This may not be 100% accurate. It is my attempt to reverse-engineer the power supply based on some old notes I have laying around. I drew it using OpenOffice.org’s “Draw” program and I was learning it as I went, so please forgive any clunkiness in the drafting.
Some History
I originally designed the power supply to be a switching supply because they are much more efficient than linear supplies, and thus run a lot cooler. However, the sales and marketing people balked at the cost, so I was instructed to essentially copy the power supply of the first generation Betalert 10 … and so I did. I was also instructed to, whenever possible, use parts that we already used on other things. This saved money by (1) saving the cost of taking out new part numbers, (2) not having to stock additional parts, and (3) increasing the volume of certain parts and thus driving down their unit cost.
Anyway, as you can see, it is a pretty straightforward two-output supply based on a dual-secondary transformer and two linear power regulators. These regulators were about as cheap as any component could possibly be. However, they were notoriously inefficient and can get quite hot (hence the heat sink). Right around the time I was designing the Betalert 10B, Linear Technology was coming out with a line of simple switching regulators that I wanted to use. However, they were about five times the cost of the linear regulators. They also had the curse of being “new fangled” and so weren’t given a warm welcome by an industry as staid and as slow-changing as the power generation industry. So, I was told to ditch the switching regulators and go with the linear regulator supply.
Transient Voltage Suppression
C27 and C28 are high-voltage capacitors used transient voltage suppression. Note their very high voltage rating of 6000 volts and relatively low capacitance (4700pF). This should allow them to capture relatively fast high-voltage transients. Note that these may degrade over time if they take a lot of transient spikes. They’re a prime candidate for replacement if you are refurbishing an old unit (and let’s face it, ALL of the units are old now!). The second line of defense for high-voltage, high-frequency transients is the transformer.
Transformer
The transformer, T1, is a split-primary, dual-secondary transformer. I don’t have any information on this. I think it was custom designed for the Betalert 10 (not 10B!) and since it was a part we were already using in large quantities on the BL10, I decided to use it on the BL10B. I can’t remember what the secondary output voltages are. I’ll get around to measuring them one of these days. I suspect the pin 6-7 tap is around 5 or 6 VAC and the pin 8-10 secondary is about 20-ish VAC.
Note the presence of the three jumper positions on the primary of the transformer. These allow the unit to be jumpered for use with either 117VAC or a 220VAC input voltage. For 117VAC operation, install jumpers A and B. For 220VAC operation, remove jumpers A and B and install Jumper C.
NOTE: The factory default was for 117VAC operation, and so jumpers A and B are permanently “installed” using PWB etch. If you wish to convert a unit to operate at 220VAC, you will need to cut the etches on jumpers A and B and then install Jumper C.
Bridge Rectifiers
Bridge rectifiers CR29 and CR30 are DB-103 type rectifiers that come in a 4-pin DIP package and are rated at 200PRV and 1 Amp. I was shocked to find that you can still buy these at places like Mouser and Digikey.
http://www.mouser.com/search/refine.aspx?N=4294966144&Keyword=DB-103
http://parts.digikey.com/1/parts/432297-rectifier-bridge-gp-200v-1a-db-1-db103.html
I can’t remember who I had specified as the original manufacturer of these parts, but since the part seems to be something of an industry standard, nearly any reputable manufacturer’s part should work as long as the ratings are equal to or better than 200PRV and 1 Amp. Verify that the pinout is the same, too! It will be very bad if they’re not.
5V Regulator
The 5V regulator is a standard 3-pin 7805 type regulator in a DO-220 package. This is very common part and can be gotten at places like Digikey and Mouser. Just do a search on “MC7805CT” and any number of them should pop up. The one I designed in was made by Motorola, but they got out of the semiconductor business several decades ago. However, any 7805 regulator from a reputable company should work just fine.
The 5V regulator has to dissipate a lot of heat, so if you replace it, make sure you re-install the heat sink. Apply thermal compound to two places:
1. Between the regulator tab (ground) and the heatsink
2. Between the heat sink and the PWB ground pad
After soldering in the regulator and applying the thermal compound, line up the screw holes in the regulator, the heatsink, and the PWB. Install using a #4 screw, lockwasher, and nut. (Note: You can use an even bigger heatsink if you want. Use any standard DO-220 heatsink. Just make sure it doesn’t physically touch any other components, the metal case of the unit, or any non-ground PWB etches.)
The input to the 5V regulator is filtered with C25, which is a 3300uF, 35V aluminum electrolytic capacitor. Aluminum electrolytics tend to become electrically “leaky” as they age, so they are always a good candidate for replacement when refurbishing old electrical equipment. If you replace it, though, make sure you use a capacitor with a 3300uF (or higher) capacitance rating and a 35V (or higher) voltage rating. If replacing this capacitor with a physically bigger device, make sure that it doesn’t touch any nearby components.
If I was designing this supply again today, I would also add a 0.1uF, 10V ceramic capacitor between pins 2 (ground) and 3 (output) of the regulator as close as possible to the two pins.
NOTE: It has recently come to my attention that the 5V regulator section was redesigned after I left the employ of Hathaway/Beta. The 7805 regulator was replaced with a LM2925T. As best I can tell, this is a 5V regulator (similar to the 7805), but with a RESET_N output that can be used to put an external device, such as a microprocessor, in reset when the output voltage is out of tolerance. I presume the reset pulse width can be programmed through the use of external components. At any rate, I don’t have any information on either the updated BL10B schematic or the LM 2925T itself, so you’re on your own if you need to service this. If I get more information, I will post it. Or, if you have any information about it, feel free to email it to me.
24V Regulator
The 24V regulator is a standard 3-pin 7824 type regulator in a DO-220 package. This is very common part and can be gotten at places like Digikey and Mouser. Just do a search on “MC7824CT” and any number of them should pop up. The one I designed in was made by Motorola, but they got out of the semiconductor business several decades ago. However, any 7824 regulator from a reputable company should work just fine.
The 24V regulator doesn’t have to dissipate as much heat as the 5V regulator, so a heatsink isn’t strictly necessary. However, if you replace it or are refurbishing a unit, it is a good idea to reapply some thermal compound between the regulators ground tab and the PWB. When done, line up the screw holes in the regulator and the PWB. Install using a #4 screw, lockwasher, and nut.
The input to the 24V regulator is filtered with C26, which is a 100uF, 50V aluminum electrolytic capacitor. Aluminum electrolytics tend to become electrically “leaky” as they age, so they are always a good candidate for replacement when refurbishing old electrical equipment. If you replace it, though, make sure you use a capacitor with a 100uF (or higher) capacitance rating and a 50V (or higher) voltage rating. If replacing this capacitor with a physically bigger device, make sure that it doesn’t touch any nearby components.
If I was designing this supply again today, I would also add a 0.1uF, 10V ceramic capacitor between pins 2 (ground) and 3 (output) of the regulator as close as possible to the two pins.
