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I have a simple switch circuit that is powered with a single 1.5V alkaline battery. I want to find a MOSFET(or other device) that will switch on when the battery voltage is applied. I can find plenty of FETs with a Vgs of 1.5V but none that seem to switch on properly if the battery is a bit flat (ie 1V).

So I'm after Vgs of less than 1V, drain current is less than 100mA, Vds is the battery voltage, ie 1 to 1.5V.

Any ideas?

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    \$\begingroup\$ Can you use a BJT instead? \$\endgroup\$
    – The Photon
    Commented Aug 22, 2018 at 16:27
  • \$\begingroup\$ There are special boost controller which can start with ~200 mV if you are willing to increase the complexity a bit. \$\endgroup\$
    – winny
    Commented Aug 22, 2018 at 16:27
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    \$\begingroup\$ What advantage is the switch if all you need is a direct contact to the battery \$\endgroup\$
    – D.A.S.
    Commented Aug 22, 2018 at 16:42

2 Answers 2

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You could use something like a MCH3383 which is a P-channel MOSFET that is guaranteed to have less than 0.5 ohm Rds(on) at 0.9V.

Or perhaps the RYM002N05T2CL which is an N-channel part that has less than 9 ohms Rds(on) at 0.9V.

There are others. Try going to a distributor website and do a parametric search, then read the datasheet thoroughly (the distributor sites often contain errors and are best used as pointers to the manufacturer's data).

Another approach that is frequently used with a single-cell power supply is to use a boost regulator to increase the cell voltage to a more appropriate voltage such as 3.3V or 2.5V. Chips such the TI TLV61225 can do this (ON Semi has a number of them too). Keep in mind that the current drawn at the higher voltage will be reflected by a proportionally higher current from the battery (eg. 100uA at 3.3V will be more than 330uA from the battery at 1.0V).

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  • \$\begingroup\$ MCH3383 onsemi.com/pub/Collateral/MCH3383-D.PDF looks like a good one. Nearly 500mA Ids at Vgs of 0.8V per chart at upper left of page 3, ID - Vs. Available at Digikey. \$\endgroup\$
    – CrossRoads
    Commented Aug 22, 2018 at 17:40
  • \$\begingroup\$ The MCH3383 looks perfect. I didn't find anything like it when I did my searches. Thank you. \$\endgroup\$
    – NickyJ
    Commented Aug 23, 2018 at 9:59
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NPN BJT could be the way to go. Current flow then is dependent on the base current vs the base voltage in a MOSFET.

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    \$\begingroup\$ If the OP wants to switch on power from the battery to a load it may be better to use a PNP BJT. \$\endgroup\$
    – Michael Karas
    Commented Aug 22, 2018 at 16:52
  • \$\begingroup\$ So a pullup resistor from the gate to the battery to hold it in the offstate, and then a switch to pull the gate low to allow current flow to the load? \$\endgroup\$
    – CrossRoads
    Commented Aug 22, 2018 at 16:55
  • \$\begingroup\$ The PNP BJT does not have a gate. \$\endgroup\$
    – Michael Karas
    Commented Aug 22, 2018 at 17:28
  • \$\begingroup\$ Well, base then, my mistake. \$\endgroup\$
    – CrossRoads
    Commented Aug 22, 2018 at 17:33
  • \$\begingroup\$ Thanks for the comments. A PNP transistor might well do the job I think I need to experiment. I need this circuit because I want to turn the unit ON when a mechanical switch is OPENED. My concern with this proposal is that when the transistor is off it might draw too much current. \$\endgroup\$
    – NickyJ
    Commented Aug 23, 2018 at 9:51

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