Simple Electronic Control Systems - Current Control with 1 transistor

RE: Simple Electronic Control Systems - Current Control with 1 transistor

ChrisDancer — Sun, 11 Oct 2020 17:11:32 GMT

One problem with this circuit is that the base- emitter voltage of the transistor and the forward voltage of the diodes will change with temperature, which will cause the current to vary. You can reduce the temperature dependence of the curcuit greatly by using an ordinary red LED in place of the 3 diodes. The change in the forward voltage of the LED with temperature is roughly equal and opposite to that of the transistor junction.

The forward drop of the LED will be about 1.8 V.

RE: Simple Electronic Control Systems - Current Control with 1 transistor

jc2048 — Fri, 18 Sep 2020 08:49:40 GMT

I had a play with Jan's circuit and this is how it gets on dynamically [the load is a blue LED and I'm using a 2N3906 transistor]:

[View:/resized-image/__size/480x234/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-aa1920d9-888a-4f9f-bc27-9b9f432c74d3/contentimage_5F00_192319.png:480:234]

I'm controlling it with an Arduino UNO output [yellow trace], via a simple diode and a resistor network, to regulate at

two currents [around 6mA and around 3mA]. The blue trace shows the LED current, measured by using a series resistor as a 'current probe'.

RE: Simple Electronic Control Systems - Current Control with 1 transistor

Jan Cumps — Fri, 11 Sep 2020 13:12:10 GMT

I called this the simplest one, but what about a diode that can keep a circuit fixed at 0.7 V. Isn't that an even simpler control circuit?

I haven't used that one because there's no active element in there, a self regulating circuit. I'd consider a diode in that configuration as a clamp instead of a control circuit.

Open for discussion and critique.

RE: Simple Electronic Control Systems - Current Control with 1 transistor

Jan Cumps — Wed, 09 Sep 2020 19:24:20 GMT

For a next post, one that's simpler but moodier:

[View:/resized-image/__size/335x366/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-aa1920d9-888a-4f9f-bc27-9b9f432c74d3/2844.contentimage_5F00_192318.png:335:366]

RE: Simple Electronic Control Systems - Current Control with 1 transistor

Jan Cumps — Tue, 08 Sep 2020 20:07:40 GMT

If you want to play along at home, and you have NPN transistors, here's the counterpart.

Components:

a common NPN, like BC547, 2N3904, 2SC945, ...
a LED. A common red one is ok.
220R, 2K2 and 10K resistors (also common values)
A current meter
A 10 V DC power supply.

[View:/resized-image/__size/620x679/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-aa1920d9-888a-4f9f-bc27-9b9f432c74d3/0361.contentimage_5F00_192313.png:620:679]

You can use the 5K resistor and 3 diodes, like in the original post (mirror them too - 3 diodes between ground and base, pointing up, and the 5K between base and 10V).

In this case, I replaced the base bias circuit with a simple voltage divider. It 'll put the base at 1.8 V same as the 3 diodes.

I chose the values deliberately.:

A 220 Ohm and a 1K combination would work too, but takes more current for no added benefit.
A 22K 100K combination would not be ok, because that's not a strong enough divider in this case.

A rule of thumb when designing voltage dividers with resistors:

The lower part of the divider should have an order of magnitude (/10) less impedance (Ohm) than the input it is driving.

Otherwise it's too compliant, and will be impacted by the load.

How do you know the input impedance of a current follower (this one in particular, without base resistor):

hFE of this transistor (if you take a common NPN) is about 100. (actually 200 for most low signal ones, but to cover whatever you have in your component box, let's settle for 100)
input impedance is output impedance / hFE
output impedance is 220R (the emittor resistor).
input impedance = 220R * 100 = 22K.
So our lower resistor in the voltage divider should be no more than the input impedance / 10 (one order of magnitude) -> 2K2 (I use E12 series values everywhere in this design, because they are the most common values)
The upper resistor is then calculated to give a 1.8V divide if supply voltage is 10 V. You can calculate this using the "rule of 3", but I used an online calculator[View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-aa1920d9-888a-4f9f-bc27-9b9f432c74d3/4834.contentimage_5F00_1.png:16:16]
[View:/resized-image/__size/620x168/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-aa1920d9-888a-4f9f-bc27-9b9f432c74d3/3007.contentimage_5F00_192314.png:620:168]
This will give 10K as the upper resistor.

I haven't tried or simulated the circuit. Only calculated.

The current meter should indicate 5.45 mA

Even if you put 2 LEDs in series - or no LED at all, without changing any other component, you should still measure 5.45 mA.

Comment below if you tried this and your DMM reports 5.45 mA... Or if it doesn't...

RE: Simple Electronic Control Systems - Current Control with 1 transistor

Andrew J — Mon, 07 Sep 2020 11:32:34 GMT

Rreally interesting read Jan, I like these fundamental demonstrations. Where has my maths gone wrong though: 8.2V / 5000ohms = 1.64mA not 160mA??

RE: Simple Electronic Control Systems - Current Control with 1 transistor

DAB — Sun, 06 Sep 2020 18:17:07 GMT

Very good explanation Jan.

Just one nit, up where you calculated the emitter current your last value should be 5.45 ma not 4.45 ma.

DAB

RE: Simple Electronic Control Systems - Current Control with 1 transistor

jw0752 — Sun, 06 Sep 2020 17:44:37 GMT

Thanks for the nice posting Jan. Now I will have a fun evening wiring it up and exploring the limitations.

John

RE: Simple Electronic Control Systems - Current Control with 1 transistor

genebren — Sun, 06 Sep 2020 16:30:29 GMT

Great write-up on a simple but elegant current control system. Very interesting.

I am a little a little curious about the power connections on your breadboard in the opening image (red and black with white heatshrink). Are the connections at the protoboard on component leads, wound spring based? Where did you source these from? Do they make a pretty solid connection?

RE: Simple Electronic Control Systems - Current Control with 1 transistor

colporteur — Sun, 06 Sep 2020 16:24:32 GMT

It has been some 40 years since tech school. I found the tutorial refreshing. Great job.