Home > DIY Bench Supply > Updated Bench Power Supply Schematic

Updated Bench Power Supply Schematic

November 10, 2010 Leave a comment Go to comments

You may have noticed that my previous post shows strange labels for the rotary switch. This is because i’ve changed the function of the rotary switch.

The ammeter causes a small voltage drop and so when the ammeter is being used as ground, the effective supply voltage is different to when the actual ground is being used. As such i’ve made the switch have the following options for the voltage measurement:

  1. Variable voltage to Ground (VG)
  2. Variable voltage to Ammeter (VA)
  3. External probe to Ground (EG)
  4. External probe to Ammeter (EA)

As such the previous schematic for the power supply needs updating. Below is the updated schematic:

I think this is a slightly more useful design as it allows me to compensate for the voltage drop across the ammeter. The only function i lose is the ability to measure the fixed outputs. But considering the external probe should be able to do this, i don’t really mind.

  1. Jeff
    December 13, 2010 at 2:57 am

    Very good looking project! I was wondering about the operation of the ammeter. It looks like the entire current of the circuit to be measured is going through the meter. I believe the ammeter works by measuring the voltage drop across a shunt resistor (as shown in the directions that came with the meter.) Am I missing something here?

    • December 13, 2010 at 8:26 am

      you’re correct. There are two ground options on the bench power supply, One that is direct to ground and the other is going to ground via the ammeter. These ammeters have the shunt resistor built in so no need to consider it in the schematic though.

  2. Ryan
    April 7, 2011 at 11:22 am

    Hi, I’m having a go at this design, although I don’t know much about electronics. How come in the schematic you have two 4P rotary switches, but in the pictures you have just one next to the variable regulator pot?

    do you think an LM338 will also be okay for the variable regulator? it can put out 5A max. My PSU is 550 watt & the 12v rail delivers 19a.

    • April 7, 2011 at 11:46 am

      The rotary switch i used is a 4P 2 way switch. You can think of this as essentially being 2 switches in 1. turning the knob moves both switches at the same time

      The LM338 will work as a variable regulator but you will have a smaller variable output range. Since the LM338 has a dropout voltage between 1.75V and 2.75V, the maximum output from your circuit would be around 10.25V (12-1.75) at 1A and 9.25V (12-2.75) at 5A. You may also need a larger heatsink if it is to provide more current.

      Let me know how you get on and if you have any more questions, I’ll offer all the help I can

  3. Ryan
    April 8, 2011 at 11:11 am

    Ah so it’s just in the schematic that the separate poles are abstracted as 2 diagrams? Are the two poles connected to one another inside the switch so that in top left position for e.g. the circuit gets closed between ground and external probe

    How about using the MIC29502WT which is the 5a version. If it has a voltage drop of 350mv so I should be on to get it to 24v ?

    also on the schematic you have black GND binding and Ammeter post, but on the compled PSU there are actually four black posts? what are the other two?

    with regards to the ammeter to ground I’m not sure I would use that feature, is the original schematic fine if so? the original one is easy to understand, but I’m not sure what VG and EG are used for. is that when you would want to connect a second black probe to ground and use the PSU exactly as you would use a multimeter ?

    feel free to respond to my e-mail address, I don’t want to clog up your comments list too match! 🙂

    • April 11, 2011 at 2:26 pm

      The two poles are mechanically connected but not electronically connected, so that when in the top left position, external probe is connected to +ve voltmeter and GND is connected to -ve volt meter, meaning the volt meter measures the voltage between the external probe and ground.

      The MIC29502WT would be perfect for this, i was considering using something similar but was restricted by availability. This will give you 12v – 0.35v = 11.65v maximum output.

      The four black posts are really two pairs of posts, so there are two posts for GND and two for the ammeter, this makes plugging multiple things in a bit easier.

      I’m not sure what you mean by “ammeter to ground”. If you mean the ammeter itself then you can easily leave out the ammeter, its corresponding binding posts and its DC/DC converter.

      VG is used to measure the variable voltage output with respect to ground and EG is used for measuring voltages with the external probe, again with respect to ground. The other two options measure voltage with respect to the ammeter +ve terminal.

      The ammeter has a small resistance between its +ve terminal and ground. This resistance causes a small voltage drop that is proportional to the current flowing through the ammeter, meaning that the voltage to the ammeter is not always the same as the voltage to ground. If you are using the ammeter as 0V then you would always measure voltage with respect to the ammeter and if you are using the actual ground as 0V then you would measure voltage with respect to ground.

      Don’t worry about posting in here, others may have similar questions and it gives them a place to quickly look it up.

  4. dave
    July 25, 2012 at 10:14 am

    the connection of the volt & amp meter doesn’t look right. the volt meter should be measuring the variable voltage output wrt ground in parallel to the load and shouldn’t be wrt to the amp meter. the amp meter should be measuring the load current in series to the load

    • July 25, 2012 at 10:48 am

      Normally yes, however with large currents going through the amp meter, the voltage drop across its internal shunt becomes significant. This voltage drop is part of the circuit. By measuring the voltage with respect to the amp meter, this voltage drop is ignored in the measurement so that you can see the true voltage across your circuit.

  5. James
    January 27, 2013 at 11:42 pm

    Are you able to recommend a voltage reg from somewhere like ebay(UK). I’ve failed to understand why it is 5 pin as opposed to 3 pin, presumably a higher voltage one is fine but you don’t want any variance. 25v would be good.

    • January 28, 2013 at 8:57 am

      the voltage regulator is adjustable by turning pot P1 on the schematic, hence the need for extra pins. I don’t have time to go through ebay trying to find something appropriate but you are looking for an adjustable low drop out regulator. The higher the current the better.

  6. James
    January 29, 2013 at 1:21 pm

    Thanks. I wouldn’t necessary expect anyone to search, but as is obvious there are a number of variables to confuse the inexperienced.

  7. BrAtKo
    March 18, 2013 at 9:27 pm

    James: I have bought MIC29152 five pieces pack from ebay, please just search through ebay and I recommend to read MIC29152 datasheet. That’s all, reading, reading… Only one variable you can use – MIC29152.

  1. November 26, 2010 at 2:04 pm
  2. December 1, 2010 at 3:07 pm

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