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7x20mm relay tests

I'm going use an insane number of 7x20mm SPST relays, so I'm testing relays and drivers.

Here's a start.

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The waveforms are drain voltage and source current.

Two surprises: that little 100 volt logic-level mosfet avalanches happily at just over 100 volts.

The drain voltage doesn't ring. The Q of that relay coil must be terrible.

The coil is about 720 ohms and 1 henry un-energized. I assume it's more when it's closed.

More testing today, contacts too.

I think I can use one mosfet per relay on the bottom side, tucked between the relay pins, and drive the fets from individual FPGA balls, with no catch diodes. Let it avalanche.

AoEx claims that modern mosfets avalanche happily as long as the dissipation is reasonable.

Reply to
John Larkin
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Paywalled. And typically useless.

Reply to
John Larkin

Well over 30 years ago I had designed an ISA bus card MCA with a 4.5kV HV source - no multiplier, straight flyback. (

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, the HV module would plug into the missing corner, a metal box with a core inside same as the one visible, totally potted which was overkill of course). I had put an IRF540 as a switch and at some point someone messed with the coil, put more windings in the primary - which made them see power reduced somewhat, testing well below 5kV. Then - shortly before I quit to start my company- I noticed the HV module was getting very hot but I ignored that, it was usable after all. I don't remember how and when I discovered the 540 was going avalanche every cycle at somewhat above 100V, hence the heat. May be it was on one of my subsequent HV sources. Then I remembered why initially I had put an IRF840 (much higher D-S max. voltage)... The moral of the story is that the IRF540 lived, no time limit to talk about. Then on a much newer HV (<15 years old) I put some other MOSFET, not looking if it had an avalanche spec, 100V D-S max again. It would barely see >100V but if it saw it would just die... So I went irf540 again, never heard of it since. The way your waveform looks I think you won't hear of your MOSFETs either (obviously I don't know that, unless they are IRF540-s... ).

Reply to
Dimiter_Popoff

That's deliberate, I trust: relays have builtin intrinsic hysteresis (that's why hold current can be lower than make current), and high frequency coil response is not required any more than recovery of stored energy. The only exception would be AC-coil relays, where AC losses could cook the steel core.

Reply to
whit3rd

I just stuck in that fet because I had some handy. But it works nicely.

It does fly back from 12 to over 100, so the inductor isn't really bad.

Reply to
John Larkin

Added a contact test.

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Not bad for a $1, 5-amp relay.

Reply to
John Larkin

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