Hello, I know that there are hordes of nice comparators in SOT-23, but just out of curiosity: is it possible to build a small voltage (relatively to GND) detector using just one transistor and no negative power supplies? If the voltage is >= VBE, i.e. 0.7V, then the task is a no-brainer, but how about half of that, say, 0.3V? Best reagrds, Piotr
Small voltage detector
Started by ●October 3, 2015
Reply by ●October 3, 20152015-10-03
On 10/3/2015 4:22 AM, Piotr Wyderski wrote:> Hello, > > I know that there are hordes of nice comparators in SOT-23, > but just out of curiosity: is it possible to build a small > voltage (relatively to GND) detector using just one transistor > and no negative power supplies? If the voltage is >= VBE, i.e. > 0.7V, then the task is a no-brainer, but how about half of that, > say, 0.3V? > > Best reagrds, PiotrAre you allowed to bias the base positive? That is, can the voltage you want to measure sink as well as supply?
Reply by ●October 3, 20152015-10-03
On 10/3/2015 4:55 AM, John S wrote:> On 10/3/2015 4:22 AM, Piotr Wyderski wrote: >> Hello, >> >> I know that there are hordes of nice comparators in SOT-23, >> but just out of curiosity: is it possible to build a small >> voltage (relatively to GND) detector using just one transistor >> and no negative power supplies? If the voltage is >= VBE, i.e. >> 0.7V, then the task is a no-brainer, but how about half of that, >> say, 0.3V? >> >> Best reagrds, Piotr > > Are you allowed to bias the base positive? That is, can the voltage you > want to measure sink as well as supply?Forgot to add an illustration: VCC + .|. | | | | '-' | | ___ | |/ In -|___|- -+------| |> | | === GND (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
Reply by ●October 3, 20152015-10-03
John S wrote:> Are you allowed to bias the base positive? That is, can the voltage you > want to measure sink as well as supply?Say the input is the secondary of a power transformer (which is my actual application, but I'll use a built-in comparator module in the real circuit). So, assume I can. Best regards, Piotr
Reply by ●October 3, 20152015-10-03
On 03.10.2015 11:22, Piotr Wyderski wrote:> Hello, > > I know that there are hordes of nice comparators in SOT-23, > but just out of curiosity: is it possible to build a small > voltage (relatively to GND) detector using just one transistor > and no negative power supplies? If the voltage is >= VBE, i.e. > 0.7V, then the task is a no-brainer, but how about half of that, > say, 0.3V? > > Best reagrds, PiotrSomething like this (assuming the source can drive 1k)? http://imgur.com/YEkCWIK Not particularly stable, but may work at room temperature... Dimitrij
Reply by ●October 3, 20152015-10-03
On Sat, 3 Oct 2015 11:22:08 +0200, Piotr Wyderski <peter.pan@neverland.mil> wrote:>Hello, > >I know that there are hordes of nice comparators in SOT-23, >but just out of curiosity: is it possible to build a small >voltage (relatively to GND) detector using just one transistor >and no negative power supplies? If the voltage is >= VBE, i.e. >0.7V, then the task is a no-brainer, but how about half of that, >say, 0.3V? > > Best reagrds, PiotrGermanium transistor? Regards, Boris Mohar Got Knock? - see: Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca void _-void-_ in the obvious place --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Reply by ●October 3, 20152015-10-03
On Sat, 03 Oct 2015 13:21:17 +0200, Piotr Wyderski <peter.pan@neverland.mil> wrote:>John S wrote: > >> Are you allowed to bias the base positive? That is, can the voltage you >> want to measure sink as well as supply? > >Say the input is the secondary of a power transformer (which is >my actual application, but I'll use a built-in comparator module >in the real circuit). So, assume I can. > > Best regards, PiotrVoltages outputed from the secondary of a power transformer are typically bipolar, low impedance and large, so your description of the measurement needing to be made doesn't make much sense. Are you trying to detect a zero crossing? a change in current flow? What? Simply offseting the base bias by a diode drop will provide first order compensation for a common base amplifier with high voltage gain, if the source impedance is low. The use of any amplifier as a comparator will always have issues. Low power, single supply comparators are avilable in 5 or 6pin transistor-sized packaging that will operate with inputs at 100s of millivolts either side of ground. RL
Reply by ●October 3, 20152015-10-03
On Saturday, 3 October 2015 11:01:41 UTC+1, John S wrote:> On 10/3/2015 4:55 AM, John S wrote: > > On 10/3/2015 4:22 AM, Piotr Wyderski wrote: > >> Hello, > >> > >> I know that there are hordes of nice comparators in SOT-23, > >> but just out of curiosity: is it possible to build a small > >> voltage (relatively to GND) detector using just one transistor > >> and no negative power supplies? If the voltage is >= VBE, i.e. > >> 0.7V, then the task is a no-brainer, but how about half of that, > >> say, 0.3V? > >> > >> Best reagrds, Piotr > > > > Are you allowed to bias the base positive? That is, can the voltage you > > want to measure sink as well as supply? > > Forgot to add an illustration: > > VCC > + > .|. > | | > | | > '-' > | | > ___ | |/ > In -|___|- -+------| > |> > | > | > === > GNDthat's the simplest option afaik. You might improve it by adding a series diode in the input, then Rin can be lower. You may also need a 2nd diode to protect against Veb >5v. NT
Reply by ●October 3, 20152015-10-03
On 10/3/2015 9:06 AM, tabbypurr@gmail.com wrote:> On Saturday, 3 October 2015 11:01:41 UTC+1, John S wrote: >> On 10/3/2015 4:55 AM, John S wrote: >>> On 10/3/2015 4:22 AM, Piotr Wyderski wrote: >>>> Hello, >>>> >>>> I know that there are hordes of nice comparators in SOT-23, >>>> but just out of curiosity: is it possible to build a small >>>> voltage (relatively to GND) detector using just one transistor >>>> and no negative power supplies? If the voltage is >= VBE, i.e. >>>> 0.7V, then the task is a no-brainer, but how about half of that, >>>> say, 0.3V? >>>> >>>> Best reagrds, Piotr >>> >>> Are you allowed to bias the base positive? That is, can the voltage you >>> want to measure sink as well as supply? >> >> Forgot to add an illustration: >> >> VCC >> + >> .|. >> | | >> | | >> '-' >> | | >> ___ | |/ >> In -|___|- -+------| >> |> >> | >> | >> === >> GND > > that's the simplest option afaik. You might improve it by adding a series diode in the input, then Rin can be lower. You may also need a 2nd diode to protect against Veb >5v. > > > NTGood observation.
Reply by ●October 3, 20152015-10-03
On 10/3/2015 8:42 AM, Boris Mohar wrote:> On Sat, 3 Oct 2015 11:22:08 +0200, Piotr Wyderski <peter.pan@neverland.mil> > wrote: > >> Hello, >> >> I know that there are hordes of nice comparators in SOT-23, >> but just out of curiosity: is it possible to build a small >> voltage (relatively to GND) detector using just one transistor >> and no negative power supplies? If the voltage is >= VBE, i.e. >> 0.7V, then the task is a no-brainer, but how about half of that, >> say, 0.3V? >> >> Best reagrds, Piotr > > Germanium transistor? > > > Regards, > > Boris Mohar >Long tail pair with a diode reference?
