Transistor hfe

[sureshot]

Ok time to make a decision, as I don't think I'm going to able to pin down an exact value for R7.

I'm going to start out with a 25 ohm 0.5 watts. As to this calculation being correct I'm unsure.

A value of 25 ohms for R7 contributes 160mW through R7 and a current of 80mA. And I think this limits the voltage of the base emitter junction to 2 Volts. It was suggested two pages back, so this seems a reasonable starting point. If I'm well of the mark there I stand to be corrected ! Thanks again for everyone's help.

 

[(*steve*)]

As I've suggested pages ago, just use 100 ohms as you already have.

R7 is required, and a wide range of values will work.

The primary limitation is that at the quiescent current of the 7812, the resistor must drop less than the Vbe required to start turning on the transistors (that's an upper bound).

The lower bound is such that the voltage drop across R7 must exceed Vbe of the transistors before you reach Imax of the regulator.

Because the Vbe for a darlington is greater than for a single transistor, it would be appropriate to use a HIGHER value for R7. Arguably, 200 ohms would be appropriate.

It's not critical, use whatever you're using now, you won't break anything unless you follow Colin's advice.

 

[sureshot]

Thank you Steve ! So it looks like I've been going in completely the wrong direction... I thought the resistance from previous comments needed to be reduced. I still don't know on paper what the circuit numbers will be using 100 ohms 0.5W for this higher power MJ11033 transistor will be. I know what it is using the TIP2955 transistors as per the original schematic component layout.

I value your opinion ! 100 ohms it is for an R7 starting point value. Measuring with a multimeter or two both current and voltage, unloaded, and loaded will tell me what work the l7812 is doing, and other test points in the circuit. Thanks again to everyone ! Hopefully I can post some real practical results of this senarios.

 

[AnalogKid]

1. You are correct.

2. I think 800 mA is unnecessarily high. For improved long-term reliability, electronic components should not be operated continuously at or above 50% of their ratings. The 7800 series of regulators is rated for 1 A continuous output current. Keeping the current through your 7812 below 500 mA is good design practice. This is one of the reasons my example sets the 7812 current at 100 mA.

3. Please don't confuse Colin's attempts at sarcasm as actual advice. Using 10 power transistors and 100 W resistors are not good ideas.

ak

 

[AnalogKid]

1. The primary limitation is that at the quiescent current of the 7812, the resistor must drop less than the Vbe required to start turning on the transistors (that's an upper bound).

2. Because the Vbe for a darlington is greater than for a single transistor, it would be appropriate to use a HIGHER value for R7. Arguably, 200 ohms would be appropriate.

3. It's not critical, use whatever you're using now, you won't break anything unless you follow Colin's advice.

1 and 2. This may be part of the source of the resistor value confusion. I don't think it is a requirement for this circuit, or even good for it, that the current through the 7812 be kept to a bare minimum level and make the transistors carry the load even for relatively small output currents. The regulator circuit's transient response and stability both are better when the 7812 is doing all of the work, so I think holding off the big bypass transistors until higher power demands require them is a good thing. As above, it is a tradeoff based on several (sometimes competing) design factors, but that is the essence of design.

3. Something we can agree on.

ak

 

[Colin Mitchell]

R7 should be a value to prevent the transistors turning on when the quiescent current for the 7812 flows.

 

[sureshot]

No I took note of some if you want to call it sarcasm, personally I didn't see as that. The few times it croped up it did leave me amused, "resistor are like a sewer in a circuit" Lol. The above comments conflict a little ( less Steve's comment) "Its good for the l7812 to be running at a minimum current (100mA)" " Its better if the regulator runs at around 800mA doing a chunk of the work" these to comments over two posts, for me seem conflicting.

I have a starting point at 100R 0.5 W for R7 how we got down to less than tens of ohms for an R7 value i've no idea. My problem I think is getting to grips with the whole circuit maths.. As in as much looking at the transistor resistor chain, along with the regulators specifactions, and the value of R7.

I'm still working on the theory side of some of this in my head, I expect it will make sense more when I take some practical real time meter measurements on voltage and current across the circuit, especially at different load levels. I have read back all 5 pages, and if this was a get out alive orienteering experience in the wilderness, I might have not made it out alive !

 

[(*steve*)]

@AnalogKid perhaps I should have said "it would not be inappropriate to use a higher value".

I see no pressing advantage in having the 7812 reg supplying more vs less of the load. However I'm prepared to be convinced. I wonder about the stability implications (in general, not specifically to any particular value of R7).

 

[sureshot]

I know your post Steve is in reply to AK. But the l7812 using the original schematic, so far with 2 x TIP2955's and powering my loads up to 8.2 Amps, stability is fairly good.

I know these aren't super high loads.. And the voltage drop is 0.3 volts at that said 8.2 Amps. Having said that this two transistor version was only meant to be a taste it and see model.

The CPU heat sink all the silicone is bolted to is only meant to keep a 60 watt CPU cool, so heatsinking could be better, but this one was only suppose to be a model. But I've ended up leaving it power radio gear for the moment.

