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This is probably a stupid question but I have been searching everywhere and cannot find a solution that works.

I am trying to come up with a circuit using transistors that acts as a SPDT switch to toggle from being powered by one supply to another. The switch enable signal is being supplied by a Raspberry Pi which has very limited current capability (16mA max) so FETs are preferred over BJTs. The device being powered draws around 100mA.

Here is what I am going for:

schematic

simulate this circuit – Schematic created using CircuitLab

I have pondered using relays, but I am going for as small of a footprint as possible and relays are rather large (and expensive, especially if using SS relays).

My original intent was actually to use a quad bilateral switch (CD4016BE) but the current capacity just isn't there.

EDIT: I think this is the route I'm going to take.

schematic

simulate this circuit

(with BS170 as NMOS device and IPP45P034L11AKSA1 as PMOS device)

Thanks for all of the help.

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  • \$\begingroup\$ Are both supplies always present together? \$\endgroup\$ – Andy aka May 26 '16 at 16:35
  • \$\begingroup\$ Yes. The reason for switching between the two is that the device runs off of the 5V supply in normal use, but when re-programming needs to occur, it switches to the secondary supply of 7V provided by a USB device with communication riding on top of it (hence the 7-9V). \$\endgroup\$ – cjswish May 26 '16 at 17:02
  • \$\begingroup\$ So why not use a small 5V regulator powered from the 7V supply permanently? No switching needed. \$\endgroup\$ – Andy aka May 26 '16 at 17:16
  • \$\begingroup\$ @Andyaka. That is within the Vdrop of a LM7805. As long as the second supply can maintain 7 volts or more. \$\endgroup\$ – user105652 May 26 '16 at 17:24
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    \$\begingroup\$ So when I asked if it were permanently present and you said "yes" that was kind of misleading and wasting my time which I give free no? \$\endgroup\$ – Andy aka May 26 '16 at 17:28
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Here is a relay that might work for you, although it does cost $1.78.

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  • \$\begingroup\$ Awesome! Did not know these types of relays were out there. I think I'm going to use this one (digikey.com/product-search/en?keywords=tlp222af). It's a little cheaper. I will probably use an NMOS and PMOS to create the SPDT effect I am going for and can keep the current WAY down. Thanks! \$\endgroup\$ – cjswish May 26 '16 at 18:48
  • \$\begingroup\$ Good find. Your's is a better choice electrically as well. If you're going to create your own SSR using MOSFETs as shown in the datasheet, use the same type for both FETs. \$\endgroup\$ – Andrew W. May 26 '16 at 19:59
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Study the voltage switching circuit on the Arduino board. Like the Uno and recent boards which automatically switch between the USB power and the barrel connector power. They use FDN340P to do the switching.

Ref: https://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf

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  • \$\begingroup\$ Thanks for the advice, I will take a look at this a little deeper when I get a chance. \$\endgroup\$ – cjswish May 26 '16 at 18:50

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