EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: dcbrown73 on August 16, 2019, 03:52:16 pm
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Hey all,
I'm assembling an Elenco XP-720 power supply kit. There is a MPSA70 PNP transistor (https://www.mouser.com/ProductDetail/Central-Semiconductor/MPSA70?qs=UMEuL5FsraCzv3%2Fgo%252BqDMA%3D%3D) and after soldering it in. I wanted to ensure I didn't create any bridges. So I used my DMM's continuity tester and I show continuity between all three legs. I don't (even using a magnifying glass) see any bridges, so I wanted to ask if this positive continuity test is correct?
(https://snag.gy/UEOH1A.jpg)
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I had to look twice when I saw that. We used to design with that transistor and I don't remember it as being able to take that much current. Well, it is a very low voltage. You should be seeing what passes for continuity on all three legs.
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Hi reading near to zero ohm is correct. This transistor is taking care of the current protection. As the values of th resistors in the circuit are very low it is hardly possible to read the values under normal situations
Regards, Eric
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I had to look twice when I saw that. We used to design with that transistor and I don't remember it as being able to take that much current. Well, it is a very low voltage. You should be seeing what passes for continuity on all three legs.
When I test it with the continuity tester, I get the following:
- Emitter -> Base: 3.1 ohm
- Emitter -> Collector: 5.8 ohm
- Base -> Collector: 2.9 ohm
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Hi reading near to zero ohm is correct. This transistor is taking care of the current protection. As the values of th resistors in the circuit are very low it is hardly possible to read the values under normal situations
Regards, Eric
Thank you!
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I had to look twice when I saw that. We used to design with that transistor and I don't remember it as being able to take that much current. Well, it is a very low voltage. You should be seeing what passes for continuity on all three legs.
When I test it with the continuity tester, I get the following:
- Emitter -> Base: 3.1 ohm
- Emitter -> Collector: 5.8 ohm
- Base -> Collector: 2.9 ohm
Those readings seem to be correct.
Most modern continuity testers us such a low voltage that the transistor itself will show as an open circuit or extremely high resistance of several MΩ.
For example, from the emitter to collector you'll get the values of R3 to R5 added in series, which is 0.18+2.7+2.7 = 5.58, which is pretty close to what you've measured, which will be a bit higher due to the resistance of the test leads. You'll probably find it doesn't read exactly zero, even if you short the test leads of the continuity tester.
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Note the PSU is missing protection diodes for the voltage reg IC's. It's in the LM317 datasheet- (https://www.onsemi.com/pub/Collateral/LM317-D.PDF) Fig. 20; one across the Vreg, one across 10uF cap, and another at the output to cover reverse polarity. You need these for a bench power supply that can take some abuse, and charge batteries. I would add them in at some point as I have wrecked every power supply that does not have them.
The 5V 3A current-limiter circuit with Q1 has R4 and R3 very low in value at 2.7 ohms so in-circuit readings would be confusing.
MSPA70 is a bit of an oddball part and substitute would be a 2N3906 or BC557B (reverse pinout). Sometimes manufacturers use whatever they have lots of laying around.
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Note the PSU is missing protection diodes for the voltage reg IC's. It's in the LM317 datasheet- (https://www.onsemi.com/pub/Collateral/LM317-D.PDF) Fig. 20; one across the Vreg, one across 10uF cap, and another at the output to cover reverse polarity. You need these for a bench power supply that can take some abuse, and charge batteries. I would add them in at some point as I have wrecked every power supply that does not have them.
The 5V 3A current-limiter circuit with Q1 has R4 and R3 very low in value at 2.7 ohms so in-circuit readings would be confusing.
MSPA70 is a bit of an oddball part and substitute would be a 2N3906 or BC557B (reverse pinout). Sometimes manufacturers use whatever they have lots of laying around.
I would just directly attach the 1n4002 directly across pins 2 & 3 of the LM317?
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Note the PSU is missing protection diodes for the voltage reg IC's. It's in the LM317 datasheet- (https://www.onsemi.com/pub/Collateral/LM317-D.PDF) Fig. 20; one across the Vreg, one across 10uF cap, and another at the output to cover reverse polarity. You need these for a bench power supply that can take some abuse, and charge batteries. I would add them in at some point as I have wrecked every power supply that does not have them.
The 5V 3A current-limiter circuit with Q1 has R4 and R3 very low in value at 2.7 ohms so in-circuit readings would be confusing.
MSPA70 is a bit of an oddball part and substitute would be a 2N3906 or BC557B (reverse pinout). Sometimes manufacturers use whatever they have lots of laying around.
I would just directly attach the 1n4002 directly across pins 2 & 3 of the LM317?
Only on the lm317.The protection diodes for the lm337 will be different.Opposite direction I mean
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For the LM317 and LM337, total six protection diodes gives full coverage for the IC's. 1N4002 is OK, I like bigger 1N5402 across the output in case I connect a battery backwards as they are harder to smoke.
Schematic for them just like here: https://sound-au.com/project44.htm (https://sound-au.com/project44.htm)
The LM7805 should not get abused as such powering digital circuits. A diode across C-E of Q2 2N6124, and a reverse polarity diode across the output would cover it.