Forums

0.3V Vbe transistor?

Started by Unknown February 4, 2014
A friend wanted to make a DC-DC converter for his daughers' science project, which involves making an LED torch out of a hand-held Peltier tile.

I suggested he look into the Joule Thief, which he did.

He says it works, but he'd prefer for it to work at 0.3V instead of 0.7V.

I then remembered the germanium transistors.  When he asked where to find one, I realized they're so old they don't come in plastic TO-92 cases, and they all seem to be PNP when I look on eBay.

Is there any modern transistor that has a Vbe of circa 0.3V?  Or are we stuck looking for decades-old NPN germanium transistors on eBay?

Thanks,

Michael
On Tuesday, February 4, 2014 2:26:34 PM UTC-5, mrda...@gmail.com wrote:
> A friend wanted to make a DC-DC converter for his daughers' science project, which involves making an LED torch out of a hand-held Peltier tile. > > > > I suggested he look into the Joule Thief, which he did. > > > > He says it works, but he'd prefer for it to work at 0.3V instead of 0.7V. > > > > I then remembered the germanium transistors. When he asked where to find one, I realized they're so old they don't come in plastic TO-92 cases, and they all seem to be PNP when I look on eBay. > > > > Is there any modern transistor that has a Vbe of circa 0.3V? Or are we stuck looking for decades-old NPN germanium transistors on eBay? > > > > Thanks, > > > > Michael
Hmm who is doing the project? Anyway googling low voltage DC-DC converter I found this, http://www.digikey.com/us/en/techzone/energy-harvesting/resources/articles/Low-voltage-DC-DC-Converters-Build-Efficient-Power-Management.html George H.
On Tue, 04 Feb 2014 11:26:34 -0800, mrdarrett wrote:

> A friend wanted to make a DC-DC converter for his daughers' science > project, which involves making an LED torch out of a hand-held Peltier > tile. > > I suggested he look into the Joule Thief, which he did. > > He says it works, but he'd prefer for it to work at 0.3V instead of > 0.7V. > > I then remembered the germanium transistors. When he asked where to > find one, I realized they're so old they don't come in plastic TO-92 > cases, and they all seem to be PNP when I look on eBay. > > Is there any modern transistor that has a Vbe of circa 0.3V? Or are we > stuck looking for decades-old NPN germanium transistors on eBay?
Turn the circuit upside down and use a PNP germanium. Make sure it has a sufficient current rating. Or, look here: https://en.wikipedia.org/wiki/Joule_thief#Competing_systems I was going to suggest a JFET, but didn't know if it'd work. I guess I have my answer. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On Tuesday, February 4, 2014 11:56:57 AM UTC-8, George Herold wrote:
> On Tuesday, February 4, 2014 2:26:34 PM UTC-5, mrda...@gmail.com wrote: > > > A friend wanted to make a DC-DC converter for his daughers' science project, which involves making an LED torch out of a hand-held Peltier tile. > > > > > > > > > > > > I suggested he look into the Joule Thief, which he did. > > > > > > > > > > > > He says it works, but he'd prefer for it to work at 0.3V instead of 0.7V. > > > > > > > > > > > > I then remembered the germanium transistors. When he asked where to find one, I realized they're so old they don't come in plastic TO-92 cases, and they all seem to be PNP when I look on eBay. > > > > > > > > > > > > Is there any modern transistor that has a Vbe of circa 0.3V? Or are we stuck looking for decades-old NPN germanium transistors on eBay? > > > > > > > > > > > > Thanks, > > > > > > > > > > > > Michael > > > > Hmm who is doing the project? > > Anyway googling low voltage DC-DC converter I found this, > > http://www.digikey.com/us/en/techzone/energy-harvesting/resources/articles/Low-voltage-DC-DC-Converters-Build-Efficient-Power-Management.html > > > > George H.
Haha, we went to Uni together and still chat every now and then, and actually he gave me a schematic very similar to what you just posted! Then I said, "Joule Thief." Much simpler than a dedicated IC... Thanks!
On Tuesday, February 4, 2014 11:59:05 AM UTC-8, Tim Wescott wrote:
> On Tue, 04 Feb 2014 11:26:34 -0800, mrdarrett wrote: > > > > > A friend wanted to make a DC-DC converter for his daughers' science > > > project, which involves making an LED torch out of a hand-held Peltier > > > tile. > > > > > > I suggested he look into the Joule Thief, which he did. > > > > > > He says it works, but he'd prefer for it to work at 0.3V instead of > > > 0.7V. > > > > > > I then remembered the germanium transistors. When he asked where to > > > find one, I realized they're so old they don't come in plastic TO-92 > > > cases, and they all seem to be PNP when I look on eBay. > > > > > > Is there any modern transistor that has a Vbe of circa 0.3V? Or are we > > > stuck looking for decades-old NPN germanium transistors on eBay? > > > > Turn the circuit upside down and use a PNP germanium. Make sure it has a > > sufficient current rating. > > > > Or, look here: https://en.wikipedia.org/wiki/Joule_thief#Competing_systems > > > > I was going to suggest a JFET, but didn't know if it'd work. I guess I > > have my answer. > > > > -- > > > > Tim Wescott > > Wescott Design Services > > http://www.wescottdesign.com
Ooh, a JFET! Thanks! https://www.google.com/patents/US4734658?dq=4,734,658&hl=en&sa=X&ei=FEfxUo-BOpiEoQTjnoCQBQ&ved=0CDMQ6AEwAA Very nice. Michael