Understanding opamp-controlled transistor driver circuit - Page 1
Forum section: electronics / beginners

author: hp3310a
author_rank: Regular Contributor
timestamp: September 01, 2025, 07:50:46 am
content: I've been playing with the innards of an Agilent 6611C precision power supply (against its will), specifically its fan driver circuit. I noticed that the 12V fan only ever gets 10.5V, and I wanted to know if that is on purpose or a fault. So, I started reversing the relevant board section and eventually arrived at the schematic below. I'm also attaching pictures of the board, though not all connections are visible, most are.The fan speed is controlled by the logic board which controls the fan through an opamp-controlled transistor. The logic board outputs 2.8V (min speed) to 3.8V (max speed). That is translated into 7.8V (min) and 10.5V (max) at the connector where the fan gets its power from (these values have been measured).The opamp in question is a MC34072 (dual) by Onsemi, I split it up into its components to make the schematic easier to read. According to the data sheet the output swing is -14.7V to +14V for a +-15V supply (which we have).Via the voltage divider R416/R413, the fan output voltages produce a voltage on the negative input of the opamp that exactly matches the positive input (for min and max input voltage). This tells me that (a) this isn't all wrong and (b) 10.5V is in fact the voltage that it is supposed to produce (not 12V).However, I really would like to understand how this works, and what type of transistor is used (due to its location it is not possible to read the markings). Evidently it works, but why and how?

author: MathWizard
author_rank: Super Contributor
timestamp: September 01, 2025, 03:27:32 pm
content: The fan voltage, will get set by the voltage divider resistors, and the op-amp output will go as high as it needs to, to make the input pins equal voltage. So the Vn pin will be about (4.87/(4.87+9.09))*Vfan, since it's a voltage divider. And looking at it another way, the Vfan be about (1+9.09/4.87)*Vp, where Vp is the non-inverting pin and Vn is the inverting pin.The supply voltage must be high enough, to allow for the desired input to output voltage gain.

author: ledtester
author_rank: Super Contributor
timestamp: September 01, 2025, 03:59:30 pm
content: This is an example of a linear regulator using an opamp.The transistor must be able to dissipate the power between the 25V rail and the fan's power supply. This will be (25-Vfan)*Ifan where Vfan and Ifan are the voltage and current of the fan.For instance, if the fan uses 100mA, the transistor will be dissipating (25-7.8 )*100mA = 1.72W in the worst case.Here's are a couple of videos explaining how an op-amp is used in a linear regulator:#1090 Basics: Voltage Regulatorhttps://youtu.be/RvNQTvdcY2wOp Amps: Linear Regulatorshttps://youtu.be/RgRbRtCyLoEThey both use a Zener diode to establish a "reference voltage" -- this is the same as the control voltage "Pin4_FAN_PROG" in your schematic.Since the supply rails for the op-amp are +-15V, the transistor is likely a NPN or N-channel MOSFET.

author: MrAl
author_rank: Super Contributor
timestamp: September 01, 2025, 07:05:35 pm
content: Quote from: hp3310a on September 01, 2025, 07:50:46 amWhat's with all the diodes?

author: hp3310a
author_rank: Regular Contributor
timestamp: September 02, 2025, 05:49:51 am
content: Thanks for the feedback. The videos where interesting, emitter-follower is the magic term I was looking for. I think NPN transistor is a good bet.However, if you look closely, the zeners have an entirely different purpose in the 6611C schematic above. The zeners in the examples provide the reference voltage for the opamp's positive input. The reference in my circuit is the input voltage from the digital board.In my regulator the zeners do something with/on the output of the opamp and the 25V supply which I don't quite comprehend. I think the diodes are there to achieve a voltage drop but to what end.In TP1, the VR304 zener voltage is 15V, the D336 diode drop say 1.5V for both. So TP1 can be at most 16.5V. Then there's VR303 and D337, causing another drop (in forward direction to the emitter) of about 2V total. So the emitter (pin 1) can't get higher than 14.5V. Is that how it works?It would be really interesting to understand what the zeners are really doing here.

author: MrAl
author_rank: Super Contributor
timestamp: September 03, 2025, 04:40:30 pm
content: Quote from: hp3310a on September 02, 2025, 05:49:51 amHi,They could be limiting the drive to within some range of values to prevent problems if something else goes out of range or during startup or shut down.Either that, or there was a huge sale on zeners and diodes so they bought a bunch, then applied a layer of glue to the PC board, then grabbed a handful and threw them at the PC board. Whatever stuck they just left there.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 04, 2025, 06:13:31 am
content: Quote from: MrAl on September 03, 2025, 04:40:30 pmGood theory, though the possibility of the traces appearing just at the right locations to connect all of this together after the diodes found their place is rather ... slim.Joking aside, if someone knows why they may have done that way, I'd be interested to hear.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 04, 2025, 11:06:25 am
content: Essentially its non-inverting amplifier with a boosted output current capability. It can only supply positve output voltages in this form. Thats whats needed here so, happy days.You can also view it as a regulator with Vfanprog as it's reference voltage. Take your pick.The output voltage is a function of the fanprog voltage and the feedback resistors R146 and R143.Vfan= Vfanprog*(1+(R146/R143)) or 2.87*Vfanprog. That gives an output range of 8-11V with the input values shown.I've added some simplified schematics by way of an explanation. If you know how a non-inveritng opamp circuit works, then it should be clear.The opamp transistor combo increases the current output. A typical opamp can only supply a few mA from it's output terminal. A few mA is enough to drive a medium power transistor. The opamp keeps it apropriately biased for a given output current. In theory, the opamp output current is multiplied by the hfe of the transistor.That might get you a fair estimate on the max output current of a practical circuit. Let's say that the MC34072 will supply 10mA, adding a typical medium power transistor like a ZTX651 with an hfe of 150, an output current of 1.5A might be possible. You need to give yourself some margin to stop your transistor cooking. In the photo, Q305 looks to be a TO220 package on a small heatsink. I'd wager its an NPN Darlington. Why? Well the opamp output current is limited by R310 and would likely not provide enough drive for typical low cost NPN power transistor (with hfe's down in the tens) in that package, but don't quote me on that!The capacitor from the inverting input to the opamp output is used to reduce the control loop bandwith.If the load isn't purely resistive, a sudden change in control voltage may cause instability and poor step response.These emitter follower boosted opamps are notoriously prone to instability if not correctly compensated.The Zeners and diodes are there to protect the opamp, they have no impact on the control function.

