1

Disclaimer: I'm trying to learn about circuits on my own. I'm reading as much as possible to avoid any basic mistakes, but if there's any, please point it out and I'll investigate further in that direction! Sometimes the problem is not lack of interest, but not knowing what you don't know (if that makes any sense). :-)

I want to build an Ammeter circuit to measure currents with an Arduino. But before I do that, I was planning to build my own current sink, in order to be able to connect it to my regulated power source and be able to draw currents to verify the measurements.

Since I have many LM358Ns, I figured I could try to reproduce the "High Compliance Current Sink" from page 18 in this datasheet.

The maximum current that I want to sink is 100mA.

I've designed a prototype and simulated connecting a 5V power source:

enter image description here

The current values in Cload1 and 2 seem to be perfectly aligned with the voltage in Vreg1 and 2.

I have a few questions though:

  • I've split the circuit in two different LM358Ns because I believe the 2n2222s are dissipating 250mW each (5V, 0.05A). Is this correct?
  • Is it ok to use a single POT like I'm using for both LM358Ns?

And last but not least: let me know if there's any improvement, or anything else that's obviously wrong with either the theory or my implementation of it!

Thanks!

diegoreymendez
  • 657
  • 1
  • 6
  • 10

1 Answers1

2

At 5V you should probably be OK. But who knows what the safe operating area is for those transistors... Why not use a 2n3055 after the 2n2222? Then the 2n3055 will never fail at least up to 10V and 2A. Also, be very careful if you replace the transistor with a mosfet in any electronic load. Check the DC SOA curve of the mosfet. [At 10V and 200mA, probably even a BD139 will be fine, and has a hole to attach a heatsink.] Make sure it is in a darlington configuration with the 2n2222, so base current (op amp output) is minimal and will not throw off readings.

As for your question, LM358 has a fairly high offset voltage. Additionally, load resistors will not be matched. So the op amps will settle at different voltages across the resistors. Meaning, different currents through the transistors. Should probably not cause a thermal runaway (as long as transistor SOA is maintained at DC). But, probably better to use a single op amp if you want any reliability between readings and linearity (i.e. single op amp driving 2 transistors, each with its own separate load resistor at emitter).

Why is it a problem? Probably not a problem if an ammeter is permanently attached to collector. Although, it will never be as accurate as placing ammeter at emitter, due to base current coming from op amp. Also, the components will have different temperature characteristics, so results may not be repeatable with respect to the position of the reference pot wiper position. Also, it is mighty inconvenient if you just want to measure voltage at the load resistor and calculate current, since there are two of them.

Since the circuit aims to calculate microcontroller current, I'm guessing accuracy is a requirement, if not at uA range, then at least in 0-5mA range. Probably best to use a TIP122, or a really suitable mosfet rated for linear use. If the circuit is never ever operated at voltages above 10V, an IRFZ44N might be fine up to 1A See my question here. Since no gate current flows into the source, this will give most accurate result.

PS: also, your schematic shows the LM358 running from the same supply. I'm assuming you have a separate regulated source for the LM358 and reference pot.... i.e. they are not connected to the transistor collectors which should be at the power supply under test.

