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DIY Workshop » ε24: Power switch driver circuit amb  Headphone Council Joined: Apr. 1, 2004 |  Message [#1] posted on: 03-24-2008 06:43 AM CST (US).   |  |  News - ε24 official website is now online. Introduction I am planning for my build of the β24 power amp, and would like to use one of the nice Bulgin vandal-resistant pushbuttons as the power switch. However, with the exception of the MP0045 series and the MPL series, all of the other ones are momentary-contact. Also, all these switches have contacts that are not adequately rated for switching the AC mains of a high-powered amp. The MP0045 series are 16mm diameter, a bit small for a large front panel. The MPL series won't fit a 10mm thick panel, which is what I have on the target case (max panel thickness for the MPL series is 4mm to 7.1mm, depending on model). I have my sights set on the MPI002 series which is 22mm, has built-in illumination, and can be mounted on a panel up to 13mm thick. But it's a momentary-contact SPST-NO switch that's rated only 24V 50mA.
Regardless of which of these to use, it is necessary to use a relay to do the actual switching. To make the relay stay in the "on" of "off" state with a momentary-action pushbutton, an always-powered latching circuit is also needed. There is also a benefit in using a relay. The relay could be mounted near the IEC inlet and the power supply, so that AC mains wiring does not need to run all the way to the front panel and back. This reduces the likelihood of inducing AC mains noise into signal wiring. Design So, I set out to make such a circuit, which I am calling the ε24. Instead if re-inventing the wheel, an internet search came up with a couple of sites with some useful circuits: One of my goals is minimum size -- I would like the board to be as small as possible, so that it could be used in other projects. This eliminates the more complex designs. The basic two-transistor bistable flip-flop, or a basic CD4013-based circuit has an unfortunate drawback of indeterministic start-up state, so additional parts must be added to ensure that the amp will always be "off" when the power cord is plugged in. I found that the MOSFET Toggle w/ Debounced Pushbutton circuit (on the first site, which is an adaptation of a circuit designed by John Lundgren), and the similar Doing it discretely circuit (by Winfield Hill, of Art of Electronics fame) are the most appealing. These circuits are simple, has deterministic start-up state, and the switch is properly debounced. So, I built and tested these on a breadboard and liked the results. I decided to base mine more closely to the "Doing it Discretely" circuit. See the site for a description of how it works. I must confess that sometimes I am as guilty as anyone when it comes to creeping featurism, but having a relay switching the power provides another opportunity that would integrate well into this circuit without much additional complexity. Namely, amplifier overheat auto-shutoff. Many high-end and pro-grade power amps have this feature, and I thought it would be useful to have it as an option here. Using Stancor disc thermostats (i.e., thermal breakers) mounted on the main heatsinks, they could enable the amp to shut itself off if the temperature exceeds a cerain threshold. Thus, I modified the circuit and worked this into the design. It needs the "normally-closed" thermal breakers (those with part numbers beginning with STO-). For my build, I chose the STO-170 which will open when the temperature reaches 170°F (77°C). The breaker will auto-reset when the temperature drops to 30°F (17°C) below the rated threshold. Note that this section is optional. If no thermal sensing capability is needed, then it could be omitted. It would be nice to have some sort of visual indication when the power is on, and if the amp shut itself off due to overheat. Thus, I want the circuit to have the ability to light up LEDs for this purpose. Bulgin's MPI001 and MPI002 series switches now has a few new models that contain two LEDs, wired in reverse-polarity parallel. For example, the MPI001/28/D4 and MPI200/28/D4 have a blue and a red LED in them, and the MPI001/28/D1 and MPI002/28/D1 have green and red. It would be extra nice to be able to use this switch. Not only does it lend a touch of class, it also eliminates any need to drill additional LED holes on the front panel. However, the way the LEDs are internally wired in the swtch makes it more complicated for the circuit. But the upside is that supporting these switches does not remove the possibility of using other switches. Discrete panel LEDs could still be used, just by wiring them in reverse-polarity parallel. In fact this reduces the number of wires from the circuit to the LEDs from four to two. I played with various discrete transistor and logic IC solutions and I didn't like any of them, because they all add too many parts to the circuit. Then I decided to use an onboard miniature DPDT relay instead. It actually would take less space on the board than a bunch of active parts. Lastly, I also opted to include a simple regulated PSU onboard, so that all that is required is a small power transformer (approximately 2VA) with a 12V secondary. Schematic diagram (Note: the following schematic is out of date. For the latest schematic see the "News" section above). Here is the schematic diagram. It might appear complex, but the actual onboard parts, after the bridge rectifier and regulator, consists of only two BJTs, one MOSFET, three diodes, one capacitor, six resistors, and a relay. Offboard, there is a power transformer, the momentary pushbutton switch, LEDs (if separate from the switch), thermal breakers (optional), and the 12VDC relay to switch the AC power. The relay that I've chosen for my β24 is the Omron G8P-1A2T-F-DC12 (chassis-mount, rated 30A).
