Author Topic: somebody can explain the tr in the rfduino relay shield?  (Read 480 times)

whiwha

  • RFduino Newbie
  • *
  • Posts: 3
  • Karma: +0/-0
    • View Profile
somebody can explain the tr in the rfduino relay shield?
« on: July 25, 2016, 11:54:03 PM »
The rfduino relay shield use

PDTC114YU transistor which R1=10k, R2=47k.

But the output of gpio from rfduino is 3.3V, that mean, (3.3v-0.7v)/10k = 0.25mA, this would be base current to the transistor.

hfe is normally 20 (on saturation situation), then the collector current would be 0.25mA x 20 = 0.5mA..am I right?? but 0.5mA would be very small current to operate relay. I measured the relay needs at least ~20mA. So it is very strange. Somebody can explain for me?

tolson

  • Global Moderator
  • *****
  • Posts: 812
  • Karma: +19/-0
    • View Profile
    • Thomas Olson Consulting
Re: somebody can explain the tr in the rfduino relay shield?
« Reply #1 on: July 26, 2016, 09:23:26 AM »
The rfduino relay shield use

PDTC114YU transistor which R1=10k, R2=47k.

But the output of gpio from rfduino is 3.3V, that mean, (3.3v-0.7v)/10k = 0.25mA, this would be base current to the transistor.

hfe is normally 20 (on saturation situation), then the collector current would be 0.25mA x 20 = 0.5mA..am I right?? but 0.5mA would be very small current to operate relay. I measured the relay needs at least ~20mA. So it is very strange. Somebody can explain for me?

Hi WhiWha,

1). 0.25mA x 20 = 5.0mA, not 0.5mA.
however...
2). The specified hFE is 100 minimum @5V, 5mA. The 20 you mention is a forced test condition configuration at the factory to obtain a parametric value for vce(sat) used for comparisons. It is not meant to be thought of as the hFE of the transistor.

So the first aproximation of available current is more like 0.25mA * 100 = 25.0mA. The maxiumum allowed collector current is 100mA. So there is plenty of gain to drive the given relays.

Have you measured the true hFE under the actual design configuration? Are you having problem getting the relays to turn on?
« Last Edit: July 26, 2016, 09:31:57 AM by tolson »