From 361f369c29ebebe13df2294d0742608a5772c2f0 Mon Sep 17 00:00:00 2001 From: Eddy Pedroni Date: Mon, 3 Nov 2025 13:19:57 +0100 Subject: Relay driver design documentation --- relay-driver-specs.md | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) create mode 100644 relay-driver-specs.md (limited to 'relay-driver-specs.md') diff --git a/relay-driver-specs.md b/relay-driver-specs.md new file mode 100644 index 0000000..a26e3df --- /dev/null +++ b/relay-driver-specs.md @@ -0,0 +1,34 @@ +# Relay Driver Specs + +## Relay Characteristics + +* Model: SRD-05VDC-SL-C +* Internal resistance: 70 Ohm +-10% +* Nominal voltage: 5 V +* Nominal current: 71.4 mA +* Minimum pull-in voltage: 3.75 V + +## RP2040 IO Characteristics + +* Logic level: 3.3 V +* Max pin current: + * 50 mA across all pins (measured here: https://forums.raspberrypi.com/viewtopic.php?t=300735) + * with 2 relays, ~20 mA per relay + +## BJT Requirements + +* Saturation Vce < 1 V +* hFE > 3 + +2N5088: Ic a bit high considering the max is 100 mA +BC549: same as 2N5088 +2N3904: seems suitable + +## 3N3904 Prototype + +With a single relay and 2k base resistor: + +* on current (emitter): ~60 mA +* base current: 1.3 mA +* on relay voltage: 4.3 V + -- cgit v1.2.3