I know its a far from perfect circuit, but I'm taken by its simplicity cost and low components count for high currents. To buy a 30 Amp linear retail supply in the UK is minimum of £120 might not sound a lot, but when you put a built unit on paper it comes out very competativly. Its also quit fun to do.

Very long term use I've no idea. Before I put this one to bed I'd like to build the best quality unit I can, adding a couple of panel meters, and using best quality components. Then just use it.
I should say I've been using the l78S12CV this is the 2Amp regulator, but for all its worth as a regulator it seems to have the same property's as the l7812 1Amp regulator. Thanks for everyone's help here ! I will post some practical results in the near future.

Edit. I did add a couple of 0.1uf decoupling ceramic capacitors close to the regulator, there is none in the original schematic.

 

[AnalogKid]

I know its a far from perfect circuit,

Not true at all. It is a classic that has been around for decades and built by tens (hundreds?) of thousands of people. Hard to find a circuit with a better pedigree. The basic regulation is not as good as a lab supply, there is no current limiting, the output noise is a bit high, blah blah blah. For what it is intended, it is excellent.

ak

 

[sureshot]

Not true at all. It is a classic that has been around for decades and built by tens (hundreds?) of thousands of people. Hard to find a circuit with a better pedigree. The basic regulation is not as good as a lab supply, there is no current limiting, the output noise is a bit high, blah blah blah. For what it is intended, it is excellent.

ak

I'm glad you thinks so AK ! As I say I am certainly fascinated by its simplicity and how it can ramp up and amplify such low currents.

 

[sureshot]

Well I've got the 6 x MJ11015 transistors, rather than just put all six transistors I'm going to build up a single transistor model.

Using the formula that AJ suggested back on page two of this thread, once built I'll power it up, load it with 50 watts or a bit more, open the circuit and measure the regulator current. Oh and voltage unloaded and loaded. So R7 25R 0.5W ballast resistor 10 Watt 0.1R ceramic. Then if that works and measurements look ok, I will build up the 6 x PSU circuit either with R7 at 150R 0.5W or as Steve said 200R 0.5R which ever value looks more favorable.

In previous pages where ever I referred to R7 I was meaning for all six transistors. But it dawned on me that AJ's comments reffered to one transistor when putting the maths up on page 2 of this thread. Think that's right. Anyway I'll be back soon with some results. After 5 pages of help and comments, it makes sense to follow up with some practical ! Thanks again all.

 

[(*steve*)]

The value of R7 does not depend on the number of transistors you're driving.

 

[sureshot]

Oh, I thought it was, only reason I thought this is because the single schematic using 1 x TIP2955 transistor has a base resistor of 10 ohms 0.5W and a ballast resistor of 1 ohm 7 watts.

Whilst the 6 x TIP2955 transistor schematic has 100 ohms 0.5W base resistor, and has 6 x 0.1 ohm 5 watt ballast resistors.

So the 25 ohms 0.5W R7 won't limit the regulator current to 0.100mA and voltage to 2.5 Volts ? I thought the resistor value was divided by the number of transistors used.

So I'm wandering why the value between R7 for the single transistor, to the six times transistor is 10 x higher ? I have no idea why this is.

 

[sureshot]

I don't have a problem using a 200R 0.5W resistor for R7, just confused by the 10 x difference I've described in the post above. And I was going on the maths for a single transistor AK suggested on page 2. It is very confusing, I know a range of base resistors for R7 vary the base current and voltage dropped across it, but there's a huge margin between 25R and 200R, I don't doubt your right Steve, but just find it hard to understand the 10 x differential in my post above.

 

[(*steve*)]

As I've said previously, the maximum value of R7 must be sufficiently small that at the regulator's quiescent current, the voltage drop is insufficient to turn on the pass transistor(s).

The value doesn't change for 1,2,or 10 pass transistors. However it does change when you use a Darlington transistor because they have a higher BE voltage required to turn them on.

The minimum value has its own calculation, and the range between them is large. 1 ohm is far smaller than I would consider using, and possibly would not work with Darlington pass transistors and a 1A regulator.

 

[sureshot]

Ok as I said I do really value your opinion ! So 200 ohm for a single transistor is perfectly suitable to turn on and forward conduct that transistor ? I think from what you're saying that it is.

The bit I'm struggling with is.. If AK's 25 ohm 0.5W resistor limits the regulator current to 0.100mA, what does a 200 ohm resistor limit the regulator current to, and the base voltage ? Can you help me understand that please, I'd be very grateful !

 

[sureshot]

So doing the maths.. A value of R7 200 ohms will give me 4 Volts dropped across R7 and a current limit of 0.020 mA for the l7812, have I worked that out right ? Help appreciated.

 

[(*steve*)]

What you're interested in is the current through the resistor when the voltage across it reaches the voltage required to begin to turn the transistor on.

lets call this 0.5v for a normal transistor and 1v for a Darlington.

If the quiescent current for the regulator is less than this you're good to go.

You need to either measure the quiescent current or use the max value from the datasheet.

1v across 200 ohms is 5 mA.

 

[sureshot]

Ok I see what's going on there.... I think... So I know the maths, is this correct... R7 100 ohms with a 2.5 Volts across it would be 25mA ? If I've got that value worked out right I think I've got it.