Transistor Vebo - Page 1
Forum section: electronics / projects

author: cv007
author_rank: Frequent Contributor
timestamp: March 03, 2025, 06:56:25 am
content: I have a pc board I'm working on which is basically a bunch of lamp dimmer circuits. It quite an old design that uses a transistor, by default via pullup, to bypass a dimmer resistor. The lamps are 28V/40ma (2ea in parallel) so around 80mA when on, are turned on via a 28v signal which also supplies the lamp power (lamps grounded in this case). To dim, the transistor base is pulled to ground (through a diode, as there are a number of circuits sharing this dim ground signal), which turns off the transistor and the current then flows only through the dimming resistor.Not a complex circuit, and I put the circuit in several simulators to play around with and did not see anything strange in simulation. But, in actual use I noticed that when a lamp is disconnected/bad there is about 70mA running through this circuit when in dim mode. The ~12mA was already a known as that is simply the 28v through the pullup resistor to the dim ground signal (when dim is on). The other ~60mA is flowing through the base and I am now assuming this is due to Vebo where the emitter voltage through the dim resistor is now without any 'pull down' from the lamp so is at a voltage where Vebo breakdown occurs. Is this correct?I did put this circuit on a breadboard and it acted the same as the real circuit. Not sure why the simulators I tried did not exhibit the same problem but I guess they do not fully simulate the transistor (a few were online simulators, and I have the Livewire program). This pc board also has negative input signals, with the lamp providing the 28v, and is basically the same circuit with a little rearranging, except that circuit has a diode on the base of the transistor before the pullup resistor. I did not understand what the purpose of that diode was, but now think it is to prevent what I am seeing on the other circuit (and apparently should have been on both). On my breadboard circuit, I added a diode to the base, and the ~60mA goes away.I guess another question is- when a lamp fails is the transistor for that circuit now in an accelerated path to failure?The circuit I am showing has the lamp disconnected, and dim on.

author: moffy
author_rank: Super Contributor
timestamp: March 03, 2025, 07:17:29 am
content: It won't show up in a SPICE sim as they often don't model breakdown voltages, except in special parts like zeners. It is not adviseable to run the transistor in reverse Vbe breakdown with so much current, but if you move the end of the 270R resistor from the emitter to the cathode end of the diode in series with the lights that should solve the problem.

author: cv007
author_rank: Frequent Contributor
timestamp: March 03, 2025, 01:00:43 pm
content: Thanks for the reply. Its a board that has to stay unmodified in its design, so its more about understanding what the cause was for the extra current. Your suggestion is good and also works. This board has a number of diodes for reverse protection- all signal inputs go through diodes and also the outputs which have lamps grounded (such as the circuit shown). The design seems to be quite protective of getting reverse signals on its inputs and protective of seeing a voltage on the outputs which are normally grounded through the lamps.I made a test pc board which connects to this board so all circuits can more easily be tested via its external connections, but I only have a single test lamp manually selecting a single circuit to test. I was not expecting the current seen when the lamp was disconnected (and signal still applied when in dim mode). I have never given Vebo much thought nor have seen it in action.Maybe another solution (if it was allowed to be modified) would be to replace the transistor with a mosfet. The transistor is a TO-39 metal can package and it doesn't seem difficult to find a mosfet which has a high Vgs max. Its only function is as a switch so needs nothing special, and when on probably only sees a 1/4W of heat (80mA with a few volts drop). Maybe there are transistors with a high Vebo but it seems this spec does not make its way into the selection options in digikey/mouser, and from I have found so far most are in the 5v-9v range.

author: moffy
author_rank: Super Contributor
timestamp: March 03, 2025, 08:30:03 pm
content: A MOSFET would be a little indeterminate in the voltage it put across the lamps, also Vgs maximums are usually +/-20V and in switching the DIM you could exceed that, though not likely.e.g.https://www.vishay.com/docs/91015/irf510.pdf

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 03, 2025, 09:27:52 pm
content: Quote from: cv007 on March 03, 2025, 01:00:43 pmThere are, but they are not common and are usually used for Audio muting.One example is DTC923T, that has a 2k2 base resistor inbuilt and a 40V VEBO rating. It also has high HFE as a bonus.

author: cv007
author_rank: Frequent Contributor
timestamp: March 04, 2025, 04:03:31 am
content: QuoteThere are about a couple dozen I found searching digikey that are both TO-92 and have Vgs +/-30v or greater. I did test a 2N7000 in place of the transistor on my breadboard test circuit, and it did work ok and the voltage drop across it was pretty close to that of the transistor. Maybe another solution would be to take a surface mount transistor and emulate a through-hole package via small/tiny pcb, incorporating a diode at the base to get the reverse breakdown out of reach. Probably will not be changing anything though, and the transistor failures do happen occasionally but not excessive. The board has some kind of conformal coating, so is a bit of a pain to deal with when it needs work.These circuits drive 2 lamps in parallel, so until both lamps fail (or are not connected, such as is the case with my test setup) these transistors do not show the breakdown current. I had to change my testing method a little bit but can continue on now that I know what was happening and why. Sometimes these datasheet numbers remain dormant until some day they decide to reveal themselves. Now I know, and also know simulation can miss revealing this detail.