author: MrAl
author_rank: Super Contributor
timestamp: September 04, 2025, 02:25:58 pm
content: Quote from: hp3310a on September 04, 2025, 06:13:31 amWell as the first line says, zeners are often used to limit the range of some voltage or current. If you place a 7 volt zener in series with a 12 volt supply you get 5 volts. Although I don't like to do it that way I did do that on one project that was not that important: an LED Christmas tree, long time ago.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 04, 2025, 05:05:51 pm
content: Quote from: Terry Bites on September 04, 2025, 11:06:25 amThanks for taking the time to go into details, much appreciated! You are right, the transistor is in a TO-220 package on a heat sink. I guess the transistor facing forward diode pair really is there to further limit the maximum voltage the transistors gets on the base thus limiting the output voltage to something the fan can handle.

author: jwet
author_rank: Frequent Contributor
timestamp: September 04, 2025, 09:43:25 pm
content: I think you have this pretty well rung out this circuit operationally but the overall reason for all this fooling around is to use the unregulated 25v rail to run the fan rather than contaminating the +15V rail- an analog supply. Fans can be a noisy load.If you draw the circuit differently, it might make more sense. Its basically a level shifter that get the op-amp swing into the right range using the 25v supply. The 15v isn't providing any current to the fan and won't get noise on it. Starting at the 25v, going through the 2k resistor and then the 7.5v zener, op-amp output then on to then on the 15v zener with the extra diode drops. The total of all that those zeners and diodes is 25v with the op-amp allways sinking current so can't put noise on +15, all its current is sinking. Its output can only go to 14.7, so it has to sink to keep the 7.5v happy and the 15 makes sure it can't exceed 15- it won't but this is HP. The two series diode drive the transistor and the loop gets closed by the feedback R's. The 10.5 max is common, fans last longer and make much less noise if not run at max voltage. The airflow is about 90% but the noise can be much higher. This is HP again- perfection. Who wants to listen to a screaming fan all day in the lab.HP and formerly Harrison labs which HP acquired for power supply expertise were artists. You can learn a lot from trying to understand these circuits.

author: hp3310a
author_rank: Regular Contributor
timestamp: September 05, 2025, 05:26:05 pm
content: Quote from: jwet on September 04, 2025, 09:43:25 pmExactly! Many thanks for explaining the motivation for doing this the way they did!! This whole mini-circuit is a lot cleverer than it appeared to be when I first looked at it.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 05, 2025, 06:12:57 pm
content: Here is the fan driver circuit from a HP6632B - they share some of the same modules with the 6611C. My guess would be that the fan driver is also common, which would make the mystery transistor a TIP 122 - NPN darlinton.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 05, 2025, 07:05:15 pm
content: Quote from: jwet on September 04, 2025, 09:43:25 pmI understand how it works, but still it does not make much sense to me to do it so complicated. What are the benefits?

author: floobydust
author_rank: Super Contributor
timestamp: September 05, 2025, 07:27:24 pm
content: The extra diodes set a minimum and a maximum speed for the fan.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 05, 2025, 09:01:58 pm
content: Quote from: Doctorandus_P on September 05, 2025, 07:05:15 pmThese power supplies are processor controlled (supervised). The fan speed (voltage) can be temporarily reduced for quieter running, or maxed-out in fault-conditions. The benefit is that the processor can be programmed to adjust the fan to suit the environment.Quote from: Doctorandus_P on September 05, 2025, 07:05:15 pmIt is essentially an interface between the processor and the fan - an "amplifier" if you like. Which components would you remove to make it simpler?

author: jwet
author_rank: Frequent Contributor
timestamp: September 05, 2025, 09:36:21 pm
content: Quote from: Andy Watson on September 05, 2025, 09:01:58 pmPlease read my post above- this transcends the clumsy design that we have all gotten used to. This is Art- note its not expensive or complex really. They want to use the 25V raw supply as not to contaminate their nice analog supplies. The op-amp is always sinking and its quiescent plus a small current for base drive will make sure that +15 won't see the base current to the fan. The -15v potential rail noise is attenuated by the PSRR of the MC34072. It's really elegant.The schematic provided by Andy Watson has the circuit drawn in a more sensible way as I mentioned- you can see its mostly just a level shifter with its own bootstrapped kind of elegance.This old stuff brings tears to my eyes- learn from the masters...

author: hp3310a
author_rank: Regular Contributor
timestamp: September 06, 2025, 05:29:33 am
content: Quote from: Andy Watson on September 05, 2025, 06:12:57 pmThanks for this one! Amazing how much different (and better) it looks and it is still exactly the same thing (except for the double diodes on the 6611C).

author: Terry Bites
author_rank: Super Contributor
timestamp: September 06, 2025, 01:52:12 pm
content: It's been designed to be ultra reliable, that is (was) the HP way.