operation is super easy, just send power (5V) to pins 6-2, then the switch between pins 8-14 (and 7-1) closes, thus closing whatever circuit they are part of; stop sending power to pins 6-2, then the switch opens (preaching to the choir, I know, just bear with me).

I programmed a loop on the PSoC microcontroller to output 5V for 2 seconds and stop for another 2, just as a test. With that I’ve been able to power an LED through the relay for 2 seconds on and 2 seconds off, just as expected – so the relay works.

However, when I change the circuit from an LED to a 1.3W (3.7V 350mA) circuit, the relay closes the circuit when it’s supposed to, but does not open again when power is removed from pins 6-2. Even if I disconnect from the PSoC completely and leave the power off from relay’s pins 6-2 it still takes a few seconds or more to open the circuit again. It’s as if the switch gets stuck in the closed position…

I thought that, perhaps, I’m just sending too much current (not that 350mA is that much) and the power is being maintained in the coil inside, but from what I understand the coil should be okay with it. I’ve read through the datasheet (attached) but it seems what I’m doing is just fine and am not exceeding any boundaries.

Would anyone happen to know of anything that can help, perhaps any parameters I’m not aware of?

Thanks in advance

Edgar

 

Reed relays have very delicate contacts that can weld and get stuck.
You generally avoid using them for power switching applications, they are best for signal switching.

Sounds like you might have surge currents in excess of the rated current.
If the thing you are driving has a large capacitance, the current surge when the contacts close can be many times the static draw.

 

You need to do some more diagnosis. Is there voltage across the relay coil when it is surposed to be on the off state but when the 1.3W LED is on ? Does the supply voltage to the microcontroller drop when the LED is on ? I am assuming that both are fed from the same supply. Have you considered using a small logic level mosfet such as an FDS8896 ? (These are claimed to be capable od switching up to 14 amps.

Les.

 

Also, check the wires to your load - do either of them run close to the reed tube? Those reeds are very sensitive and 350 mA is 35X the coil current.

 

Sounds like you might have surge currents in excess of the rated current.

I find that due to the Sales Department [Cheet sheet Fudge factor] I only design at 25% of rated.
if there is any capacitance in the ckt your switching you will be way over the rated switching current of .5a.

 

Everyone, thank you so much for all your replies!

Based on what you guys and my professor have been saying, I think using a bypass cap is the way to go, to help with surges when the power comes on for the load - at least at this testing stage. We've been careful not to let the load wire come close to the relay chip, but not on purpose - we didn't know that could potentially cause trouble - thanks for the heads up!

If we move on with our project (I'm an EE college student) we will probably try a different approach for the switch, as opposed to this reed-relay, like the small logic level mosfet - though I gotta say transistors are were not my favorite subject...

Thanks again, guys!

Edgar