Unexpected high voltage. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 29, 2020, 09:08:57 pm
content: I've tried looking carefully for a problem I have. What I've got is a 7812 linear positive regulator, and a pair of TIP36C transistors, it's putting out 12.80 Volts as expected. I have one silicone diode in the negative rail to get the voltage up just above 12 volts. The circuit can power 50 watts and 100 watts, all is as expected. Although I did note a little volts drop at 100 watts, not tragic though. Yesterday I thought I would swap out the TIP36C transistors for two TIP147 darlington transistors. But something odd is going on, I'm getting virtually double my output voltage with these TIP147 transistors. I have other TIP147 transistors from other reputable suppliers, and it's the same. About 23.00 Volts, I did try a 12 Volt halogen lamp expecting it to blow, it didn't, but dropped the voltage to about 17.00 volts. When I swap back to the TIP36C transistors, all is fine at 12.80 volts. I've no idea what is going on here, I am at a complete loss to be able to work it out.I had started an earlier thread on a zener diode going short, but I though that was my only problem. But it's not, I know transistors amplified current and power, but not the voltage by twice it's expected output. If anyone knows what is going on, that would be great. Thanks for reading.The circuit is the schematic below.

author: Renate
author_rank: Super Contributor
timestamp: August 29, 2020, 09:59:02 pm
content: I'm concerned about that 470 µF.If your load drops suddenly, the output voltage will spike.The 7812 takes current just by itself.That leakage (internal references, bias, etc) 8.5 mA is enough to hit the darlingtons.The darlingtons are more sensitive than the simple BJT.Unless you load it heavily you'll never get to 12 V.You really don't need any gain (or darlingtons) because the 7812 by itself can pull an ampere of base current.Moreover, with the darlingtons you lose headroom on your input supply.Stick to simple BJT.Edit: If you want more than 12.0 V put in an LM317, a resistor and a trimmer.There's no need to add diodes somewhere.Oh, yeah, even with BJT you will probably need a load of 100 mA just to take it down to regulated voltage.That's 8.5 mA quiescent of the 7812 (or LM317) * the beta of the BJT.Or you could switch to a switch mode supply and save the heat.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 29, 2020, 11:23:16 pm
content: Thank you for the reply, I haven't stuck to the schematic values exactly. I have a transformer of 15 Volts RMS at 300 VA bit over kill for 10 Amps maybe. It's 10 Amp per secondary winding, and they are in parallel. I have a copacitance of 25000 uf and 6800 uf on the output rail. The regulator has 0.1 uf ceramic disks for decoupling. In previous builds I've used the PNP darlington MJ11015 TO3 type package, similar gain to the TIP147, that power supply gave 12.80 Volts or close to it with no voltage over shoot. I only need 12 Volts for the application I'm using. I have used variable linear regulators in the past with good results. I just don't understand how the same circuit as that when I used MJ11015 transistors, but TIP147 darlington transistors doubles my output voltage. Is it really possible they can push up the voltage that high ? I knew darlington transistors can reduce voltage sag, but had no idea it could double my voltage output.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 30, 2020, 01:47:13 am
content: I would use a much lower value for the 100 Ohm resistor.Dimension it in such a way that if the current is below a few hundred mA, then the transistors are completely closed and the 7812 does the job on it's own.Then, when current increases the BJT's go over their Base-Emitter threshold and start helping the poor 7812.If you use Darlingtons for the BJT's (single bjt's would suffice, as there is plenty of base drive available) then the BJT's start helping at 1200mV Ube.So if you want to start above 300mA, then:1.2V / 300mA = just over 4 Ohm.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 30, 2020, 01:54:14 am
content: Silicon is a pretty hard substance used for making tansistors.Silicone is a soft rubbery materialhttps://en.wikipedia.org/wiki/SiliconeAlso, if you want good help, it would be nice if you put in some effort to post the actual schematic you are using.There are several ways to lift the voltage of a LM7812 a bit.A simple way is to use a small signal diode in the GND leg.You can also use it in the same way as the LM317. It just has a reference voltage of 12V instead of 1.25V.