If the thermal breakers won't be installed, then the following parts can be omitted: The red LED (if separate from the pushbutton switch), TB1, TB2, R4, Q1, D2 and K1. Jumper across the T1 and T2 pads, and jumper the K1 relay socket as if the DPDT switches are in their "unpowered" state. Your comments about the circuit are also welcome. [Edited by amb on 03-17-2009 at 01:58 PM.] Attachment: C3613.png |
Dougigs HeadWize Fanatic Joined: Sep. 21, 2005 | Message [#2] posted on: 03-24-2008 08:09 AM CST (US). | | I use the "Doing it Discretely" circuit in one of my amps, and it works quite nicely - - I'm planning on putting one in my β22 when I get to the casing. I've been thinking of mating this circuit with the music-detecting auto on/off circuit I was playing with a couple years ago (it ran into difficulties with the virtual grounds on battery-powered circuits, which prevent a proper turn-off, but that wouldn't be a problem with line-powered amps). Anyway, there are a few of us here who've adapted those vandal-proof switches this way, and a proper PCB would be a welcome addition... I'd snap up a couple.
[Edited by Dougigs on 03-24-2008 at 08:32 AM.] |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#3] posted on: 03-24-2008 08:48 AM CST (US). | |
Isn't that what RLY+ and RLY- are in the schematic? |
| Dougigs HeadWize Fanatic Joined: Sep. 21, 2005 | Message [#4] posted on: 03-24-2008 10:47 AM CST (US). | | Yes, you're right - - if you wanted to have the +12V supply and the main power relay located in an external chassis, you could just attach the 12V supply to RLY+... The only difference would be that the thermal-shutdown indicator light wouldn't work (though the thermal relay itself would). [Edited by Dougigs on 03-24-2008 at 10:49 AM.] |
Ferrari HeadWizer Joined: Apr. 29, 2006 | Message [#5] posted on: 03-24-2008 12:13 PM CST (US). | | Defenitely interested in PCBs for this little cicuit with a simple regulated PSU onboard. Since the 12V - 2VA transformer is relative small, I think that it may be more convenience to have the transformer integrated on the PCB (something like my little PS). The stray field of such a small size transformer is minimum and will probably can't harm anything. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#6] posted on: 03-24-2008 07:12 PM CST (US). | | Dougigs, I think I misunderstood you. Rather than powering the circuit with its onboard regulator, you want to be able to use an existing 12VDC source from the PSU case. If that is what you want, then yes, I agree that having a extra 12VDC line to the board might be useful. Ferrari, I had considered PCB-mount transformers, but that means less flexibility. You'll have to use a specific model of transformer, and it might or might not be readily available on a worldwide basis. Onboard mounting of the transformer also means that a lot of board space is taken up by the transformer, and a larger board will cost more. Even though I have not yet decided to make this board in quantity, I am designing it as if I would. |
| Ferrari HeadWizer Joined: Apr. 29, 2006 | Message [#7] posted on: 03-25-2008 03:25 PM CST (US). | | amb, you are right! I focused too much on "all in one" unit. PCB-mount transformer is indeed less flexibility, also part availability is important too. |
| Dougigs HeadWize Fanatic Joined: Sep. 21, 2005 | Message [#8] posted on: 03-26-2008 04:43 AM CST (US). | | My interest in using this switch with the β22 (and thus with an external AC chassis) is partly because the thermal breakers you've sourced seem perfectly designed to mount on the tops of TO-220 heat sinks (approximately 1" spacing between screws - - just need to tap the heatsink's top and screw these on). The β22, if used with speakers and onboard heat sinks, is an even better candidate for this, since it's in greater danger of thermal overload. I'm using my β22 as both headphone and speaker amp, with 2.5" high heat sinks on three channels (and a pair of σ22 with same size heat sinks - - not sure if this will be mounted in amp chassis or AC chassis). It powers medium-efficiency speakers to fairly loud levels - - gets hot, but not up to the danger level. Still, if anything needs a circuit to monitor its thermal levels, it's this amp - - there's no way to know who will be using it, and how. Even if you don't decide to mass-produce this PCB (an understandable decision for such a single-use item), I think I'll pick up two of those thermal breakers and bolt one to the top of a heat sink on the L and R channels (ground channel is not being used for speakers). The IRFZ24N claims it's good to 175°C. I'm assuming, given the thermal resistances, that we don't want to go much above half that value... so the breaker you've selected, or one perhaps 10 degrees hotter, sounds right, doesn't it? [Edited by Dougigs on 03-26-2008 at 04:45 AM.] |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#9] posted on: 03-26-2008 04:58 AM CST (US). | |
The 175°C rating is junction temperature, which will be a lot hotter than heatsink surface temperature. Half is probably a good number to work with, I chose slightly lower for extra safety. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#10] posted on: 03-26-2008 05:11 AM CST (US). | | I made a PCB layout for this circuit. The board size is tiny: 2" x 1.2". Here is the layout, a 3D rendering, and an updated schematic to match the minor changes I made. The 3D render is missing the relay because Eagle3D's library doesn't come with it, and I'm too lazy at this time to make one.
