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Old 15th Apr 2019, 9:45 pm   #1
poppydog
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Default Replacement power transistors hfe.

I am looking for replacement power transistors for my heathkit ip-20 power supply, replacements that are in my budget have a lower hfe than the originals or their equivalents. The originals/equivalents have hfe of 100 min and the replacements that I have looked at are 20 hfe min.
Will the lower gain transistors have any adverse effects or require modifications(which I don't want to do)? They are to3 packaged Ge pnp types.
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Old 15th Apr 2019, 10:54 pm   #2
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Default Re: Replacement power transistors hfe.

Hi little diode list Motorola 2n2147 @ 8.99 they have 2 Mick
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Old 15th Apr 2019, 11:16 pm   #3
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Default Re: Replacement power transistors hfe.

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Originally Posted by mickm3for View Post
Hi little diode list Motorola 2n2147 @ 8.99 they have 2 Mick
Thanks for that mickm3for, what i am after really is conformation that i can use a lower gain transistor than the originals.

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Old 15th Apr 2019, 11:20 pm   #4
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Default Re: Replacement power transistors hfe.

It doesn't normally matter much in these sorts of circuits, but a minimum hfe of 100 is very high for a Ge power transistor, so they may have been specified for a good reason.
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Old 16th Apr 2019, 12:17 am   #5
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Default Re: Replacement power transistors hfe.

Many designs for power supplies with vintage Ge power transistors (pass transistors etc) make the assumption the the power transistor's hfe is 10 min. The circuit values are chosen with this in mind. This way all specimens of the part are likely to work.

Later on the assumption for Silicon types in similar power applications became 15 or 20.

This assumption (or rule of thumb) was also applied if the transistor was used as a saturated switch (though strictly speaking the hfe has little meaning in this application) So, for example, to likely ensure saturation, if your collector current is 2A you need at least 0.2A base current. There are better methods to ensure saturation (the C-E voltage lower than the B-E voltage) but the hfe of 10 "rule of thumb" usually works well for all specimens for power switching transistors too.
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Old 16th Apr 2019, 3:24 am   #6
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Default Re: Replacement power transistors hfe.

The answer is that it depends on how the supply was designed.

A lower Hfe device will demand more base current from the device driving it. This device may overheat and fail as a result or it may be fine.

Post the circuit and we can calculate the effects. Also, a cheaper Si part may be a possibility.

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Old 16th Apr 2019, 4:05 am   #7
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Default Re: Replacement power transistors hfe.

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The answer is that it depends on how the supply was designed.

A lower Hfe device will demand more base current from the device driving it. This device may overheat and fail as a result or it may be fine.

David
Its hard to know that without knowing the design of the supply. In a typical pass transistor situation where the base current is part of the output current (like an emitter follower) then the transistor's base current will increase for any output load if the hfe is lower, putting more demand on the driver, but that demand normally comes via a resistor which turns the pass transistor on, not the driver transistor that turns it off, unless of course there is an extra transistor that turns the pass transistor into a Darlington, then that transistor would have increased demands/dissipation.

In addition, if the output from the supply comes from the pass transistor's collector, a lower gain pass transistor has little effect on the demands on the driver circuit, all that happens in that case (the hfe too low) is that the feedback loop runs out of dynamic range and the driver saturates (protecting itself too) in an attempt to get the output voltage up to the normal regulated voltage.

Most manufacturers appear to set the design parameters/values so that even the poorest hfe specimen of a transistor will work in the circuit (or at least they should). Most Germanium power transistor I have tested appear to have an hfe in the range of 18 to 50, so if the design allows for one with an hfe of 10, they will all work.

Also, of note, for transistor switching in Automotive applications (with early transistors), the base currents Lucas chose for their power transistors were mostly very close to 1/10 the collector currents, which is interesting.
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Old 16th Apr 2019, 9:35 am   #8
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Default Re: Replacement power transistors hfe.

At a guess, in a power supply of that vintage and with a TO-3 output device, a Darlington arrangement is likely, but not certain.

Some supplies were well-designed, others were bodge-ups. We have to see the circuit to tell.

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Old 16th Apr 2019, 10:03 am   #9
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Default Re: Replacement power transistors hfe.

Circuit taken from here:-

https://www.vintage-radio.info/heathkit
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Old 16th Apr 2019, 11:04 am   #10
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Default Re: Replacement power transistors hfe.

So..... a couple of paralleled outputs in a darlington configuration.

Quite easy to siliconize I would think.......

Or perhaps not. TO111 package is nicely obscure! Doing anything but direct replacement without drilling holes could be "interesting".
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Old 16th Apr 2019, 11:32 am   #11
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Default Re: Replacement power transistors hfe.