author: Renate
author_rank: Super Contributor
timestamp: August 30, 2020, 01:59:38 am
content: It's a question of minimum load.Without a load you're going to get the full input voltage due to leakage.If you remove the 7812 there should be very minimal current flowing through the transistors.With the 7812 removed a 1k resistor should be able to take the output to zero.If there isn't, that's your problem right there.TIP147 has Hfe of 1000 (minimum)MJ11015 has Hfe of 1000 (minimum)MJ2955 has Hfe of 20-70TIP36C has Hfe of 10-50The 7812 leaks 8.5 mAThat means theminimumload for that circuit using the TIP147 or MJ11015 will be 8.5 AmperesActually, it's even worse than that, since the maximum Hfe is over 5000!If you don't load it enough it will be running higher than 12 V and the 7812 will be outputing zero current, not regulating, but sucking down that 8.5 mAUsing the MJ2955 theminimumload will be somewhere between 170-600 mAOkay, this was not a Spice analysis, it was back of envelope ignoring the 100 ohm resistor.If you lower that resistor, it will lower the minimum load requirement, but also lower the maximum current.Quote from: Doctorandus_P on August 30, 2020, 01:47:13 amHey! I wrote that before I saw your post!Summary: You don't want a gain of 1000. More is not better. A gain of 20 is plenty for 15 amps output.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 30, 2020, 03:40:18 am
content: Thank you for the replys, the schematic above is the circuit i am using. the only difference is my filter capacitance is quite high, as is my final output capacitor. i can see the gain of the darlington transistors is not helping much, i was just amazed that just switching transistors with little to no load doubled my output voltage. What i was trying to achieve was stiff output voltage that had very little voltage drop under heavy loads. I thought darlington transistors would do this, as i thoght the huge gain would amplify the current and power better. My weak point in electronics is my maths, i can work out a fare bit of the figures, but once it gets really complicated i get lost in the equation. For me the poor math is areal handicap, especially when you need these figures to make adjustments that get the job done. I think for now i will stick with single BJT transistors, and look in to lowering the base bios resistor. If i can get my head around the formula to select a better value resistor for the transistors base. I only want the 7812 regulator to stay with in a comfortable margin, or safe operating area, and the power transistors take up the heavier work load.

author: eblc1388
author_rank: Frequent Contributor
timestamp: August 30, 2020, 05:57:00 am
content: TP147 has an internal diode between collector and emitter. This would cause the high voltage seen in your case. As the load current is passing via this diode instead of the transistor junction, it is also possible that the load voltage will drop to 17V under load.I trusted that you would not have made the silly error of swapping the Collector and Emitter pin. Are you sure?

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 30, 2020, 10:01:16 am
content: Quote from: CJay on August 30, 2020, 09:34:31 am+1.I guess 7812 is dead now, so it doesn't work. But there is no strong sense to guess because TS definitely has a multimeter and can measure voltage levels in different points of his circuit and make a conclusion (or at least he can bring some measurements onto the forum).

author: mariush
author_rank: Super Contributor
timestamp: August 30, 2020, 11:33:58 am
content: You could just connect two LM1084 to-220 regulators in parallel, each is capable of 5A so in parallel you have 10A.Example... adjustable version :https://www.digikey.com/product-detail/en/texas-instruments/LM1084IT-ADJ-NOPB/LM1084IT-ADJ-NOPB-ND/363557See page 15:https://www.analog.com/media/en/technical-documentation/data-sheets/108345fh.pdf

author: Renate
author_rank: Super Contributor
timestamp: August 30, 2020, 11:40:17 am
content: I was a bit too quick to ignore the importance of the value of the 100 ohm resistor.If we make that 50 ohm, then the 8.5 mA quiescent will cause .425 V, not enough to activate the pass transistors.That should make the minimal load zero (approximately).In normal loaded operation there would then be 12 mA through that resistor.The ratio of current sharing between the pass transistors and the 7812 will then be the Hfe of the transistors.Using darlingtons, the threshold should be twice as high and should theoretically not be activated by the 7812 queiscent current.So we must conclude that the circuit is oscillating.Quote from: CJay on August 30, 2020, 10:15:27 amWell, then it seems that reducing the loop gain by a factor of 50 would be prudent.(i.e. use regular BJTs, not darlingtons)