Attachment: C3632.png,C3630.jpg,C3631.png |
| Dougigs HeadWize Fanatic Joined: Sep. 21, 2005 | Message [#11] posted on: 03-26-2008 05:20 AM CST (US). | | Nice! I should note that there are thermal breakers available in somewhat more elegant TO-220 packages: http://uk.farne...d=en/212179.xml Datasheet: http://www.farne...eets/112733.pdf [Edited by Dougigs on 03-26-2008 at 05:23 AM.] |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#12] posted on: 03-26-2008 05:53 AM CST (US). | | Yup, those would work. Instead of FastOn connectors, you could use a 3P Molex KK .100 series plug (leave the center position unused) to wire these up without having to solder to the TO-220 pins. Digikey and Newark carries these in the US. Part number 67L075 or 67L080 would be good to use. |
| masantos HeadWizer Joined: Dec. 13, 2005 | Message [#13] posted on: 03-26-2008 06:46 AM CST (US). | | Amb, looks great! This kind of circuit is very useful indeed. There's one thing I think could be an useful addition and should be simple to implement, which is a slow start circuit. it could be optional and shouldn't add many parts and board size right? What's your opinion? |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#14] posted on: 03-26-2008 07:39 AM CST (US). | |
It depends on how you want to implement soft-start. The simplest method is to use an ICL (inrush current limiter) in series with the AC line. This is basically a NTC thermistor. When the amp is off, the ICL is cold and has some resistance. When you turn the power on, the resistance will absorb the turn-on surge. The ICL then warms up from the current flowing through it and its resistance drops to a nominal operating value. This scheme does not involve the ε24 circuit at all. It works best with a device that has constant current draw at or near its maximum, so that the ICL would always maintain a low resistance after warm-up. On an amp such as this one, the difference between "idle" and "maximum" is big, so it's hard to pick an appropriate ICL. Also, if you were to turn off the power and then turn in on again within a short time, the ICL might not have cooled down enough to provide adequate soft-start the second time. Another solution is to use a power resistor in series with the AC mains, which will absorb the turn-on surge just like the ICL would. After a few seconds, a relay is activated to bypass that resistor. This scheme does not have the drawbacks of the ICL, but it requires yet another power relay (separate from the one used with the ε24), plus a time-delay circuit. As you could imagine, that would add complexity to the ε24. The delay could itself be done in various ways, some requiring active circuitry and some not. At this point I don't think I want to add this feature into ε24. This is because we could simply use a slow-blow fuse in conjunction with a high-current relay to switch the power (e.g., I am using the Omron G8P-1A2T-F-DC12 with contacts rated at 30A). The relay contacts should handle the current surges well, and I don't think anything else need further protection. If you want some ideas about soft-start schemes, see this diyhifi.com thread. |
| Dougigs HeadWize Fanatic Joined: Sep. 21, 2005 | Message [#15] posted on: 03-26-2008 07:54 AM CST (US). | | Masantos, Rod Elliott sells a nice PCB for exactly this soft-start circuit - - I use it in my power conditioner (which has a 1500VA toroid and therefore big inrush problems), and it works great. Might be worth using in this amp. |
n_maher HeadWize Fanatic Joined: Dec. 15, 2004 | Message [#16] posted on: 03-26-2008 07:56 AM CST (US). | | Ti, I would think that there would be enough interest to warrant a run of boards. I know that I've often shied away from momentary switches simply because of the lack of an elegant implementation. |
| thrice Member Joined: Mar. 20, 2004 | Message [#17] posted on: 03-26-2008 08:28 AM CST (US). | | I second a run of boards. I know for certain that I could use multiples of these. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#18] posted on: 03-26-2008 10:25 AM CST (US). | | Here is a preliminary BOM: AMB ε24 BOM Qty / Ref / Description / Recommended part(s) / Mouser, Digikey or other part # Notes: [Edited by amb on 04-21-2008 at 11:42 PM.] |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#19] posted on: 03-26-2008 11:58 AM CST (US). | | OK, I went ahead and ordered 20 boards. I believe there is enough interest that I wouldn't be sitting on them forever. ETA is around April 9. PM me if you want some boards. I will add you to my list, first-come, first-served. Thanks. |
| fierce_freak Member Joined: Jun. 4, 2007 | Message [#20] posted on: 03-26-2008 12:11 PM CST (US). | | I have a build planned with a 24vac transformer that will have an unused secondary. Would it be ok to use that secondary to power the e24, or will there be too much heat created? If it's possible, I'd be in for 2-3 boards. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#21] posted on: 03-26-2008 12:15 PM CST (US). | |
If you're planning to use this for a power switch, then the transformer must be always-on. If that transformer is supposed to power the main circuit, which you want the ε24 to turn on/off, then you've got a problem...  |
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