Author Topic: Anyone experience with 28V microwave relays ? Bought one, coil current ~ 150mA ?  (Read 1045 times)

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Offline prutser

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I have no expererience  with microwave relais. Bought 2 used 18GHz relays. One from Sivers/Philips, draws about 40mA  @ 28V
The other one from Microwave Associates (brand seems to be sold regularly on Ebay, but I cannot find any details)
Coil does 160mA @ 28V which is over 4 Watts.

Does anyone knows if this is usual for those kines of relays ?

Offline TheMG

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It can be normal depending on the design and size of the relay. Some have quite large and multiple mechanical moving parts that require a stronger coil.

Offline Bud

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Maybe it was designed for fast switching with higher energizing voltage and then to lower holding voltage.
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Offline radar_macgyver

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Some include a freewheel diode internally (HP/Keysight relays in particular), so polarity matters for those. If forward biased, current draw will be high.

Offline Whales

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Are the eBay ones cheap?  Crack one open and see what's inside.

I'd also try and verify that their coils have been wound correctly (thickness of wire + number of turns) for 28V.  Try hooking up the coil inputs to a power supply and slowly turn up the voltage until the relay clicks.  If it moves at only 3V then you know it's wound incorrectly, if it latches at a bigger fraction (eg 12V?) of the full 28V then it's probably wound correctly for 28V.

Offline cj

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It might also be a bi-stable relais which only requires a short pulse to switch after that no power is required anymore. The polarity of the pulse determines which port is selected.

160mA @ 28V is excessive, as Bud says it may require a lower holding voltage.

I just measured a 28V SMA relay (Transco) and it draws 80mA @ 24V which should be about 90mA @ 28V and it holds fine at 12V 40mA.


« Last Edit: January 19, 2022, 10:15:26 pm by cj »

Offline jonpaul

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Most of the RF relays I have  do not include any  diode.

Check the resistance of the coil.

Perhaps the high current relay is not 28V but 12?


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Offline Joel_Dunsmore

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Check if it is a latching relay (someone mentioned bi-stable).  You will can switch it then remove the drive voltage and see if it holds the state.  If it does, then you can just send a pulse and to switch and switch back.  Some have both latching RF and DC (Keysight switches work this way) where there is also a relay on the DC side so when you apply the drive voltage, the relay snaps across and the DC contact is broken as the switch is moving. A couple of tests will sort out if you need to drive them all the time.

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