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 30, 2020, 12:36:44 pm
content: No to make poor excuses, if my math was better, I would better understand what's going on with the TIP147 transistors. I have taken readings with a multimeter around the circuit etc, but haven't noted them or written them down. I should have took some readings on paper, the circuit is out of the PSU case at the moment. I've used 16 Amp terminal blocks for quick disconnect from the circuit board. Using molex female sockets for regulator and transistor connection, just something I though made dismantling any semiconductor really easy. My leads from these have been kept as short as possible, I'm aware of voltage drops over lengthy cable, so used good AMG gauge wire for the purpose.I have had a linear regulator oscillation in the past with a circuit, so aware of what that looks like with voltages all over the place. The 100 ohm resistor was recommended to me from a forum member, in the same circuit but with 6 X transistors. I asked back then about changing that resistor, with the use of just 2 X transistors. The guy I spoke with said it would be fine at 100 ohms with 2 X transistors. I can connect the circuit back up later today and take some readings with the TIP147 transistors in place. One piece of information above mentioned diodes inside the TIP147 transistors, something I hadn't been aware of. I should have looked at the package arrangement before using the darlington TIP147 transistors, I know some scenarios with diodes can act by increasing voltage in certain configurations. I was stumped by the voltage increase by almost twice expected output, because I had used darlington transistors MJ11015 with good results. I think I should have looked more closely at the TIP147 data sheet before using them. The regulator 7812 is fine, I've removed all transistors, and noted a voltage of 12.08 in a steady state. I have reconnected the TIP36C transistors and it's just fine, behaving as expected. I remove the TIP36C transistors, and change them for the TIP147, and my output voltage is higher than my input voltage. My input voltage is 21.00 Volts with either TIP36C or TIP147 connected, my output voltage is 12.89 with the TIP36C transistors connected, and 23.00 with the TIP147 transistors connected.If I can, later today I will connect both types of transistors up, and take voltage notations of each type, unloaded, and with a 50 watt load across the circuit form input to output, covering all the transistor junctions. In linear regulator circuits, both variable and fixed voltage, I've never seen an output voltage twice as high as expected, and even higher than the filtered DC before the regulator. As I said, the only thing I can see that might explain this high voltage from the use of the TIP147 transistors, is the internal diodes they contain in the package. I will report some voltages back in a bit if I can. Thank you for the help and suggestions with this problem I have.

author: Renate
author_rank: Super Contributor
timestamp: August 30, 2020, 12:51:17 pm
content: Quote from: davelectronic on August 30, 2020, 12:36:44 pmWell, that tells you something's wrong right there.A voltmeter takes a complicated signal and reduces it to a single number.If the signal is not DC or not a perfect sine (with the meter in AC mode) the number will be dependent on the design of the voltmeter.As CJay said, measure the AC component.If you're not sure how your voltmeter works, measure the AC in series with a capacitor.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 30, 2020, 05:25:51 pm
content: I've not got the voltages yet, hopefully in a bit I can post measurements. Looking at the TIP147, and MJ11015, data sheets, they both types of transistors have this internal diode. So it can't be that pushing the voltage up, I have used the MJ11015 on at least two previous circuits, and both times the voltage was just fine. The bit I don't understand is how can a 7812 positive regulator put out 23 volts, because that is the voltage across the negative and positive output rails. Should have some voltage readings in a bit.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 30, 2020, 08:10:27 pm
content: Yes i will measure through out the board with both types of transistors. And with that large value filter capacitor removed. That sureshot guy seems to ask similar questions to me, strange one that.I think i've seen him on a UK radio forum, two threads in particular stand out to me on there. He's got a lot of hits on there... Anyway its been a trying day, but i will try an get some measurements asap.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 30, 2020, 10:05:19 pm
content: Quote from: davelectronic on August 30, 2020, 05:25:51 pmIs there some one more series connected power supply? Or it is only a dead battery in the muktimeter?

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 30, 2020, 10:09:55 pm
content: No, nothing in series with this power supply circuit. Multimeter battery is fairly new. I have used two other meters, they read the same. My first thought was the multimeter was playing up.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 31, 2020, 09:11:17 am
content: Magic always play games with newbies. That's ok.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 31, 2020, 08:08:54 pm
content: So I've got some numbers, I have no idea if this will show any anomalies, be it undesirable. I had a pair of TIP2955 these behaved very similar to the TIP36C's. The same high voltage from the TIP147's. It seemed easier to right down the numbers on both transistor / regulator tests. I took numbers unloaded, and a 20 watt halogen lamp load. I photographed the two sheets, hopefully it will post with sufficient clarity. Thank you for looking.

author: Renate
author_rank: Super Contributor
timestamp: August 31, 2020, 10:49:15 pm
content: The TIP36C works fine.Your 7812 apparently has less quiescent current than the max (8.5 mA) speced.It looks like 3.4 mAIt seems at this temperature that the 100 ohm is ok.The TIP 147 setup is all screwed up.What happens to this setup with the 7812 removed?Either that transistor is blown or backwards.Since you are measuring the C-E voltage and it's always almost a diode drop, I say that it's backwards.In any case, since it's always measuring low I don't think it's oscillating because spikes would make it read much higher.The other measurements (like the output) it's not as easy to tell because spikes of zero would not affect the reading as much.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 31, 2020, 11:08:43 pm
content: Thank you for your insight, the TIP147 transistors are connected the correct way round. I measure the diode drop with a meter. I know is only a very basic test. I've got a 60Mhz analogue scope, I don't know if that would be of some use in testing this or not. I just can't understand why it's putting out almost twice the expected voltage. I've use darlington transistors before, but never had this happen. I have other 12 positive voltage regulators, the LM2940 LDR, might try one of them, just to see if it is the L7812 that's at fault.

