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Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179
The second circuit.
Can I replace both NPN transistors 2N3055 with a MJ29055 PNP transistor and a 2N2222 with an old germanium transistor?
I want to do this in order to reduce the minimum start voltage of the Joule Thief. The emitter-base Voltage of a germanium transistor is 0.4V or less. Germanium transistors are obsolete but they make sense in Joule Thieves. Is there a Germanium transistor that has a specially low Veb? (I will buy an old Germanium transistor on Ebay for this circuit for around $7. NTE27 is a very expensive.)
Would it make sense to add a tap to the secondary of this circuit for the feedback?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It should work fine. The only issue is that most older germanium transistors are very slow with a ft in the hundreds of kHz. I don't know of any fast ones that can handle significant power.
Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179
I found one but I will abandon this idea because the collector current of this transistor is so small that I will burn it by driving the MJ29055 with it.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
if your intention is to light the led with the signal/antenna power you will probably have more success with a simple bridge rectifier with small output capacitor.
with a simple dipole and 4x 1N4148 I can light an led at several meters using my 144 MHz handheld. I didn't have them at the time but if I do it again I'd use 1N5711, lower voltage, faster, and in my stock.
A typical random length wire receiving antenna would have a fairly high source impedance, I think that it would be more suitable for a simple bridge rectifier than a 'joule thief'
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Joule thief. Can I convert this to PNP from NPN entirely + switch polarity? (I am ordering parts.)