I have found that most bench power supplies rely on the VBE of a transistor often the output one for the current limit sensing.
The output and sensing transistors would need to changed as a pair.
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Old 16th Apr 2019, 1:03 pm   #12
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Default Re: Replacement power transistors hfe.

Would silicon work if the TO3 power transistor X1 the current limiter, and TO3 power transistors X4, X5, the series pass transistors changed to MJ2955 and the two smaller transistors X2, X3, to BFX29 or similar. Would the three series pass resistors need changing from their 0.33 Ohms 2Watts. If all the diodes where changed round could NPN transistors be substituted?
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Old 16th Apr 2019, 1:08 pm   #13
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Default Re: Replacement power transistors hfe.

I've never found that! For starters, Vbe is temperature dependent, so as the thing gets hotter, Vbe falls and you'd think it was passing less current.

For a bench supply, the output current is often metered and a fairly accurate current-limit control added, so current sensing needs to be a low-error method. The bench supplies I have seen, have all used resistors.

Radiomuseum shows the 2N2147 as being TO3, but other sites TO111. Can't comment here! But if the output transistors are low hfe, then yes the driver will work harder, as David RW says
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Old 16th Apr 2019, 2:29 pm   #14
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Default Re: Replacement power transistors hfe.

Quote:
Originally Posted by kalee20 View Post
I've never found that! For starters, Vbe is temperature dependent, so as the thing gets hotter, Vbe falls and you'd think it was passing less current.
Actually you can do it (across a B-E junction or a rectifier junction as a current sensing element)see attached, one of my designs for a current sensor for a dynamo regulator (detecting the current by the voltage drop across a 180NQ035 power rectifier). One advantage of it is that you don't waste heat in a current sensing resistor. But it requires a thermally bonded diode (as indicated by the orange line in the diagram) to neutralize the temperature dependence you pointed out.
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Old 16th Apr 2019, 7:41 pm   #15
poppydog
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Default Re: Replacement power transistors hfe.

Thanks to Stationx for posting the schematic.

I was hoping I could maybe use 2N458A in place of the 2N2147 however I'm still unsure whether or not I can use it regarding the HFE.

https://alltransistors.com/transisto...ransistor=4205

https://alltransistors.com/transisto...ransistor=1647

The power transistor line up that was in it was a pair of OC35 and a OC20 meaning that it already has been looked at...

The pics below show what I was greeted with when I removed them, one of them was missing its pins, that might explain why the current limiter doesn't work and its very unstable.

Regards
Poppydog
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Old 16th Apr 2019, 11:23 pm   #16
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Default Re: Replacement power transistors hfe.

If you want to stick to germanium, I can see your dilemma, the 2N2147 does have a high range hfe, and a high-ish collector voltage rating, so ideally you would use a replacement that was at least similar. One possibility is the 2N1906, hfe min75, these appear cheaper than the 2N2147:

https://www.ebay.com/itm/2N1906-TRAN...YAAOSwCGVX2Vhr

but it might be a pest for you to have to get them from the USA.

An AD142 and AD143 also comes in high hfe variants, grade 5 & 6, but these may be harder to get. Check out the stock at Halbleiter:

https://www.sh-halbleiter.de
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Old 17th Apr 2019, 1:11 am   #17
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Default Re: Replacement power transistors hfe.

Pondering this some more. Somebody decided the OC35 would work . I don't know the max voltage the transistor is exposed to in your circuit.

A lower voltage rated (than the 2N2147) but with a good minimum hfe of 50 is the vintage (and classic) NKT404, by Newmarket. These were one of the most widely used TO-3 germanium power transistors in the UK in the 1960's and used for many amateur radio analog psu projects and even switching supplies such as Royer DC converters. These may be an option if the voltage ratings are not exceeded by your circuit.
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Old 17th Apr 2019, 8:41 am   #18
poppydog
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Default Re: Replacement power transistors hfe.

Thanks for the replies but I am still none the wiser as to whether I can use a lower gain one ??

Regard poppydog
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Old 17th Apr 2019, 10:50 am   #19
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Default Re: Replacement power transistors hfe.

I have got some replacements, so the mods can close.

regards poppydog
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Old 17th Apr 2019, 11:11 am   #20
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Default Re: Replacement power transistors hfe.

What type did you end up using?

About the original question: Using a lower gain transistor can indeed cause problems. As a rule of thumb: you don't do that. It depends a bit on the circuit, though. In linear applications with feedback, there's often a large margin. In switching circuits the transistor may fail to go into saturation and get hot fast.
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