author: Renate
author_rank: Super Contributor
timestamp: August 31, 2020, 11:18:00 pm
content: Don't start going down the rabbit hole on other designs, finish this one first!Disconnect the 7812 on the TIP147 setup.The output should go to zero.If it doesn't the TIP147 is to blame.Yes, if you have a scope, use it!It doesn't have to be the accurate.You need your electronic eyes to see what is going on.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 01, 2020, 12:57:54 am
content: Ok, yes i will see what happens removing the L7812 first. Maybe 100 ohms for a base resistor is to high, would halving its value be worth a try ? Its the math with this scenario that i often get hung up on.

author: Renate
author_rank: Super Contributor
timestamp: September 01, 2020, 01:27:38 am
content: We're not dealing with a subtle problem, those transistors are full on/dead short/passing whatever they can.We know the 7812 is ok.The 100 ohm is a finer nuance that we can see is not a problem with the TIP36C.I'm just trying to show you that with no 7812, just the TIP147 in circuit it's like a dead short.I don't understand why if this circuit works fine with the TIP36C you keep on trying to make it work with the TIP147.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 01, 2020, 02:13:55 am
content: From a while ago now, an experienced guy told me darlington transistors suitable as series pass transistors have less voltage sag under load due to there high gain. i was told about the diode drop across these type of transistors, also oscillation could be a problem in this application, again due to the high gain. I have had great results with the MJ11015 darlington transistors. I will try removing the L7812 just to see if the output drops to zero. If i still can't figure it out i'm going to re build the circuit, and improve high current paths. As its strip board i use, i'm going to reinforce high current paths with 1mm annealed copper wire. Where iv'e gone wrong in the past, expecting to much current through insufficient path ways, to fine AWG. Something tells me iv'e stuffed up the circuit some how, yet another tells me the TIP36C's work fine. so have i stuffed something up ? i just don't know now.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 01, 2020, 09:23:11 pm
content: Well that's it.... I Looked over the circuit very closely. I took the advice about disconnecting the regulator. There was 23.00 volts on the output. I went over the circuit for what seemed like ages, with no short I could see anywhere. Then I began cutting link up wires, in the vain hope I'd eventually cut the offending link. I ended up with 7.83 volts with no wire jumpers. Go figure, I can only conclude that one of the terminal blocks I used for transistors, regulator, and input output must be shorted with in the terminal blocks itself.I'm not discouraged for use these terminal blocks for remote connections. But I will check each one carefully before using it in a circuit. The terminal block and molex line socket just makes a neat connect disconnect solution, if a failure occurred it makes replacement really quick and easy. I've yet to desolder the terminal blocks, but I'm fairly certain that there must be a short in one of them.

author: Renate
author_rank: Super Contributor
timestamp: September 01, 2020, 10:01:49 pm
content: If it were the terminal blocks, why does it all work fine with the TIP36C?What does your ohmmeter say on the TIP147s?

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 02, 2020, 12:27:20 am
content: That's something i can not answer, as i have no idea. What scenario could give me 23.00 Volts on the final output, with no regulator ? I can only imagine the input was making it down the final output rails, but iv'e no idea where. I tested with the multimeter as went along each section of the board. Its almost embarrassing, i make no excuses for my poor maths, but iv'e rarely screwed up a simple single sided strip board project. iv'e drawn out another piece of strip board and wil start over, but much more careful this time. I know the TIP36C x 2 worked great, even quite good under an 8.4 Amp load, i have no idea what went wrong with the TIP147's and i did try several different pairs of TIP147's as i have some from different suppliers. It explains the zener diode failure in the other thread i started. Thanks for all the help, I have learned quite a bit more from the posts you all wrote. I'm just a novice hobbyist that has an interest in anything power supply or related to power supply stuff.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 02, 2020, 06:14:31 am
content: If you need only 8 Aps then an only one LM1083 can do the job (it depends on input voltage too. It mustn't be too high under load).And yes - that circuit with BJTs is intended to work with regular BJTs, but not with super-h21 ones.