Multimeter

Thread Starter

gregg_a_g

Joined Oct 31, 2020
10
I'm a relative newbie to this forum. In a previous thread I inquired about a Hit and Hold Solenoid driver (https://forum.allaboutcircuits.com/threads/hit-and-hold-solenoid-driver.173857/).

I have built this little circuit on a breadboard to test it out before I put it on a PCB stripboard (also, my soldering isn't too good yet, so the breadboard really helps me get things figured out)!

1606519178465.png

Anyway, after assembling the breadboard I used a "cheap Harbor Freight" multimeter to check the voltages.
I have a 13.8V wall wart that is supplying power to the board. The idea of this circuit is for the "output" (bottom connector)
to "hit" a little CO2 solenoid with about 12V at turn-on to energize and open the valve. After about 25-30ms, the
voltage decays to about 6V to "hold" the solenoid valve open. By halving the voltage, the power dissipated in the
solenoid is reduced by 75% - so the solenoid should last a really long time.

When I use the "cheap" multimeter to measure output (without the solenoid connected) it reads 13.8V (same
as the direct voltage from the wall wart. After experimenting a while (different multimeter, checking connections,
trying different resistors) I got pretty consistent results (always 13.8V).

Even when I just put a 330ohm resistor in-line with my wall wart, I still got 13.8V. I would have guessed
that I would get about 6V, but I don't think I'm understanding how the multimeter works. I guess the
impedance, etc., of the multimeter has a big effect, yes?

Finally, I just connected the output to the solenoid, and lo and behold, it energized (so I'm assuming it got
a long enough "hit" of over 12V), and then I probed the solenoid connection with the multimeter and read
about 6V!

Okay, so I guess I have a really crappy multimeter, and a poor understanding of how the multimeter works.

Can someone explain my measurements, and recommend a "good" multimeter that will provide better results?

FYI, the resistor is 330ohm, cap is 680 micro farad.

Even the cheap units work okay to tell you if 110V is "on" or "off", but this is the first time I've tried to build
a "time-dependent" circuit and am in need of a better multimeter.

Thanks a lot!
Gregg G.
 

Thread Starter

gregg_a_g

Joined Oct 31, 2020
10
Or perhaps your meter is right but you don't quite understand how the circuit works.

Ha, ha! I'm sure you're right about that!

I think the capacitor never gets "charged" when I just measure with the multimeter, because there isn't enough current flowing.
So that makes sense to me, but I don't understand why I read "13.8V" with just the 330ohm resistor in line with the 13.8V wall wart.

I'm glad the circuit is able to "energize" the solenoid and drop down the voltage to about 6V, but I'd like to understand more
about the measurement process. My fish and plants will be happy (the solenoid valve controls CO2 injection into a planted aquarium),
but now it's all about learning what I can about circuits and electronics. It's really a lot of fun for me!

Kind regards,
Gregg G.
 

MrChips

Joined Oct 2, 2009
23,515
Ok, here goes.

When you connect the voltmeter at the output, zero current flows (well, almost zero).
The voltage across the resistor is 0V. Hence the meter sees the full 13.8V from the power supply.

With the solenoid connected and the circuit at steady-state, the solenoid takes current. How much current?
Assuming that is RED-RED-BROWN on the resistor = 220Ω

I = (13.8V - 6V) / 220Ω = 35mA

Now that current is flowing, the voltage across the resistor is 7.8V and the solenoid sees 6V.
The power dissipated by the resistor is

P = I x V = 0.035A x 7.8V = 0.27W
The resistor will get warm. Make sure that the power rating of the resistor is at least ½W.

Before you connect the solenoid the capacitor is discharged. The instant you connect the solenoid there is an inrush of current flowing attempting to charge the capacitor. This is enough to engage the solenoid. I cannot tell you how much current flows initially without knowing the inductance of the solenoid.
 

dl324

Joined Mar 30, 2015
12,871
FYI, the resistor is 330ohm, cap is 680 micro farad.
The resistor in the picture is 220 ohms.

Do yourself and us a favor and learn how to draw a real schematic. Those Fritzing cartoons are awful to read. Especially when you don't take the time to plan the wires.

EDIT:
This is what I transcribed from your Fritzing layout:
1606525318148.png
Makes no sense to me.

You're misusing the busses.
 
Last edited:

Thread Starter

gregg_a_g

Joined Oct 31, 2020
10
Mr Chips - Thanks for the explanation. Yes, I have a 2W resistor. The solenoid draws about 56mA with 12V.
So the multimeter has very high impedance which leads to "almost zero current flow". I think I get it now.


Dennis - Sorry for my bad "breadboard" picture. Here is the actual schematic.

1606526525412.png

I did create the schematic first, and then moved to the "breadboard" to try and figure out how to connect it. What do you mean when
you say "misusing the busses"? Sorry for my ignorance, but I am just getting started with a lot of this!

The idea is that the capacitor allows "full voltage" to flow for about 25-30ms. Once the capacitor charges all of the current should then
flow through the resistor and thus reduce the voltage across the solenoid to about 6V. Clippard (the solenoid mfg) suggests about 20-25ms
at 12V, then reduce to about 6V to "hold" and extend life of the solenoid.

I had a 13.8V wall wart handy, so I am using that. When I run KiCad Spice simulator, it shows about 12V for 25ms and then drops off
to 6V.

The circuit seems to be performing as expected. I am laying out the "stripboard" design, but wanted to try it with the breadboard before
I started trying to solder.

Thanks for your replies!

Kind regards,
Gregg G.
 

dl324

Joined Mar 30, 2015
12,871
What do you mean when
you say "misusing the busses"?
You didn't need to use the buses at all. They're normally used for power.

What you had on the breadboard could have been drawn like this:
clipimage.jpg

It doesn't match the schematic you were trying to prototype.
 
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