Reed Relays

hippy

Technical Support
Staff member
The answer will depend upon the relay coil resistance and the in-rush and constant current drawn.

If current drawn is within pin, port and chip limits then it should be possible to drive a reed relay directly.

That advert suggest the coil resistance is 500R so should draw 10mA at 5V so should be suitable for PICAXE use but it would be worth checking the full datasheet.
 

ZOR

Senior Member
Thanks hippy. I will look for manufacturers data sheet as you suggest to make sure. I was going to use standard relays/transistors with Picaxe, but just want very low current momentary switch contacts just as a pushbutton switch would do. I haven't used reed relays before, but if they work saves me board/components space/save.
 

inglewoodpete

Senior Member
The HAMLIN HE721A0510 is a good choice for driving directly from a PICAXE pin - it has a built-in protection diode. Due to the inclusion of the diode in the relay package, you must ensure that you connect the coil the correct way around. Otherwise, you could damage the PICAXE's internal pin drivers.
 

ZOR

Senior Member
Thanks inglewoodpete, I found a datasheet but overlooked it had a protection diode, thanks for pointing that out, avoiding a disaster.
 

erco

Senior Member
Be sure to wire it active LOW to ensure more current flow. Lower impedance that way. Connect one side to B+. It might not even work connected to ground. BTW another nice relay for direct drive is DPDT, an Aromat or NAIS TF2-5E. Watch coil polarity. 1A contact capacity is twice that of most Reed relays.
 
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ZOR

Senior Member
Thanks grim_reaper, just looked at site/prices. Item cheap but want £12.00 shipping + VAT. I managed to get 6, total price including postage for £9, so did okay as long as work when I get them. However thanks for taking time to look. Regards
 

ZOR

Senior Member
Thanks lbenson, sorry I missed your reply earlier. Just told grim_reaper #9 about shipping/vat price on UK site. Thanks for looking. Regards
 

ZOR

Senior Member
Thanks erco, yes they serve a purpose and cheap. However for my application I need small relays on my main board.

What does this mean (from Ebay page/spec)

5V 4-Channel Relay interface board, and each one needs 50-60mA Driver Current.
When they say driver, do they mean from Picaxe output? or just power supply requirement.

Thanks
 

grim_reaper

Senior Member
Difficult to tell from that description Zor. Could mean the whole board needs 50-60mA, could mean per channel, etc.

I've got a 2 channel version of that board at home - I'll check what chip the optoisolator is. I vaguely remember it drawing next to nothing when I tested it though. Obviously the relays themselves draw more power, but not from the PICAXE pin.

EDIT: Looking at a cruddy photo of the board, I think it's a K817P optocoupler. They are indeed rated 60mA at 1.6V. The datasheet shows test conditions of 50mA at 1.6V, but also shows other tests conducted with the current at 10mA... so I guess unless you're pushing it to it's limits, it'll run at < 20mA directly off a PICAXE pin (with a safety resistor too).
 
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hippy

Technical Support
Staff member
What does this mean (from Ebay page/spec)

5V 4-Channel Relay interface board, and each one needs 50-60mA Driver Current.
Most microcontrollers would not be able to deliver 60mA through a digital signal line, would need a driver to do that, which would render having a driver on the module rather pointless.

I would take it that this 50mA-60mA is what the relay coil and driver draws from the power rail when activated.

5V and 50mA would suggest a 100R coil for the relay which would seem in the right ballpark. The only datasheet I could find for an SRD-05VDC-SL-C relay however suggests it's lower than that, 70R, and would draw 72mA. But perhaps the driver makes for the difference.
 

RexLan

Senior Member
Why can't you use a simple FET?

I don't see what it is you are trying to do other than you said a momentary contact.
 

ZOR

Senior Member
Thanks hippy, I will order 1 just to play with and check before sending a Picaxe to the promised land.

Thanks RexLan, I am just wanting to short out something externally. Needs to be totally free of any voltages, just a short circuit, hence relay equivelent contacts closing.
 

erco

Senior Member
Yes, 60 mA sounds like coil current. That particular relay board does use optocouplers, and most of those would use 5-10 mA max through the internal LED to switch the relay through a phototransistor. Other relay boards such as

http://www.ebay.com/itm/5V-4-Channel-Relay-Module-For-Arduino-ARM-PIC-relay-board-new-/231940526299

use switching transistors in lieu of optocouplers, and once again the current requirement to saturate the transistor is low, typ 10 mA or less. That listing shows a schematic with 1K series resistors into the base of 8550 switching transistors, so the current is under 5 mA.
 
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