Indraneel
  • 2,117
  • 1
  • 17
  • 24
  • How does different currents through the two loads cause a problem? They just add together to make a load for the supply under test. – The Photon Aug 23 '18 at 04:51
  • Can you explain why a FET is not suitable for an electronic load? virtually all modern electronic loads are FET based. – Jack Creasey Aug 23 '18 at 05:01
  • Good questions. Edited answer to address comments. – Indraneel Aug 23 '18 at 05:06
  • You are still discouraging from using mosfet. Why is that? – Chupacabras Aug 23 '18 at 05:20
  • I'm fairly sure the mosfet doesn't have to be "rated for linear use", because of the negative feedback going to the opamp – BeB00 Aug 23 '18 at 05:48
  • 1
    Like I said above, check the DC SOA curve. Also see the link to my question. The important thing is that if the mosfet is fried, there may not be much resistance between drain and gate, so the rest of the circuit, op amps etc, may get fried too. This is more likely with a mosfet, and less with a BJT. I am not discouraging use of a mosfet. Mosfet will give the most accurate result. – Indraneel Aug 23 '18 at 05:54
  • Yes, mosfet has to be rated for linear use. An electronic load is a linear device. The mosfet is never being operated as a switching device. – Indraneel Aug 23 '18 at 05:55
  • 1
    *mosfet has to be rated for linear use* Show us an example of such a MOSFET. My bet: you can't because these don't exist. Except for RF MOSFETs basically all discrete MOSFETs are intended for switching applications (low Rdson). But even a MOSFET for switching can be used in a linear circuit, I know because I have done so myself. As long as Rdson can be controlled by Vgs, the MOSFET will work as a linear device. Even if the relation Rdson(Vgs) isn't linear that doesn't matter, the opamp can still control the MOSFET and therefore the current. – Bimpelrekkie Aug 23 '18 at 06:59
  • 3
    Please see this link first: https://electronics.stackexchange.com/questions/361407/dc-operation-for-irfz44-mosfet-soa-curve-max-current-at-40v Also this link for LINEAR POWER MOSFETS WITH EXTENDED FBSOAS http://ixapps.ixys.com/family.aspx?i=5 and also http://ixapps.ixys.com/family.aspx?i=54 and http://ixapps.ixys.com/family.aspx?i=56 – Indraneel Aug 23 '18 at 07:01
  • Essentially, this may also include mosfets specifically made with planar technology for audio power amplifiers like the common and inexpensive http://www.mouser.com/ds/2/149/FQP50N06-1009503.pdf but I have not tested it. They do say it is for fast switching however, so probably not recommended. – Indraneel Aug 23 '18 at 07:08
  • The above question/answer is irrelevant here as it talks about operation outside the SOA (Safe Operating Area). That's always dangerous and common knowledge that MOSFET's don't handle that very well. **But** one should not be operating the MOSFET outside SOA. For the circuit discussed here that's easy to do, just select the right MOSFET. Since power is quite low, that should be easy. For example: IRFZ44N can handle 49A and 94 W it will be difficult to destroy it as long as you limit the maximum current from your power supply. – Bimpelrekkie Aug 23 '18 at 07:09
  • 1
    I suggest actually testing out what you say, instead of quoting theory from the datasheet. Please read the link I have posted. The IRFZ44N will be destroyed at only 500mA at 40V DC. Other common mosfets may not fare that well. I do agree that for the intended operation in this question (5V 100mA), the IRFZ44N will work fine. – Indraneel Aug 23 '18 at 07:11
  • *mosfets specifically made with planar technology for audio power amplifiers* I do not see anything special about that FQP50N06, I do not see **any** sign that this is **not** a switching MOSFET. show me another example. – Bimpelrekkie Aug 23 '18 at 07:12
  • 1
    Switching is a mosfet's claim to fame. But larger dies (less mosfets per die, as in planar) common in old mosfets will handle linear operation better. Please see the IXYS links I posted above. – Indraneel Aug 23 '18 at 07:15
  • *The IRFZ44N will be destroyed at only 500mA at 40V DC* Sure even 100 V 10mA it will not survive but we're using a low voltage DC supply here. You haven't shown any real proof of the false claims you made. Like why a MOSFET needs to be a "linear type" for a linear circuit. Or why a MOSFET isn't a good idea for a current load while 9 out of 10 active loads use MOSFETs. There's a teardown by EEVBlog on Youtube of a professional type active load, what do they use: MOSFETs. – Bimpelrekkie Aug 23 '18 at 07:16
  • Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/82117/discussion-between-indraneel-and-bimpelrekkie). – Indraneel Aug 23 '18 at 07:18
  • *old mosfets will handle linear operation better.* Better in what sense? If I want a good common drain stage (like in Audio amp output) I want large W/L to get a low output impedance. Hey presto, switching MOSFETs have high W/L to keep Rdson low. So these work perfectly in an Audio amp. – Bimpelrekkie Aug 23 '18 at 07:20
  • Oh and the 2N3055 suggested has an **extremely** low beta so the output current (at the collector) will have a certain error as the base current from the opamp will be large (might also be a problem). A MOSFET will solve all this :-) – Bimpelrekkie Aug 23 '18 at 07:56
  • 1
    For mosfet safe operation and thermal runaway, see also my question here: https://electronics.stackexchange.com/questions/357636/is-it-okay-to-use-a-mosfet-in-its-resistive-region-with-a-heat-sink – piojo Aug 23 '18 at 08:02
  • Good link. The problem in an electronic load (not this one which operates only at 5V) is that at higher voltages, the problem is greater. This is the Spirito effect. A mosfet is many parallel transistors on the same die. A bjt is only one transistor per die. Even with active feedback (the op amp), some of the parallel transistors in the mosfet may run away. This is why we use emitter resistors for paralleling bjt. These resistors are missing in the individual transistors on the mosfet die. – Indraneel Aug 23 '18 at 09:01
  • Thanks for your reply! I'm still not sure about the first question. If the currents shown in the simulation above were real: is each transistor dissipating 250mW as I think? Or in the case of a single one, would it be dissipating 500mW? I want to know if my understanding is correct or completely off. Also, is there any other thermal consideration in the other components? – diegoreymendez Aug 23 '18 at 19:22
  • PS: I'll consider the switch to a TIP122 as suggested, and I'll split the source for the load and for the sink. – diegoreymendez Aug 23 '18 at 19:24
  • 1
    Yes, you are right about the 250mW per transistor. Actually it is 2.5mW less, because 2.5mW is dissipated in the resistor. Your understanding, as well as the circuit is theoretically correct. The op amp will not dissipate much, as the 2n2222 operates at a gain close to 100. 2n2222 is not a great choice, you need high gain to minimize reading error. With TIP122 or mosfet IRFZ44N you need only 1 op amp. Power the circuit from a separate regulated 5V. Connect the supply under test to the transistor (collector or drain) through a 200mA fuse. – Indraneel Aug 24 '18 at 02:26
  • 1
    To clarify, actually you will need about 6V for the op amp if IRFZ44 is used, as LM358 max output is 1.5V less than VCC, and IRFZ44 is not logic level. For TIP122 even 4V is enough for op amp. Power supply under test can be up to 20V, but preferably under 10V. That will leave a wide margin of safety. – Indraneel Aug 24 '18 at 05:28